Vibrational analysis and conformational study of 3-methylamino-2-acetyl propenenitrile and 3-methylamino-2-methylsulfonyl propenenitrile

Vibrational analysis and conformational study of 3-dimethylamino-2-acetyl

propenenitrile and 3-dimethylamino-2-methylsulfonyl propenenitrile

J. Polovkova a, A. Gatial a,*, V. Milata b, P. Cernuchova b, N. Pronayova c, T. Liptaj c, P. Matejka d

a Department of Physical Chemistry, Slovak University of Technology, 81237 Bratislava, Slovakiab Department of Organic Chemistry, Slovak University of Technology, 81237 Bratislava, Slovakia

c Central Laboratories, Faculty of Chemical and Food Technology, Slovak University of Technology, 81237 Bratislava, Slovakiad Department of Analytical Chemistry, Institute of Chemical Technology, 16628 Prague, Czech Republic

Received 18 July 2005; received in revised form 14 September 2005; accepted 20 September 2005

Available online 4 November 2005

Abstract

The IR, Raman and NMR spectra of similar push–pull ethylenes 3-dimethylamino-2-acetyl propenenitrile (DAAPN) (H3C)2N–CHaC(CN)

(COCH3) and 3-dimethylamino-2-methylsulfonyl propenenitrile (DASPN) (H3C)2N–CHaC(CN)(SO2CH3) were measured. According to NMR

spectra both compounds were prepared as pure E-isomers.

The observed IR and Raman bands were compared with harmonic vibrational frequencies, calculated using ab initio MP2 and B3LYP density

functional methods in 6-31G** basis set, and assigned on the base of potential energy distribution. In addition, the geometries and relative

energies of possible conformers and isomers of the studied compounds were also evaluated on the same levels.

Vibrational spectra revealed that in polar solutions E-DAAPN exists as two conformers with Z or E orientation of acetyl group, the Z conformer

being about 2.2G0.3 kJ molK1 more stable than the E one. The influence of environment polarity on this conformational equilibrium is discussed

with respect to the SCRF solvent effect calculations using IPCM and PCM models.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Vibrational spectra; Conformational analysis; Push–pull enamines; Ab initio calculations; Solvent effect calculations

1. Introduction

Push–pull ethylenes with diverse electron donor and

acceptor groups connected through a double CaC bond

represent an interesting group of organic molecules with

many applications [1,2]. Mainly enamines, with general

formula R1R2NCR3aCXY, where R1, R2, R3 can be hydrogen,

alkyl or hetero(aryl) group and X,Y are strong electron

acceptor groups (such as –CN, –COR, –COOR, –NO2), are

frequently used as reactants or intermediates in chemical

syntheses of drugs, polymers and dyes [3,4]. The polar

character of push–pull ethylenes and electronic interactions

between substituents and the double bond are responsible for

their non-linear optical properties and their use as new electro-

optics materials [5,6].

In past decades, intensive research has been carried out to

study the isomeric equilibria, conformations and solvent

0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2005.09.028

* Corresponding author. Tel.: C421 2 59325460; fax: C421 2 52926032.

E-mail address: [email protected] (A. Gatial).

influence on rotational barrier and reactivity of these

compounds [7–10]. However, only few works are devoted to

the vibrational analysis and interpretation of their vibrational

spectra [6,11–15] although the vibrational spectroscopy is also

an important method used for the characterization and

identification of molecules. On the other hand, the unambigu-

ous assignment of the spectral bands of these highly conjugated

and polarized systems is complicated mainly due to consider-

able mutual coupling of some stretching and deformation

bands. Therefore, the use of theoretical methods including

normal coordinate calculations is helpful tool to solve this

problem.

Previously, the detailed spectroscopic and conformational

study supported by theoretical ab initio calculations of

aminomethylene propanedinitrile [H2N–CHaC(CN)2] (AM)

and 1-aminoethylidene propanedinitrile [H2N–C(CH3)

aC(CN)2] (AE) and their N-methyl derivatives has been

performed [12,13]. We have decided to extend these studies on

enamines with cyano and acetyl or methylsulfonyl electron

withdrawing groups, respectively. Recently, some alkenyl

sulfones [16–18] and a, b unsaturated sulfones have been used

as reactants in preparation of several biologically active

Journal of Molecular Structure 785 (2006) 85–97

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J. Polovkova et al. / Journal of Molecular Structure 785 (2006) 85–9786

sulfone molecules with anticancer [19] and carcinogenesis-

supressing activity [20]. Despite the large interest in synthesis

of suitable alkenyl sulfones, there is the lack of information on

their structural parameters and spectroscopic data. The most

works are focused on the study of simple vinyl sulfones without

push–pull character [21,22] that has a significant influence on

geometry, polarization and conjugation of the whole molecule.

This work presents the vibrational spectra, conformational

study and theoretical structure investigation of 3-dimethy-

lamino-2-acetyl propenenitrile (DAAPN) (H3C)2N–

CHaC(CN)(COCH3) and 3-dimethylamino-2-methylsulfonyl

propenenitrile (DASPN) (H3C)2N–CHaC(CN)(SO2CH3).

2. Experimental and computational details

The preparation of samples has been described in our

previous paper [23]. After the isolation and purification

processes pure E-isomers (the first letter in the conformers

labelling) of both compounds were obtained and confirmed by

NMR spectroscopy. The purity of both samples was better than

97% and melting points of DAAPN and DASPN, determined

by differential scanning calorimetry on Perkin–Elmer DSC-7

calorimeter, were 68 and 130 8C, respectively. The infrared

spectra in the region 4000–400 cmK1 were recorded with a

Nicolet model NEXUS 470 FTIR spectrometer at room

temperature. The solid phase measurements were performed

after mixing the powdered samples with KBr and pressing into

a pellet. The far infrared spectra in the region 600–50 cmK1

were recorded with a Nicolet model MAGNA 750 FTIR

spectrometer in polyethylene pellet at room temperature. The

solutions of samples in acetonitrile and chloroform were

measured in cell equipped with KBr windows. Temperature-

dependent infrared spectra were recorded in acetonitrile

solution using variable temperature cell Specac in temperature

Fig. 1. Infrared (top) and Raman (bottom) spect

range 258–328 K. Raman measurements were performed using

a Bruker RFS 100 Raman spectrometer with Nd3C:YAG laser

at the wavelength of 1064 nm.

Ab initio MP2 computations and the DFT ones employing

B3LYP functional were performed using GAUSSIAN 98 [24] and

GAUSSIAN 03 [25] programs. The 6-31G** basis set was used in

both MP2 and DFT calculations. The vibrational analysis has

been also performed to convince that all normal vibrational

modes are positive and calculated structures are the ground

states (not saddle points). The SCRF theory via IPCM and

PCM model including the effect of environment was applied to

correct the relative ground state energies obtained for isolated

molecules in gas phase.

In order to obtain zero point energies (ZPE) of the studied

compounds as well as their dipole moments and harmonic

vibrational frequencies, the normal coordinate calculations

with geometries optimized at MP2 and DFT levels were also

performed. The calculated frequencies were scaled with the

scaling factors developed by the fitting the experimental and

theoretical frequencies of the model compounds. The

description of normal modes was done according to the

potential energy distribution (PED).

3. Results and discussion

3.1. IR and Raman spectra

The mid- and far-infrared and Raman solid phase spectra of

DAAPN are depicted in Fig. 1 and the same spectra of DASPN

are given in Fig. 2. Since, both samples have the small vapour

pressure, only the measurements in solid phase and in solvents

with different polarity combined with temperature IR measure-

ments have been performed. The comparison between the solid

phase IR and Raman spectra of DAAPN with those in

rum of solid DAAPN at room temperature.

Fig. 2. Infrared (top) and Raman (bottom) spectrum of solid DASPN at room temperature.

J. Polovkova et al. / Journal of Molecular Structure 785 (2006) 85–97 87

acetonitrile and in chloroform solutions is shown in Figs. 3 and

4. As can be seen, better from the Raman spectra than from the

IR ones, there are few bands appeared in acetonitrile solution

which are only slightly visible in chloroform solution and are

totally missing in the spectra of solid DAAPN. These are

positioned at 1652, 1614, 1300, 949 and 634 cmK1. The band

located at the 1652 cmK1 in Raman spectrum is lacking in the

IR one. This fact can be explained by small absorption

coefficient of DAAPN at this frequency or by the overlap of the

bands of responsible conformers.

These experimental results confirm that the conformational

conversion between the both conformers of DAAPN (with the

orientation of the carbonyl oxygen towards or from the CaC

double bond labelled by the second letter Z or E) occurs mainly

in more polar environment. Moreover, the increase of the

intensities of the mentioned bands with respect to the

Fig. 3. Details of infrared spectra of DAAPN in solid state (dash-dot line) and in

acetonitrile solution (solid line) at room temperature. Dot line represents

infrared spectrum of pure acetonitrile.

increasing solvent polarity implies that the less polar

conformer crystallizes preferentially and thus only this one

exists in solid phase.

Table 1 summarizes the bands frequencies of DAAPN

observed in both IR and Raman spectra. Observed IR and

Raman frequencies of DASPN are given in Table 2.

3.2. Quantum-chemical calculations

To support the experimental results we have extended our

study also by the theoretical calculations. The numbering and

structure of studied compounds are depicted in Fig. 5. Table 3

summarizes the MP2 and DFT total and relative energies of the

found isomers and conformers of both compounds as well as

their dipole moments. The obtained structural parameters of

EZ- and EE-DAAPN and E-DASPN are included in Table 4.

We have expected nearly planar structure of amino group

due to strong electron withdrawing groups and possible

Fig. 4. Details of Raman spectra of DAAPN in solid state (dash-dot line) and in

acetonitrile solution (solid line) at room temperature. Dot line represents

Raman spectrum of pure acetonitrile.

Table 1

Infrared and Raman spectral data of 3-dimethylamino-2-acetyl propenenitrile

Infrared Raman Interpretation

Liquid Solid Liquid Solid EZ EE

CH3CNa CH3Cl KBr Nujol CH3CN

3076 vw, bb n1

3029 w 3031 w, sh n2, n3

3018 w, sh 3018 m n4

2989 w n5, n6

2970 w 2970 m n7

2923 vw 2925 w 2927 s n8, n9

2880 vw 2886 w, sh n10

2815 vw 2814 vw 2816 w

2197 vs 2200 m 2194 vs 2193 vs 2196 vs 2193 vs n11

2138 vw, sh 2139 vw, sh

1672 vs 1669 s 1658 vs 1657 vs 1672 s 1654 s n12

1652 m * n12

1617 vs,sh * * * 1614 m, sh * n13

1605 vs 1599 vs 1596 vs 1600 vs 1603 s 1586 s n13

1478 vw 1481 vw, sh n14

1453 w, sh 1452 w, sh n15, n16

1427 m 1432 s 1432 m, sh n17, n18, n19

1420 s n20, n21

1414 w, sh 1411 w, sh 1411 vw, sh 1413 vs 1409 s n22

1386 m, sh * n23

1367 s 1368 m 1366 s 1368 s 1357 m 1354 w n23

1346 m, sh 1345 w, sh 1341 m, sh 1341 m, sh 1344 m, sh 1339 w n24

1300 s, sh * * * 1300 w * n25

1286 vs 1286 s 1282 s 1281 s 1287 w 1286 w n25

1222 s 1218 m 1218 m 1221 vw 1221 vw n26

1139 s 1135 w 1145 m 1145 m 1139 w 1142 w n27, n28

1105 vw, sh n29

1059 vw 1056 w 1055 w 1059 w, sh n30

1041 vw, sh 1042 vw, sh 1045 m 1043 vw n31

1024 vw, sh 1023 vw, sh n32

993 w 993 w, sh 993 vw n33

970 m 971 w 973 m 971 s 969 w 973 vw n34

949 w * * * 948 w * n33

852 w,sh 854 vw 854 vw

834 vw 830 vw 829 vw 841 w 833 vw n35

807 vw 803 vw 803 vw 807 s 806 m n36

634 w, sh * 634 w, sh * n37

622 m, sh 623 s, sh n37

620 m 621 w 617 m 617 s 620 s 618 m n38

552 vw, sh 552 m 553 vw, sh n39

544 w 546 vw 541 w 541 m 545 m 543 w n40

507 vw * n40

466 w n41

453 vw 454 vw 451 w 451 vw n42

426 w 426 w 428 vw n43

366 vw 362 m, sh 369 w n44

285 vw 293 m 293 w n45, n46

180 w 177 s 187 s n47

163 w, sh n48

158 w, sh n49

111vw n50, n51

84 vvw, sh n52

76 vw n53

Weak bands in the regions 4000–3200, 2800–2300 and 2000–1800 cmK1 have been omitted. * denotes bands vanishing in the solid phase.a Solvents used.b Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; b, broad.


connection of the lone electron pair on nitrogen into the

conjugation. Unfortunately, the experimental structural data

for studied molecules are still not available. Therefore, it is

difficult to decide, which of the theoretically obtained

structures is more reliable. Nevertheless, we can make two

conclusions from the calculated geometries:

Table 2

Infrared and Raman spectral data of 3-dimethylamino-2-methylsulfonyl propenenitrile

Infrared Raman Interpretation

Liquid Solid Liquid Solid

CH3CNa CH3Cl KBr Nujol CH3CN

3087 vvwb

3040 vw, b n1, n2, n3

3013 w 3013 s n4, n5

3003 w 3003 s n6

2969 w 2942 w, b 2947 m n7

2930 w 2923 w 2922 vs n8

2881 vw, b n9, n10

2853 vw

2819 vw 2818 vw 2815 w

2202 m 2203 s 2193 s 2192 m, sh 2199 s 2194 s n11

2182 s 2180 m 2186 s, sh 2182 s

2135 vw 2134 vw

1638 vs 1635 vs 1645 vs 1643 s 1638 w n12

1478 w 1474 w n13

1447 vw n14, n15

1436 s 1434 m 1436 w n16, n17

1423 m 1423 m 1424 w 1423 w,sh n18

1413 m 1417 m 1416 w 1414 s 1417 m,sh n19, n20

1411 w,sh n21

1372 m 1379 m 1379 w,sh n22

1370 w,sh

1319 m 1318 vw n23

1309 s 1312 vs 1300 vs 1300 s 1308 w 1297 w n24

1282 vw, sh 1281vw, sh

1248 w 1248 w n25

1164 w 1153 w n26

1146 m, sh 1144 w n27

1140 s 1139 vs 1137 vs 1136 s 1139 s 1140 w n28

1126 m, sh 1122 w n29

1062 vw 1060 w 1060 w n30

1035 m 1037 m 1036 m 1038 w 1038 w n31

973 m 973 m 974 w n32

962 m 961 w 962 w n33

957 w 956 s 955 m 954 w 958 vw 955vw,sh n34

841 w 840 m 841 m 841 m 840 vw 842 vw,b n35

823 vw, sh 834 vw, b

811 vw 810 vw 819 vw 814 w n36

755 s 754 s 756 m 757 m n37

583 m 582 s 581 s 581 s 584 w 586 m n38

565 m 563 s 565 s 564 s 565 w 564 w n39

500 w 501 w 498 m 497 m 502 w 500 w,sh n40

496 w 492 w 494 w,sh 495 m n41

456 vw n42

418 w 419 w n43

411 vw 412 vw 414 w n44

364 w 364 m 364 m n45

330 w 327 w,sh n46

306 vw 304 w 314 m n47

263 vw 267 w,b 265 w,b n48

243 vw 249 w,b n49, n50

162 w 156 s 164 s n51

138 w 138 m n52, n53

106 m n54

89 m n55

52 vw n56, n57

Weak bands in the regions 4000–3200, 2800–2300 and 2000–1800 cmK1 have been omitted.a Solvents used.b Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; b, broad.


Table 3

Calculated MP2 and DFT relative energies and dipole moments of conformers

and isomers of DAAPN and DASPN

Conformer E(MP2)

kJ molK1

E(B3LYP)

kJ molK1

mMP2

D

mB3LYP

D

DAAPN

EZ 0.00 0.00 4.97 5.15

EE 18.75 24.39 8.40 8.89

ZZ 19.56 23.55 5.61 5.74

DASPN

E 0.00 0.00 6.89 7.00

Z 23.08 23.10 5.02 4.89


(i) Both MP2 and DFT methods give the more planar

structure of amino group for DAAPN than for the

DASPN conformers. The calculated structure from DFT

indicates entirely planar character of the amino group

mainly for DAAPN.

(ii) The bond lengths as well as the dihedral angles imply

that for both compounds the DFT method predicts more

conjugated structures than MP2.

The length of double CaC bond, which reflects the extent of

conjugation in such type of compounds, was found to be higher

for DAAPN as for DASPN. This is in accordance with slightly

lower CaC stretching frequency of DAAPN in comparison

with DASPN. This phenomenon can be explained by stronger

electron acceptor properties of the acetyl group than of the

methylsulfonyl one. On the contrary, the length of aC–N bond

in these compounds becomes shorter and the bond order

increases due to the connection of electron pair on nitrogen into

the conjugation with CaC bond.

Due to the conjugation effect the aC–C(O) bond length also

becomes somewhat shorter than in non-conjugated molecules

or in comparison with the experimental length of this bond of

1.494 A in parent methyl vinyl ketone [26]. This conjugation

apparently stabilizes the Z or E in-plane arrangement of CaC

and CaO bonds in both found conformers of DAAPN what is

in agreement with the series of a, b-unsaturated aldehydes and

ketones [27,28]. As it was mentioned above, DFT method gives

entirely planar structure of DAAPN whereas MP2 gives about

28 deviation from the planar arrangement. Although the main

bond lengths show on large conjugation in DAAPN, relatively

small permanent dipole moment of the most stable conformer

Fig. 5. The structure and numbering of the studied compounds.

(ca. 5 D) does not indicate that this molecule might posses

marked non-linear optical properties as for example previously

studied malononitrile derivatives [6,14].

The only stable conformation of DASPN was found at the

dihedral angle between one of the oxygen and C2 atom of

double CaC bond at approximately 38. Similar result was

obtained previously from ab inito study of 2-methylsulfonyl-2-

butene [29]. Such arrangement, when CaC and SaO bonds are

eclipsed in the same plane, is usually denoted as anticlinal

conformation and stabilizes the given configuration. According

to all preceding studies, the anticlinal conformation is the most

stable one in vinyl sulfones and in E-isomers of b-substituted

vinyl sulfones [21,29–34].

In the previous work [12] slightly non-planar character of

amino group for (dimethylamino)methylene propanedinitrile

[(H3C)2N–CHaC(CN)2] (DMAM) has been found by geome-

try optimization at Hartree–Fock level of theory in DZP basis.

Due to the different approach we could not compare the

calculated geometries of this similar compound with DAAPN

and DASPN. Therefore, we have recalculated the geometry of

DMAM by both MP2 and B3LYP methods in 6-31G** basis

set. The obtained values are 1.376 and 1.384 A for CaC and

1.346 and 1.342 A for aC–N bond lengths, respectively. The

CaC bond length of DMAM does not differ substantially from

DAAPN. It implies the similar electron acceptor properties of

both cyano and acetyl group. On the other side, the calculated

non-planarity of amino group of DMAM is slightly higher than

that obtained for our studied compounds.

The relative energies of single conformers obtained in vacuo

were corrected by including solvent effects into the calcu-

lations. For this purpose, Isodensity Surface Polarizable

Continuum Model (IPCM) and Polarizable Continuum Model

(PCM) were used for the evaluation of energy differences

between the conformers and isomers of DAAPN. The results

will be discussed later.

3.3. Conformational analysis

The conformational possibilities of the studied compounds

are evidently restricted only to the nature of electron acceptor

groups. The above mentioned high conjugation expanding

through the whole molecule can significantly influence the

rotational barriers of single and double bonds as well as the

ground and excited states energies of single conformers. So the

conformational behaviour of such types of compounds is not

easy to predict. Particularly the polarity of solvent has the great

influence on the charge distribution inside these molecules [8–

9].

As reported previously, the rotation of methylsulfonyl group

is characterized by two minima with the same energy near the

anticlinal conformations corresponding to the CaC–S–C

dihedral angles of 120 and 2408, when one of the SaO bonds

is parallel with double CaC bond [29]. So, the only stable

conformation of this group should be considered. Our

theoretical calculations also revealed only one stable confor-

mation of E-DASPN with C2aC1–S6–C9 dihedral angle of

K1188 obtained by the MP2 method. The infrared and Raman

Table 4

Optimized geometric parameters of DAAPN and DASPN (with 6-31G** basis set)

Coordinatea EZ-DAAPN EE-DAAPN E-DASPN

MP2 B3LYP MP2 B3LYP MP2 B3LYP

Bonds (A)

C1aC2 1.375 1.382 1.375 1.380 1.371 1.376

C1–C7 1.424 1.420 1.430 1.427 1.418 1.412

C2–N3 1.342 1.340 1.350 1.347 1.344 1.342

C2–H6 1.085 1.088 1.083 1.085 1.084 1.087

N3–C4 1.457 1.460 1.457 1.460 1.457 1.460

N3–C5 1.456 1.460 1.455 1.458 1.456 1.460

C4–H9 1.091 1.096 1.091 1.096 1.092 1.096

C4–H10 1.088 1.096 1.088 1.096 1.087 1.096

C4–H11 1.085 1.086 1.084 1.085 1.085 1.086

C5–H12 1.086 1.090 1.087 1.091 1.092 1.097

C5–H13 1.092 1.097 1.093 1.097 1.086 1.090

C5–H14 1.090 1.097 1.090 1.097 1.089 1.096

C16–H17 1.086 1.090 1.086 1.090 1.088 1.092

C16–H18 1.090 1.095 1.090 1.096 1.087 1.091

C16–H19 1.090 1.095 1.090 1.096 1.086 1.090

C7bN8 1.186 1.168 1.185 1.165 1.187 1.168

C1–X15 1.486 1.491 1.480 1.486 1.775 1.800

X15–C16 1.511 1.516 1.518 1.527 1.783 1.806

X15aO20 1.234 1.226 1.233 1.222 1.473 1.476

S15aO21 1.467 1.468

Angles (8)

C1aC2–N3 131.2 133.3 130.8 133.0 130.1 131.9

C2–N3–C4 123.3 125.8 123.4 126.1 122.8 125.5

C2–N3–C5 119.8 119.9 119.4 119.9 119.5 119.9

C1aC2–H6 113.8 112.3 116.6 115.2 115.0 113.9

N3–C4–H9 111.6 109.7 111.9 109.6 111.4 110.0

N3–C4–H10 108.1 109.7 107.8 109.6 108.2 109.6

N3–C4–H11 110.1 111.3 110.1 111.4 110.3 111.3

N3–C5–H12 109.5 110.2 109.6 110.5 110.9 110.4

N3–C5–H13 110.8 110.4 111.2 110.5 109.5 110.2

N3–C5–H14 109.8 110.4 109.7 110.5 109.7 110.3

C2aC1–C7 125.4 125.7 124.2 124.4 128.3 128.9

C1–C7bN8 177.3 178.2 179.2 179.8 176.7 177.3

X15–C16–H17 108.7 108.9 107.5 107.5 106.8 106.4

X15–C16–H18 110.5 110.8 111.3 111.8 108.9 108.8

X15–C16–H19 110.5 110.8 111.4 111.8 108.9 109.1

C2aC1–X15 115.9 115.3 120.9 120.7 116.3 115.5

C1–X15–C16 117.0 117.4 119.4 119.6 103.4 103.9

C1–X15aO20 121.6 121.4 120.3 120.5 107.2 106.8

C1–S15aO21 108.6 109.1

Dihedral angles (8)

C1aC2–N3–C4 K1.5 0.0 K0.9 0.0 K3.9 K1.6

C1aC2–N3–C5 K170.4 K180.0 K166.6 K180.0 K169.5 K178.6

N3–C2aC1–X15 K177.3 K180.0 K175.6 K180.0 179.5 176.7

N3–C2aC1–C7 5.7 0.0 7.6 0.0 2.6 K0.8

H6–C2–N3–C7 K179.9 180.0 K178.8 180.0 K178.1 179.5

C2–N3–C4–H9 K79.7 K120.6 K78.7 K120.6 K75.8 K114.9

C2–N3–C4–H10 160.3 120.7 161.3 120.7 164.2 126.2

C2–N3–C4–H11 41.4 0.0 42.4 0.0 45.3 5.8

C2–N3–C5–H12 K13.0 0.0 K17.9 K0.0 104.5 117.8

C2–N3–C5–H13 107.2 120.2 102.4 120.2 K15.7 K2.4

C2–N3–C5–H14 K132.7 K120.2 K137.3 K120.2 K135.3 K122.5

C2aC1–X15–O20 2.0 0.0 K178.1 K180.0 2.7 4.4

C2aC1–X15–O21 K129.6 K127.3

C2aC1–X15–C16 K177.9 K180.0 2.1 0.0 116.6 118.5

C1–C15–C16–H17 179.8 K180.0 179.6 180.0 177.1 176.5

C1–C15–C16–H18 K59.4 K59.1 K61.0 K60.6 58.1 57.6

C1–C15–C16–H19 59.0 59.1 59.9 60.6 K63.4 K64.1

Coordinates describing linear part of molecule (8)

C1C7N8X15 177.3 178.2 179.5 180.2 177.2 177.9

C1C7N8X15 180.3 180.0 181.1 180.0 177.3 177.1



measurements in solid phase and in solvents of different

polarity also pointed out on the existence of single conformer

of E-DASPN either in solid and condensed phase.

In the case of DAAPN we have supposed the existence of

two E-conformers arising from the rotation of whole acetyl –

COCH3 group about the single aC–COCH3 bond and, as

already mentioned, denoted by the second Z or E letter

according to the orientation of the carbonyl oxygen (towards or

from the double CaC bond).

NMR spectroscopy, except hindered rotation, is not very

suitable method to detect the different conformations of

molecules at the room temperature due to fast exchange

between them. We have successfully used 1H NMR spec-

troscopy in our previous study of (methylamino)methylene

propanedinitrile [H3C–NH–CHaC(CN)2] [12] and 1-(methy-

lamino)ethylidene propanedinitrile [H3C–NH–C(CH3)

aC(CN)2] [13], where in DMSO solutions the rotation around

aC–N bond was sufficiently hindered even at the room

temperature. Both the anti and syn conformers arising by this

rotation could be then distinguished also by NMR. The same

behaviour was observed also for both the studied compounds.

The rotation around the aC–N bond is slow enough and two

separate signals for both methyls in amino group are visible in

NMR spectra [23].

On the contrary, the rotation of acetyl group is much faster

and results only in averaged unresolved signals in 13C NMR

spectra at ambient temperature. This means that the coalesc-

ence temperature for this conformational transition lies below

the room temperature. Therefore, we have used both IR and

Raman spectroscopy as the more suitable techniques for

conformational analysis. Carbonyl group and CaC double

bond in DAAPN molecule give very strong well separated

bands in IR and Raman spectra in the region 1680–1550 cmK1

which belong to the so-called group frequencies allowing to

observe the conformational transitions (Figs. 3 and 4).

Fig. 6. The comparison of IPCM and PCM model in evaluation of relative

energy differences between EE and EZ conformers of DAAPN depending on

relative permittivity of environment: C IPCM/MP2/6-31G**, 6 PCM/DFT,

, PCM/MP2. PCM computations were carried out in 6-31CCG** basis set

and simultaneously with geometry optimization.

Theoretical calculations show that the most stable con-

former should be the EZ-one with dipole moment of w5 D,

while the calculated dipole moment for the second EE-

conformer is about 8.5 D. The calculated relative energy of

EE-DAAPN was found to be rather high, about w19 and

w24 kJ molK1 according to MP2 and DFT methods, respect-

ively. These values are on the same level as the relative energy

for the only found ZZ conformer of Z-DAAPN isomer. The

higher dipole moment of EE-conformer comparing to ZZ-one

causes that the relative energy of EE conformer decreases and

ZZ conformer increases in more polar solvent (Fig. 7 in Ref.

[23]). Due to the presented facts and probably also due to the

height of the rotational barrier about CaC bond the probability

of the isomerization of E-DAAPN is negligible. In accordance

with it, no evidence for isomerization process of E-DAAPN in

the solvents of various polarity was found by NMR

measurements.

However, the relative energy differences between the EZ

and EE conformers in highly polar solutions obtained by IPCM

model are still somewhat overestimated. The energy difference

of w15 kJ molK1 for DMSO solution is rather high as could be

expected from experiment. It can be explained by the fact that

IPCM model is static only. It means that during calculations no

further geometry optimisation is performed. However, accord-

ing to previous reports, the polarity of solvent can significantly

affect also the geometry of some systems [35,36]. So we

employed another PCM model, which provides also the

geometry optimisation in the presence of environment.

Simultaneously, expanded 6-31CCG** basis set including

also the diffuse functions on heavy atoms and hydrogens was

used. Using such approach, the energy difference between the

EZ and EE conformers lowered to w8 kJ molK1 at DFT level

and nearly to 5 kJ molK1 at MP2 level, as depicted in Fig. 6.

Described theoretical results correlate with the behaviour of

bands intensities belonging to single conformers of DAAPN.

The identity of the appropriate conformers may be

confirmed also by Dn difference between the CaO and CaC

stretching frequencies [37]. As it has been found for series of

acyclic a, b unsaturated aldehydes and ketones, the difference

in position of these two stretching modes is higher than

60 cmK1 for Z and below the 60 cmK1 for E-conformers

according to the orientation of oxygen with respect to the

double CaC bond.

In DAAPN, Dn is 69 cmK1 for bands present both in

acetonitrile solution and in solid phase and 38 cmK1 for those,

which are present only in acetonitrile solution. Both Dn values

are taken from Raman measurements in acetonitrile solution. It

implies again that the EZ-conformer will crystallize preferen-

tially (i.e. the only solid phase conformer) while in polar

solvents there will be a mixture of both EZ and EE conformers

of DAAPN.

To obtain the experimental enthalpy difference between

both conformers we have used the change of band intensities

ratio from the temperature measurements of infrared spectrum

of DAAPN in acetonitrile. This is depicted in Fig. 7 for the

bands at 617 and 634 cmK1 related to the CaO rocking

deformation mode of EZ and EE conformers, respectively. The


same trend was found for the next visible bands belonging to

both conformers. Temperature dependence of CaO rocking

mode was also used for the evaluation of experimental enthalpy

difference between these conformers in acetonitrile since this

cannot be obtained from NMR measurements as mentioned

above. This mode gives also the most suitable peaks of both

conformers for mathematical decomposition. The value of

2.2G0.3 kJ molK1 was obtained from the slope of linear

dependence of the logarithm of band areas ratio on 1/T which

results from van’t Hoff equation. The ratio of bands areas at

every temperature was achieved by the deconvolution of the

overlapped bands into two Lorentz functions. Comparing this

experimentally determined enthalpy difference with those

calculated by IPCM and PCM models indicates that the latter

(together with geometry optimisation in every solvent) gives

better results than static IPCM model.

3.4. Normal coordinate analysis

The calculations of vibrational frequencies at MP2 or DFT

level of theory include electron correlation and improve their

prediction substantially. Nevertheless, they are still several

percent higher than the experimental values. For better

correlation between the experimental and theoretical frequen-

cies the ab initio force field needs to be scaled by appropriate

scale factors Fij(scaled)ZFij(ab initio)(xixj)1/2, where xi and xj

are the scale factors for i and j internal coordinates,

respectively [38]. Generally, one scale factor can be used for

all internal coordinates. However, if there are some modes in

vibrational spectra, which are clearly separated from the other

modes, it is possible to use the individual scale factors for the

internal coordinates connected with these modes. Therefore, in

the studied molecules we have used the individual scale factors

for C–H stretching, CbN stretching, CaO stretching, CaC

stretching, H–C–H bending and aCH out of plane bending.

The special scaling factor was used also for C–CbN bending,

because (as can be seen from geometry calculations for the

CbN bond length) there are the highest discrepancies between

ab initio MP2 and DFT methods. The individual scale factor

was used also for all bending coordinates in methylsulfonyl

group. All other internal coordinates have been scaled by one

common scale factor. The values of the mentioned nine scale

Fig. 7. Temperature dependence of infrared spectrum of DAAPN in the region

of 500–660 cmK1 in acetonitrile solution.

factors for MP2 and DFT B3LYP frequencies in 6-31G** basis

set have been obtained by a least square fitting procedure of the

279 experimental and calculated vibrational frequencies at the

same level of the theory for parent compounds of our

molecules: acrylonitrile [39], vinylidene cyanide [40], mal-

eonitrile and fumaronitrile [41] containing cyano groups on

ethylene, methyl vinyl ketone [26,28,42], acraldehyde [27,43],

methacrolein [28,44] and trans-crotonaldehyde [28,45] con-

taining carbonyl group in E and Z conformations towards the

CaC double bond, N-methylvinylamine [46], dimethylamine

[47,48], d0 and d6 dimethylsulfone [49,50]. The obtained scale

factors are collected in Table 5. The calculated scaled

frequencies and the experimental ones are compared in Tables

6 and 7. The assignment of harmonic vibrational modes was

done on the base of potential energy distribution (PED).

The highest vibrational frequencies for both the investigated

compounds should belong to the aC–H stretching vibration of

ethylene group, usually positioned above 3000 cmK1. But

according to the potential energy distribution, this band was

found to be localized somewhere between the asymmetric

stretching vibrations of methyl groups. It might be explained

by weakening CaC double bond due to high conjugation inside

the molecules resulting in the shift of aC–H stretching mode to

lower frequencies. The C–H asymmetric and symmetric

vibrations of three methyl groups in the region around

3000 cmK1 are also difficult to assign more accurately without

isotopic studies due to the overlap of some of them. Normal

coordinate calculations show certain differences between the

used MP2 and B3LYP method. Probably due to immediate

vicinity of these bands, their positions are sometimes

interchanged.

The almost about 0.02 A shorter CbN bond calculated for

both compounds by DFT method is connected with the

substantially higher non-scaled frequency of CbN stretching

mode than the MP2 method (about 2200 cmK1 for MP2 and

2300 cmK1 for DFT). On the other hand, the scaled stretching

modes of cyano group are about 40 cmK1 overestimated by

using MP2 method, whereas DFT method gives only small

deviation from experimental values for both compounds. Both

methods use the scaling factors developed on the same model

compounds and therefore, based on the above frequency

differences, the DFT method better reflects the changes

Table 5

Scaling factors for MP2 and B3LYP force field

Internal coordinate Scale factor

MP2 B3LYP

C–H stretch 0.866 0.916

CbN stretch 1.063 0.900

H–C–H deformation 0.872 0.931

C–CbN deformation 1.196 0.969

SO2 deformation 1.091 1.237

CaC stretch 0.900 0.902

CaO stretch 0.944 0.902

aC–H wagging 0.964 0.930

All other 0.928 0.966

Table 6

Comparison of calculated and observed IR vibrational frequencies for EZ and EE conformers of DAAPN

EZ-DAAPN EE-DAAPN

No. MP2 B3LYP Observed MP2 B3LYP Observed Fundamental PED(MP2)a

n1 3046 3071 3076 3049 3077 CH3 asym stretch 86 (N)CH3 as

n2 3037 3043 3029 3042 3073 CH3 asym stretch 98 (C)CH3 as

n3 3030 3034 3029 3034 3031 CH3 asym stretch 75 (N)CH3 as, 20 aC–H s

n4 3021 3025 3018 3020 3016 C–H stretch 79 aC–H s, 19 (N)CH3 as


n6 2998 2959 2989 2996 2957 CH3 asym stretch 100 (C)CH3 as


n8 2916 2922 2925 2913 2915 CH3 sym stretch 98 (C)CH3 ss

n9 2912 2909 2925 2912 2907 CH3 sym stretch 91 (N)CH3 ss

n10 2899 2902 2880 2895 2899 CH3 sym stretch 93 (N)CH3 ss

n11 2241 2197 2197 2241 2216 CbN stretch 87 CbN s, 13 aC–CN s

n12 1717 1692 1672 1690 1680 1652 CaO stretch 72 CaO s, 11 CaC s

n13 1648 1616 1605 1667 1634 1614 CaC stretch 32 CaC s, 38aC–N s, 21 CaO s

n14 1486 1489 1481 1490 1500 CH3 asym bend 49 (N)CH3 ad, 14 (N)CH3 r

n15 1461 1464 1453 1461 1466 CH3 asym bend 76 (N)CH3 ad



n18 1433 1439 1427 1440 1448 CH3 sym bend 81 (N)CH3 sd

n19 1433 1438 1427 1437 1439 CH3 asym bend 87 (C)CH3 ad

n20 1425 1429 1420 1435 1439 CH3 asym bend 83 (C)CH3 ad

n21 1420 1424 1420 1423 1433 CH3 sym bend 57 (N)CH3 sd

n22 1416 1410 1413 1410 1417 aC–N stretch 12 aC–N s, 40 (N)CH3 sd

n23 1375 1377 1367 1393 1388 1386 aC–H rock 58 aC–H r

n24 1366 1364 1346 1357 1352 CH3 sym bend 83 (C)CH3 sd

n25 1292 1270 1286 1307 1285 1300 aC–CO stretch 30 aC–CO s, 25 NC2 as, 13 CaC–N d

n26 1233 1217 1222 1231 1211 NC2 asym stretch 26 NC2 as, 16 aC–CO s, 15 (N)CH3 r

n27 1143 1154 1139 1144 1154 CH3 rock 85 (N)CH3 r

n28 1133 1136 1139 1128 1130 CH3 rock 40 (N)CH3 r, 11 C–N s

n29 1106 1110 1105 1105 1109 CH3 rock 82 (N)CH3 r

n30 1069 1064 1056 1070 1067 CH3 rock 68 (N)CH3 r, 23 NC2 as

n31 1050 1042 1041 1048 1040 CH3 rock 24 (C)CH3 r, 14 C–CN s, 12 CC2 r

n32 1021 1027 1024 1018 1023 CH3 rock 67 (C)CH3 r, 14 CaO u

n33 967 969 993 935 926 949 CO–CH3 stretch 25 CO–CH3 s, 37 (C)CH3 r

n34 962 963 970 909 916 aC–H wag 75 aC–H u

n35 838 834 852 852 854 NC2 sym stretch 66 NC2 ss, 10 CO–CH3 s

n36 803 807 807 804 806 aC–CN stretch 23 C–CN s, 29 CaC–N d, 14 NC2 r

n37 612 636 622 621 645 634 CaO rock 28 CaO r, 18 aC–CO s

n38 571 612 617 589 622 CaO wag 54 CaO u, 24 CC2 u,?23 C–CbN d

n39 539 542 552 524 536 C–CbN bend 13 CC2 d, 23 CaO r, 19 C–CbN d

n40 528 535 544 494 488 507 C–CbN bend 37 C–CbN d, 27 CaO u

n41 463 464 466 476 468 NC2 deformation 40 NC2 d, 18 CC2 d

n42 440 437 453 399 406 NC2 rock 16 NC2 r, 18 NC2 d, 24 C–C–C d

n43 398 400 426 388 377 CaC torsion 19 CaC t,?22 C–CbN d, 20 C–N t

n44 338 342 362 364 377 C–C(O)–C deformation 48 C–C–C d, 16 NC2 r

n45 292 298 293 290 294 CaC–N deformation 15 CaC–N d,?27 NC2 r, 20 CC2 r

n46 238 275 285 230 274 CC2 wag 18 CC2 u, ?35 NC2 u, 16 N–CH3 t

n47 172 176 180 175 170 N–CH3 torsion 58 N–CH3 t, 36 CC2 u

n48 165 161 163 169 165 CC2 rock 29 CC2 r, 17 N–CH3 t, 16 NC2 d

n49 157 153 158 167 161 CC2 deformation 47 CC2 d,?42 C–CbN d

n50 135 136 112 161 159 C–CH3 torsion 87 C–CH3 t, 13 (C)CH3 r

n51 114 124 112 125 133 NC2 wag 52 NC2 u, 48 N–CH3 t

n52 99 79 84 110 74 N–CH3 torsion 70 N–CH3 t, 15 CaC–N d

n53 63 65 76 62 60 aC–CO torsion 91 aC–CO t

n54 51 51 44 49 C–N torsion 44 C–N t, 38 CaC t, 13 CC2 u

a s, symmetric; a, asymmetric; s, stretch; d, deformation; r, rocking; u, wagging; t, torsion.


between the model and the studied compounds. The CbN

stretching modes were found at the same frequency of

2194 cmK1 for both the compounds, what is about 10–

20 cmK1 lower than for the AE and its N-methyl derivatives

[13] and 15–30 cmK1 lower than for the AM and its N-methyl

derivatives [12]. This downshift indicates the higher conju-

gation through a cyano group in DAAPN and DASPN than in

previously studied compounds resulting in decreasing multiple

Table 7

Comparison of calculated and observed IR vibration frequencies of E-DASPN

No. MP2 B3LYP Observed Fundamental PED (MP2)a

n1 3049 3068 3040 CH3 asym stretch 92 (S)CH3 as

n2 3042 3053 3040 CH3 asym stretch 91 (N)CH3 as

n3 3040 3052 3040 CH3 asym stretch 92 (S)CH3 as

n4 3034 3045 3040 C–H stretch 72 aC–H s, 22 (N)CH3 as

n5 3026 3023 3013 CH3 asym stretch 60 (N)CH3 as, 27 aC–H s



n8 2928 2944 2922 CH3 sym stretch 100 (S)CH3 ss

n9 2911 2911 2881 CH3 sym stretch 90 (N)CH3 ss

n10 2900 2903 2881 CH3 sym stretch 93 (N)CH3 ss

n11 2242 2197 2202 CbN stretch 87 CbN s, 14 aC–CN s

n12 1674 1647 1638 CaC stretch 47 CaC s, 41 aC–N s, 12 aC–H r

n13 1487 1496 1474 CH3 asym bend 43 (N)CH3 ad, 13 (N)CH3 r

n14 1461 1467 1458 CH3 asym bend 68 (N)CH3 ad

n15 1448 1457 1458 CH3 asym bend 80 (N)CH3 ad

n16 1438 1446 1434 CH3 sym bend 50 (N)CH3 sd, 36 (N)CH3 ad

n17 1437 1439 1434 CH3 asym bend 54 (N)CH3 ad, 36 (N)CH3 sd

n18 1421 1431 1423 CH3 sym bend 87 (N)CH3 sd

n19 1417 1421 1417 CH3 asym bend 85 (S)CH3 ad

n20 1413 1417 1417 CH3 asym bend 47 (S)CH3 ad, 14 (N)CH3 sd

n21 1409 1414 1411 aC–N stretch 39 (S)CH3 sd, 11 aC–N s

n22 1383 1387 1379 aC–H rock 59 aC–H r, 10 aC–CN s

n23 1335 1324 1319 CH3 sym bend 100 (S)CH3 sd

n24 1311 1290 1309 SO2 asym stretch 93 SO2 as

n25 1263 1239 1248 NC2 asym stretch 52 NC2 as, 17 (N)CH3 r,

n26 1144 1153 1153 CH3 rock 82 (N)CH3 r

n27 1129 1132 1144 CH3 rock 39 (N)CH3 r, 13 aC–N s

n28 1121 1110 1137 SO2 sym stretch 72 SO2 ss

n29 1106 1099 1122 CH3 rock 79 (N)CH3 r

n30 1069 1065 1060 CH3 rock 60 (N)CH3 r, 24 NC2 as

n31 1038 1009 1037 aC–S stretch 26 aC–S s, 22 C–CN s, 17 aC(S)C r

n32 984 978 973 CH3 rock 67 (S)CH3 r

n33 980 975 962 CH3 rock 62 (S)CH3 r

n34 958 958 957 aC–H wag 80 aC–H u

n35 845 839 841 NC2 sym stretch 63 NC2 ss, 11 aC–S s

n36 813 816 811 aC–CN stretch 29 CaC–N d, 21 aC–CN s, 13 NC2 r

n37 763 741 755 S–CH3 stretch 58 S–CH3 s, 15 SO2 u

n38 596 609 583 C–CbN bend 41 C–CbN d, 20 aC(S)C u, 12 SO2 u

n39 569 558 565 SO2 scissors 30 SO2 d, 17 aC–S s, 13 SO2 u

n40 509 500 500 SO2 wag 23 SO2 u, 16 C–CbN d

n41 497 481 496 C–CbN bend 36 SO2 d, 16 C–CbN d

n42 445 438 456 NC2 deformation 55 NC2 d

n43 415 425 418 SO2 rock 40 SO2 r, 20 NC2 r

n44 400 393 414 CaC torsion 23 CaC t, 18 aC–H u, 16 aC–N t

n45 362 367 365 NC2 rock 27 SO2 r, 18 NC2 r

n46 318 330 330 SO2 twist 76 SO2 t, 14 C–S–C d

n47 288 306 306 C–S–C deformation 43 C–S–C d, 14 SO2 t, 14 NC2 r

n48 267 264 263 CaC–N deformation 13 CaC–N d, 13 aC–S s, 11 aC(S)C r

n49 221 246 243 S–CH3 torsion 81 S–CH3 t

n50 216 207 243 NC2 wag 19 NC2 u, 34 N–CH3 t, 16 S–CH3 t

n51 156 147 162 aC(S)C rock 43 aC(S)C r, 22 NC2 d, 20 N–CH3 t

n52 145 142 138 aC(S)C deformation 32 aC(S)C d, 30 N–CH3 t,17 C–CbN d

n53 133 137 138 N–CH3 torsion 31 N–CH3 t, 22 C(S)C d, 22 NC2 u

n54 100 114 106 aC(S)C wag 25 aC(S)C u, 34 N–CH3 t, 14 NC2 u

n55 96 68 89 N–CH3 torsion 56 N–CH3 t, 13 CaC–N d, 12 NC2 u

n56 56 47 52 aC–N torsion 48 aC–N t, 36 CaC t

n57 46 30 52 C–S torsion 78 aC–S t, 22 CaC t

a s, symmetric; a, asymmetric; s, stretch; d, deformation; r, rocking; ,u wagging; t, torsion.


character of this bond. This can be caused by the different

nature of investigated compounds due to the presence of acetyl

and methylsulfonyl group, respectively. There is also the small

contribution of aC–CN stretching vibration to this mode.

The CaO stretching mode for EZ-DAAPN and EE-DAAPN

in solution was found at 1672 and 1658 cmK1. The higher

frequency of CaO mode of EZ-conformer is in agreement with

the band positions of methyl vinyl ketone: 1704 cmK1 for EZ-


conformer and 1676 cmK1 for the EE-one [42]. Normal

coordinate calculations indicate the slight coupling of this

mode with CaC stretching vibration and 20–30 cmK1 down-

shift indicates higher conjugation in comparison with methyl

vinyl ketone.

The CaC stretching frequency is almost about 50 cmK1

higher for DASPN than for DAAPN. This observation

indicates higher conjugation in DAAPN than in DASPN. The

CaC frequency of DAAPN at 1596 cmK1 is significantly

lower than that of previously described DMAM and DMAE

[12,13] as well as methyl vinyl ketone at 1620 cmK1 [42]. The

strong coupling was found for CaC stretching mode and

according to PED calculations there is high contribution of

aC–N stretching mode for both compounds. As a consequence

of particularly multiple character of aC–N bond in such

conjugated molecules, the appropriate stretching vibration can

be supposed to be shifted to higher wavenumbers. We have

found this band at 1411 cmK1 for DAAPN and at 1379 cmK1

for DASPN what is also in accordance with calculated more

conjugated structure of DAAPN in comparison with DASPN.

The intense bands at 1286 cmK1 for DAAPN and

1300 cmK1 for DASPN have different origins. While for

DAAPN this band can be assigned to the aC–CO stretching

vibration coupled with N–CH3 asymmetric stretching, for

DASPN it belongs to asymmetric SO2 stretching mode.

Symmetric SO2 stretching vibration was found at

1140 cmK1. Both these values for SO2 modes are close to

that for methyl vinyl sulfone at 1303 and 1135 cmK1 [50].

The next common vibrations for both compounds are NC2

asymmetric and symmetric stretching frequencies, which

appear at 1218 and 830 cmK1 for DAAPN and about 30 and

10 cmK1, respectively, higher at 1248 and 841 cmK1 for

DASPN. Similarly, the aC–H rocking and wagging vibrations

for both the compounds occur in the same region. The former

one at 1367 and 1379 cmK1 and the latter one at 970 and

955 cmK1 for DAAPN and DASPN, respectively, indicate no

dramatic influence of acetyl and methylsulfonyl groups on

these vibrations.

The isolated band at 755 cmK1 with strong intensity can be

assigned to S–CH3 stretching frequency. On the contrary, the

aC–S stretching mode was found at rather higher frequency of

1035 cmK1. For the earlier studied methyl phenyl sulfone these

bands are at 786 and 1084 cmK1, respectively, and the latter is

strongly coupled with C–C stretching vibrations of aromatic

nucleus and C–H skeletal deformation vibrations [51]. In

DASPN the aC–S stretching mode is mixed with aC–CN

stretching vibration and aC(S)(C) rocking deformation. For

SO2 deformation vibrations usually accepted intervals are

535G40 cmK1 for scissoring, 485G50 cmK1 for wagging,

405G65 cmK1 for twisting and 320G40 cmK1 for rocking

[52]. In agreement with this the first two modes at 565 cmK1

for scissoring and at 498 cmK1 for wagging have been

calculated and assigned. Interchanged positions were obtained

for the next two modes: 418 cmK1 for rocking and 330 cmK1

for twisting. Such opposite assignment was also recently

calculated for methanesulfonic acid monomer [53].

Methyl rocking vibrations of –SO2CH3 group are according

to both MP2 and B3LYP methods separated from N(CH3)2

rocking vibrations and are located at lower frequencies. For

example, the methyl symmetric deformation mode was found

in the expected region [52] at 1346 and 1319 cmK1 for

DAAPN and DASPN, respectively, while the corresponding –

N(CH3)2 modes are over 1400 cmK1.

The assignment of other deformation bands of DAAPN and

DASPN is difficult and not very unambiguous due to large

coupling between single deformation modes and relatively

small contributions to PED. Moreover, NC2 rocking and CaC

torsion mode were determined for both the compounds at

approximately similar frequency about 400 and 360 cmK1,

respectively. In the assignment of other bands the relatively

high uncertainty can exist since the experimental frequencies

for similar compounds had not been reported and described yet.

4. Conclusions

The assignment of normal vibrational modes of two simple

push–pull ethylenes was performed. The vibrational analyses

supported by theoretical calculations in vacuo and in the

environment of various polarity revealed the existence of two

conformers of DAAPN originating in the rotation of acetyl

group. Unlike the previous study of methyl vinyl ketone, EZ-

DAAPN was proven to be the most stable conformer.

The experimental enthalpy difference in acetonitrile

solution was obtained from infrared measurements at various

temperatures. The value of 2.2G0.3 kJ molK1 is more close to

those obtained by PCM solvent effect calculations performed

with the geometry optimization at ab initio MP2 level.

Comparing with stationary IPCM model, the PCM method

seems to be more suitable in predicting energy changes

occurring due to the interactions between these compounds and

various solvents.

Acknowledgements

This work has been supported by Slovak Grant Agency

(Projects VEGA No. 1/0052/03 and 1/0058/03). We thank the

IBM Slovakia for computing facilities. Dr. Martin Breza is

gratefully acknowledged for helpful discussion.

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Documents

Vibrational analysis and conformational study of 3-methylamino-2-acetyl propenenitrile and 3-methylamino-2-methylsulfonyl propenenitrile