Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignment of 5-chloropent-2-yne

Infrared and Raman spectra, conformational stability, ab initio

calculations, and vibrational assignment of 5-chloropent-2-yneq

Gamil A. Guirgis, Barry R. Drew, Nida J. Luangjamekorn, Shiyu Shen, James R. Durig*

Department of Chemistry, University of Missouri-Kansas City, 5100 Rochill Road, 64110-2499 Kansas City, MO, USA

Received 8 January 2002; accepted 31 January 2002

Abstract

The infrared (3400–50 cm21) and/or Raman (3400–10 cm21) spectra of gaseous, xenon solution, liquid and solid 5-

chloropent-2-yne, CH2ClCH2CCCH3, have been recorded. These data indicate that the molecule exists in the anti (the C–Cl

bond is trans to the CxC bond) and the gauche conformations in the vapor and liquid but only the anti conformer remains in the

solid state. From a variable temperature infrared study of the xenon solution, the anti conformation has been determined to be

more stable than the gauche form by 233 ^ 23 cm21 (2.79 ^ 0.28 kJ/mol) and it is estimated that 39% of the sample is in the

gauche form at ambient temperature. The optimized geometries, conformation stabilities, harmonic force fields, Raman

activities, depolarization ratios, and infrared intensities have been obtained from ab initio MP2/6-31G(d) calculations with full

electron correlation. These predicted quantities are compared to the corresponding experimental quantities when appropriate.

Equilibrium geometries and energies for both conformers have been obtained from ab initio MP2/6-311G(d,p), MP2/6-

311G(2d,2p) and MP2/6-311G(2df,2pd) calculations. Vibrational assignments for the 24 normal modes for the anti conformer

are proposed and several of the fundamentals for the gauche conformer are assigned. The sub-band structure on the pseudo-

degenerate vibrations of the methyl group indicates that it is almost free internal rotation. From this fine structure, the Coriolis

coupling constants, j, have been determined. These experimental and theoretical results are compared to the corresponding

quantities of some similar molecules. q 2002 Elsevier Science B.V. All rights reserved.

Keywords: Infrared and Raman spectra; Conformational stability; Xenon solutions; Ab initio calculations; 5-Chloropent-2-yne

1. Introduction

The 1,2-disubstituted ethane molecules, XCH2-

CH2Y, have been of interest to chemists for a number

of years both because of their structural parameters as

well as their conformational stabilities. For example,

when X and Y are the same atoms or groups, there are

significant differences in their conformational stab-

ilities. If X and Y are both a chlorine atom,

ClCH2CH2Cl, or a methyl group, CH3CH2CH2CH3

(n-butane), which have similar sizes, then the more

stable conformer is the trans form [1,2]. However, if

X is a chlorine atom and Y is a methyl group,

ClCH2CH2CH3 (1-chloropropane), then the gauche

conformer is the more stable form [3] by 52 ^ 3 cm21

(0.62 ^ 0.04 kJ/mol). In contrast, the 1,2-difluor-

oethane, FCH2CH2F [4], and 1-fluoropropane, FCH2-

CH2CH3 [5], molecules both have the gauche

0022-2860/02/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.

PII: S0 02 2 -2 86 0 (0 2) 00 1 25 -4

Journal of Molecular Structure 613 (2002) 15–35

www.elsevier.com/locate/molstruc

q Taken in part from the dissertation of B.R. Drew, which has

been submitted to the Department of Chemistry in partial fulfillment

of the PhD degree.* Corresponding author. Tel.: þ1-816-235-6038; fax: þ1-816-

235-2290.

E-mail address: [email protected] (J.R. Durig).

http://www.elsevier.com/locate/molstruc

conformer as the more stable form whereas 1-fluoro-

2-chloroethane, FCH2CH2Cl [6], has the trans con-

former as the more stable form by 126 ^ 8 cm21

(1.51 ^ 0.09 kJ/mol). It is clear that the size of the X

or Y substituent is not the dominant factor in

determining the stability of the 1,2-disubstituted

ethanes since FCH2CH2Cl and FCH2CH2CH3 have

nearly the same size substituents, yet they have

different conformers as their more stable rotamers.

As a continuation of our conformational studies we

have investigated the conformational stability of 5-

chloropent-2-yne, ClCH2CH2CCCH3, with Y the

propyne group, CxCCH3. To the best of our knowl-

edge, there have been no previous spectroscopic

studies reported for this molecule. Therefore, we have

recorded the Raman spectra of the liquid and solid

along with the infrared spectra of the gas, solid and

xenon solution using variable temperatures. We have

also carried out ab initio calculations employing the 6-

31G(d) basis set at the level of Moller–Plesset to the

second order (MP2) by the perturbation method with

full electron correlation to obtain equilibrium geome-

tries, force constants, vibrational frequencies, infrared

and Raman intensities, and conformational stabilities.

Structural parameters and conformational stabilities

have also been obtained from the larger basis sets of 6-

311G(d,p), 6-311G(2d,2p), and 6-311G(2df,2pd) at

the MP2 level also with full electron correlation. The

results of this spectroscopic and theoretical study are

reported herein.

2. Experimental section

The sample of 5-chloropent-2-yne was prepared by

the reaction of 2-pentyn-5-ol with thionyl chloride,

SO2Cl2, in pyridine at 70 8C for 2 h. The sample was

frozen with boiling liquid nitrogen and degassed. It

was then warmed to room temperature and the volatile

material was collected, washed first with 5% sodium

bicarbonate, then with distilled water. The sample was

finally purified by a low-temperature and low-

pressure sublimation column. The purity of the

sample was checked with NMR and mass spectral

data.

The mid-infrared spectra (Fig. 1) of the gas and

solid were recorded on a Perkin–Elmer model 2000

Fourier transform spectrometer equipped with a Ge/

CsI beamsplitter and DTGS detector. Atmospheric

water vapor was removed from the spectrometer

housing by purging with dry nitrogen. The spectrum

of the gas was obtained with the sample contained in a

10 cm cell. The spectrum of the solid was obtained by

condensing the sample on a CsI substrate held at the

Fig. 1. Mid-infrared spectra of 5-chloropent-2-yne: (A) gas; (B) annealed solid.

G.A. Guirgis et al. / Journal of Molecular Structure 613 (2002) 15–3516

temperature of boiling liquid nitrogen, housed in a

vacuum cell fitted with CsI windows. The sample was

condensed as an amorphous solid and repeatedly

annealed until no further changes were observed in the

spectra.

The mid-infrared spectrum of the sample dissolved

in liquified xenon (Fig. 2) was recorded as a function

of temperature on a Bruker model IFS Fourier

transform spectrometer equipped with a globar

source, a Ge/KBr beamsplitter and a DTGS detector.

For all spectra, 100 interferograms were collected at

1.0 cm21 resolution, averaged and transformed with a

boxcar truncation function. For these studies, a

specially designed cryostat cell was used. It consists

of a copper cell with a path length of 4 cm with a

wedged silicon windows sealed to the cell with

indium gasket. The cell was cooled by boiling liquid

nitrogen and the temperature was monitored with two

Pt thermoresistors. The complete cell was connected

to a pressure manifold, allowing the filling and

evacuation of the cell. After the cell has cooled to

the desired temperature, a small amount of the

compound was condensed into the cell. Next, the

pressure manifold and the cell were pressurized with

the noble gas, which immediately started to condense

in the cell, allowing the compound to dissolve.

The Raman spectra were recorded on a SPEX

model 1403 spectrometer equipped with a Spectra-

Physics model 164 argon ion laser operating on the

514.5 nm line. The laser power used was 0.5 W for the

liquid and the solid with a spectral band pass of

3 cm21. The spectrum of the liquid was recorded with

the sample sealed in a Pyrex glass capillary.

Depolarization measurements were obtained for the

liquid sample using a standard Ednalite 35 mm

camera polarizer with 38 mm of free aperture affixed

to the SPEX instrument. Depolarization ratio

measurements were checked by measuring the state

of polarization of the Raman bands of carbon

tetrachloride immediately before depolarization

measurements were made on the liquid sample. The

Raman frequencies are expected to be accurate to

^2 cm21 and typical spectra are shown in Fig. 3.

The far infrared spectrum of gaseous 5-chloropent-

2-yne (Fig. 4) was recorded on a Bomem model

DA3.002 Fourier transform spectrometer equipped

with a vacuum bench, 6.25 and 12.5 m mylar

beamsplitters, and a liquid helium-cooled Si bol-

ometer. The spectrum was obtained with an effective

resolution of 0.10 cm21 from the sample contained in

a 1 m folded path cell equipped with mirrors coated

with gold, and fitted with polyethylene windows. The

spectra of the amorphous and crystalline solids (Fig.

4) were obtained with a Perkin–Elmer model 2000

infrared Fourier transform spectrometer equipped

with a metal grid beamsplitter and a DTGS detector.

The sample was deposited on a Si substrate held at

77 K with boiling liquid nitrogen which was con-

tained in a cryostat cell equipped with polyethylene

windows. All of the bands of significant intensity

observed in the infrared and Raman spectra with their

proposed assignments are available from the authors

and the wavenumbers for the fundamentals for both

the anti and gauche forms in the gaseous, solution in

liquid xenon, liquid and solid states are listed in

Tables 1 and 2.

Fig. 2. Infrared spectra of 5-chloropent-2-yne: (A) experimental

spectrum of 5-chloropent-1-yne in liquified xenon at 275 8C; (B)

MP2/6-31G(d) ab initio calculated spectrum of the anti and gauche

mixture with a DH of 233 cm21 (C) calculated for the pure gauche

conformer; and (D) calculated for the pure anti conformer.

G.A. Guirgis et al. / Journal of Molecular Structure 613 (2002) 15–35 17

3. Ab initio calculations

Ab initio calculations were performed with the

GAUSSIAN 98 program [7] using Gaussian-type basis

functions. The energy minima with respect to nuclear

coordinates were obtained by simultaneous relaxation

of the geometry parameters using the gradient method

of Pulay [8]. The structural optimizations for the anti

and gauche conformers were carried out with initial

parameters taken from those obtained from the ab

initio MP2/6-31G(d) calculations of 1-butyne [9] and

1-chloropropane [3]. The structural parameters as

determined by different basis sets for the anti and

gauche conformers of 5-chloropent-2-yne are listed in

Table 3.

The energies obtained by these ab initio calcu-

lations of the anti and gauche conformers are given in

Table 3. The anti conformer is the more stable form in

every case, with the gauche conformer having higher

energy by 393 cm21 than the anti form from the MP2/

6-31G(d) calculation. The lowest estimate of this

energy difference is 292 cm21 which was obtained

from the MP2/6-311 (2df,2pd) calculation.

The force fields in Cartesian coordinates were

obtained by the GAUSSIAN 98 program [7] from the

MP2/6-31G(d) calculation. Internal coordinates (Fig.

5) were used to calculate the G and B matrices using

the structural parameters given in Table 3. Using the B

matrix [10], the force fields in Cartesian coordinates

were then converted to force fields in internal

coordinates, and the pure ab initio vibrational

frequencies were reproduced. The force constants

for the two conformers can be obtained from the

authors. Subsequently, scaling factors of 0.88 for C–

H stretches, 0.9 for heavy atom stretches and C–H

bends, 1.4 for the CxC–C in-plane bend, 1.5 for

CxC–C out-of-plane bend and 1.0 for CxC stretch

Fig. 3. Raman spectra of 5-chloropent-2-yne: (A) liquid and (B) solid.

Fig. 4. Far infrared spectra of 5-chloropent-2-yne: (A) gas; (B)

amorphous solid; and (C) polycrystalline solid. Bands marked with

an asterisk are due to HCl impurity.


Table 1

Observed and calculated frequencies (cm21) and PEDs for the anti conformer of 5-chloropent-2-yne

Species Vib.

No.

Fundamentala Ab

initiob

Fixed

scaledc

IR

int.dRaman

act.dIR

gas

IR solid Raman

liquid

Raman

solid

Xe

soln.

PED

A0 n1 CH3 antisymmetric

stretch

3190 2992 8.0 85.2 2976 2976 2965 2977 2977 100S1

n2pCH2 symmetric stretch 3166 2970 13.1 58.3 2971 2960 2962 2963 2962 100S2

n3 CH2 symmetric stretch 3108 2916 10.2 94.2 2937 2917 2922 2917 2937 99S3

n4 CH3 symmetric stretch 3104 2912 18.1 222.2 2922 2917 2922 2922 2923 99S4

n5 CxC stretch 2311 2289 1.9 138.3 2241 2233 2236 2233 2241 83S5

n6pCH2 deformation 1552 1473 4.9 3.9 1458 1446 1449 1447 1450 80S6, 20S8

n7 CH3 antisymmetric

deformation

1546 1466 7.7 20.2 1453 1431 1439 1431 1441 93S7

n8 CH2 deformation 1543 1465 0.9 24.5 – 1431 1439 1431 1441 79S8, 20S6

n9 CH3 symmetric defor-

mation

1472 1396 2.5 35.0 1377 1383 1380 1381 1379 97S9

n10 CH2 wag 1419 1347 3.9 10.7 1334 1338 1339 1335 1336 45S10, 46S11

n11pCH2 wag 1320 1253 28.1 5.6 1247 1254 1248 1250 1244 55S11, 39S10

n12 C4C5 stretch 1203 1142 0.8 0.3 1143 1142 1142 1143 1144 41S12, 42S16, 12S10

n13 C1C2 stretch 1089 1043 2.4 4.3 1035 1032 1025 1032 1027 64S13, 19S14

n14 CH3 rock 1073 1023 1.5 1.1 1027 – 1025 1019 1027 71S14, 18S13

n15 CCl stretch 833 806 31.4 20.6 794 796 795 794 794 38S15, 19S17, 15S16,

14S18

n16 C2C3 stretch 757 722 22.4 6.0 712 699 705 701 708 25S16, 44S15, 22S12,

10S5

n17 C1C2C3 bend 417 434 4.4 6.8 451 451 458 452 456 33S17, 40S20, 11S15

n18 CCCl bend 281 279 3.7 3.6 – 301 294 298 – 68S18, 10S15

n19 C2C3C4 in-plane bend 232 269 7.0 2.7 268 279 274 279 – 100S19

n20 C3C4C5 in-plane bend 84 93 0.01 1.1 – – – – – 63S20, 34S17

A00 n21pCH2 antisymmetric

stretch

3237 3036 5.6 33.9 3010 3019 3009 3018 3006 98S21


stretch

3191 2993 7.8 92.0 2976 2976 2965 2977 2977 100S22


stretch

3159 2964 4.2 77.4 – 2953 2950 2950 2938 99S23


deformation

1545 1466 6.6 20.9 1453 1431 1439 1431 1441 93S24

n25 CH2 twist 1348 1279 0.02 14.1 – 1281 1278 1281 1283 48S25, 40S26

n26pCH2 twist 1214 1154 1.2 1.6 1150 1158 1158 1158 1147 48S26, 43S25

(continued on next page)

G.A

.G

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Mo

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tructu

re6

13

(20

02

)1

5–

35

19

and all other coordinates along with the geometric

average of the scaling factors for interaction force

constants were used to obtain the fixed scaled force

field and resultant wavenumbers (Tables 1 and 2). The

set of symmetry coordinates is given in Table 4 and

they were used to determine the corresponding

potential energy distribution (PED). The calculated

infrared intensities, Raman activities and PEDs are

also given in Tables 1 and 2.

Infrared spectra were calculated based on the

dipole moment derivatives with respect to the

Cartesian coordinates. The derivatives were taken

from the ab initio calculations at the MP2/6-31G(d)

level and transformed to normal coordinates by

›mu

›Qi

� �¼X

j

›mu

›Xj

!Lij

where Qi is the ith normal coordinate, Xj is the jth

Cartesian displacement coordinate and the Lji is the

transformation matrix between Cartesian displace-

ment coordinates and normal coordinates. The

infrared intensities were then calculated by:

Ii ¼Np

3c2

›mx

›Qi

� �2

þ›my

›Qi

� �2

þ›mz

›Qi

� �2" #

:

The predicted infrared spectra of the pure anti and the

gauche conformers are shown in Fig. 2D and C,

respectively. The combination of the two spectra for

the conformers with DH of 233 cm21 between the

most stable anti conformer and the less stable gauche

rotamer is shown in Fig. 2B. The DH value used was

taken from the temperature dependent measurements

of the xenon solution performed in this study. The

predicted infrared spectrum was very useful for

identifying the bands due to the two conformers in

the infrared spectrum of the sample dissolved in liquid

xenon.

The predicted Raman spectra (Fig. 6) for the two

conformers of 5-chloropent-2-yne and the room

temperature mixture were calculated using scattering

activities determined from the MP2/6-31G(d) ab initio

calculations. The GAUSSIAN 98 program [7] with the

option of calculating the polarizability derivatives

was used. The evaluation of Raman activity by using

the analytical gradient methods has been developedTab

le1

(co

nti

nued

)

Sp

ecie

s

Vib

.

No

.F

un

dam

enta

la

Ab

init

iob

Fix

ed

scal

edc

IR int.

d

Ram

an

act.

d

IR gas

IRso

lid

Ram

an

liq

uid

Ram

an

soli

d

Xe

soln

.P

ED

n27

CH

3ro

ck1

08

41

03

60

.50

.41

03

51

03

7–

––

64S

27,

12S

28,

11S

29

n28

pC

H2

rock

10

56

10

05

1.3

2.2

99

91

00

0–

10

00

99

93

2S

28,

30S

29,

25S

27

n29

CH

2ro

ck7

94

75

51

.80

.87

53

76

1/7

53

74

97

52

75

04

5S

29,

43S

28

n30

C3C

4C

5o

ut-

of-

pla

ne

ben

d

28

43

35

0.9

11

.6–

–3

74

38

0–

66S

30,

20S

31,

12S

32

n31

C2C

3C

4o

ut-

of-

pla

ne

ben

d

21

52

55

9.2

0.4

22

32

47

–2

41

–7

9S

31,

10S

32

n32

Asy

mm

etri

cto

rsio

n8

08

41

.10

.16

6–

––

–7

5S

32,

23S

30

n33

CH

3to

rsio

n1

31

30

.03

0.1

––

––

–1

00S

33

aT

he

aste

risk

on

the

CH

2g

rou

pin

dic

ates

the

gro

up

wit

hth

eC

lat

om

atta

ched

.b

Fro

mM

P2/6

-31(d

)ca

lcula

tion.

cS

cali

ng

fact

or

of

0.8

8fo

rC

–H

stre

tch

es,

0.9

for

hea

vy

ato

mst

retc

hes

and

C–

Hb

end

s,1

.4fo

rth

eCx

C–

Cin

-pla

ne

ben

d,

1.5

for

Cx

C–

Co

ut-

of-

pla

ne

ben

dan

d1

.0fo

rCx

C

stre

tch

and

all

oth

erco

ord

inat

es.

dIn

frar

edin

tensi

ties

inkm

/mol

and

Ram

anac

tivit

ies

inA

4/a

mu

from

MP

2/6

-31(d

)ca

lcula

tion.


Table 2

Observed and calculated frequencies (cm21) and PEDs for the gauche conformer of 5-chloropent-2-yne

Species Vib. No. Fundamentala Ab initiob Fixed

scaledc

IR int.d Raman act.d IR gas Raman liquid Xe soln. PED

A0 n1 CH3 antisymmetric stretch 3189 2992 8.7 88.6 2976 2965 2977 51S1, 49S22

n2pCH2 symmetric stretch 3156 2961 22.2 87.9 – 2952 2962 85S2, 14S23

n3 CH2 symmetric stretch 3090 2899 14.7 139.4 – 2908 2900 93S3

n4 CH3 symmetric stretch 3104 2911 20.6 175.3 – 2922 2938 100S4

n5 CxC stretch 2314 2292 2.1 107.7 2241 2292 2241 83S5

n6pCH2 deformation 1543 1465 5.1 9.1 – 1439 1441 97S6

n7 CH3 antisymmetric deformation 1546 1466 6.1 22.6 – 1439 1441 69S7, 22S24

n8 CH2 deformation 1531 1452 5.7 17.9 – 1425 1424 98S8

n9 CH3 symmetric deformation 1472 1396 2.2 31.4 1377 1380 1379 97S9

n10 CH2 wag 1414 1343 15.1 20.7 1305 – 1330 64S10, 14S26

n11pCH2 wag 1396 1325 31.5 2.2 1297 1299 1300 86S11, 12S25

n12 C4C5 stretch 1204 1143 1.2 1.1 1143 1144 1147 34S12, 35S16

n13 C1C2 stretch 1088 1040 1.9 2.6 1034 1025 1027 18S13, 37S14, 16S28, 12S29

n14 CH3 rock 1070 1019 2.2 2.0 1027 1025 1027 50S14, 19S13, 12S28

n15 CCl stretch 732 698 10.9 5.6 682 – 680 46S15, 13S12, 12S16, 11S29

n16 C2C3 stretch 705 675 9.9 8.2 669 659 666 16S16, 37S15, 20S12

n17 C1C2C3 bend 504 511 8.1 3.1 515 514 512 40S17, 15S20, 11S18

n18 CCCl bend 318 334 2.1 5.0 341 346 – 27S18, 27S20, 16S32, 10S30

n19 C2C3C4 in-plane bend 231 266 3.7 3.5 268 – – 92S19

n20 C3C4C5 in-plane bend 131 146 4.6 1.7 130 – – 64S20, 23S32, 15S17

n21pCH2 antisymmetric stretch 3225 3026 7.9 64.5 – 3015 – 98S21

n22 CH3 antisymmetric stretch 3190 2993 7.7 79.6 – 2965 2977 51S22, 49S1

n23 CH2 antisymmetric stretch 3144 2949 3.6 106.4 – 2951 – 80S23, 14S2

n24 CH3 antisymmetric deformation 1546 1466 6.5 21.2 – 1439 1441 71S24, 23S7

n25 CH2 twist 1285 1222 0.6 14.5 – 1220 1222 43S25, 28S26

n26pCH2 twist 1230 1168 0.6 4.4 – 1170 1168 41S26, 32S25, 10S10

n27 CH3 rock 1077 1029 2.3 0.5 1027 – 1027 86S27

n28pCH2 rock 940 904 3.8 2.0 899 894 896 29S28, 30S29, 10S17, 10S18

n29 CH2 rock 991 942 8.7 3.7 936 937 935 25S29, 33S13, 16S28

n30 C3C4C5 out-of-plane bend 257 298 1.4 8.5 – – – 21S30, 56S31, 16S18

n31 C2C3C4 out-of-plane bend 203 234 3.9 0.2 213 210 – 47S31, 20S18, 17S30


G.A

.G

uirg

iset

al.

/Jo

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al

of

Mo

lecula

rS

tructu

re6

13

(20

02

)1

5–

35

21

[11,12] and the activity Sj can be expressed as

Sj ¼ gj 45a2j þ 7b2

j

� where gj is the degeneracy of the vibrational mode j,

aj is the derivative of the isotropic polarizability, and

bj is that of the anisotropic polarizability. The Raman

scattering cross sections, ›s/›V which are pro-

portional to the Raman intensities, can be calculated

from the scattering activities and the predicted

frequencies for each normal mode using the relation-

ship [13,14]

›sj

›V¼

24p4

45

!ðn0 2 njÞ

4

1 2 exp2hcnj

kT

�0BBB@

1CCCA h

8p2cnj

!Sj

where n0 is the exciting frequency, nj is the vibrational

frequency of the jth normal mode, h, c and k are the

universal constants, and Sj is the corresponding

Raman scattering activity. To obtain the polarized

Raman scattering cross section, the polarizabilities are

incorporated in Sj by Sjð1 2 rjÞ=ð1 þ rjÞ where rj is

the depolarization ratio of the jth normal mode. The

Raman scattering cross section and calculated fre-

quencies were used together with a Lorentzian line

shape function to obtain the calculated spectrum. The

predicted Raman spectra of the anti and the gauche

conformers individually are shown in Fig. 6D and C,

respectively, along with the combination of the two

spectra with DH of 233 cm21 (Fig. 6B). The

agreement between the predicted spectrum (Fig. 6B)

and the observed one (Fig. 6A) is not as good as

usually found, partially because the DH value may be

significantly different in the liquid. Thus the predicted

intensities of several of the lines is obviously very

poor.

4. Vibrational assignment

The assignment of the carbon–hydrogen stretches

is rather challenging since the CH3 stretches are

predicted at nearly the same frequencies as the CH2

stretches, whereas those for the pCH2 group (methyl-

ene group with the chlorine atom attached) are

predicted at significantly different frequencies. How-

ever, because of the nearly free rotation of the methylTab

le2

(co

nti

nu

ed)

Sp

ecie

sV

ib.

No

.F

und

amen

tala

Ab

init

iob

Fix

ed

scal

edc

IRin

t.d

Ram

anac

t.d

IRg

asR

aman

liq

uid

Xe

soln

.P

ED

n32

Asy

mm

etri

cto

rsio

n6

67

21

.43

.15

6–

–4

5S

32,

37

S30,

14S

17

n33

CH

3to

rsio

n1

61

60

.10

.0–

––

10

0S

33

aT

he

aste

risk

on

the

CH

2gro

up

indic

ates

the

gro

up

wit

hth

eC

lat

om

atta

ched

.b

Fro

mM

P2

/6-3

1(d

)ca

lcu

lati

on

.c

Sca

ling

fact

or

of

0.8

8fo

rC

–H

stre

tches

,0.9

for

hea

vy

atom

stre

tches

and

C–

Hben

ds,

1.4

for

the

Cx

C–

Cin

-pla

ne

ben

d,

1.5

for

Cx

C–

Co

ut-

of-

pla

ne

ben

dan

d1

.0fo

rCx

C

stre

tch

and

all

oth

erco

ord

inat

es.

dIn

frar

edin

ten

siti

esin

km

/mo

lan

dR

aman

acti

vit

ies

inA

4/a

mu

fro

mM

P2

/6-3

1(d

)ca

lcu

lati

on

.


Table 3

Structural parameters (bond distances in A, angles in degrees, rotational constants in MHz, dipole moments in Debye, and energies in Hartree) for anti and gauche 5-chloropent-2-

yne from ab initio calculations

MP2/6-31G(d) MP2/6-311G(d,p) MP2/6-311G(2d,2p) MP2/6-311G(2df,2pd)

anti gauche anti gauche anti gauche anti gauche

r(C1–C2) 1.524 1.523 1.525 1.524 1.519 1.518 1.517 1.517

r(C2–C3) 1.462 1.460 1.462 1.461 1.461 1.460 1.454 1.453

r(C3xC4) 1.221 1.220 1.219 1.219 1.214 1.214 1.212 1.212

r(C4–C5) 1.461 1.461 1.462 1.461 1.461 1.461 1.454 1.454

r(C1–Cl) 1.786 1.783 1.783 1.781 1.796 1.793 1.780 1.778

r(C1–H1) 1.090 1.090 1.090 1.090 1.082 1.082 1.085 1.085

r(C1–H2) 1.090 1.091 1.090 1.091 1.082 1.082 1.085 1.085

r(C2–H3) 1.095 1.096 1.095 1.095 1.088 1.088 1.089 1.087

r(C2–H4) 1.095 1.098 1.095 1.097 1.088 1.090 1.089 1.087

r(C5–H5) 1.094 1.093 1.093 1.093 1.086 1.086 1.087 1.087

r(C5–H6) 1.093 1.093 1.093 1.093 1.086 1.086 1.087 1.087

r(C5–H7) 1.093 1.093 1.093 1.093 1.086 1.086 1.087 1.087

/C1C2C3 110.5 113.2 110.8 113.0 110.1 112.7 110.4 112.7

/C2C3C4 180.0 179.3 180.0 178.6 180.0 179.4 180.0 179.3

/C3C4C5 180.0 179.8 180.0 179.7 180.0 179.8 180.0 179.8

/C2C1Cl 110.8 111.7 110.6 111.0 110.7 111.3 110.6 111.3

/C2C1H1 110.9 110.7 110.6 110.5 111.1 111.1 111.0 111.0

/C2C1H2 110.9 110.4 110.6 110.4 111.1 110.8 111.0 110.8

/H1C1H2 109.0 109.2 109.5 109.8 109.2 109.7 109.1 109.5

/ClC1H1 107.6 107.4 107.8 107.6 107.3 106.9 107.4 107.0

/ClC1H2 107.6 107.2 107.8 107.4 107.3 106.9 107.4 107.0

/C1C2H3 109.2 109.1 109.2 108.9 109.3 109.1 109.2 109.0

/C1C2H4 109.2 107.4 109.2 107.7 109.3 107.6 109.2 107.8

/H3C2H4 106.9 106.8 107.2 107.4 107.2 107.2 107.0 107.1

/C4C5H5 110.9 111.1 110.8 110.9 110.8 111.0 110.9 110.9

/C4C5H6 110.9 110.8 110.8 110.7 110.8 110.7 110.9 110.8

/C4C5H7 110.9 110.9 110.8 110.7 110.8 110.8 110.9 111.0

/H5C5H6 108.0 108.0 108.2 108.2 108.1 108.1 108.0 108.0

/H5C5H7 107.9 108.0 108.1 108.2 108.1 108.1 108.0 108.0

/H6C5H7 107.9 107.9 108.1 108.1 108.1 108.0 108.0 108.0


G.A

.G

uirg

iset

al.

/Jo

urn

al

of

Mo

lecula

rS

tructu

re6

13

(20

02

)1

5–

35

23

group, one observes the strong, weak, weak alterna-

tions of the Q-branches (Fig. 7) for the essentially

degenerate CH3 antisymmetric stretch. The K ¼ 0

transitions must be either the 3010 or 2981 cm21 Q-

branch, since these are the two strongest ones (Fig. 7)

in this spectral region. The infrared spectrum of the

xenon solution seems more consistent with the choice

of the 2981 cm21 band as the K ¼ 0 transition. The

corresponding symmetric stretch, n4, is predicted to be

the strongest Raman line in this region with it being

observed at 2922 cm21 (2923 cm21 in the xenon

solution). With these assignments for the CH3 group,

the possible assignments for the two CH2 stretches are

significantly reduced to 2962 or 2950 cm21 for the

CH2 antisymmetric stretch. However, the 2962 cm21

seems more appropriately assigned as the pCH2

symmetric stretch based on the ab initio predictions,

particularly since the corresponding antisymmetric

stretch is observed at a rather high frequency of

3009 cm21 in the Raman spectrum of the liquid. This

leaves the CH2 symmetric stretch assigned at

2937 cm21, which appears to shift to a considerably

lower frequency in the solid.

There are some uncertainties in the assignments for

the CH3 and CH2 deformations. The pCH2 defor-

mation can be clearly assigned to the Q-branch at

1458 cm21, but the K ¼ 0 transition is either at

1460.4 or 1418.5 cm21 since these are the strongest

Q-branches. In the infrared spectrum of the xenon

solution there are bands at 1450, 1441, 1437 and

1424 cm21, so one is left with the problem of taking

the 1418.5 as the K ¼ 0 transition with a shift to

1424 cm21 in the xenon solution, or the 1460.4 Q-

branch as the K ¼ 0 transition which gives a rather

small difference with the ab initio predicted value of

1466 cm21. We have chosen this latter alternative

since the predicted infrared intensities of n7 and n24

indicate that they should give rise to the strongest

infrared band in this spectral region. Since the band

center will be below the K ¼ 0 transition Q-branch

(Table 5) the shift of the band center to 1454 in the gas

to 1450 cm21 in the krypton solution is consistent for

the rather small changes in the frequencies normally

found in going from the vapor to the rare gas solution.

This assignment indicates that the earlier assignment

of this fundamental for the corresponding fluoride

molecule, 5-fluoropent-2-yne, is in error, since the

1418.66 cm21 Q-branch was chosen as the K ¼ 0Tab

le3

(co

nti

nued

) MP

2/6

-31

G(d

)M

P2

/6-3

11

G(d

,p)

MP

2/6

-31

1G

(2d

,2p

)M

P2

/6-3

11

G(2

df,

2pd

)

an

tig

au

che

an

tig

au

che

an

tig

au

che

an

tig

au

che

/C

lC1C

2C

31

80

.06

5.4

18

0.0

66

.71

80

.06

5.7

18

0.0

65

.3

A1

98

65

55

68

19

99

75

63

01

99

24

55

05

20

03

65

53

8

B8

30

12

80

83

11

27

78

33

13

00

84

01

31

4

C8

09

10

91

81

01

09

28

11

11

03

81

81

11

4

lmal

2.6

13

1.0

07

2.6

68

1.0

69

2.5

92

0.9

97

2.6

06

0.9

90

lmbl

0.5

01

1.8

13

0.4

97

1.8

16

0.4

84

1.7

96

0.4

61

1.7

41

lmcl

0.0

00

0.6

86

0.0

00

0.7

13

0.0

00

0.7

02

0.0

00

0.6

51

lmtl

2.6

61

2.1

83

2.7

14

2.2

25

2.6

37

2.1

71

2.6

46

2.1

06

2(E

þ6

53

)0

.645

53

90

.64

37

47

0.9

09

91

80

.90

83

90

0.9

97

63

20

.99

61

36

1.1

06

34

31

.10

50

14

DE

(cm

21)

39

33

35

32

82

92


band [15]. This assignment then leaves the 1441 cm21

band in the xenon solution as n8, the CH2 deformation.

The assignment of the remaining fundamentals in

the fingerprint region (Fig. 8) follows rather straight-

forwardly from the ab initio predicted values, group

frequencies, the infrared band contours, and the

spectra of the solid. Of particular interest was the

CH3 rocks where the ab initio predictions have the A0

and A00 modes separated by 13 cm21, i.e. 1023 and

1036 cm21, respectively. However, the infrared

spectrum of the gas clearly indicates the strong,

weak, weak alternation for these CH3 rocking modes

with the K ¼ 0 transition about 1035 cm21. There-

fore, these rocking modes are assigned (Table 5) as

pseudo-degenerate vibrations like the CH3 antisym-

metric stretches and deformations.

Fig. 5. Internal coordinates for 5-chloropent-2-yne.

Fig. 6. Raman spectra of 5-chloropent-2-yne: (A) experimental

spectrum of the liquid; (B) MP2/6-31G(d) MP2/6-31G(d) ab initio

calculated spectrum of the anti and gauche mixture with a DH of

233 cm21 (C) calculated for the pure gauche conformer; and (D)

calculated for the pure anti conformer.

Fig. 7. Comparison of (A) infrared spectrum of the annealed solid,

(B) infrared spectrum of the gas showing the free internal rotation of

the terminal methyl group; and (C) Raman spectra from 2800 to

3100 cm21 of the annealed solid for 5-chloropent-2-yne.


The low frequency spectral region is where the most

pronounced evidence for the conformers should be

observed, and this is, in fact, the case. For example, the

C3C4C5 ‘in-phase’ bend is predicted between 131

(unscaled) and 146 cm21 (scaled) for the gauche

conformer, with a predicted infrared intensity of

4.6 km/mol, whereas the corresponding mode for the

anti conformer is predicted between 84 (unscaled) and

93 cm21 (scaled) with essentially zero intensity. There

is a relatively strong band at 130 cm21 in the infrared

spectrum of the gas which is not present in the infrared

spectrum of the solid (Fig. 4). Similarly, the C1C2C3

and the CCCl bends (A0 modes for the anti conformer)

for the gauche conformer are predicted at 511 and

334 cm21 with no fundamental for the anti conformer

predicted within 50 cm21 of the higher frequency

band. The C3C4C5 bending mode of the anti conformer

is predicted at 335 cm21 but it is clearly observed at

374 cm21 in the Raman spectrum of the liquid. This

fundamental is predicted to have an infrared intensity

of 0.9 km/mol. Therefore the bands observed at 515

and 341 cm21 in the infrared spectrum of the gas which

must be assigned to these fundamentals for the gauche

conformer. Both of these bands disappear from the

Table 4

Symmetry coordinate for 5-chloropent-2-yne

Species Descriptiona Symmetry coordinateb

A0 CH3 antisymmetric stretch S1 ¼ 2r7 2 r5 2 r6pCH2 symmetric stretch S2 ¼ r1 þ r2

CH2 symmetric stretch S3 ¼ r3 þ r4

CH3 symmetric stretch S4 ¼ r5 þ r6 þ r7

CxC stretch S5 ¼ SpCH2 deformation S6 ¼ 412 b1 2 b2 2 a1 2 a2

CH3 antisymmetric deformation S7 ¼ 2f2 a5 2 a6

CH2 deformation S8 ¼ 4h2 b3 2 b4 2 a3 2 a4

CH3 symmetric deformation S9 ¼ b5 þ b6 þ b7 2 a5 2 a6 2 f

CH2 wag S10 ¼ b3 þ b4 2 a3 2 a4pCH2 wag S11 ¼ b1 þ b2 2 a1 2 a2

C4C5 stretch S12 ¼ T

C1C2 stretch S13 ¼ U

CH3 rock S14 ¼ 2b7 2 b5 2 b6

CCl stretch S15 ¼ V

C2C3 stretch S16 ¼ Q

C1C2C3 bend S17 ¼ u

CCCl bend S18 ¼ p

C2C3C4 in-plane bend S19 ¼ j1

C3C4C5 in-plane bend S20 ¼ l1

A00 pCH2 antisymmetric stretch S21 ¼ r1 2 r2

CH3 antisymmetric stretch S22 ¼ r5 2 r6

CH2 antisymmetric stretch S23 ¼ r3 2 r4

CH3 antisymmetric deformation S24 ¼ a5 2 a6

CH2 twist S25 ¼ b3 2 b4 2 a3 þ a4pCH2 twist S26 ¼ b1 2 b2 2 a1 þ a2

CH3 rock S27 ¼ b5 2 b6pCH2 rock S28 ¼ b1 2 b2 þ a1 2 a2

CH2 rock S29 ¼ b3 2 b4 þ a3 2 a4

C3C4C5 out-of-plane bend S30 ¼ l2

C2C3C4 out-of-plane bend S31 ¼ j2

Asymmetric torsion S32 ¼ t2

CH3 torsion S33 ¼ t1

a The pCH2 refers to the carbon with the chlorine atom attached.b Not normalized.


infrared spectrum of the solid (Fig. 4). On these bases it

is concluded that the anti conformer is the rotamer

remaining in the polycrystalline solid state. Thus, the

assignment of skeletal bending fundamentals can

readily be made from the ab initio predictions and the

infrared and Raman spectra of the solid. It should be

noted that the C3C4C5 out-of-plane bend which is

observed more than 30 cm21 higher than the predicted

value (335 versus 374 cm21 for the liquid) is only

observed in the Raman spectrum which shows the

importance of having both Raman and infrared data for

making vibrational assignments.

The asymmetric torsional transitions for both the

anti and gauche conformers are observed at 66 and

56 cm21, respectively. The predicted values are 80

(unscaled) and 66 cm21 (unscaled), respectively,

which is a much larger difference than normally

found for these modes. However, the relative

intensities of these two peaks is much more consistent

with the proposed assignment than the alternative of

assigning the 66 cm21 band as the asymmetric

torsional fundamental of the gauche conformer and

leaving the corresponding mode for the anti rotamer

unassigned.

Table 5

Coriolis structure of nearly-degenerate CH3 antisymmetric vibrations

CH3 antisymmetric stretch CH3 antisymmetric deformation CH3 antisymmetric rock

m nobs nobs 2 ncalc nobs nobs 2 ncalc nobs nobs 2 ncalc

12 3093.96 20.41 1112.88 20.54

11 3085.01 20.14 1106.23 20.59

10 3075.97 20.08 1101.69 1.43

9 3066.70 0.11 1093.21 20.51

8 3057.46 20.21 1086.88 20.34

7 3048.12 0.25 1554.40 0.41 1081.64 0.89

6 3038.67 0.23 1540.93 20.02 1074.64 0.33

5 3029.19 0.22 1527.37 20.43 1068.42 0.52

4 3019.32 20.14 1513.35 21.21 1061.48 20.05

3 3009.98 0.07 1055.00 20.19

2 3000.30 20.01 1488.88a 1048.64 20.25

1 2990.71 0.03 1476.24 1.99 1041.83 20.78

0 2980.91 20.09 1461.41 0.79 1035.40 20.97

21 2971.58 0.30 1447.02 0.13

22 2961.21 20.31 1432.01 21.05

23 2951.00 20.71 1418.42 20.72 1018.22 0.37

24 2941.43 20.43 1404.98 20.14 1013.48 1.73

25 2931.97 20.06 1390.55 20.45 1004.62 21.05

26 2922.10 20.05 1377.26 0.57 999.34a

27 2912.76 0.68 1362.99 0.02

28 1347.94 20.12

29 1333.79 0.23

210 1319.00a

St. dev. 0.33 0.85 0.88

c0 2981.00 ^0.10 1460.62 ^0.34 1036.4 ^0.31

c1 9.699 ^0.0176 13.6800 ^0.047 6.224 ^0.074

c2 20.0264 ^0.0016 20.0487 ^0.0099 0.016 ^0.0087

F001 6.0653 6.0653 6.0653

z 0.2005 ^0.0014 20.128 ^0.005 0.487 ^0.007

n0 2976.1 ^0.1 1453.7 ^0.2 1034.8 ^0.3

a Not used in fit.


5. Conformational stability

A variable temperature study of the infrared

spectrum of 5-chloropent-2-yne dissolved in liquified

xenon was carried out to determine the energy

difference between the two stable conformers. An

important advantage to this temperature study is that

the conformer peaks are better resolved and the area

under them is more easily measured than bands

observed in the infrared spectrum of the gas. In Fig. 9,

a portion of the infrared spectrum of the sample

dissolved in xenon is shown. These data clearly show

how well the bands are separated and identified for the

individual conformers. Spectral data were obtained at

10 different temperatures ranging from 255 to

2100 8C of the infrared spectrum from 3500 to

400 cm21. The bands chosen for the anti conformer

for the enthalpy difference determination were 456,

708, 749 and 1244 cm21 and those for the gauche

conformer at 513, 665, 680, 896, 936 and 1300 cm21.

A typical spectral change over this temperature range

is shown in Fig. 10. Hence the following conformer

pairs were used in the enthalpy determination: 456/

513, 456/665, 456/896, 456/936, 456/1300, 708/513,

708/665, 708/896, 708/936, 708/1300, 749/513, 749/

665, 749/896, 749/936, 749/1300, 1244/665, 1244/

680, 1244/896, 1244/936 and 1244/1300 cm21. The

enthalpy difference between the anti and gauche

Fig. 8. Comparison of (A) infrared spectrum of 5-chloropent-2-yne

in liquified xenon at 275 8C; (B) Raman spectra of the liquid; (C)

infrared spectra of the annealed solid; and (D) Raman spectra of the

solid for 5-chloropent-2-yne from 650 to 1350 cm21.

Fig. 9. Mid-infrared spectrum of 5-chloropent-2-yne in liquified

xenon at 275 8C from 400 to 1100 cm21.

Fig. 10. Temperature dependant infrared spectrum (425–525 cm21)

of 5-chloropent-2-yne in liquid xenon for the anti (456 cm21) and

the gauche C1C2C3 bend (512 cm21).


Table 6

Temperature and intensity ratios from the conformational study of 5-chloropent-2-yne dissolved in liquid xenon

T (8C) 1000/T (K) I456/513 I456/665 I456/896 I456/936 I456/1300 I708/513 I708/665 I708/896 I708/936 I708/1300

255 4.59 2.331 0.535 2.114 1.177 0.608 15.689 3.603 14.229 7.924 4.092

260 4.69 2.070 0.540 2.037 1.146 0.583 15.798 4.123 15.548 8.749 4.449

265 4.81 2.293 0.579 1.968 1.247 0.625 16.592 4.190 14.244 9.023 4.523

270 4.93 2.080 0.561 2.234 1.233 0.614 14.112 3.803 15.154 8.361 4.163

275 5.05 2.136 0.559 2.183 1.185 0.622 16.764 4.389 17.133 9.297 4.883

280 5.18 1.713 0.557 2.238 1.216 0.638 14.451 4.699 18.882 10.257 5.381

285 5.32 2.553 0.650 2.463 1.336 0.745 18.795 4.782 18.132 9.837 5.485

290 5.46 2.677 0.783 2.857 1.532 0.824 18.222 5.330 19.451 10.426 5.612

295 5.62 2.554 0.725 3.148 1.653 0.927 17.701 5.022 21.821 11.460 6.428

2100 5.78 2.931 0.841 3.116 1.691 0.923 20.226 5.805 21.503 11.666 6.371

DHa (cm21) 159 ^ 77 263 ^ 44 283 ^ 38 232 ^ 37 293 ^ 39 140 ^ 50 244 ^ 32 265 ^ 29 214 ^ 24 274 ^ 29

T (8C) 1000/T (K) I749/513 I749/665 I749/896 I749/936 I749/1300 I1244/665 I1244/680 I1244/896 I1244/936 I1244/1300

255 4.59 16.298 3.743 14.782 8.232 4.251 2.711 23.393 10.706 5.962 3.079

260 4.69 16.260 4.244 16.003 9.005 4.579 3.063 26.299 11.551 6.500 3.305

265 4.81 14.930 3.770 12.817 8.119 4.070 3.074 25.057 10.452 6.621 3.319

270 4.93 14.415 3.884 15.480 8.540 4.253 3.010 24.213 11.997 6.619 3.296

275 5.05 18.229 4.772 18.630 10.109 5.310 3.190 24.737 12.453 6.757 3.549

280 5.18 12.803 4.164 16.729 9.087 4.768 3.124 23.919 12.552 6.818 3.577

285 5.32 20.764 5.283 20.032 10.868 6.060 3.440 31.086 13.042 7.076 3.945

290 5.46 22.829 6.678 24.368 13.061 7.030 3.698 34.262 13.494 7.233 3.893

295 5.62 19.124 5.426 23.576 12.381 6.944 3.136 43.243 13.629 7.157 4.014

2100 5.78 20.635 5.922 21.938 11.902 6.500 3.699 34.144 13.702 7.434 4.060

DHa (cm21) 200 ^ 89 304 ^ 64 324 ^ 64 273 ^ 49 334 ^ 58 135 ^ 35 298 ^ 72 155 ^ 23 104 ^ 13 165 ^ 16

a Average value of DH is 233 ^ 11 cm21 with the anti conformer the more stable form.

G.A

.G

uirg

iset

al.

/Jo

urn

al

of

Mo

lecula

rS

tructu

re6

13

(20

02

)1

5–

35

29

conformers was calculated by using the van’t Hoff

equation, 2ln K ¼ ðDH=RTÞ2 DS=R: A plot of

2 ln K versus 1/T, where K is the ratio of the intensity

of a band due to the anti conformer to one due to the

gauche conformer, has a slope which is proportional

to the enthalpy difference. The data given in Table 6

for the aforementioned conformer pairs of 5-chloro-

pent-2-yne dissolved in xenon yield enthalpy values

that range from a low value of 104 ^ 13 cm21 to a

high value of 334 ^ 58 cm21 with an average value

of 233 ^ 11 cm21. While the range of 230 cm21

between the low and high values may at first seem

large, most of the values of the 20 conformer pairs fall

within the range of the average value. In addition, it is

the conformer pairs that contain the higher frequency

anti band observed at 1244 cm21 that falls outside the

range of the average value of 233 ^ 11 cm21. As one

may observe from Table 6, most of the DH values

obtained with the anti band at 1244 cm21 are lower in

value than the conformational values obtained by the

other three bands of the anti conformers. This could

be due in part to an overtone or combination band

lying underneath the band at 1244 cm21 of the anti

conformer which is too weak to be observed, yet

intense enough to interfere with the intensity of the

band of the anti conformer. The listed uncertainties

are statistical uncertainties, which cannot take into

account any underlying overtone or combination

bands. Therefore, we believe a more realistic

uncertainty is ten percent, i.e. 23 cm21. This value

should be close to the value for the vapor [2,6,15–18]

since the volumes of the two conformers and their

dipole moments do not differ significantly.

6. Asymmetric torsional potential

The asymmetric torsion for the anti conformer is

predicted at 80 cm21 (84 cm21, scaled value) but the

fundamental is assigned at 66 cm21. The intensity of

this band is consistent with this assignment (Fig. 4).

The corresponding fundamental for the gauche

conformer is assigned at 56 cm21 although the ab

initio calculations predict a separation of 14 cm21 for

these two fundamentals.

The potential function for conformational inter-

change has been determined by first beginning with

the DH value of 233 cm21 which was obtained from

the rare gas solutions, the dihedral angle 114.38

(180 2 65.78 which is the value of the dihedral angle

ClCCC) of the gauche rotamer from MP2/6-

311G(2d,2p) calculation, and the asymmetric tor-

sional transitions of the anti and gauche conformers.

The torsional dihedral angular dependence of the

internal rotational constant, FðfÞ; can be represented

as a Fourier series:

FðfÞ ¼ F0 þX7

k¼1

Fk cos kf:

The relaxation of the structural parameters, BðfÞ;during the internal rotation can be incorporated into

the above equation by assuming that they are small

periodic functions of the torsional angle of the general

type:

BðfÞ ¼ a þ b cos fþ c sin f:

The structural parameters (Table 3) obtained from the

optimized geometries for both the anti and gauche

conformers, were used to obtain the kinetic constants.

The torsional potential is also represented as a

Fourier cosine series in the internal angle (f ):

VðfÞ ¼X6

i¼1

ðVi=2Þð1 2 cos ifÞ:

The kinetic terms, the asymmetric torsional frequen-

cies for both conformers, the experimental enthalpy,

Table 7

Potential function coefficients (cm21) for asymmetric torsion of 5-

chloropent-2-yne and barriers to interconversion (cm21)

Parameter Experimental valuea Ab initiob

V1 736 ^ 38 863

V2 2431 ^ 32 2334

V3 1572 ^ 17 1729

V4 44

DH (cm21) 196 ^ 40 393

Barriers

anti/gauche 1433 1726

gauche/anti 1237 1333

gauche/gauche 2112 2199

Dihedral angle 114.3 114.6

a Calculated using F0 ¼ 0:424431; F1 ¼ 20:035809; F2 ¼

0:003985; F3 ¼ 20:000359; F4 ¼ 0:000398; F5 ¼ 20:000359;

F6 ¼ 0:003985; F7 ¼ 20:035809 cm21:b Obtained from the MP2/6-31G(d) calculation.


and the gauche dihedral angle were used to fit the

potential function utilizing a computer program

developed in our laboratory [19]. As an iterative

process, this calculation was continued until the

differences between the observed and calculated

wavenumbers, as well as the dispersions in the

potential constants, were minimized (Table 7). From

the calculated potential function, the anti to gauche

and gauche to gauche barriers are determined to be

1433 and 2112 cm21, respectively, with the enthalpy

difference between the two conformers of

196 ^ 40 cm21. These results are listed in Table 7

and the determined potential function is shown in Fig.

11. The values for the V1 and V2 terms are relatively

large and we have omitted the V4 term since there are

only four pieces of data for determining the potential

function. The V1 and V2 terms are strongly correlated

and the use of the gauche dihedral angle for the

potential function determination reduces the corre-

lation between these two terms.

Utilizing MP2/6-31G(d) ab initio calculations, we

have also determined the barriers to internal rotation.

The calculated potential constants, V1, V2 and V3, have

similar values to those obtained from the experimental

data. However the anti to gauche barrier is predicted

to be about 20% (296 cm21) larger than the value

obtained from the experimental data. The MP2/6-

31G(d) ab initio calculation gives the values of the

torsional fundamentals of 84 and 72 cm21 for the anti

and gauche conformers, respectively, which are larger

than the observed values of 66 and 56 cm21,

respectively. In addition, this calculation predicted a

rather large energy difference of 393 cm21 between

the two conformers, which is 160 cm21 larger than the

experimental enthalpy difference. Therefore the two

potential functions are quite similar with the major

differences in the energy difference between the

conformers and slightly higher anti to gauche barrier

for the predicted values (Fig. 11).

7. Coriolis interaction

In the infrared spectrum of the gas where the

antisymmetric CH3 stretching, CH3 deformational

and rocking modes are expected, sub-band structure

with strong–weak–weak–strong intensity alternation

is observed which is typical of perpendicular bands of

symmetric-top molecules such as the methyl halides.

In symmetric-top molecules, this structure is caused

by the Coriolis interaction of the overall rotational

motion of the molecule with the angular momentum

of the E vibrations of the methyl group. Similar sub-

band structure has been observed in the corresponding

bands in the infrared spectra of 2-pentyne [20] and the

1-halo-2-butynes [21,22]. The Q-branches of the

different sub-bands agree well with the formula nsub0 ¼

c0 ^ c1m þ c2m2:For a perpendicular band of a symmetric-top

molecule such as the degenerate fundamentals of a

Fig. 11. Potential function governing the internal rotation of the asymmetric torsion of 5-chloropent-2-yne as determined by ab initio

calculations (dotted line) and by the observed asymmetric torsional transitions from the far infrared spectrum of the gas (solid line). The

torsional dihedral angle of 08 refers to the anti conformer.


methyl group, the formula for the sub-band structure

with the type of intensity alternation already men-

tioned has been given [23,24] by

nsub0 ¼ n0 þ ½A0ð1 2 zÞ2 2 B0�^ 2½A0ð1 2 zÞ2 B0�K

þ ½ðA0 2 B0Þ2 ðA00 2 B00Þ�K2;

where z is the Coriolis coupling constant and the

factor (1 2 z )2 is used [23,24] rather than (1 2 2z ) as

in Herzberg [25] and Nyquist [26]. The single prime

refers to the upper state and double prime to the lower

state.

However, in the case of 5-chloropent-2-yne, the

sub-band structures are due to the interaction of the

vibrational angular momentum of the three pseudo-

degenerate vibrations with the internal rotational

angular momentum of the nearly free rotating internal

top and not with the overall rotation of the molecule.

A quadratic equation can be derived from first

principles using the appropriate Hamiltonian, but in

order to retain the z notation, the following equation

can be derived from the sub-band formula for overall

rotation of symmetric-top molecules by elimination of

the kinetic constants for overall rotation, B or

(B þ C )/2, and by replacing the A constants with F1

which is h=8p2cIredt ; the inverse reduced moment of

inertia for internal rotation of the methyl top:

nsub0 ¼ ½n0 þ F0

1ð1 2 zÞ2� þ 2F01ð1 2 zÞm

þ ðF01 2 F00

1Þm2:

In 5-chloropent-2-yne, the anti conformer is shown to

be the lower energy form with a complete vibrational

analysis given above. Thus, the sub-band structure

undoubtedly arises from the near degeneracy of

n1/n22, n7/n24 and n14/n27 of the anti form. Values

for the kinetic constant F001 in the ground vibrational

state are obtained from ab initio optimized geometric

structures of the anti conformer given in Table 3.

Additionally, from the energies of several ab initio

calculations of eclipsed and staggered configurations

of 5-chloropent-2-yne, it is evident that the barrier to

internal rotation, V3, of the methyl group is essentially

negligible, i.e. a few cm21 from all calculations.

The measurements of the observed sub-bands are

given in Table 5. Fits to the sub-band peaks have been

made by the least-squares method to the formula

nsub0 ¼ c0 ^ c1m þ c2m2 and the coefficients are also

given in Table 6. For each of the bands, the quadratic

coefficient c2 is small; it is the change in the internal

rotational constant, F, between the ground and excited

states. Taking F00 ¼ 6:0653 cm21 from the MP2

geometric structure calculation for the CH3 pseudo-

degenerate stretching fundamentals, n1/n22, we obtain

z ¼ 0:2005 ^ 0:0014 from the coefficient of the linear

term, 2F0ð1 2 zÞ; and from n0 þ F0ð1 2 zÞ2 the band

center, n0, is 2976.1 ^ 0.1 cm21. In the same way, for

the deformational fundamentals, n7/n24, we obtain z ¼

20:128 ^ 0:005 from the linear term and n0 is

1453.7 ^ 0.2 cm21 for the band center. For the two

rocking modes, n14/n27 z is 0.487 ^ 0.007 with the

band center at 1034.8 ^ 0.3. These values of both z

and n0 for the stretch and deformation are similar to

those derived from the infrared spectrum of 1-fluoro-

2-butyne in the same manner [22]. However for the

fluoride no fine structure was observed on the rocking

modes. The main reason for employing the z notation

in the fitting equation for Coriolis coupling between

the internal rotation and the angular momentum of the

degenerate CH3 vibrations is that the resulting

magnitudes of z are also comparable with those

obtained for Coriolis coupling with overall rotation in

symmetric-top molecules, the halomethanes [25] and

the 1-halo-propynes [23,26].

8. Discussion

The conformational stability determined from the

xenon solution with the anti conformer more stable by

233 ^ 23 cm21 (2.79 ^ 0.28 kJ/mol) is expected to

be close to the value in the gas phase [2,6,17,18], since

the size of the two conformers as well as their dipole

moments do not differ appreciably. This experimental

value is considerably lower than the predicted energy

differences which range from 292 cm21 (MP2/6-

311G(2df,2pd)) to 393 cm21 (MP2/6-31G(d)). It

should be noted that the rather large experimental

result is rather surprising since 1-chloropropane has

the gauche conformer as the more stable rotamer by

52 ^ 3 cm1 (0.62 ^ 0.04 kJ/mol) from variable tem-

perature FT-IR spectral studies of a xenon solution

[3]. The difference in these two molecules (YCH2-

CH2Cl) is the substitution of a propynic group as Y for

the methyl group. This difference in Y results in the


C1–C2 bond distance being predicted to be about

0.007 A shorter in 1-chloropropane (1.517 A) than the

corresponding bond distance (1.524 A) in 5-chloro-

pent-2-yne. The corresponding cyanide, ClCH2CH2-

CN, is a good model [27] for comparison since the CN

and CxCH groups have similar electronegativities

[28] and are expected to effect the C1–C2 bond

distances about the same amount. In fact the ab initio

predicted value for the C1–C2 bond for the 3-

chloropropionitrile molecule is 0.002 A longer than

this bond in 5-chloropent-2-yne. However for the

nitrile molecule the gauche conformer was deter-

mined to be the more stable rotamer by 609 ^ 31 cm1

(7.29 ^ 0.37 kJ/mol) but this result was for the liquid

[27], which could be the result of the very large dipole

moment for the gauche conformer. Nevertheless the

ab initio predicted stability gives a larger value 451

versus 393 cm21 from MP2/6-31G(d) than that for 5-

chloropent-2-yne for the energy difference between

the conformers but with the anti rotamer being the

more stable form. A re-investigation of the confor-

mational stability of ClCH2CH2CN in the gas phase or

in the rare gas solutions would be of interest.

Undoubtedly the C1–C2 bond distance is an important

factor in determining the conformational stability of

YCH2CH2Cl molecules and one expects the enthalpy

differences to be very similar for 3-chloropropionitrile

and 5-chloropent-2-yne.

The heavy atom distances are predicted to be

essentially the same for the two conformers as are

most of the angles except the C1C2C3 angle which is

predicted to be 2.28 larger for the gauche conformer

[MP2/6-311G(d,p) calculations] compared to the

similar angle for the anti form. However, most of

the other parameters have similar values for the two

conformers with most of the angles differing by only

0.1–0.48. For the other three heavy atom angles these

differences are 0.38 for one angle and 0.48 for the other

two angles.

Most of the force constants have nearly the same

values for the two conformers, i.e. within 1–2%.

However, there are two notable exceptions where the

in-plane C2C3C4 bending force constant for the trans

rotamer is 10% larger than the corresponding force

constant for the gauche conformer (0.414 versus

0.372 mdyn/A2) and the C1C2C3 bend where the force

constant for the trans rotamer is 7% smaller than the

corresponding force constant for the gauche form

(0.690 versus 0.741 mdyn/A2). The C1C2C3 angle is

2.28 larger for the gauche conformer than this angle

for the trans form. The HCC bending force constants

for the CH2 group differ by about 3% depending on

the relative position of the CCl bond, i.e. whether it is

splitting the HCH angle or in the gauche position.

Many of the remaining force constants differ by about

1% and even the ClC1C2 angle bend differs by only

2% between the force constant values for the two

conformers. Therefore most of the frequency differ-

ences for the fundamentals for the two conformers are

the result of differences in the mixing, i.e. the PEDs.

Most of the carbon–hydrogen stretches and bends

have very similar frequencies for the two conformers

with the exception of three of the bending modes, i.e.pCH2 wag, pCH2 rock, and CH2 rock (the asterisk

indicates the carbon atom with the chlorine atom

attached). For the pCH2 wag, the band is observed at

1247 cm21 for the anti conformer, whereas the

corresponding mode is observed at 1297 cm21 for

the gauche conformer. This difference is mainly due

to the difference in mixing of the modes between the

two conformers where there is a 39% contribution

from the CH2 wag for the anti conformer, whereas the

corresponding mode for the gauche conformer has

12% contribution from the CH2 twist and 86%

contribution from the pCH2 wag. The pCH2 and CH2

rocks are observed at 999 and 753 cm21 for the anti

conformer, respectively, whereas these modes are

observed at 904 and 942 cm21, respectively, for the

gauche form. The almost 200 cm21 difference for the

CH2 rock arises from an almost equal contribution of

45% for the CH2 rock for the anti form from both the

CH2 and pCH2 rocks, whereas for the gauche form the

contribution from the CH2 rock is only 29% with

contributions of 33% from the C1–C2 stretch and only

16% from the CH2 rock. There are also some

significant differences in the observed frequencies

for several of the heavy atom vibrations such as the

CCl and C2C3 stretches as well as the C1C2C3, CCCl

and C3C4C5 bends. Most of these modes differ by

about 50 cm21 except the CCl stretch which is

observed at 794 cm21 for the anti conformer and at

682 cm21 for this mode for the gauche conformer.

Again, this difference in frequencies is due to mixing

rather than force constants differences for these

stretches.

For chloro- and hydrocarbon compounds, one can


usually predict the wavenumbers of the fundamentals

to within 1% from MP2/6-31G(d) calculations utiliz-

ing only two scaling factors of 0.88 for the carbon–

hydrogen stretches and 0.9 for the carbon–hydrogen

bends and heavy atom stretches while keeping the

other force constants with the predicted values. Such

predictions usually give fundamental wavenumbers

with average errors of 10–12 cm21. However for the

acetylene group, the in- and out-of plane CxC–C

bends need to be scaled by factors of 1.4 and 1.5,

respectively, to give reasonable results from the

MP2/6-31G(d) calculations. By using these four

scaling factors, the fundamentals frequencies are

predicted with errors of 0.8% (11 cm21) for the A0

modes and 1.0% (13 cm21) for the A00 modes for the

anti conformer. Much of the error for the A00 modes

arises from the poor predictions for the lowest three

observed vibrations. Similar errors are also found for

the fundamentals for the gauche conformer. Even

with the two different scaling factors for the

acetylenic group, the low frequency bending modes

for this group are not very well predicted. In fact, DFT

calculations by the B3LYP method predict these

bending modes much better and if these frequencies

are needed for the vibrational assignment for

molecules with the acetylenic group the DFT

calculations are preferred.

With the DH value of 233 ^ 23 cm21 the

abundance of the gauche conformer is predicted to

be 39 ^ 2% at ambient temperature so it should be

possible to obtain microwave spectra of both

conformers. The assignment of the microwave lines

for the anti conformer should be relatively routine

since its microwave spectrum will be mainly a-type,

whereas that for the gauche conformer will be

mainly b-type which is more difficult to assign.

Since the carbon–hydrogen parameters are expected

to be well predicted from the ab initio MP2/6-

311G(d,p) calculations, the 12 rotational constants

for the two conformers with the two isotopomers

from the naturally occurring chlorine atom should be

sufficient to obtain the heavy atom structural

parameters if they are combined with the ab initio

predicted values. It would be of interest to obtain

such data so the parameters could be compared to the

corresponding ones for the 1-chloropropane mol-

ecule. The CxC bond is predicted too long by

0.010 A by the MP2/6-311G(d,p) calculations but it

is expected that the remaining heavy atom par-

ameters should be accurate to 0.006 A for distances

and 0.58 for angles. Alternatively, electron diffraction

data could be used for the structural determination.

These data could then be used to verify that the

relative C2–C3 bond distance is the major factor

determining the conformational stability of YCH2-

CH2Cl molecules.

Acknowledgments

J.R. Durig acknowledges partial support of these

studies by the University of Missouri-Kansas City

Faculty Research Grant program.

References

[1] Y.E. Youssoufi, M. Herman, J. Lievin, J. Mol. Phys. 94 (1998)

461.

[2] W.A. Herrebout, B.J. van der Veken, A. Wang, J.R. Durig,

J. Phys. Chem. 99 (1995) 578.

[3] J.R. Durig, X. Zhu, S. Shen, J. Mol. Struct. 570 (2001) 1.

[4] J.R. Durig, J. Liu, T.S. Little, V.F. Kalasinsky, J. Phys. Chem.

96 (1992) 8224.

[5] G.A. Guirgis, X. Zhu, J.R. Durig, Struct. Chem. 10 (1999) 445.

[6] W.A. Herrebout, B.J. van der Veken, J. Phys. Chem. 100

(1996) 9671.

[7] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A.

Robb, J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery,

R.E. Stratmann, J.C. Buran, S. Dapprich, J.M. Millam, A.D.

Daniels, K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi,

V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.

Adamo, S. Clifford, J. Ochterski, G.A. Petersson, P.Y. Ayala,

Q. Cui, K. Morokuma, D.K. Malick, A.D. Rabuck,

K. Raghavachari, J.B. Foresman, J. Cioslowski, J.V. Ortiz,

B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi,

R. Gomperts, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham,

C.Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,

P.M.W. Gill, B.G. Johnson, W. Chen, M.W. Wong, J.L.

Andres, M. Head-Gordon, E.S. Replogle, J.A. Pople, GAUS-

SIAN 98, Revision A.7, Gaussian, Inc., Pittsburgh PA, 1998.

[8] P. Pulay, Mol. Phys. 17 (1969) 197.

[9] G.A. Guirgis, J.R. Durig, S.J. Bell, J. Mol. Struct. 196 (1989)

101.

[10] G.A. Guirgis, X. Zhu, Z. Yu, J.R. Durig, J. Phys. Chem. A 104

(2000) 4383.

[11] M.J. Frisch, Y. Yamagushi, J.F. Gaw, H.F. Schaefer III, J.S.

Binkley, J. Chem. Phys. 84 (1986) 531.

[12] R.D. Amos, Chem. Phys. Lett. 124 (1986) 376.

[13] P.L. Polavarapu, J. Phys. Chem. 94 (1990) 8106.


[14] G.W. Chantry, in: A. Anderson (Ed.), The Raman Effect, vol.

1, Marcel Dekker, New York, NY, 1971, Chapter 2.

[15] S. Bell, X. Zhu, G.A. Guirgis, J.R. Durig, Phys. Chem. Chem.

Phys. 39 (2001) 776.

[16] B.J. van der Veken, F.R. DeMunck, J. Chem. Phys. 97 (1992)

3060.

[17] M.O. Bulanin, J. Mol. Struct. 347 (1993) 73.

[18] M.O. Bulanin, J. Mol. Struct. 19 (1973) 59.

[19] P. Groner, R.D. Johnson, J.R. Durig, J. Mol. Struct. 142 (1986)

363.

[20] S. Bell, G.A. Guirgis, S.W. Hur, J.R. Durig, Spectrochim. Acta

55A (1999) 2361.

[21] R.D. McLachlan, Spectrochim. Acta 23A (1967) 1793.

[22] G.A. Guirgis, S. Bell, R.J. Durig, Spectrochim. Acta 57A

(2001) 1235.

[23] D.W. Davidson, H.J. Bernstein, Can. J. Chem. 33 (1955) 1226.

[24] E.W. Jones, R.J.L. Popplewell, H.W. Thomson, Spectrochim.

Acta 22 (1966) 639.

[25] G. Herzberg, Molecular Spectra and Molecular Structure: II.

Infrared and Raman Spectra of Polyatomic Molecules, Van

Nostrand, Princeton, NJ, 1945, p. 429.

[26] R.A. Nyquist, Spectrochim. Acta 21 (1965) 1245.

[27] J.R. Durig, G.A. Guirgis, A.S. Drew, J. Mol. Struct. 327

(1994) 55.

[28] N. Inamoto, S. Masuda, Chem. Lett. (1982) 1003.


Documents

Infrared and Raman spectra, conformational stability, ab initio calculations, and vibrational assignment of 5-chloropent-2-yne