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Raman study of the conformational equilibrium of ethylene glycol in dimethyl sulfoxide
Spectiochimica Aeta, Vol. 33A, pp. 1025 to 1032. Pergamon Press 1977. Printed in Great Britain
Raman study of the conformafional equilibrium of ethylene glycol in dimethyl sulfoxide M. SCHWARTZ Department of Chemistry, North Texas State University, Denton, TX 76203, U.S.A. (Received 26 October 1976; receivedfor publication 25 March 1977) Abstract--The Raman spectrum of liquid ethylene glycol (EG) was investigated as a function of concentration in dimethyl sulfoxide (DMSO). Utilizing previous vibrational assignments of the various bands to the gauche and trans conformers, it was observed that the equilibrium appeared to shift almost entirely to the latter form in dilute DMSO solution. This result permitted a quantitative estimate of the fraction of molecules in the trans configuration in EG-DMSO mixtures. Extrapolation of the measurements yielded a calculated value of 0.35 for the fraction of trans conformers in pure ethylene glycol, which was compared with the result obtained from earlier NMR investigations. A qualitative picture of the molecular structure and bonding in pure EG and in EG-DMSO mixtures, consistent with the observed data has been proposed. The Raman spectrum of an aqueous solution of ethylene glycol was also recorded, and compared to the spectrum of the pure glycol. In contrast to the conclusions of earlier workers, no significant shift in the gauche-trans equilibrium was detected.
l. INTRODUCTION
the gauche form, stabilized by intramolecular hydrogen bonding, or if an equilibrium exists between The realization of a better understanding of the con- gauche and trans conformers. formational structure and bonding interactions in On the basis of vibrational spectra of the pure liquid and aqueous ethylene glycol (1,2 ethanediol), liquid, some investigators [6, 17] have concluded that the simplest of the polyalcohols, is a subject of rele- (as in the other phases), all molecules are present in vance to many areas of chemistry. In addition to this the gauche form, while others [11-14] feel that there molecule's tendency to form bidentate ligands with is significant spectroscopic evidence to indicate that transition metals and Group II cations, [1--4"1 ethyl- gauche and trans conformers exist in equilibrium in ene glycol and other polyols (such as glycerol and this phase. There have also been at least two Raman 1,2 propanediol) have been found to aid effectively •investigations of ethylene glycol in aqueous solution, in the stabilization of proteins and nucleic acids the first by MATSULrP.A et al. [14], who concluded against denaturation by substances such as urea and that, while there is a finite fraction of trans conformer guanidine hydrochloride [5]. Furthermore, a clearer in the pure liquid, the equilibrium is shifted entirely picture of the conformation and bonding of this to the gauche form in dilute aqueous solution. In disimple polyol is of fundamental significance in the rect contrast, WILLIAMSand ATALLA[4] suggest that understanding of extensively hydrogen-bonded the predominant conformation is trans in a five mole aqueous systems, since, like water, and in contrast percent solution of EG in water. to the simple mono-alcohols, ethylene glycol and With an aim towards obtaining a more quantitative other diols contain two protons capable of hydrogen measure of the gauche-trans equilibrium in pure EG bonding. and in aqueous solution, and to learn the effects of Over the past years, there have been numerous in- strong proton accepting solvents on the equilibrium, frared [6-11] and Raman [4, 11-17] spectroscopic in- we have undertaken a study of the Raman spectra vestigations of the molecular conformation in the of ethylene glycol at various mole fractions in vapor, liquid and solid phases as well as in solution dimethyl sulfoxide (DMSO), concentrating, in particuin such solvents as CC14 and water. There appears lar, on the ratio of the intensity of the 480 cm- 1 line to be a general acceptance that ethylene glycol (EG) (attributed to the trans conformer) relative to other exists primarily in the gauche conformation in both vibrational bands in the mixtures. The data obtained the solid and vapor phases. It is in the liquid and from this study have been utilized to estimate the in aqueous solution, however, that there remains fraction of trans molecules in the neat liquid and in some controversy over whether all molecules are in the various EG-DMSO mixtures. 1025
1026
M. SCHWARTZ
The Raman spectrum of a ten mole percent aqueous solution of EG has also been recorded and is compared to the spectrum of pure ethylene glycol.
procedure will be discussed below). The DMSO peak was lound to have a negligible contribution at the higher mole fractions studied, and no correction was considered necessary at these concentrations.
I1. EXPERIMENTAL PROCEDURE |II. RESULTS
Chromatoquality ethylene glycol (99 + %) was purchased from MCB Chemical Company; analytical reagent grade DMSO (H20 impurity <0.2%) was obtained from Mallinkrodt Chemical Co., and used without further purification. The mixtures, ranging from Xe6 = 0.1 to pure EG (XEG is the mole fraction of ethylene glycol) were prepared gravimetrically. The samples were contained in thin-walled 5 mm NMR tubes, which were illuminated transversely by light from a Coherent Radiation CR-3 laser operated at 5145 A. The laser power was measured before and after each spectrum (typical powers used were in the range from 0.4 to 0.6 watts). The scattered light was collected at 90° to the incident beam and dispersed with a Spex 1401 double monochromator (with 1800gr/mm holographically ruled gratings). The signals were then amplified by an electrically cryostatted RCA C31034 PM tube and Spex PC-1 photon counting detection system, and the resultant spectra were displayed on a recorder directly coupled to the monochromator drive. The monochromator slits were set at 5 cm- 1 resolution (220 #). All intensities reported below represent the band area (A), obtainecl by multiplication of the peak maximum by the full-width at half-height (A). This procedure was adopted in order to correct for possible band width variations in different mixtures. (It should be noted that the area of both Lorentzian and Gaussian bands is proportional to this product.) In the case of somewhat overlapped spectral lines or where one side appeared to display secondary structure, the band was assumed to be naturally symmetrical, and A was recorded as twice the half-width of the nonoverlapped side of the band. One further correction was applied to one of the vibrational bands in some of the mixtures. DMSO has a very intense peak located at 390 cmz 1, with a full-width of approximately 15 cm -~. It was found that for the most dilute solutions of EG in DMSO, the long tail of this Lorentzian shaped band appeared to give a small, but finite, contribution to the total scattering intensity in the region around 480 cm-1, where the much weaker ethylene glycol peak is located. Therefore, at the three lowest mole fractions (XE~ = 0.10, 0.17 and 0.25), its contribution was subtracted from the observed spectrum. This correction was found to alter the area of the 480 cm- 1 EG peak by about 15-20%. (The implications of this correction
A. Vibrational assignments The complete Raman spectrum of ethylene glycol has been reported in other papers I l l , 13, 14]; the spectra obtained in this work were found to be in quite good agreement with earlier results and are not reproduced, except for regions of particular interest to this study. As discussed in the introduction, vibrational studies of EG have resulted in conflicting conclusions with respect to the presence of any trans isomers in the liquid phase. This has been largely because of difficulties encountered in assignment of the various bands in the vibrational spectrum. We direct ourselves to four regions of interest for the present study: 1) 1400-1500 cm- 1, 2) 1000-1100 cm- 1, 3) 800-900 cm- 1, and 4) 400-550 em- 1. The first region is straightforward to interpret as there is only one EG vibration, located at 1460 cm-1, which is unambiguously assigned to the CH2 scissoring vibration [13]. The solvent, DMSO, has one very strong band centered at 1420 era-1, which is due to the CH3 deformation mode [18]; this latter DMSO peak will be utilized in the analysis to be presented below. Region (2) from 1000 to l l 0 0 c m - t contains three overlapping lines in the Rarnan spectrum of the liquid at 1042(dep.), 1069(pol.) and 1093(pol.)cm-l[13] while the i.r. spectrum has strong lines at 1046 and 1094 cm -a [6], with no observable absorption corresponding to the 1069 cm-1 Raman line (This band is also absent in the gas phase Raman spectrum). The observed spectra in this region have been given various assignments [6, 13, 14]. However, based on the polarization of the Raman lines and the absence of the middle band from the i.r. spectrum, it is felt that the most plausible interpretation of this region is that of KRISHNANand KRISHNAN [13], who assign the 1093 and 1044 cm-1 lines to stretching modes (of symmetries A and B respectively) of the gauche isomer, and the band at 1069 cm- 1 to the trans C - - O stretch (Ag), which one expects, as observed, to be i.r. inactive. DMSO also contains a single band in this region, located at 1042cm -~ and assigned to the S==O stretch [18]. The third region, from 800 to 900 cm-~ has also been subject to uncertainties in assignment. The Raman spectrum of liquid ethylene glycol exhibits an intense,
Raman study strongly polarized line at about 865 cm-1 (with weak components at 883 and 803 cm-1)[13], while the i.r. spectrum shows a strong doublet with peak frequencies at 866 and 887cm -x [6]. While at least one group [6-1, on the basis of studies of the partially deuterated glycol (HOCD2CD2OH), has concluded that the 865 cm-x band in the vibrational spectra arises primarily from the CHz rocking vibration of the gauche conformer, other workers[10, 13-1 contend that this band is due to the C----C stretch (and is coincident for both conformers). Based on the high intensity and low depolarization ratio of the 865 cm-1 peak in the Raman spectrum (CH2 rocking vibrations are typically weak and depolarized in Raman spectra), this investigator feels that the latter assignment is more likely to be correct. Unfortunately, there is some contradictory evidence, and a conclusive assignment of this vibration is therefore not possible at present. Region (4), which is of crucial importance to this study) contains only two peaks in the Raman spectrum of the liquid, located at 525 and 480 cm-1. The latter band is absent from the i.r. spectrum, which, in addition to a line at 515cm - I , contains a very weak absorption at 430 cm- 1. BUCKLEY and GIGUERE [6-1, assuming that only the gauche conformation is present in the liquid phase, assign these lines in the i.r. to the two C---C--O bending vibrations of this conformer, and say that they correspond to the 525 and 480 crn-1 Raman bands. This investigator finds it difficult to reconcile the 50 cm-1 discrepancy of the lower frequency line between the Raman and i.r. spectra (480 vs. 430 cm-t). In contrast, Kalsh~Atq and KglsrrtqAtq [13], on the basis of a normal coordinate analysis, have assigned the 525 cm-1 Raman vibration to the antisymmetric (B) mode of the gauche conformer and the 480 cmband to the symmetric (Ag) C - - C - g ) bending mode of the trans conformer. This same assignment has been given in an independent study of MATSUURA[14-1. In addition, FUKUSHIMA and ZwoLINSKI [19], performed a complete analysis of the vibrations in n-propanol (which has a skeletal structure very similar to that of EG), and assigned a 458 cm- 1 vibration in this molecule to the symmetric C - - C - - O bend of the trans conformer, which lends additional support for the above assignment of the 480 cm-1 peak in EG to the trans form. B. Calculation of the gauche-trans equilibrium tn Figs. 1 and 2 are displayed the Raman spectra of EG at various concentrations in DMSO in the 456-550crn-1 region and in the range from 1000 to l 1 5 0 c m - k It is quite apparent that one
1027 i
f
i
540
i
500
FREOUENCY
i
,
E
460
(CM - I )
Fig. 1. The Raman spectrum of ethylene glycol as a function of mole fraction in DMSO (and in water) in the range from 450 to 550 cm-1. (A) Pure EG, (B) X~G = 0.60 in DMSO, (C) XE~ = 0.10 in DMSO (the broken curve represents the peak corrected for residual intensity from the lower frequency DMSO peak--see text), (D) EG in water (XEG = 0.10).
observes dramatic changes in the spectra in these regions as ethylene glycol is diluted in DMSO. The band at 525 c m - 1, assigned to the gauche conformer's C----C---O bend diminishes in intensity relative to the trans conformer's bending vibration at 480 cm- 1 (Fig. 1). Similarly (Fig. 2), the 1093 cm -1 band, assigned to the gauche C---O stretch is decreased markedly in intensity upon dilution in the solvent. (The presence of the 1042 cra-1 DMSO peak in the mixture precludes the observation of any increase in the intensity of the trans peak in this frequency region.) In the author's view, the most plausible explanation of the observed spectral changes is that the gauchetrans equilibrium is shifting more towards the latter conformation upon dilution in DMSO (a strong proton accepting solvent). Indeed, the great decrease in the intensity of the peaks assigned to the gauche conformer would suggest that a very high fraction of EG molecules exist in the trans configuration in very dilute DMSO solution. In order to obtain a quantitative measure of the fraction of molecules in the trans conformation (Fr), the ideal proc,edure would be to find a band in the
1028
M. SCHWARTZ ,
,
~
,
,
,
,
z
I100 1060 1020 FREQUENCY (CM -I) Fig. 2. The Raman spectrum of ethylene glycol as a function of mole fraction in DMSO (and in water) in the range from 1000 to l l 0 0 c m - L (A) Pure EG, (B) Xe6 = 0.10 in DMSO, and (C) EG in water (Xz6 = 0.10).
Raman spectrum of ethylene glycol whose scattering intensity is independent of the molecular conformation, to which the area of the 480 cm-1 trans vibration could be referenced. Then, any observed decrease in the ratio of the 480 c m - 1 band area relative to that of the reference peak would be indicative of a decrease in the fraction of trans isomer. The only EG vibration that can be safely assigned to both con° formers is the CH2 scissoring mode at 1460 c m - 1 ; unfortunately, its scattering intensity is too weak to be used as a quantitative reference (particularly at the lower concentrations, which are of major importance in the calculational procedure). The 865 c m - 1 band could be used safely only if it were conclusively assigned to the C----C stretch of both conformers. Since, as discussed above, the assignment of this vibration remains ambiguous, the above procedure, utilizing this peak as a reference, was utilized only as a check on the method of calculation adopted below. It was decided that the most reliable procedure is to use the D M S O CH3 deformation mode, which is located at 1420 c m - 1 and is very strong in the Raman spectrum, as a reference peak. One expects that any intermolecular interactions between D M S O and EG
will have little, if any, effect on the intensity of this vibration. One does encounter a slight added difficulty when using a solvent band as the reference, since the ratio, R = ,~4so/,~A~/~0~so142o('A' indicates peak area) will necessarily increase with increasing mole fraction of EG, independent of any variation in the fraction of trans conformers. However, this may be accounted for, as shown below. Presented in Table 1 are the spectral parameters of the trans EG band as a function of mole fraction (XEo), including the band areas. Table 2 contains the area of the 1420 c m - 1 D M S O peak and the observed ratio of peak areas (R °~' = A4so/A142o) as a function of mole fraction (as expected, this ratio increases with increasing concentration of ethylene glycol). In order to proceed fur.ther in the calculation, one must make the assumption that in the limit of very low mole Table 1. Spectral parameters of the 480 cm- 1 band of EG in DMSO XE~
Vo(Cm - 1)
I max*
A**
A4ao( x 10- a)
0.10 0.17 0.25 0.40 0.50 0.60 0.70 0.80 0.90 1.00
480 480 481 481 481 482 482 483 484 484
50 73 82 155 176 190 184 204 208 215
11.0 12.0 13.2 12.0 13.2 13.6 14.4 14.6 16.4 17.2
0.55 0.88 1.08t 1.8 6
2.32 2.58 2.65 2.9s 3.41 3.7o
* Intensity at band maximum (divided by laser power). ** Full-width at half-height (twice the half width of the low frequency side of the band). t Lower case digits are subject to uncertainty. Table 2. Calculation of the fraction of trans conformers utilizifig the 1420 cm- ~ DMSO band as a reference XE(;' 0.10 0.17 0.25 0.40 0.50 0.60 0.70 0.80 0.90 1.00
A1420 ( X
10 -4)
1.9s1" 1.82 1.5o 1.3o 1.15 0.9a 0.7a 0.5,1. 0.35 - -
R °bs*
R r**
Fr
O.02a 0.04s 0.072 0.144 0.201 0.252 0.363 0.550 0.98a
0.030 0.056 0.091 0.182 0.274 0.41o 0.63a 1.09 2.46
0.92 0.86 0.79 0.79 0.73 0.62 0.57 0.50 0.40
- -
- -
0.36~;
A480/A1420 (see Table 1 for A 480). ** R r is the predicted area ratio for 100% trans conformer (see text for discussion). I" Lower case digits are subject to uncertainty. :~Extrapolated value. * R °bs =
Raman study fraction of EG in D M S O , practically all of the molecules are in the trans form. In view of the great reduction in the gauche band intensities relative to the trans peak upon dilution (as seen in Figs. 1 and 2), this appears to be a not unreasonable approximation. Fitting the observed intensity ratios (R) at X ~ Z 0.10 and X ~ = 0.17 to the empirical relation, R = A . X + B . X 2 (which is constrained to yield R = 0 at X = 0, as must be the case), it was found that A = 0.276 and B = 0.0529. This equation then allows one to interpolate values of the area ratios at various low concentrations. Using the parameters above, one obtains at X~t; = 0.01, R0.ot = 2.76 x 10-3. Assuming that almost all molecules are trans at sufficiently low concentrations, the above result can be used to predict the area ratio, Rr(480/1420), that would be expected at higher mole fractions, if all the molecules were to remain in the trans form. We use the simple equation: Rr(XeG) = Ro.ol x
where the factor on the right corrects for the naturally expected increase in Rr(480/1420) with increasing Set;.
The results are shown in the fourth column of Table 2. (It was found that using X = 0.02 or 0.03 as a reference mole fraction gave virtually no change in the calculated results.) It is apparent that the observed ratio of peak ar'eas (R °bs) decreases relative to the predicted value for 100~ trans (R r) as the mole fraction of E G is increased, indicating that the fraction of trans conformer (Fr) decreases with increasing concentration. The actual fraction of trans conformer at each mole fraction is obtained simply as R°bS/Rr, and is displayed in col. (5) of Table 2. A graph of the experimental values of F r versus mole fraction is shown in Fig. 3. The solid line represents a least-squares fit of the data to the equation, Fr= 1 +A'X+B'X 2. We obtained values of A = - 0 . 5 8 and B = - 0 . 0 6 (with an rms deviation of 0.04). The relatively low value for B indicates, as seen in the graph, that the data is essentially linear in mole fraction (although we attach no theoretical significance to this result). An extrapolation of this fitted curve to XEG = 1.0 predicts that in pure ethylene glycol, F r = 0.36 (A fit of the data to an unconstrained straight line yields an extrapolated value of 0.38). As mentioned above, an alternative (though less reliable) procedure is to compare the integrated area of the 480 c m - 1 line to that of the ethylene glycol peak at 865 c m - 1 (assuming the intensity of this vibS.A.A.33ql
I
0.8
z
I.-
0.6~
Z 0 I"- 0.4
0.2
0.0 0
'
0'. 2
'
0 4i
'
0 6i
.
. 0.8.
.
1.0
MOLE FRACTION (XEG)
X~/0.01 XDmo/0.99
1029
Fig. 3. The fraction of trans conformers of ethylene glycol as a function of mole fraction in DMSO.
ration to be conformation independent). In this case, R r was obtained by a simple linear extrapolation of R °b~ (at X = 0.10 and 0.17) to X = 0. It was found that R r = 0.327. A direct comparison of this ratio with R °bs at each mole fraction yielded results for F r shown in Table 3. These values are in qualitative agreement with those obtained from the first procedure (particularly) for pure ethylene glycol, where the fraction of trans conformers is identical to that obtained above). It was discussed in an earlier section that the 480cm -1 band at the two lowest mole fractions Table 3. Calculation of the fraction of trans conformers utilizing the 865 cm-~ EG band as a reference get;
A86s( x 10 -4)
R °bs*
Fr**
0.10 0.17 0.25 0.40 0.50 0.60 0.70 0.80 0.90 1.00
0.204t 0.383 0.519 0.690 1.26 1.64 1.9a 2.4s 2.7a 3.11
0.259
0.82 0.70 0.99 0.88 0.57 0.46 0.42 0.37 0.38 0.36
0.229 0.323 0.289
0.185 0.151 0.137 0.122 0.12 a 0.119
* R °b~ = A480/A865. ** F r = R°b'/R r (R r = 0.327).
t Lower case digits are subject to uncertainty.
1030
M. SCHWARTZ
(crucial to the above calculations) was corrected to account for the long tail of the 390 cm- 1 D M S O peak (see the dotted curve in Fig. 1). If this correction is not performed, the same calculations yield a value of 0.25 fraction trans in the pure liquid. The author believes the value of 0.36 obtained using the corrected data to be more reliable; however, use of the unadjusted data furnishes a qualitative measure of the uncertainty in the calculated results. One further note is in order. The assumption that F r approached 1.0 at sufficiently low concentrations, used in both methods of calculation may not strictly be valid. The presence of a small, but finite, concentration of gauche conformers in the limit of infinite dilution would tend to slightly lower the values of Fr at the higher concentrations. It is estimated that if there remained ten mole percent of gauche conformers, the calculated value of F r for pure EC would be lowered to approximately 0.32. IV. D I S C U S S I O N
As the results in Fig. 3 show, ethylene glycol in D M S O exists as an equilibrium mixture of the gauche and trans conformers, which is steadily shifted toward the latter as the EG is diluted in the strongly basic solvent. Pure liquid ethylene glycol appears to have a gauche-trans ratio of approximately 2:1 (Fr = 0.35 + 0.1), in contrast to the vapor and solid phases, where all molecules are in the gauche configuration. It must be emphasized that the results obtained in this study apply only to the configuration of the molecular skeleton--that is, the O - - C - - C - - O dihedral angle, which has three equilibrium values, as shown below
H H
I H H qauche
H H [ H H qauche
H
H
O
H
tran$
As pointed out by PODO et al. 1-20] In a theoretical analysis of ethylene glycol and similar ethane derivatives, a complete characterization of the molecular configurat!on requires the specification of the two C - - C - - - O - - H dihedral angles as well as the skeletal configuration (shown above). Furthermore, we note that the experimental results cannot be used to predict what fraction of those molecules in the gauche configuration are intramolecularly hydrogen bonded.
Given the above restrictions, it is still possible to propose a model for the molecular structure in the pure liquid and in dilute D M S O solution which is consistent with the results obtained here. The generally accepted presumption that all EG molecules are gauche in the vapor is easily understood, since this form is stabilized by intramolecular hydrogen bonds. In the liquid, on the other hand, the molecules can also form intermolecular hydrogen bonds with their neighbors. These bonds are generally found to be stronger than intramolecular bonding [21] because of the non-linearity of the bond (due to steric hindrance). In the pure liquid, we can picture three simplified molecular configurations:
H. H-.O~ "',o.~H---O C--C / \ \ / H- -O\ ..,O~.H___O O"-H~..O .-H C--C O---H~o .-'H H \ (O) \0-- H C--C \ / \ C--C .O--H---O (b)
i-i
(c)
The first picture represents EG in the gauche form, stabilized by one intramolecular hydrogen bond and possibly two additional intermolecular hydrogen bonds. Picture (b) shows the possibility of a cyclic dimer (with both EG molecules in the gauche conformation) containing two intermolecular bonds connecting the monomers with as many as four additional hydrogen bonds with other molecules in the liquid. The third picture is that of a molecule in the trans configuration, capable of participation in as many as four hydrogen bonds with neighboring molecules. In contrast to the vapor, where structure (a) is expected to be the most stable form, in the liquid, the trans configuration is also stabilized by hydrogen bonding (picture (c)). This may explain the presence of a finite fraction of trans conformers in this phase. We turn now to discuss the apparent diminishment of gauche conformers in dilute D M S O solution. In contrast to the pure liquid, where molecules can act both as proton donors and acceptors, D M S O can only accept protons in hydrogen bond formation. Thus one can picture the following two possible structures: \
/ o ~ H " ' . O ~ HyO-~S\
\
/
/S~o"'H~ 0 \
c--c
C--C (o)
O.~.H_ O ~ S / (b)
\
Raman study In this case, the gauche isomer (a) is stabilized by one, relatively weak, intramolecular hydrogen bond and one intermolecular bond, while the trans conformer is free to form two, stronger, intermolecular hydrogen bonds with the solvent. This may account for the apparent preference of EG molecules to assume a trans conformation in DMSO. It should be pointed out that, while the above (relatively simplistic) picture of hydrogen bonding in EGDMSO mixtures offers a satisfactory explanation of the results, it is, of necessity, somewhat speculative in nature (due to the complexity of intermolecular interactions in hydrogen bonded liquids), and other interpretations of the observed data may be possible. The ca]rculated value obtained here for the fraction of trans conformers in the pure liquid may be compared with earlier studies of ethylene glycol using NMR spectroscopy. PACnLER and WESSELS[22], in a study of the solvent dependence of vicinal protonproton coupling constants in various substituted ethanols including ethylene glycol, found a trans population of approximately 0.2 for the neat liquid, which is somewhat outside the limits of error estimated for this study (0.25 < F r < 0.45). However, in order to obtain coupling constants for each conformation, their analysis required the assumption that the fraction of the more polar gauche conformer increases with increasing dielectric constant of the solvent (independent of its specific bonding properties). While this assumption may well have been valid for earlier NMR studies of dihalo ethanes [23], it is felt that it cannot fairly be made for the case of alcohols and their derivatives, where hydrogen bonding interactions with the solvent (rather than polarity alone) can be expected to exert a strong influence on the solute conformation. The questionable nature of their assumption is borne out by the results obtained in the present investigation, where the molecular equilibrium appears to shift towards the trans conformer upon dilution in DMSO, a solvent with a fairly high dielectric constant (~ = 44.7)[24]. Our results are in agreement with the NMR coupling constant study of VITI et al. [25] on dimethoxyethane and methoxyethanol, who found that, for the latter substance (quite similar to EG), the "trans form is greatly stabilized" in solvents such as DMSO, DMF and ketones, all of which have high dielectric constants (and which are all strong Lewis bases). Their results appear to support the contention that in hydrogen bonding liquids, the specific nature of the solvent may have a greater influence on the conformational equilibrium than simply the solvent's dielectric constant. One other NMR investigation of EG was performed by CONNORand MCLAUCHLAN[26], who util-
1031
ized the[13] C - - H proton satellite side bands to determine F r in ethylene glycol, and obtained values around 0.25-0.30 for the pure liquid extracted (from Fig. 5 of their paper), in reasonable agreement with the result obtained here. Finally, we turn to a brief discussion of the effect of dilution in water on the conformation of ethylene glycol. KRISHNAN and KRISHNAN[13] studied the Raman spectrum of dilute aqueous solutions of EG in the region from 1000 to ll00cm-1. They observed an apparent decrease in the intensity of the 1069 cm -~ band (C---O stretch of the trans conformer) relative to the 1042 cm -~ gauche peak, and concluded that EG molecules in dilute aqueous solution are all in the gauche conformation. However, they were unable to obtain the Raman spectrum in the important 4(K)-550cm -1 interval, due to what they termed broad, diffuse scattering from the water in the solutions. On the other hand, ATALLAand WILLIAMS[4], who examined the spectra of five mole percent EG in H 2 0 , suggested that the molecular conformation is totally trans in aqueous solution. We have re-examined the spectrum of an aqueous solution of ethylene glycol (with XnG = 0.10) in both regions of interest, and observed that a new maximum does seem to appear at around 1050cm -1 (Fig. 1C) in aqueous solution. However, we were also able to study the low frequency region, and, aside from a constant background due to n 2 0 scattering, there were no significant differences in the intensity of the 481cm -~ trans peak relative to the gauche band located at 525 cm-1, when compared to that in pure EG (see Figs. lA and ID). A major shift of the equilibrium towards either conformer in dilute aqueous solutions would be expected to show up quite clearly in this portion of the spectrum. It would therefore appear likely that the relatively minor intensity variations in the higher region of the spectrum arise from a solvent shift in the intensity maxima in this region, rather than from a large change in the conformational equilibrium. In conclusion, the results of this investigation have illustrated that the trans configuration of ethylene glycol appears to be increasingly stabilized upon dilution in the highly polar (but Lewis basic) solvent, dimethyl sulfoxide; this permitted the determination of a semiquantitative estimate of the relative population of conformers in the pure liquid, which is in reasonable agreement with values obtained earlier from NMR techniques. A simplified qualitative interpretation of bonding in EG-DMSO mixtures has been offered to explain the experimental data. However, the question of intermolecular interations in hydrogen bonded liquids remains a complex one,
1032
M. SCHWARTZ
and further experiments are currently in progress in this laboratory to study the conformational equilibrium of other glycols and substituted ethanols in solvents with varying properties. The results of these experiments will be reported upon their completion.
REFERENCES
[1] D. KNETSCH and W. L. GROENEVELD, lnorg. Nucl. Chem. Lett. 12, 27 (1976). [2] D. KNETSCH and W. L. GROENEVELD, Inorg. Chim. Acta 7, 81 (1973). [3] R. F. EVILIA and C. N. REILLEY,J. Coord. Chem. 3, 7 (1973). [4] R. M. WILLIAMSand R. H. ATALLA,J. Chem. Soc.-Perkin Trans II, 1155 (1975). [5] H. J. SAGE and S. J. SINGER, Biochemistry 1, 305 (1962). [6] P. BUCKLEYand P. A. GIGUERE, Can. J. Chem. 45, 397 (1967). [7] P. J. KRUEGERand H. D. METTEE, J. Mol. Spectry. 18, 131 (1965). [8] D. J. MORANTZand M. S. WAITE, Spectrochim. Acta 27A, 1133 (1971). [9] W. K. BUSFmLD, M. P. ENNIS and I. J. MCEWEs, Spectrochim. Acta. 29A, 1259 (1973). [10] W. SAWOONY,K. NmDENZUand J. W. DAWSON,Spectrochim. Acta 23A, 799 (1967).
[11] M. KuI~, W. LUTTr~ and R. Z. MECr~, Anal. Chem. 107, 106 (1959). [12] T. A. HARm^N, J. Indian Inst. Sci. 36, 224 (1954). [13] K. KRISHNANand R. S. KRISrtNAN,Proc. Indian Acad. Sci. A64, 111 (1966). [14] J. MATSUtmA,M. Hmxtsm and T. MPtAZAW^, Spectrochim. Acta 28A, 2299 (1972). [15] M. MASSON,J. ROYER, R. DUPEYRAT,C.R. Acad. Sci. 2"/4B, 62 (1972). [16] A. MANISSE, J. BEAUDOIN and M. HAm~AND C.R. Acad. Sci. 277B, 659 (1973). [17] U. KA~mAVASm and K. NUKADA, Nippon Kayaki Zasshi 84, 297 (1963). [18] W. D. HORROCKS,Jn. and F. A. COTTON,Spectrochim. Acta 17, 134 (1961). [19] K. FUKUSHnVlAand B. J. ZWOLINSKI,J. Mol. Spectry. 26, 368 (1968). [20] F. Pooo, G. NEMETHY, P. L. INOOVINA, L. RADIES and V. Vm, Molec. Phys. 27, 521 (1974). [21] G. C. PIMENTELand A. L. MCCLELLAN,The Hydrogen Bond, W. H. Freeman, San Francisco (1960). [22] K. G. R. PACHLERand P. L. WESSELS,J. Mol. Structure 6, 471 (1960). [23] R. J. ABRAHAM,L. CAVALLIand K. F. R. PACHLER, Molec. Phys. 11, 49 (1966). [24] M. D. JOHNSTON,JR. and M. BARFIELD,J. Chem. Phys. 54, 3083 (1971). [25] V. VITI, P. L. INDOVINA,F. PODO, L. RADIES and G. NEMETHY, Molec. Phys. 27, 541 (1974). [26] T. M. CONNOR and K. A. MCLAUCHLAN,J. Phys. Chem. 69, 1888 (1965).
Raman study of the conformafional equilibrium of ethylene glycol in dimethyl sulfoxide M. SCHWARTZ Department of Chemistry, North Texas State University, Denton, TX 76203, U.S.A. (Received 26 October 1976; receivedfor publication 25 March 1977) Abstract--The Raman spectrum of liquid ethylene glycol (EG) was investigated as a function of concentration in dimethyl sulfoxide (DMSO). Utilizing previous vibrational assignments of the various bands to the gauche and trans conformers, it was observed that the equilibrium appeared to shift almost entirely to the latter form in dilute DMSO solution. This result permitted a quantitative estimate of the fraction of molecules in the trans configuration in EG-DMSO mixtures. Extrapolation of the measurements yielded a calculated value of 0.35 for the fraction of trans conformers in pure ethylene glycol, which was compared with the result obtained from earlier NMR investigations. A qualitative picture of the molecular structure and bonding in pure EG and in EG-DMSO mixtures, consistent with the observed data has been proposed. The Raman spectrum of an aqueous solution of ethylene glycol was also recorded, and compared to the spectrum of the pure glycol. In contrast to the conclusions of earlier workers, no significant shift in the gauche-trans equilibrium was detected.
l. INTRODUCTION
the gauche form, stabilized by intramolecular hydrogen bonding, or if an equilibrium exists between The realization of a better understanding of the con- gauche and trans conformers. formational structure and bonding interactions in On the basis of vibrational spectra of the pure liquid and aqueous ethylene glycol (1,2 ethanediol), liquid, some investigators [6, 17] have concluded that the simplest of the polyalcohols, is a subject of rele- (as in the other phases), all molecules are present in vance to many areas of chemistry. In addition to this the gauche form, while others [11-14] feel that there molecule's tendency to form bidentate ligands with is significant spectroscopic evidence to indicate that transition metals and Group II cations, [1--4"1 ethyl- gauche and trans conformers exist in equilibrium in ene glycol and other polyols (such as glycerol and this phase. There have also been at least two Raman 1,2 propanediol) have been found to aid effectively •investigations of ethylene glycol in aqueous solution, in the stabilization of proteins and nucleic acids the first by MATSULrP.A et al. [14], who concluded against denaturation by substances such as urea and that, while there is a finite fraction of trans conformer guanidine hydrochloride [5]. Furthermore, a clearer in the pure liquid, the equilibrium is shifted entirely picture of the conformation and bonding of this to the gauche form in dilute aqueous solution. In disimple polyol is of fundamental significance in the rect contrast, WILLIAMSand ATALLA[4] suggest that understanding of extensively hydrogen-bonded the predominant conformation is trans in a five mole aqueous systems, since, like water, and in contrast percent solution of EG in water. to the simple mono-alcohols, ethylene glycol and With an aim towards obtaining a more quantitative other diols contain two protons capable of hydrogen measure of the gauche-trans equilibrium in pure EG bonding. and in aqueous solution, and to learn the effects of Over the past years, there have been numerous in- strong proton accepting solvents on the equilibrium, frared [6-11] and Raman [4, 11-17] spectroscopic in- we have undertaken a study of the Raman spectra vestigations of the molecular conformation in the of ethylene glycol at various mole fractions in vapor, liquid and solid phases as well as in solution dimethyl sulfoxide (DMSO), concentrating, in particuin such solvents as CC14 and water. There appears lar, on the ratio of the intensity of the 480 cm- 1 line to be a general acceptance that ethylene glycol (EG) (attributed to the trans conformer) relative to other exists primarily in the gauche conformation in both vibrational bands in the mixtures. The data obtained the solid and vapor phases. It is in the liquid and from this study have been utilized to estimate the in aqueous solution, however, that there remains fraction of trans molecules in the neat liquid and in some controversy over whether all molecules are in the various EG-DMSO mixtures. 1025
1026
M. SCHWARTZ
The Raman spectrum of a ten mole percent aqueous solution of EG has also been recorded and is compared to the spectrum of pure ethylene glycol.
procedure will be discussed below). The DMSO peak was lound to have a negligible contribution at the higher mole fractions studied, and no correction was considered necessary at these concentrations.
I1. EXPERIMENTAL PROCEDURE |II. RESULTS
Chromatoquality ethylene glycol (99 + %) was purchased from MCB Chemical Company; analytical reagent grade DMSO (H20 impurity <0.2%) was obtained from Mallinkrodt Chemical Co., and used without further purification. The mixtures, ranging from Xe6 = 0.1 to pure EG (XEG is the mole fraction of ethylene glycol) were prepared gravimetrically. The samples were contained in thin-walled 5 mm NMR tubes, which were illuminated transversely by light from a Coherent Radiation CR-3 laser operated at 5145 A. The laser power was measured before and after each spectrum (typical powers used were in the range from 0.4 to 0.6 watts). The scattered light was collected at 90° to the incident beam and dispersed with a Spex 1401 double monochromator (with 1800gr/mm holographically ruled gratings). The signals were then amplified by an electrically cryostatted RCA C31034 PM tube and Spex PC-1 photon counting detection system, and the resultant spectra were displayed on a recorder directly coupled to the monochromator drive. The monochromator slits were set at 5 cm- 1 resolution (220 #). All intensities reported below represent the band area (A), obtainecl by multiplication of the peak maximum by the full-width at half-height (A). This procedure was adopted in order to correct for possible band width variations in different mixtures. (It should be noted that the area of both Lorentzian and Gaussian bands is proportional to this product.) In the case of somewhat overlapped spectral lines or where one side appeared to display secondary structure, the band was assumed to be naturally symmetrical, and A was recorded as twice the half-width of the nonoverlapped side of the band. One further correction was applied to one of the vibrational bands in some of the mixtures. DMSO has a very intense peak located at 390 cmz 1, with a full-width of approximately 15 cm -~. It was found that for the most dilute solutions of EG in DMSO, the long tail of this Lorentzian shaped band appeared to give a small, but finite, contribution to the total scattering intensity in the region around 480 cm-1, where the much weaker ethylene glycol peak is located. Therefore, at the three lowest mole fractions (XE~ = 0.10, 0.17 and 0.25), its contribution was subtracted from the observed spectrum. This correction was found to alter the area of the 480 cm- 1 EG peak by about 15-20%. (The implications of this correction
A. Vibrational assignments The complete Raman spectrum of ethylene glycol has been reported in other papers I l l , 13, 14]; the spectra obtained in this work were found to be in quite good agreement with earlier results and are not reproduced, except for regions of particular interest to this study. As discussed in the introduction, vibrational studies of EG have resulted in conflicting conclusions with respect to the presence of any trans isomers in the liquid phase. This has been largely because of difficulties encountered in assignment of the various bands in the vibrational spectrum. We direct ourselves to four regions of interest for the present study: 1) 1400-1500 cm- 1, 2) 1000-1100 cm- 1, 3) 800-900 cm- 1, and 4) 400-550 em- 1. The first region is straightforward to interpret as there is only one EG vibration, located at 1460 cm-1, which is unambiguously assigned to the CH2 scissoring vibration [13]. The solvent, DMSO, has one very strong band centered at 1420 era-1, which is due to the CH3 deformation mode [18]; this latter DMSO peak will be utilized in the analysis to be presented below. Region (2) from 1000 to l l 0 0 c m - t contains three overlapping lines in the Rarnan spectrum of the liquid at 1042(dep.), 1069(pol.) and 1093(pol.)cm-l[13] while the i.r. spectrum has strong lines at 1046 and 1094 cm -a [6], with no observable absorption corresponding to the 1069 cm-1 Raman line (This band is also absent in the gas phase Raman spectrum). The observed spectra in this region have been given various assignments [6, 13, 14]. However, based on the polarization of the Raman lines and the absence of the middle band from the i.r. spectrum, it is felt that the most plausible interpretation of this region is that of KRISHNANand KRISHNAN [13], who assign the 1093 and 1044 cm-1 lines to stretching modes (of symmetries A and B respectively) of the gauche isomer, and the band at 1069 cm- 1 to the trans C - - O stretch (Ag), which one expects, as observed, to be i.r. inactive. DMSO also contains a single band in this region, located at 1042cm -~ and assigned to the S==O stretch [18]. The third region, from 800 to 900 cm-~ has also been subject to uncertainties in assignment. The Raman spectrum of liquid ethylene glycol exhibits an intense,
Raman study strongly polarized line at about 865 cm-1 (with weak components at 883 and 803 cm-1)[13], while the i.r. spectrum shows a strong doublet with peak frequencies at 866 and 887cm -x [6]. While at least one group [6-1, on the basis of studies of the partially deuterated glycol (HOCD2CD2OH), has concluded that the 865 cm-x band in the vibrational spectra arises primarily from the CHz rocking vibration of the gauche conformer, other workers[10, 13-1 contend that this band is due to the C----C stretch (and is coincident for both conformers). Based on the high intensity and low depolarization ratio of the 865 cm-1 peak in the Raman spectrum (CH2 rocking vibrations are typically weak and depolarized in Raman spectra), this investigator feels that the latter assignment is more likely to be correct. Unfortunately, there is some contradictory evidence, and a conclusive assignment of this vibration is therefore not possible at present. Region (4), which is of crucial importance to this study) contains only two peaks in the Raman spectrum of the liquid, located at 525 and 480 cm-1. The latter band is absent from the i.r. spectrum, which, in addition to a line at 515cm - I , contains a very weak absorption at 430 cm- 1. BUCKLEY and GIGUERE [6-1, assuming that only the gauche conformation is present in the liquid phase, assign these lines in the i.r. to the two C---C--O bending vibrations of this conformer, and say that they correspond to the 525 and 480 crn-1 Raman bands. This investigator finds it difficult to reconcile the 50 cm-1 discrepancy of the lower frequency line between the Raman and i.r. spectra (480 vs. 430 cm-t). In contrast, Kalsh~Atq and KglsrrtqAtq [13], on the basis of a normal coordinate analysis, have assigned the 525 cm-1 Raman vibration to the antisymmetric (B) mode of the gauche conformer and the 480 cmband to the symmetric (Ag) C - - C - g ) bending mode of the trans conformer. This same assignment has been given in an independent study of MATSUURA[14-1. In addition, FUKUSHIMA and ZwoLINSKI [19], performed a complete analysis of the vibrations in n-propanol (which has a skeletal structure very similar to that of EG), and assigned a 458 cm- 1 vibration in this molecule to the symmetric C - - C - - O bend of the trans conformer, which lends additional support for the above assignment of the 480 cm-1 peak in EG to the trans form. B. Calculation of the gauche-trans equilibrium tn Figs. 1 and 2 are displayed the Raman spectra of EG at various concentrations in DMSO in the 456-550crn-1 region and in the range from 1000 to l 1 5 0 c m - k It is quite apparent that one
1027 i
f
i
540
i
500
FREOUENCY
i
,
E
460
(CM - I )
Fig. 1. The Raman spectrum of ethylene glycol as a function of mole fraction in DMSO (and in water) in the range from 450 to 550 cm-1. (A) Pure EG, (B) X~G = 0.60 in DMSO, (C) XE~ = 0.10 in DMSO (the broken curve represents the peak corrected for residual intensity from the lower frequency DMSO peak--see text), (D) EG in water (XEG = 0.10).
observes dramatic changes in the spectra in these regions as ethylene glycol is diluted in DMSO. The band at 525 c m - 1, assigned to the gauche conformer's C----C---O bend diminishes in intensity relative to the trans conformer's bending vibration at 480 cm- 1 (Fig. 1). Similarly (Fig. 2), the 1093 cm -1 band, assigned to the gauche C---O stretch is decreased markedly in intensity upon dilution in the solvent. (The presence of the 1042 cra-1 DMSO peak in the mixture precludes the observation of any increase in the intensity of the trans peak in this frequency region.) In the author's view, the most plausible explanation of the observed spectral changes is that the gauchetrans equilibrium is shifting more towards the latter conformation upon dilution in DMSO (a strong proton accepting solvent). Indeed, the great decrease in the intensity of the peaks assigned to the gauche conformer would suggest that a very high fraction of EG molecules exist in the trans configuration in very dilute DMSO solution. In order to obtain a quantitative measure of the fraction of molecules in the trans conformation (Fr), the ideal proc,edure would be to find a band in the
1028
M. SCHWARTZ ,
,
~
,
,
,
,
z
I100 1060 1020 FREQUENCY (CM -I) Fig. 2. The Raman spectrum of ethylene glycol as a function of mole fraction in DMSO (and in water) in the range from 1000 to l l 0 0 c m - L (A) Pure EG, (B) Xe6 = 0.10 in DMSO, and (C) EG in water (Xz6 = 0.10).
Raman spectrum of ethylene glycol whose scattering intensity is independent of the molecular conformation, to which the area of the 480 cm-1 trans vibration could be referenced. Then, any observed decrease in the ratio of the 480 c m - 1 band area relative to that of the reference peak would be indicative of a decrease in the fraction of trans isomer. The only EG vibration that can be safely assigned to both con° formers is the CH2 scissoring mode at 1460 c m - 1 ; unfortunately, its scattering intensity is too weak to be used as a quantitative reference (particularly at the lower concentrations, which are of major importance in the calculational procedure). The 865 c m - 1 band could be used safely only if it were conclusively assigned to the C----C stretch of both conformers. Since, as discussed above, the assignment of this vibration remains ambiguous, the above procedure, utilizing this peak as a reference, was utilized only as a check on the method of calculation adopted below. It was decided that the most reliable procedure is to use the D M S O CH3 deformation mode, which is located at 1420 c m - 1 and is very strong in the Raman spectrum, as a reference peak. One expects that any intermolecular interactions between D M S O and EG
will have little, if any, effect on the intensity of this vibration. One does encounter a slight added difficulty when using a solvent band as the reference, since the ratio, R = ,~4so/,~A~/~0~so142o('A' indicates peak area) will necessarily increase with increasing mole fraction of EG, independent of any variation in the fraction of trans conformers. However, this may be accounted for, as shown below. Presented in Table 1 are the spectral parameters of the trans EG band as a function of mole fraction (XEo), including the band areas. Table 2 contains the area of the 1420 c m - 1 D M S O peak and the observed ratio of peak areas (R °~' = A4so/A142o) as a function of mole fraction (as expected, this ratio increases with increasing concentration of ethylene glycol). In order to proceed fur.ther in the calculation, one must make the assumption that in the limit of very low mole Table 1. Spectral parameters of the 480 cm- 1 band of EG in DMSO XE~
Vo(Cm - 1)
I max*
A**
A4ao( x 10- a)
0.10 0.17 0.25 0.40 0.50 0.60 0.70 0.80 0.90 1.00
480 480 481 481 481 482 482 483 484 484
50 73 82 155 176 190 184 204 208 215
11.0 12.0 13.2 12.0 13.2 13.6 14.4 14.6 16.4 17.2
0.55 0.88 1.08t 1.8 6
2.32 2.58 2.65 2.9s 3.41 3.7o
* Intensity at band maximum (divided by laser power). ** Full-width at half-height (twice the half width of the low frequency side of the band). t Lower case digits are subject to uncertainty. Table 2. Calculation of the fraction of trans conformers utilizifig the 1420 cm- ~ DMSO band as a reference XE(;' 0.10 0.17 0.25 0.40 0.50 0.60 0.70 0.80 0.90 1.00
A1420 ( X
10 -4)
1.9s1" 1.82 1.5o 1.3o 1.15 0.9a 0.7a 0.5,1. 0.35 - -
R °bs*
R r**
Fr
O.02a 0.04s 0.072 0.144 0.201 0.252 0.363 0.550 0.98a
0.030 0.056 0.091 0.182 0.274 0.41o 0.63a 1.09 2.46
0.92 0.86 0.79 0.79 0.73 0.62 0.57 0.50 0.40
- -
- -
0.36~;
A480/A1420 (see Table 1 for A 480). ** R r is the predicted area ratio for 100% trans conformer (see text for discussion). I" Lower case digits are subject to uncertainty. :~Extrapolated value. * R °bs =
Raman study fraction of EG in D M S O , practically all of the molecules are in the trans form. In view of the great reduction in the gauche band intensities relative to the trans peak upon dilution (as seen in Figs. 1 and 2), this appears to be a not unreasonable approximation. Fitting the observed intensity ratios (R) at X ~ Z 0.10 and X ~ = 0.17 to the empirical relation, R = A . X + B . X 2 (which is constrained to yield R = 0 at X = 0, as must be the case), it was found that A = 0.276 and B = 0.0529. This equation then allows one to interpolate values of the area ratios at various low concentrations. Using the parameters above, one obtains at X~t; = 0.01, R0.ot = 2.76 x 10-3. Assuming that almost all molecules are trans at sufficiently low concentrations, the above result can be used to predict the area ratio, Rr(480/1420), that would be expected at higher mole fractions, if all the molecules were to remain in the trans form. We use the simple equation: Rr(XeG) = Ro.ol x
where the factor on the right corrects for the naturally expected increase in Rr(480/1420) with increasing Set;.
The results are shown in the fourth column of Table 2. (It was found that using X = 0.02 or 0.03 as a reference mole fraction gave virtually no change in the calculated results.) It is apparent that the observed ratio of peak ar'eas (R °bs) decreases relative to the predicted value for 100~ trans (R r) as the mole fraction of E G is increased, indicating that the fraction of trans conformer (Fr) decreases with increasing concentration. The actual fraction of trans conformer at each mole fraction is obtained simply as R°bS/Rr, and is displayed in col. (5) of Table 2. A graph of the experimental values of F r versus mole fraction is shown in Fig. 3. The solid line represents a least-squares fit of the data to the equation, Fr= 1 +A'X+B'X 2. We obtained values of A = - 0 . 5 8 and B = - 0 . 0 6 (with an rms deviation of 0.04). The relatively low value for B indicates, as seen in the graph, that the data is essentially linear in mole fraction (although we attach no theoretical significance to this result). An extrapolation of this fitted curve to XEG = 1.0 predicts that in pure ethylene glycol, F r = 0.36 (A fit of the data to an unconstrained straight line yields an extrapolated value of 0.38). As mentioned above, an alternative (though less reliable) procedure is to compare the integrated area of the 480 c m - 1 line to that of the ethylene glycol peak at 865 c m - 1 (assuming the intensity of this vibS.A.A.33ql
I
0.8
z
I.-
0.6~
Z 0 I"- 0.4
0.2
0.0 0
'
0'. 2
'
0 4i
'
0 6i
.
. 0.8.
.
1.0
MOLE FRACTION (XEG)
X~/0.01 XDmo/0.99
1029
Fig. 3. The fraction of trans conformers of ethylene glycol as a function of mole fraction in DMSO.
ration to be conformation independent). In this case, R r was obtained by a simple linear extrapolation of R °b~ (at X = 0.10 and 0.17) to X = 0. It was found that R r = 0.327. A direct comparison of this ratio with R °bs at each mole fraction yielded results for F r shown in Table 3. These values are in qualitative agreement with those obtained from the first procedure (particularly) for pure ethylene glycol, where the fraction of trans conformers is identical to that obtained above). It was discussed in an earlier section that the 480cm -1 band at the two lowest mole fractions Table 3. Calculation of the fraction of trans conformers utilizing the 865 cm-~ EG band as a reference get;
A86s( x 10 -4)
R °bs*
Fr**
0.10 0.17 0.25 0.40 0.50 0.60 0.70 0.80 0.90 1.00
0.204t 0.383 0.519 0.690 1.26 1.64 1.9a 2.4s 2.7a 3.11
0.259
0.82 0.70 0.99 0.88 0.57 0.46 0.42 0.37 0.38 0.36
0.229 0.323 0.289
0.185 0.151 0.137 0.122 0.12 a 0.119
* R °b~ = A480/A865. ** F r = R°b'/R r (R r = 0.327).
t Lower case digits are subject to uncertainty.
1030
M. SCHWARTZ
(crucial to the above calculations) was corrected to account for the long tail of the 390 cm- 1 D M S O peak (see the dotted curve in Fig. 1). If this correction is not performed, the same calculations yield a value of 0.25 fraction trans in the pure liquid. The author believes the value of 0.36 obtained using the corrected data to be more reliable; however, use of the unadjusted data furnishes a qualitative measure of the uncertainty in the calculated results. One further note is in order. The assumption that F r approached 1.0 at sufficiently low concentrations, used in both methods of calculation may not strictly be valid. The presence of a small, but finite, concentration of gauche conformers in the limit of infinite dilution would tend to slightly lower the values of Fr at the higher concentrations. It is estimated that if there remained ten mole percent of gauche conformers, the calculated value of F r for pure EC would be lowered to approximately 0.32. IV. D I S C U S S I O N
As the results in Fig. 3 show, ethylene glycol in D M S O exists as an equilibrium mixture of the gauche and trans conformers, which is steadily shifted toward the latter as the EG is diluted in the strongly basic solvent. Pure liquid ethylene glycol appears to have a gauche-trans ratio of approximately 2:1 (Fr = 0.35 + 0.1), in contrast to the vapor and solid phases, where all molecules are in the gauche configuration. It must be emphasized that the results obtained in this study apply only to the configuration of the molecular skeleton--that is, the O - - C - - C - - O dihedral angle, which has three equilibrium values, as shown below
H H
I H H qauche
H H [ H H qauche
H
H
O
H
tran$
As pointed out by PODO et al. 1-20] In a theoretical analysis of ethylene glycol and similar ethane derivatives, a complete characterization of the molecular configurat!on requires the specification of the two C - - C - - - O - - H dihedral angles as well as the skeletal configuration (shown above). Furthermore, we note that the experimental results cannot be used to predict what fraction of those molecules in the gauche configuration are intramolecularly hydrogen bonded.
Given the above restrictions, it is still possible to propose a model for the molecular structure in the pure liquid and in dilute D M S O solution which is consistent with the results obtained here. The generally accepted presumption that all EG molecules are gauche in the vapor is easily understood, since this form is stabilized by intramolecular hydrogen bonds. In the liquid, on the other hand, the molecules can also form intermolecular hydrogen bonds with their neighbors. These bonds are generally found to be stronger than intramolecular bonding [21] because of the non-linearity of the bond (due to steric hindrance). In the pure liquid, we can picture three simplified molecular configurations:
H. H-.O~ "',o.~H---O C--C / \ \ / H- -O\ ..,O~.H___O O"-H~..O .-H C--C O---H~o .-'H H \ (O) \0-- H C--C \ / \ C--C .O--H---O (b)
i-i
(c)
The first picture represents EG in the gauche form, stabilized by one intramolecular hydrogen bond and possibly two additional intermolecular hydrogen bonds. Picture (b) shows the possibility of a cyclic dimer (with both EG molecules in the gauche conformation) containing two intermolecular bonds connecting the monomers with as many as four additional hydrogen bonds with other molecules in the liquid. The third picture is that of a molecule in the trans configuration, capable of participation in as many as four hydrogen bonds with neighboring molecules. In contrast to the vapor, where structure (a) is expected to be the most stable form, in the liquid, the trans configuration is also stabilized by hydrogen bonding (picture (c)). This may explain the presence of a finite fraction of trans conformers in this phase. We turn now to discuss the apparent diminishment of gauche conformers in dilute D M S O solution. In contrast to the pure liquid, where molecules can act both as proton donors and acceptors, D M S O can only accept protons in hydrogen bond formation. Thus one can picture the following two possible structures: \
/ o ~ H " ' . O ~ HyO-~S\
\
/
/S~o"'H~ 0 \
c--c
C--C (o)
O.~.H_ O ~ S / (b)
\
Raman study In this case, the gauche isomer (a) is stabilized by one, relatively weak, intramolecular hydrogen bond and one intermolecular bond, while the trans conformer is free to form two, stronger, intermolecular hydrogen bonds with the solvent. This may account for the apparent preference of EG molecules to assume a trans conformation in DMSO. It should be pointed out that, while the above (relatively simplistic) picture of hydrogen bonding in EGDMSO mixtures offers a satisfactory explanation of the results, it is, of necessity, somewhat speculative in nature (due to the complexity of intermolecular interactions in hydrogen bonded liquids), and other interpretations of the observed data may be possible. The ca]rculated value obtained here for the fraction of trans conformers in the pure liquid may be compared with earlier studies of ethylene glycol using NMR spectroscopy. PACnLER and WESSELS[22], in a study of the solvent dependence of vicinal protonproton coupling constants in various substituted ethanols including ethylene glycol, found a trans population of approximately 0.2 for the neat liquid, which is somewhat outside the limits of error estimated for this study (0.25 < F r < 0.45). However, in order to obtain coupling constants for each conformation, their analysis required the assumption that the fraction of the more polar gauche conformer increases with increasing dielectric constant of the solvent (independent of its specific bonding properties). While this assumption may well have been valid for earlier NMR studies of dihalo ethanes [23], it is felt that it cannot fairly be made for the case of alcohols and their derivatives, where hydrogen bonding interactions with the solvent (rather than polarity alone) can be expected to exert a strong influence on the solute conformation. The questionable nature of their assumption is borne out by the results obtained in the present investigation, where the molecular equilibrium appears to shift towards the trans conformer upon dilution in DMSO, a solvent with a fairly high dielectric constant (~ = 44.7)[24]. Our results are in agreement with the NMR coupling constant study of VITI et al. [25] on dimethoxyethane and methoxyethanol, who found that, for the latter substance (quite similar to EG), the "trans form is greatly stabilized" in solvents such as DMSO, DMF and ketones, all of which have high dielectric constants (and which are all strong Lewis bases). Their results appear to support the contention that in hydrogen bonding liquids, the specific nature of the solvent may have a greater influence on the conformational equilibrium than simply the solvent's dielectric constant. One other NMR investigation of EG was performed by CONNORand MCLAUCHLAN[26], who util-
1031
ized the[13] C - - H proton satellite side bands to determine F r in ethylene glycol, and obtained values around 0.25-0.30 for the pure liquid extracted (from Fig. 5 of their paper), in reasonable agreement with the result obtained here. Finally, we turn to a brief discussion of the effect of dilution in water on the conformation of ethylene glycol. KRISHNAN and KRISHNAN[13] studied the Raman spectrum of dilute aqueous solutions of EG in the region from 1000 to ll00cm-1. They observed an apparent decrease in the intensity of the 1069 cm -~ band (C---O stretch of the trans conformer) relative to the 1042 cm -~ gauche peak, and concluded that EG molecules in dilute aqueous solution are all in the gauche conformation. However, they were unable to obtain the Raman spectrum in the important 4(K)-550cm -1 interval, due to what they termed broad, diffuse scattering from the water in the solutions. On the other hand, ATALLAand WILLIAMS[4], who examined the spectra of five mole percent EG in H 2 0 , suggested that the molecular conformation is totally trans in aqueous solution. We have re-examined the spectrum of an aqueous solution of ethylene glycol (with XnG = 0.10) in both regions of interest, and observed that a new maximum does seem to appear at around 1050cm -1 (Fig. 1C) in aqueous solution. However, we were also able to study the low frequency region, and, aside from a constant background due to n 2 0 scattering, there were no significant differences in the intensity of the 481cm -~ trans peak relative to the gauche band located at 525 cm-1, when compared to that in pure EG (see Figs. lA and ID). A major shift of the equilibrium towards either conformer in dilute aqueous solutions would be expected to show up quite clearly in this portion of the spectrum. It would therefore appear likely that the relatively minor intensity variations in the higher region of the spectrum arise from a solvent shift in the intensity maxima in this region, rather than from a large change in the conformational equilibrium. In conclusion, the results of this investigation have illustrated that the trans configuration of ethylene glycol appears to be increasingly stabilized upon dilution in the highly polar (but Lewis basic) solvent, dimethyl sulfoxide; this permitted the determination of a semiquantitative estimate of the relative population of conformers in the pure liquid, which is in reasonable agreement with values obtained earlier from NMR techniques. A simplified qualitative interpretation of bonding in EG-DMSO mixtures has been offered to explain the experimental data. However, the question of intermolecular interations in hydrogen bonded liquids remains a complex one,
1032
M. SCHWARTZ
and further experiments are currently in progress in this laboratory to study the conformational equilibrium of other glycols and substituted ethanols in solvents with varying properties. The results of these experiments will be reported upon their completion.
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