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RAMAN STUDIES OF CONFORMATIONAL ENERGIES
AND HYDROGEN BONDING IN ALCOHOLS
THESIS
Presented to the Graduate Council of the
North Texas State University in Partial
Fulfillment of the Requirements
For the Degree of
MASTER OF SCIENCE
By
Simindokht Maleknia, B.S.
Denton, Texas
August, 1982
AlO, 5O
Maleknia, Simindokht, Raman Studies of Conformational
Energies and Hydrogen Bonding in Alcohols. Master of
Science (Chemistry), August, 1982, 52 pp., 8 tables,
9 illustrations, bibliography, 20 titles.
The conformational energy differences have been deter-
mined for ethylene glycol, 2-chloroethanol, and 2,2-
dichloroethanol in the neat liquid, DMSO, and H20 with
Raman spectroscopy. Spectra in the 0-H valence region were
utilized to determine the energy difference between inter-
and intramolecularly hydrogen bonded species. It was found
that the solvent effect on the relative stabilities of the
gauche and trans rotamers of the alcohols differ signifi-
cantly. The results also indicate that, unlike ethylene
glycol, there is significant intramolecular hydrogen bond
formation in the halogenated alcohols in the neat liquid
phase. Stronger intramolecular hydrogen bond formation
was observed in dichloroethanol than in 2-chloroethanol.
TABLE OF CONTENTS
PageLIST OF TABLES.. ............... . . . iv
LIST OF ILLUSTRATIONS .0... .... .......... v
Chapter
I. CONFORMATIONAL EQUILIBRIUM ANALYSIS WITHVIBRATIONAL SPECTROSCOPY . . .. .....
IntroductionConformational EnergiesExperimental
II. CONFORMATIONAL ENERGIES OF 2-CHLOROETHANOLAND ETHYLENE GLYCOL IN HYDROGEN BONDINGSOLVENTS . ..... . . .......... . 6
IntroductionExperimentalResults and Discussion
III. CONFORMATIONAL ENERGIES AND HYDROGEN BONDINGIN DICHLOROETHANOL.......... ......... 27
IntroductionExperimentalResults and Discussion
APPENDIX.................................. . .* . 47
BIBLIOGRAPHY .............. ..................... 51
iii
LIST OF TABLES
Table Page
I. Area Ratios for Neat 2-Chloroethanol . . . . . 8
II. Spectroscopic Parameters for 10 MolePercent 2-Chloroethanol in Water . . . . 9
III. Spectroscopic Parameters for 40 MolePercent 2-Chloroethanol in DMSO . . . . . 10
IV. Spectroscopic Parameters for 65 MolePercent 2-Chloroethanol in DMSO . . . . . 11
V. Calculated Equilibrium Constant for 10 MolePercent Ethylene Glycol in Water . . . . 13
VI. Solvent Dependence of AH (=HTrans-HGauche- 16
VII. Spectroscopic Parameters of the HydroxylStretching Modes ... .. ......... 34
VIII. Comparison of Energy Differences . . . . . . . 35
iv
LIST OF ILLUSTRATIONS
Figure Page
1. Temperature Dependence of Area Ratios forNeat 2-Chloroethanol (Squares), 40% inDMSO (Open Circles), 65% in DMSO (SolidCircles), and 10% in Water (Triangles) . . 15
2. High Frequency Raman Spectrum of2-Chloroethanol in the Neat LiquidPhase at Room Temperature ........ 20
3. High Frequency Raman Spectrum of 40%2--Chloroethanol in H20 . *... ....... 22
4. High Frequency Raman Spectrum of EthyleneGlycol in the Neat Liquid Phase . . . . . 24
5. High Frequency Raman Spectrum of DCE at-140C; Circles--Experimental Spectrum;Solid Line--Theoretical Curve . . . . . 31
6. High Frequency Raman Spectrum of DCE at+470C; Circles--Experimental Spectrum;Solid Line--Theoretical Curve......... 33
7. Temperature Dependence of Peak Frequenciesof DCE (Solid Circles) and Chloroethanol(Open Circles).................. . . . 39
8. Temperature Dependence of Line Widths ofDCE (Solid Circles) and Chloroethanol(Open Circles)............. . ........41
9. Temperature Dependence of RelativeScattering Intensities in DCE (SolidCircles) and Chloroethanol (OpenCircles) . . . . a . . . . . - -0 . . . . . 43
CHAPTER I
CONFORMATIONAL EQUILIBRIUM ANALYSIS WITH
VIBRATIONAL SPECTROSCOPY
Introduction
The techniques of infrared and Raman spectroscopy have
long provided a means for studying the conformational
behavior of various hydrocarbons and their derivatives in
different chemical environments. Distinct peaks arising
from the vibrations of the individual gauche and trans con-
formers can be observed because the time scale of molecular
vibrations (103-104 sec) is considerably shorter than
that for rotation about carbon-carbon single bonds. Thus,
variations in peak areas can be correlated to the influence
of temperature and environment (i.e. solvent) on the enthalpy
differences between the gauche and trans rotamers (1,2).
In this work, the conformational energies of some
alcohols have been investigated by Raman spectroscopy. The
ability of these molecules to form intermolecular and intra-
molecular hydrogen bonds influences the relative enthalpies
of the conformational states. The results of hydrogen
bonding effects in the liquid phase and in solution are
discussed. The conformational energy differences for
2
ethylene glycol, 2-chloroethanol and 2,2-dichloroethanol in
the neat liquid and some solvent systems with varying hydro-
gen bonding capacities are also presented.
Conformational Energies
The conformational energy differences (AH) are deter-
mined spectroscopically by measuring the temperature and
the solvent dependence of the relative intensities of
vibrational peaks assigned to the trans and gauche conformers
(3). In the gas phase this technique is best employed with
infrared spectroscopy because of the inherently greater
intensity of the IR absorptions. Raman spectroscopy is
preferred for the liquid phase and in solution due to the
small sample size and facile variation of temperature over
a broad range.
For the conformational equilibrium in disubstituted
ethanes, where the two gauche rotamers are equivalent, the
equilibrium constant can be expressed by:
2XT -AH/RT AS/RK =X G=e
XG
where XT and XG represent the mole fraction of the two
conformers.
In Raman spectroscopy, where the concentration is
directly proportional to the peak intensities, the equili-
brium constant is given by:
3
2NTAK- -A(2)
NGAGG G
where AT and AG are the integrated areas of the two bands;
NT and NG are the Raman scattering cross sections. By
substituting equation 2 into equation 1, the Van't Hoff
equation is obtained after taking the natural logarithm:
LN(AT/AG) = -AH/RT + C .
Enthalpy differences (AH) may be determined directly
-lfrom the slope of a plot of LN(AT/AG) vs T , where the
constant C is a function of both the entropy difference and
relative scattering cross sections of the two conformers.
Experimental
The samples of ethylene glycol, 2-chloroethanol, and
2,2-dichloroethanol were obtained commerically and were
purified by vacuum distillation prior to use. Solutions of
alcohols in the solvents DMSO and H20 were prepared gravi-
metrically. The samples were contained in sealed melting
point capillary tubes and were inserted in a standard
Harney-Miller cell. The temperature was regulated by either
heated air flow or liquid nitrogen boil-off through the
cell, and was measured with an iron-constantan thermo-couple.
The associated accuracy in temperature measurement was
determined to be +10C.
4
The excitation source was a Coherent Radiation CR-3
0
argon ion laser operated at 4800 A. Typical powers used
were in the range of 20 to 500 milliwatts. The scattered
light was collected at 900 to the incident radiation and
was analyzed through a Spex 14018 double monochromator,
followed by a RCA C31034 photomultiplier tube, Spex photon
counting electronics and a Linear Instruments 9011 recorder.
For determination of conformational energies, the
integrated areas of the peaks were calculated by multiplying
the height by the width at half height.
For hydrogen bonding studies, the Raman spectra in the
O-H valence region (3000-3700 cm 1 ) were acquired automa-
tically by an interfaced microprocessor (APPLE II+). In
this region, the two peaks were overlapping, and they were
resolved by a non-linear curve fitting procedure (4,5),
outlined in the Appendix, prior to area ratio calculations.
CHAPTER BIBLIOGRAPHY
1. Mizushima, S., Structure of Molecules and InternalRotation, New York, Academic Press, 1954.
2. Orville-Thomas, W. J., Internal Rotation in Molecules,New York, John Wiley and Sons, 1974.
3. Barnes, A. J., and Orville-Thomas, W. J., VibrationalSpectroscopy Modern Trends, New York, Elsevier,1977.
4. Horak, M., and Vitek, A., Interpretation and Processingof Vibrational Spectra, New York, John Wiley andSons, 1978.
5. Maddams, W. F., Appl. Spectrosc. 34, (1980), 245.
5
CHAPTER II
CONFORMATIONAL ENERGIES OF 2-CHLOROETHANOL AND
ETHYLENE GLYCOL IN HYDROGEN BONDING SOLVENTS
Introduction
Ethylene glycol and 2-chloroethanol differ from the
simple alcohols in their capacity to form intramolecular
hydrogen bonds; it is this common feature that is believed
responsible for their similar behavior in dilute carbon
tetrachloride solution (1) as well as in the vapor phase
(2,3). In both cases, the gauche molecular conformation is
observed to predominate, stabilized by this form of hydrogen
bonding.
Quantitative studies on the structural equilibrium of
ethylene glycol revealed the trans conformer to be greatly
stabilized by dilution in dimethylsulfoxide (4,5), a Lewis
base solvent. In contrast, a Raman spectrum of this mole-
cule in aqueous solution (4) appeared to indicate no signi-
ficant shift in the gauche-trans equilibrium; however,
quantitative experiments were not performed.
In order to determine whether the above mentioned
similarity between the two molecules exists in active,
hydrogen bonding solvents, the conformational energy
6
7
difference (AH=HT-HG) has been determined for 2-chloroethanol
at two concentrations in DMSO, and for both compounds in
dilute aqueous solution.
Experimental
Energy differences were calculated from the Van't Hoff
equation, utilizing 14 to 18 intensity (area) ratios mea-
sured for each sample at various temperatures throughout
the liquid range (typically from -100 C to +900 C).
Area ratios (AT/AG), for 2-chloroethanol (10 mole
percent) in water were obtained from the bands (7) at
750 cm~1 (trans C-Cl stretch) and 660 cm~1 (gauche C-Cl
stretch). Interference from solvent vibrations prohibited
the use of these bands in DMSO. However, it was found that,
in DMSO-d6 , area measurements on the C-C-0 bending modes
(7) at 395 cm~1 (trans) and 475 cm~1 (gauche) could be
obtained down to moderate concentration. The spectroscopic
parameters and area ratios are listed in Tables I to IV.
For ethylene glycol, the study of direct area ratios
(AT/AG) of the two conformers was limited due to solvent
interaction. However, the equilibrium constant (2XT/XG
was obtained from the bands (4) at 865 cm (C-C stretching
of both conformers) and 480 cm-1 (gauche, symmetric C-C-0
bending). At sufficiently low concentration in solvent,
the integrated intensity ratio, (A4 8 0/A86 5 ), approaches a
Y) (N O o % H ' LO H H(N qzr 0)(N HO Y 0000o
4H H*1 r4, IHHr4
o m r H r-001 m l (N ON M L)
r- r- r, r-01 kD N q:
H Ho o' e N H yH H I I
8
,40z
Pipq0
P
0
CN
z
O0
U)0
H-
E-1
4 U
ior4
OrdLH
rj
c4-)u
OH A0U
rd u
4
r.4 -)
z C)
y I)
rd UO4H
-I ci
w r-
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UC)o
H- eV o C% tO tor o NO oq 0 0 r 0 -0 0
V LO Iu L IC) N N Om
N Ns Ce H oh w H- CO COS 0 0 0 - 0 . 0 0
rr) C0) CC m C?) N qr H qi'
N% CN N CN N N N N N
E-4
z
0z
0
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zP4
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E-Q
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P40
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.
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co N N r-m LO
N N H H Hr-r- N
( co r% N CO 'ONll2 qq qcv N N 0 c o o
00HH40
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rN D L0 )C 0 CO H COm) CO 0% L OmC N N H-
0- 1
C ~
SCH
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H
U
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-- -
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Cm n k. N m r-I r-I )mL0 Lf) CO N '.0 H (N O'I 00
4c4 o N (N (N N N H Hr-
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(N '.Jo CC) 0 'V0 (N '0
o m c O O ) CO CONN H (N H H H H Hr- H
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1
LO)
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C- r-4 C4 r- CH H H riH
) C) Ci L0 C)
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HH1
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(N
r-")
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--- 1- 1
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NLC LO N O WCWO .
OC'n O M 00C0 N 00 e0 m 0m0 0 0 0 0H H o'- o 0 I H H
(N H H H (N HH H H
co ~ L H mL m m 0 0 mC4 0 0 0C N 0
H H HCN (N N C')CM m
___ __I_ _ _ _
NOH
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0
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4)
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E-q
LO LOr- H
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000
A _________-4_
LO 00 LOt,0 rl
00 F" a3 L
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o 0 0O0 H O N (N U'0 C 0 C) m N H H (
I I
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H
E-H
H
C)
C)
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4100
110r--
12
constant value (0.327) corresponding to a trans mole fraction
of unity (4), represented as:
A480A8 6 5T 0.327
XG T= -XT
and
K = 2XT/Xeq T G.
The area ratios and corresponding equilibrium constant are
listed in Table V.
Results and Discussion
The linear temperature dependence of the AT/AG ratio
for 2-chloroethanol in the neat liquid and at various concen-
trations in the two solvents, is displayed in Figure 1. The
calculated gauche-trans energy differences for ethylene
glycol and 2-chloroethanol are presented in Table VI.
Earlier studies (4,5) show that dilution of ethylene
glycol in DMSO has a most striking effect on the skeletal
equilibrium, stabilizing the trans conformer (or, alterna-
tively,- destabilizing the gauche form) by an amount greater
than 1.5 kcal/mole at only moderate dilution. The precise
opposite trend is observed in 2-chloroethanol, where dilution
in the same solvent actually induces a significant enhance-
ment of the relative stability of the gauche rotamer.
00 o 191, ) (
ON H H H H H
O 1000 0 m0 N%-0e LO MCIm n m10 0 OOCHHHHNH H H
0000000 o
E-1HwUP4
0
-40
HZOr4
E- Z
2H
zO
H 4
z
w
HE-
CI
E-
13
N oCO c\ H H
E-
E-H
4I-)
Co4-)
0 xUN
PH
PH
'H 0
01
0'H 10)(dO(d O
1) 00N 4t
0Go
-E-4Q-)H
14
Fig. 1--Temperature dependence of area ratios forneat 2-chloroethanol (squares), 40% in DMSO(open circles), 65% in DMSO (solid circles),and 10% in water (triangles).
15
1-0 1
03
10
3.6 4.6
10OO/T
0.0
40AP --,zo
-1-.o
-20
2.6: 1I a A
(k~ 1)
16
r-i0 ro ro o o
W 0 0 0 0C:) C)I 0 C
4)o 0 0 00
0 N +1 -H +1 +1
0 d -00 HLOr-O N 0 0 N
0 H H H
E1 r-V
m 00 C) ( D L0
UI+ .1 +1 + +1
0- -
H -r-I e00e
U>4<H H
u4 4 + + 1 +l +0 T0
E- 0 ri
O-O CtI N r- LIO
> 000 0 . 0. 0 0-0 C4 f4 P4P 4P H
v ++ I I 0
z +z a4 )r
[4 4
E-t a oa 0
D> c O O4-44-)4-4---
Hr ' o O o o~ O,
-a -r-* -i-
U) U 0\0W\0 0\0U)C01 LC)rnz -o l~
17
In aqueous solution, a marked difference in the behavior
of the two alcohols is observed. The conformational energy
difference of aqueous ethylene glycol is of the same magni-
tude as that in the neat liquid. (The small variation lies
within the range of experimental error in the measurements).
2-Chloroethanol, on the other hand, exhibits a 4 to 5 fold
increase in AH relative to the liquid phase value.
Although the complexity of these strongly hydrogen
bonded systems precludes any unique interpretation of the
above results, it is possible to furnish a tentative expla-
nation of the behavior of ethylene glycol in the two solvents.
In DMSO, the formation of two intermolecular hydrogen bonds
with solvent molecules is the likely cause for the shift in
equilibrium to the trans conformation (4). By contrast,
since the glycol contains two hydrogen bonding protons and
no hydrophobic groups, the relative invariance of the equili-
brium in water may reflect the ability of these molecules to
form water-like pseudolattice structures in the liquid as
well as in aqueous solution.
The observed stability of the gauche conformer of
2-chloroethanol in both solvents could be concomitant with
an increase in the degree of intramolecular hydrogen bonding,
similar to that observed in solutions of the haloethanols
in carbon tetrachloride (1). To investigate this possibility
further, the room temperature Raman spectrum of 2-chloro-
ethanol in the O-H stretching region (3200-3700 cm) as
18
obtained. In the neat alcohol (Fig. 2), the extremely
broad band centered around 3370 cm~1 is due to intermole-
cularly hydrogen bonded hydroxyl groups, while the narrower
peak in the vicinity of 3580 cm arises from intramolecu-
larly bonded species (8). Any increase in intramolecular
hydrogen bonding would be manifested clearly by a rise in
the intensity of this latter band. However, this peak is
diminished greatly, if present at all, in aqueous solution
(Fig. 3). A similar marked diminution in this band also
is exhibited in solution with DMSO. This indicates that
there is a decrease in intramolecular hydrogen bonding even
though the equilibrium is shifted towards the gauche confor-
mation. Thus, there is no simple relationship between the
conformational equilibrium and the hydrogen bonding properties
of 2-chloroethanol in complex solvents.
The major differences in conformational behavior of
the two alcohols extend also to the intra-intermolecular
hydrogen bonding equilibria. In stark contrast to neat
2-chloroethanol, the high frequency spectrum of ethylene
glycol (Fig. 4) reveals the virtually complete absence of
any intramolecular hydrogen bonds, both in the neat liquid
and in either solvent.
In conclusion, it may be seen that the apparent simi-
larities between 2-chloroethanol and ethylene glycol exhibited
in dilute CC 4 solution and in the vapor phase do not exist
in interactive solvent systems. From this, it may be
19
Fig. 2--High frequency Raman spectrum of2-chloroethanol in the neat liquidphase at room temperature.
20
3600 3400 3200
FREQUENCY (CM 1 )
"now
21
Fig. 3--High frequency Raman spectrum of40% 2-chloroethanol in H2 0.
22
3600 3400 3200
FREQUENCY (CM 1 )
23
Fig. 4--High frequency Raman spectrum ofethylene glycol in the neatliquid phase.
24
3600 3400 3200
FREQUENCY (CM )
25
inferred that differences in the properties of substituted
alcohols such as the two studied here from those of simple
alcohols are due, not directly to intramolecular hydrogen
bond formation, but rather arise from more complex factors.
CHAPTER BIBLIOGRAPHY
1. Krueger, P. J., and Mettee, H. D., J. Mol. Spectry. 18,(1965), 131.
2. Buckley, P., and Giguere, P. A., Can. J. Chem. 45,(1967), 397.
3. Buckley, P., Giguere, P. A., and Schneider, M., Can.J. Chem. 47, (1969), 901.
4. Schwartz, M., Spectrochim. Acta 33A, (1977), 1025.
5. Pruettiangkura, P., Ho, S., and Schwartz, M., Spectrosc.Letters 12, (1979), 679.
6. Davenport, D., and Schwartz, M., J. Molec. Struct. 1,(1978), 259.
7. Wyn-Jones, E., and Orville-Thomas, W. J., J. Molec.Struct. 1, (1967), 79.
8. Gupta, A., Davenport, D., and Schwartz, M., Spectrochim.Acta 36A, (1980), 601.
9. Kastha, G. S., Roy, S. B., and Nandy, S. K., Indian J.Phys. 46, (1972), 293.
26
CHAPTER III
CONFORMATIONAL ENERGIES AND HYDROGEN BONDING
IN DICHLOROETHANOL
Introduction
Vibrational spectroscopy has proven to be of consider-
able utility in the study of rotational isomerism about the
carbon-carbon bond in disubstituted ethanes (1,2). One
series of particular interest is the halogenated ethanols
(3-10) since they are among the simplest of molecules with
the capacity to form both intra and intermolecular hydrogen
bonds. Earlier studies have employed Raman spectroscopy to
determine the factors affecting conformational (gauche/trans)
energy differences in the 2-haloethanols (XCH2CH2 OH, X=F,
Cl, Br, I) in the neat liquid (7) and in hydrogen bonding
solvents (9), and also to analyze the inter/intramolecular
hydrogen bonding .equilibrium in these systems (10).
Analogous results have been obtained with 2,2-dichloro-
ethanol (DCE). Like 2-chloroethanol, this molecule has
three equilibrium skeletal conformations, of which two are
spectroscopically equilvalent (11):
27
28
OH
C1 C CI CI C C
OH HO
Gauche (G) Gauche (G) Gauche' (G')
However, in contrast to the former alcohol, the hydroxyl
group in dichloroethanol is always gauche to at least one
halogen atom. Another factor expected to influence the
conformational and hydrogen bonding' behavior in this mole-
cule is the increased interelectronic repulsion between the
oxygen and the two halogen atoms on the molecular skeleton.
Experimental
The calculation of conformational energy differences
from the Van't Hoff equation requires measurement of the
temperature dependence of relative intensities of modes
assigned to the two rotamers. In the neat liquid and in
aqueous solution, the 570 cm~1 C-C-0 deformation (G') and
790 cm~1 C-Cl asymmetric stretch (G) were used (6). This
latter band was obscured by solvent interference in DMSO.
Therefore, the 420 cm- C-C-0 deformation (6) was used in
its place in these solutions (12). As in 2-chloroethanol,
the presence of nearby DMSO vibrations prohibited accurate
29
measurements at concentrations below approximately 40 mole
percent.
The high frequency 0-H valence region in the Raman
spectrum of DCE is qualitatively similar to that of the
monohalogenated alcohols (10), containing a very broad band
at approximately 3400 cm and a higher frequency, narrower
peak, assigned to the O-H stretching modes of the inter-
and intramolecularly hydrogen bonded species, respectively.
In addition, there is a small residual intensity at the
lower frequencies (3000-3100 cm1 ), arising from the C-H
scattering modes. There was no evidence for non-hydrogen
bonded 0-H groups at any of the temperatures studied.
By the method described in the Appendix, a non-linear
least-squares curve fitting algorithm was used (13) to
resolve the band parameters of the inter- and intramolecu-
larly hydrogen bonded hydroxyl modes. Fits performed both
with and without convolution to the experimental slit
function gave essentially the same linewidths (14). The
experimental and theoretical spectra at two typical temper-
atures are displayed in Fig. 5 and 6. The numerical values
of the calculated band parameters in DCE at the various
temperatures are contained in Table VII.
Results and Discussion
The calculated conformational energy differences for
DCE and 2-chloroethanol (7,9) are contained in Table VIII.
A precise comparison of the two sets of results is, of
30
Fig. 5--High frequency Raman spectrum of DCE at-140 C; circles--experimental spectrum;solid line--theoretical curve.
31
Cc
IJ
3050 3240 3430 3620 381[FREQUENCY (CM-1)
32
Fig. 6--High frequency Raman spectrum of DCE at+470 C; circles--experimental spectrum;solid line--theoretical curve.
33
S
co
cE
-U):nLLJ
3050 3240 3430 3620 3810FREQUENCY (CM-1)
(N 0) C CN C) N O LL iOH r- H H 0 0 0 0
0 C; 0 0 0 0 0 0
-Hk.0 L LO o t o Li -O) N
S N (N ( (N (N ( N
tC
+i1C) N N C f-CY O V- N ci 0iLi L L ) % D O %O L C)- H H H H-r H-i H-A H r-
U)
0
z
H
E-)
E-i
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A4
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U)0
O4
E-uw)04Hn
(N
LO(Y
C)
+1ko
R:34(Y
N " 'rV C H ' (N) 00 00 00 .00- N N
Li L Lin LOO L ifn Lm Ci cY io cq Y
re Y H 0- to(N 00 N ' Y 0 mC4
Co4 m Ci ci c( )c (Ni ci i ci ci ci c
r- % 11V o r-- c o o) 0 or c00Hn O t -r N r- 0 m0O
I I
34
0)
dH.
Cd4
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ri
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c ,. -H
-r-
x r4 r-IM ,.4 00)rd r- p H:1a) -H HP4 '-
-r- i -
.. I i
i
i
35
r-)
H HH C
o0 H0
CD 0CC)
C C) CD
CO+0 1 - 0 0
H ti o o o
z <r-4 r-
I H HN N
0000
N + 1 +
U4
zH U 0
H CHHH ,.C C
H 0 -iJ M> 0\0
C0 0 0 C- qO co
E-' 0 < +1 +1 -HI 00 H LOnz (N 0 (N
0 - - .U) 0 H H
H + + +
rd O 4-1 4-4 -r-
o (Ncr-)
36
course, not possible since DCE contains no skeletal confi-
guration directly analogous to the trans rotamer of the
former molecule (in which no intramolecular hydrogen bond
formation is possible). However, the molecules in the neat
liquid phase exhibit energy differences that are of the same
magnitude, but somewhat lower than typical values obtained
in other, non-hydrogen bonded substituted ethanes (1,2).
In solution, on the other hand, quite striking differences
between the two alcohols are observed. Whereas the gauche
form in chloroethanol is greatly stabilized (by approximately
a factor of four) in both DMSO and H20, the energy differ-
ences between the G and G' configuration remain low in DCE.
Although no unambiguous interpretation is possible at
present in these complex systems (10,15), one may still
offer a tentative explanation of the above results. It is
not unreasonable to speculate that the increased energy
differences of chloroethanol arise, at least in part, from
dielectric stabilization of the more polar gauche form in
these two solvents (E:(H20)=78.5 (16) and s(DMSO)=44.7 (17)
at 250C). One would at first expect a similar stabilization
of the G' conformation of DCE, which calculations have shown
to have a much greater dipole moment than the G rotamer (6).
However, it should be noted from the high frequency spectra
(18) that the hydrogen bonding in this alcohol in both sol-
vents is almost exclusively intermolecular in nature. As
pictured below, this would result in greater halogen-oxygen
37
repulsion in the G' configuration.
Cl 0- H -- O H O-H--OC1_ __C1I, \
/ H NH
H H Cl H
Gauche' (0') Gauche (G)
Thus, the relative insensitivity of AH to dilution in the
two solvents may result from a competition between dielectric
stabilization and interelectronic repulsion.
Comparisons of the temperature dependence of the cal-
culated peak frequencies and linewidths of the inter- and
intramolecularly hydrogen bonded 0-H modes in DCE and
chloroethanol (10) are displayed in Fig. 7 and 8, respec-
tively. The results are almost identical in the two alcohols.
Indeed, the only significant difference is that the line-
width of the intramolecularly bonded species are slightly
lower in DCE. It is interesting to note that the same phe-
nomenon was observed in early investigations of both alcohols
at low concentrations in CCl4 (3).
In contrast to the above results, one does observe
differences in the band area ratios between the two alcohols.
As shown in Fig. 9, 1I /I. is noticeably greater inintra inter
DCE than in chloroethanol. Furthermore, the calculated value
of AH(=Hintrainter) was found to be 2.4+0.13 kcal/mole,
38
Fig. 7--Temperature dependence of peak frequenciesof DCE (solid circles) and chloroethanol(open circles) .
39,
3580 -
INTRA
-F 3480:E
33800'0
3280 INTER
-30 0 30 60 90
Temperature (*C)
40
Fig. 8--Temperature dependence of line widths ofDCE (solid circles) and chloroethanol(open circles).
41
32 1
26
am"0O
201
1801
1601
14O
30
Temperature
0*
INTRA
INTER
3Oa-30 0 60
(0c)
42
Fig. 9--Temperature dependence of relative scatteringintensities in DCE (solid circles) andchloroethanol (open circles) .
43
1-
0
0 0
3.0 3.5 4.0
1000/'T K
-2.0
z-i
-3.01-
-4.0 F
I1'. - I I I a
(K~ 1 )
44
compared to 2.9+0.3 kcal/mole in chloroethanol (10).
The overall enthalpy change should, in principle,
correspond to the difference between the heats of formation
of the inter- and intramolecular hydrogen bonds. If it is
assumed that the intermolecular hydrogen bonds in these
molecules are formed primarily through the oxygen atom, AHF
(inter) should remain approximately constant for halogenated
ethanol (19), with a value on the order of -5 kcal/mole
(reported for ethanol itself). Given this assumption, the
above results imply that the intramolecular hydrogen bond
strength is somewhat greater in DCE than in chloroethanol.
It is satisfying to note that this conclusion is consistent
with the results obtained in earlier investigations of the
two alcohols in dilute solution (3).
CHAPTER BIBLIOGRAPHY
1. Mizushima, S.,, Structure of Molecules and InternalRotation, New York, Academic Press, 1954.
2. Orville-Thomas, W. J. , Internal Rotation in Molecules,New York, John Wiley and Sons, 1974.
3. Krueger, P. J., and Mettee, H. D., Can. J. Chem. 42,(1964), 326.
4. Wyn-Jones, E., and Orville-Thomas,, W. J. , J. Mole'c.Struct. 1, (1967), 79.
5. Kastha, G. S., Roy, S. B., and Nandy, S. K., IndianJ.,Phys.6, (1972), 293.
6. Hirokawa, T., Sumida, Y., Hayashi, M., and Murata, H.,J.;Sci. Hiroshima Univ. '38, (1974), 281.
7. Davenport, D., and Schwartz, M., J. Molec. Struct. 50,(1978), 259.
8. Pertilla, M., Spectrochim. Acta 35A, (1979) , 37.
9. Maleknia, S., Friedman, B. Ri., Abedi, N., andSchwartz, M., Spectrosc. Letters 13, (1980), 777.
10. Gupta, A., Davenport, D., and Schwartz,, M., Spectrochim.Acta 36A, (1980) , 601.
11. The conformation termed G' in this paper has alterna-tively been labelled as the trans configurationin some earlier work (ref. 6) .
12. Determination of AH in the neat liquid utilizingvarious pairs of band intensities, as expected,yielded values that were the same to withinexperimental error.
13. Maddams, W. F., Appl. Spec'trosc. 34, (1980) , 245.
14. Savitzky, A., and Golay, M. J. E.,' Anal. Chem. _36,(1964) , 1627.
45
46
15. Schwartz, M., Spectrochin. Acta 33A, (1977), 1025.
16. Table of Dielectric Constants of Pure Liquids, NBSCircular 514, (1951).
17. Johnston, M. D., Jr., and Barfield, M. , J. Chem.Phys. 54, (1971), 3083.
18. Although not pictured, Raman spectra in the 0-H valenceregion of DCE dissolved in both solvents showthe virtually complete absence of the bandassigned to intramolecularly hydrogen bondedhydroxyl groups. This behavior was also observedfor 2-chloroethanol in the same solvents (Fig. 1of ref. 9).
19. This assumption is felt to be reasonable, sincevarious thermodynamic studies have shown thatintermolecular hydrogen bonds of alcohols withalkyl halides are significantly weaker than withethers; e.g. West, R., Powell, D. L., Whatley,L. S., Lee, M. K. T., and Schleyer, P. Von R.,J. Am. Chem. Soc. 84, (1962) , 3221.
APPENDIX
Characteristic to the analysis of high frequency Raman
spectral data is the problem of resolving overlapping bands
into individual components. In these investigations, this
difficulty was overcome by using the computer program T3BW
(1), written specifically to input digitized spectra and
output resolved peaks derived from a non-linear least-squares
algorithm.
The high frequency Raman spectra of halogenated ethanols
contains two bands due to inter- and intramolecular hydrogen
bonding. The former is a very broad band centered around
3400 cm and the latter is a narrow band at approximately
3600 cm . In addition, there exists a contribution from
the residual intensity arising from the C-H stretch, cen-
tered at 3000 cm~. This residual intensity was subtracted
from the intensity of hydrogen bonding peaks using a simple
exponential function of the form:
Residual = wA * EXP(-wE * (w-o ))
where Iresidual is the intensity at the frequency w, wow is
the center frequency of the C-H stretch, wA is the peak
intensity, and wE is an exponential parameter. The spectral
47
48
bands due to inter- and intramolecular hydrogen bonding
were assumed to be a combination of Gaussian and Lorentzian
lineshapes described by:
(3-Lk 2-LN2 ()
I = Ae DGaussian
-A
Lorentzian _+(' )1+ (W-WQ)
D
where wO is the center frequency, A is the height, and D is
the half width at half height of the bands. In these parti-
cular studies a pure Gaussian lineshape was found to be an
optimum model for the hydrogen bonding bands. The computer
program then adjusts the exponentail function for the
residual C-H stretch intensity and the Gaussian lineshape
for the unresolved bands until the sum of the three model
functions approximate the experimental data to the desired
degree of accuracy. The resulting functions are inspected
to obtain the height, width, and frequency for the hydrogen
bonding bands.
The input data set for this computer program consists
of the following:
1) 201 equally spaced intensity values representing the
experimental high frequency Raman spectrum.
2) The starting frequency and the frequency increment
of the spectrum.
49
3) Initial estimates of woo, wA, wE, and wo, A, and D
for each of the bands.
4) Mask array, specifying which parameters should be
initially held constant.
The estimates for wo, A, and D are obtained by inspec-
tion of the experimental spectrum. The center frequency
(wow) for these studies was estimated at 3700 cm . However,
the results were found to be insensitive to the precise
origin of the exponential function. Initial estimates for
wA and wE were determined from the first two points of the
recorded spectrum using the formulae:
A = yl e lE (wow-OW)
wE = LN (y2/yl)
where wo1 and wo2 are the frequency of the points; y1 and Y2
are the corresponding intensities. In the case where the
initial estimates deviate greatly from the final values,
the parameters for the narrow peak may be greatly in error
due to its small size relative to the neighboring band.
These errors are minimized by initially fixing the narrow
band parameters using the mask array.
The output of the program is the height, width, and
center frequency of the resolved bands, and the parameters
of the residual intensity due to the C-H stretch.
APPENDIX BIBLIOGRAPHY
1. Burrill, J. H., Jr., Progran ' 66, Quan tum Chemi'strProgram Exchange, University of Indiana,Bloomington, Indiana.
50
BIBLIOGRAPHY
Books
Barnes, A. J., Vibrational Spectroscopy Modern Trends,New York, Elsevier, 1977.
Horak, M., Interpretation and Processing of VibrationalSpectra, New York, John Wiley and Sons, 1978.
Mizushima, S., Structure of Molecules and Internal Rotation,New York, Academic Press, 1954.
Orville-Thomas, W. J., Internal Rotation in Molecules,New York, John Wiley and Sons, 1974.
Articles
Buckley, P., and Giguere, P. A., Can. J._ Chem. 45, (1967) ,397.
Buckley, P., Giguere, P. A., and Schneider, M., Can. J.Ch'em. 47, (1969) , 901.
Davenport, D., and Schwartz, M., J. Molec. Struct. 50,(1978) , 259.
Gupta, A., Davenport, D., and Schwartz, M., Spectrochim.Acta 36A, (1980) , 601.
Hirokawa, T., Sumida, Y., Hayashi, M., and Murata, H.,J. Sci. Hiroshima Univ. 38, (1974) , 281.
Johnston, M. D., and Barfield, M., J. Chem. Phys. 54,(1971) , 3083.
Kastha, G. S., Roy, S. B., and Nandy, S. K., Indian J.Phys. 46, (1972) , 293.
Krueger, P. J., and Mettee, H. D., Can. J. Chem. 42,(1964) , 326.
Maddams, W. F., Appl. Spectrosc. 34, (1980), 245.
51
52
Maleknia, S., Friedman, B. Ri., Abedi, N., and Schwartz, M.,Spectrosc. Letters 13, (1980) , 777.
Perttila, M., Spectrochim. Acta 35A, (1979), 37.
Pruettiangkura, P., Ho, S., and Schwartz, M., Spectrosc.Letters 12, (1979) , 679.
Savitzky, A., and Golay, M. J. E., Anal. Chem.' 36, (1964) ,1627.
Schwartz, M., Spectrochim. Acta 33A, (1977), 1025.
Wyn-Jones, E., and Orville-Thomas, W. J., J. Molec. Struct.
1, (1967), 79.
Reports
Burrill, J. H., Jr., Program 66, Quantum Chemistry ProgramExchange, University of Indiana, Bloomington, Indiana.