Solvent-Induced Frequency Shifts in the Infrared Spectrum of Dimethyl

induced frequency shifts in the infrared spectrum of DMSO. The results are explained in terms of the solvent effect on the equilibrium of two resonanc...
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J. Phys. Chem. 1996, 100, 2019-2024

2019

Solvent-Induced Frequency Shifts in the Infrared Spectrum of Dimethyl Sulfoxide in Organic Solvents W. Ronald Fawcett* and Alla A. Kloss Department of Chemistry, UniVersity of California, DaVis, California 95616 ReceiVed: June 23, 1995; In Final Form: October 26, 1995X

Solvent-induced frequency shifts in the infrared spectrum of dimethyl sulfoxide (DMSO) and deuterated dimethyl sulfoxide (DMSO-d6) as dilute solutes have been studied in a wide variety of organic solvents. The results obtained show that intermolecular interactions affect vibrations of all the groups in the molecules of the solute, but in different ways and to different extents. The dipole related SdO stretching mode in both DMSO and DMSO-d6 appears to be the most sensitive to the molecular environment. Bands corresponding to the νa(CSC) A′′ asymmetric stretch in DMSO and the r|(CD3) A′ rocking vibration in DMSO-d6 were also chosen as probe modes because they are more clearly observed in all the studied solutions. Complications in the spectrum of DMSO due to the strong overlaps of the bands were avoided by using DMSO-d6. The relative contributions of specific and nonspecific solvation effects were evaluated using a linear solvation energy relationship (LSER). Simple linear equations were obtained for evaluation (prediction) of the solvent induced frequency shifts in the infrared spectrum of DMSO. The results are explained in terms of the solvent effect on the equilibrium of two resonance forms of DMSO in solution.

Introduction It is well-known that the vibrational spectrum of a molecule is markedly affected by environmental factors. Intermolecular interactions modify infrared spectra in a number of ways, including the frequency shifts of the normal vibrational modes of the molecule, alteration of the intensities and half-widths of the bands.1 During recent years some advances have been made in applying the measurement of solvent induced infrared spectral changes to studies of solute-solvent interactions in solutions made up of different organic solvents.2-6 These studies were focused mostly on observation of solvent induced frequency shifts (SIFS) of the vibrations associated with the dipole of the probe molecule. The first theoretical treatment of infrared SIFS was given by Kirkwood7 and by Bauer and Magat8 and led to the KirkwoodBauer-Magat equation (KBM). This equation was derived on the basis of Onsager’s reaction field theory,9 using the simple model of a diatomic oscillator within a spherical cavity in the solvent which was represented as a dielectric continuum. Later, the KBM equation was modified in different ways for better correlation with experiments, tested with a large number of compounds, and found to be valid only in a very limited range for dilute solutions of nonpolar solvents where specific interactions can be neglected.1,10 In a series of recent works, Nyquist2-5 used a different approach in which specific local interactions were emphasized and attempted to correlate the observed SIFS with an empirical solvent parameter, measuring solvent electrophilicity (Gutmann’s acceptor number (AN)11). This approach was applied to solutions of aliphatic and aromatic ketones in organic solvents, but in most cases a good correlation was not observed. In a later work, Fawcett et al.6 used a more general approach to explain the SIFS for solutions of acetonitrile in various organic solvents: Both the electrophilicity and electrodonicity (Gutmann’s donor number (DN)12,13) of the solvent were considered with respect to those of the solute. This work was later extended10 using the Koppel and Palm linear solvation X

Abstract published in AdVance ACS Abstracts, January 1, 1996.

0022-3654/96/20100-2019$12.00/0

energy relationship.14,15 It was shown that a precise quantitative description of SIFS is possible only when one considers both specific solvation effects (electrophilicity and electrodonicity), and nonspecific effects which depend on the bulk dielectric properties of the solvent.10 The important advantage of the LSER method is that it also allows one to evaluate the relative contributions of each property of the solvent in the SIFS and makes it possible to correlate these contributions to the properties of the solute. From this point of view, the study of dimethyl sulfoxide (DMSO) as a solute is especially interesting because of its very high electrodonicity,1 as well as its strongly nonideal behavior in solutions.16-18 In the present paper, we report a study of SIFS in solutions of DMSO and deuterated DMSO (DMSO-d6) in a number of organic solvents with a wide range of properties. Experimental Section Purified DMSO (HPLC grade Aldrich , 99.9% pure), and DMSO-d6 (Cambridge Isotope Laboratories, 99.9% pure) were used for the experiments. Other organic solvents were HPLC grade (Aldrich), at least 99.9% pure with a residual water level less then 0.005%. Solutions of DMSO and DMSO-d6 in other solvents were made with a constant mole ratio of 1 mol of solute to 12 mol of solvent. The IR path length cells (Graseby Specac) were made of NaCl and of ZnSe, with a Mylar spacer, providing a 0.005 mm path length. Spectra were collected using the Mattson Research Series FTIR Spectrometer with a resolution of 0.5 cm-1. The cells were cleaned with acetone, dried in a flow of nitrogen gas, and washed with the test solution before each experiment. All the experiments were performed at a constant temperature of 22 °C. The reported spectra were obtained by subtracting the spectrum of the pure solvent from that of the solution containing DMSO. The subtrahend multiplying factor ranged from 0.9 to 1.1 and was chosen to ensure that a minimum of solvent features remained in the difference spectrum. Overlapping bands were © 1996 American Chemical Society

2020 J. Phys. Chem., Vol. 100, No. 6, 1996

Figure 1. (a) IR spectrum of pure liquid DMSO; (b) IR spectrum of pure liquid DMSO-d6.

resolved using the Curvefit program (Spectrocalc, Galactic), calculating pure Lorenzian bands in order to resolve overlapping peaks. Other spectral manipulations were made using Spectrocalc (Galactic) or Enhanced First Fourier Infrared (Mattson) software. Results and Discussion The DMSO molecule belongs to the Cs point group, possessing only one plane of symmetry containing S and O atoms.19 The methyl groups are equivalent, but within the groups the hydrogen atoms are located differently with respect to the sulfur and oxygen atoms.20 The 24 fundamental vibrations are divided into two types of symmetry of the point group Cs: 13 A′ and 11A′′. All of them are both Raman- and IR-active.19 IR spectra of pure DMSO and DMSO-d6 in the frequency range 600-1200 cm-1 are shown in Figure 1. In the spectrum of pure liquid DMSO (Figure 1a) a very intense band is observed at 1058 cm-1, which has two shoulders, a rather intense one at 1044 cm-1, and another one arising from the overlapping peaks at 1027 and 1013 cm-1 due to the rocking vibrations of the methyl groups.19 Results of the curve fit in this region of the spectrum for both DMSO and DMSO-d6 are presented in Figure 2. The positions of the peaks at 1058 and 1044 cm-1 are almost the same in both DMSO and DMSO-d6, indicating that these peaks are associated with the SO vibrations. Studies of the change of the shape and position of these bands when DMSO is diluted with carbon tetrachloride19 allow one to identify the corresponding vibrations. As the concentration of DMSO decreases, the peak at 1044 cm-1 disappears. This fact together with the absence of the analogous peak in the spectrum of gaseous DMSO19 shows that it corresponds to autoassociates of DMSO molecules. At the same time the 1058 cm-1 peak becomes more intense with respect to the one at 1044 cm-1

Fawcett and Kloss

Figure 2. (a) Results of the curve fit of the SO stretching region of the spectrum of pure liquid DMSO-d6 showing four bands between 1000 and 1100 cm-1. (b) Similar results of the curve fit of the SO stretching region of the spectrum of pure liquid DMSO-d6.

and shifts in the blue direction with dilution. In very dilute solutions it appears at 1070 cm-1. On the portion of the spectrum of DMSO-d6 shown in Figure 1b, one can see three very intense peaks at 1059, 1030, and 1008 cm-1. Dilution with CCl4 strongly modifies them.19 The first peak shifts from 1059 to 1069 cm-1, while the other two bands only decrease in intensity. Results of the curve fit (Figure 2b) show that the spectral features of DMSO-d6 arise from the above peaks and the peak at 1044 cm-1 corresponding to the SO stretching vibration of DMSO-d6, from self-associates. This peak is difficult to see because of the strong overlaps with the bands at 1030 and 1008 cm-1 arising from CD3 deformational vibrations, and the band at 1059 cm-1 due to the SO stretching vibrations of the unassociated molecules of DMSO-d6. The self-association of DMSO must be viewed within the context of the association of DMSO with other solvent molecules. In this connection the infrared spectral characteristics of DMSO relating changes in the stretching frequency of the SdO oscillator with perturbances induced at either terminal of the bond are of special interest. Table 1 summarizes the present experimental data for the frequencies of the SO stretching mode of DMSO and DMSO-d6 in different organic solvents. The data were examined, using four independent parameters to describe solvent properties. These are the solvent’s acceptor and donor numbers for specific solvation effects, and solvents polarity and polarizability for nonspecific effects. Solvent polarity is defined from the static dielectric constant as

Y ) (s - 1)/(s + 2)

(1)

and is related to the molecule’s polar properties as described in

Dimethyl Sulfoxide in Organic Solvents

J. Phys. Chem., Vol. 100, No. 6, 1996 2021

TABLE 1: Experimental Data for the Characteristic Frequencies (cm-1) of DMSO and DMSO-d6 in Different Organic Solvents solvent acetic acid acetone acetonitrile benzyl alcohol benzene benzonitrile tert-butyl alcohol carbon tetrachloride chloroform dichloroethane dichloromethane diethyl ether N,N-dimethyl acetamide N,N-dimethyl formamide dimethyl sulfoxide ethanol formamide methanol N-methyl formamide nitrobenzene nitromethane 2-propanol trifluoroacetic acid

freq ν(SO) DMSO DMSO-d6 1002.2 1063.9 1060.3 1028.8 1067.5 1059.8 1057.5 1069.8 1055.8 1061.0 1058.4 1061.0 1062.8

1002.0 1062.7 1059.8 1028.1 1066.5 1061.3 1054.3 1065.8 1054.6 1059.9 1057.6 1059.9 1061.7

1056.3

1057.8

1058.0 1024.7 1023.9 1022.4 1028.1 1060.6 1060.1 1027.0 941.2

1059.0 1025.5 1023.3 1022.0 1028.7 1060.2 1057.5 1027.4 939.1

freq νa(CSC), freq r|(CD3) A′′, of DMSO of DMSO-d6 711.3 694.0 697.9 704.0 688.0 697.8 701.0 692.9 697.8 695.7 695.8 692.5 696.4

826.2 820.7 820.9 823.1 817.0 818.9 823.7 818.0 820.0 819.1 819.7 818.0 820.9 822.0

698.4 704.7 706.6 705.0 703.4 690.5 699.2 704.0 725.4

822.3 824.7 826.6 826.0 824.8 818.6 821.4 824.0

TABLE 2: Independent Solvent Parameters Used To Describe Specific and Nonspecific Solvation Effects solvent

AN

DN

Y

Π

acetic acid acetone acetonitrile benzyl alcohol benzene benzonitrile tert-butyl alcohol carbon tetrachloride chloroform dichloroethane dichloromethane diethyl ether N,N-dimethyl acetamide N,N-dimethyl formamide dimethyl sulfoxide ethanol formamide methanol N-methyl formamide nitrobenzene nitromethane 2-propanol trifluoroacetic acid

52.9 12.5 18.9 34.5 8.2 15.5 27.1 8.6 23.1 16.7 20.4 3.9 13.6 16.0 19.3 37.9 39.8 41.3 32.1 14.8 20.5 33.8 105.3

12.7 17.0 14.1 15.8 0.0 11.9 21.9 0.0 4.0 0.0 1.0 19.2 27.8 26.6 29.8 19.2 24.0 19.1 27.0 4.4 2.7 21.1 7.1

0.634 0.868 0.924 0.801 0.302 0.890 0.793 0.286 0.565 0.758 0.725 0.524 0.925 0.922 0.938 0.887 0.973 0.914 0.984 0.918 0.921 0.865 0.716

0.226 0.218 0.211 0.313 0.293 0.307 0.234 0.273 0.265 0.265 0.254 0.215 0.261 0.257 0.283 0.220 0.267 0.202 0.258 0.319 0.231 0.234 0.365

the continuum Debye model.22 Solvent polarizability comes from the Lorentz-Lorenz 23 model for molecular polarizability and is estimated from the solvent’s refractive index using the relationship

Π ) (n2 - 1)/(n2 + 2)

(2)

Table 2 summarizes these parameters for the solvents considered in this study. Analysis of solvation effects on the frequency of the SO stretching mode was conducted using the LSER method previously applied for other systems.10 The equation used to describe solvent effect on a given vibrational band is

ν ) ν0 + ∑(RiPi)

(3)

i

where ν is the frequency of the band in the particular solution,

Figure 3. (a) Plot of the frequency of the SO stretch of DMSO-d6 against solvent acceptor number. (b) Same for DMSO-d6.

and the parameter Pi represents one of the solvent parameters. The coefficients Ri describe the response of the observed effect to the respective solvent parameter and are obtained using multiple linear regression analysis. ν0 is the value of the frequency for the case that all Pi are zero. When the LSER analysis was performed on the data for the frequencies of the SO stretch of DMSO presented in the Table 1, the strongest correlation was obtained with the solvent’s acceptor number (correlation coefficient R ) 0.977), correlations with each of the other three parameters being much weaker. The standard deviation (SD) for this simple one parameter fit was 6.3 cm-1. Addition of any one of the remaining parameters did not result in a significant improvement of the fit. It is clear that AN is the factor of the prime importance influencing the intermolecular interactions in solutions of DMSO. Therefore, a rather precise description of the solvent induced changes in the SO stretching vibrations of DMSO molecules can be achieved by using the simplified equation

ν ) ν0 + R1AN

(4)

Linear regression analysis of the data yields R1 ) -1.375 and ν0 )1080 cm-1. Analogous results were obtained for DMSO-d6, AN being the most important parameter. A one parameter fit using eq 4 resulted in a correlation coefficient R ) 0.981 and standard deviation SD ) 5.8 cm-1. No improvement was obtained by adding any other parameter. Results of the linear regression analysis are R1 ) -1.360 and ν0 ) 1080 cm-1. The special importance of the electrophilicity of the solvent can be related to the fact that DMSO is a strong Lewis base (electron donor).10 Figure 3 illustrates the dependence of the SO-stretching frequency on solvent acceptor number (AN) and

2022 J. Phys. Chem., Vol. 100, No. 6, 1996

Fawcett and Kloss SCHEME 1

Figure 4. SO stretching region of the spectra obtained in DMSO-d6 solutions in dichloromethane (1), chloroform (2), and acetonitrile (3).

demonstrates rather good linear relationships for both DMSO and DMSO-d6. From the analysis of these data one concludes that the direction of the shift of the SO stretching frequency in solutions from that in pure liquid DMSO (DMSO-d6) changes. It is positive (or blue), for the solvents whose AN is less than that of DMSO, and negative (or red) in the other case. The positive shift does not exceed 12 cm-1, while the negative one for some of the solvents considered is large, being -120 cm-1 in the case of trifluoroacetic acid. The solvents shifting the ν(SO) band in the blue direction are relatively nonpolar and poor electron acceptors (Tables 1 and 2). The absolute values of the SIFS in these solvents are small and decrease as the solvent AN approaches that of DMSO. The solvents whose AN is close to that of DMSO do not perturb the SO stretching frequency of DMSO to a very significant extent (Table 1). The values of the SIFS in these solutions usually do not exceed a few wavenumbers. It is interesting that the other parameters characterizing intermolecular interactions (DN, polarity and polarizability) for these solvents vary significantly (Table 2). The fact that these parameters do not influence SIFS to any significant extent may be related to the very strong electron-donor properties of DMSO making the electrophilicity of the solvent the factor of prime importance. It also should be noted that in these solutions a strong tendency of DMSO molecules to form self-associates is observed: the intensities of the peak at 1044 cm-1 are rather high. A few examples are shown in the Figure 4, where a significant peak at 1044 cm-1 appearing as a shoulder in the SO stretching region of the spectrum can be seen without curve fitting. For instance, in acetonitrile whose AN is very close to that of DMSO, the SIFS observed was one of the smallest. This shift can be attributed to the formation of the intermolecular associates such as the antiparallel dipolar conformation shown in Scheme 1. However, it should be noted that the small value of the shift indicates that these interactions are weak. This also explains

the rather large peak corresponding to the SO stretch of selfassociates of DMSO (Figure 4). The increase of the solvent AN results in large negative SIFS of the SO stretch, especially in hydrogen bonding solvents. The hydrogen bond accepting properties of DMSO are evident from the fact that it is an excellent solvent for materials that contain hydroxyl and other hydrogen bond donating groups, even when the molecular weights of these solvents are relatively high.18 The peak due to the DMSO self-associates is not observed at all in primary alcohols, acids and formamide, while the SIFS of the SO stretch in these solvents is quite large (Table 1). Therefore, our data indicate that in these solutions interactions between solvent and solute successfully compete with the interactions between molecules of DMSO. Another important detail is that the values of ν0 obtained in LSER analysis of the of the SO stretching frequencies for both DMSO and DMSO-d6 solutions are exactly the same and close to the frequencies of the SO stretch of DMSO and DMSO-d6 when they are dilute solutes in inert solvents.19 These values are lower then those for DMSO (DMSO-d6) in the gas phase (1101 and 1099 cm-1, respectively) where the molecule is not perturbed by intermolecular interactions. The somewhat low ν0 value can be related to the strong interactions between molecules of DMSO in solutions. Replacement of solvent molecules in the environment of a given DMSO solute molecule by other DMSO molecules is expected to take place in solvents with low acceptor numbers. To assess the role of selfassociation on the present analysis, it was repeated considering only those solvents with values of AN greater than that of DMSO itself. Excluding trifluoroacetic acid whose AN is very high, there are nine such solvents with values of AN varying from 23.1 (chloroform) to 52.9 (acetic acid; see Table 2). For this limited group of solvents, linear regression with AN gives

ν ) 1096 - 1.85AN

(5)

with a correlation coefficient R ) 0.933 and standard deviation SD ) 6.6 cm-1. The value of the limiting frequency in the absence of a solvent effect is now much closer to the gas phase result (1101 cm-1). This result suggests that the observed blue shifts of the SO band in solvents with values of AN lower than that of DMSO would be larger if self association of DMSO did not occur in these systems. It also suggests that the SO band would shift somewhat to the blue in more dilute solutions with a mole ratio of solvent to solute greater then 12. It was also noted that a slight improvement in the description of the SIFS for this limited solvent group is achieved by adding solvent polarity as a second parameter. The value of ν0 then increases to 1110 cm-1, and the correlation coefficient to 0.946. This result suggests that a nonspecific solvation effect is present in the observed SIFS, but it is not large. Until present very little was known about the shifts of the bands associated with the CSC and CH3 vibrations of DMSO due to intermolecular interactions in solutions. First of all the 600-1000 cm-1 frequency range of the spectrum of pure DMSO

Dimethyl Sulfoxide in Organic Solvents

J. Phys. Chem., Vol. 100, No. 6, 1996 2023 LSER analysis of the observed SIFS of the νa(CSC) A′′ and the r|(CD3) A′ bands was performed using eq 3, the procedure being analogous to that described above for the ν(SO) band. Analysis of the data for the νa(CSC) A′′ frequencies (Table 1) showed that the solvent AN is the most important parameter influencing the frequency shifts. A one-parameter fit results in the equation

νa(CSC) ) 690.3 + 0.361AN

(6)

with R ) 0.965 and SD ) 2.1 cm-1. Including the second most important parameter (solvent DN) significantly improves the fit. The equation is then

νa(CSC) ) 688.5 + 0.357AN + 0.136DN Figure 5. Plot of the frequency of the νa (CSC) A′′ vibration of DMSOd6 against solvent acceptor number (1); Same for the r|(CD3) A′ vibration of DMSO-d6 (2).

depicted in Figure 1a is considered further. The bands at 953, 931, and 896 cm-1 refer to the r⊥(CH3)A′, the r⊥(CH3)A′′, and the r|| (CH3)A′′ motions, respectively. The bands at 953 and 931 cm-1 are rather intense, but partially overlap. These bands undergo SIFS in solutions. As a result, they can overlap with the bands due to the r||(CH3) A′ vibration, the ν(SO) vibration of the molecules in DMSO self-associates and the ν(SO) vibrations of unassociated DMSO molecules, which are observed at 1027, 1044, and 1058 cm-1, respectively, in pure DMSO (see Figure 1a). Two bands, an intense one at 698 cm-1 and a weak one at 668 cm-1, correspond to the νa(CSC) A′′ and to the νs(CSC) A′ stretching modes of the DMSO skeleton. The peak at 668 cm-1 is rather weak and hard to see in solution because of the solvent induced changes and the proximity of this band to the cutoff frequency of the IR cell material. For this reason the 698 cm-1 peak was chosen as a probe band to study the solvent effect on the CSC vibrations of DMSO. In the 600-1000 cm-1 frequency range of the IR spectrum of pure DMSO-d6 shown in Figure 1b, two intense bands are observed at 822 and 759 cm-1 corresponding to the r|(CD3) A′ and the r⊥(CD3) A′′ rocking vibrations. The bands at 773 and 679 cm-1 due to the r⊥(CD3) A′ and the r|(CD3) A′′ motions are weak and also hard to see because of the overlaps in this region. Bands at 623 and 612 cm-1 corresponding to the vibrations of the DMSO-d6 skeleton appear very close to the cutoff frequency of the IR cell material and are also hard to see. Since the r|(CD3) A′ band of DMSO-d6 is more clearly observed in most of the solutions, it was selected as a band to examine for a SIFS. The data for the frequencies of the νa(CSC) A′′ of DMSO and the r|(CD3) A′ band of DMSO-d6 in the organic solvents studied here are presented in Table 1. It is clear from these results that the solvent induced changes in the frequencies of the CSC and CD3 vibrations are less pronounced than in the case of the SO stretch. The changes are somewhat greater in the case of the CSC stretch, whereas the SIFS of the r|(CD3) A′ band do not exceed a few wavenumbers. Nevertheless, these shifts follow the change in the solvent’s AN (Figure 5). The observed SIFS are negative for solvents whose AN is less than that of DMSO and positive when the AN of the solvent is greater than that of DMSO. It is interesting to note that the sign of the observed SIFS for these two modes is opposite to that for the SO band. This tendency shows that the interaction of DMSO with a strong electron acceptor strengthens the C-S and C-H (or C-D) bonds whereas it weakens the SdO bond.

(7)

with R ) 0.979 and SD ) 1.7 cm-1. Analysis of the statistics 10 for this fit shows that ∼85% of the explained correlation in ν is due to the solvent AN with ∼15% due to the solvent DN. Although the second parameter (solvent DN) is less important in the SIFS, its influence is statistically significant. Addition of other parameters does not improve the fit. In analogy to the case of the SO stretch, the largest SIFS for the νa(CSC) A′′ band is observed in the solution of trifluoroacetic acid. The effect of this datapoint on the results of the LSER analysis was examined. It was found that exclusion of the datapoint corresponding to trifluoroacetic acid as a solvent does not change either the slope or the intercept of the plot to any significant extent. LSER analysis of the data presented in Table 1 for the frequencies of the r|(CD3) A′ vibration in solutions of DMSOd6 shows that a one-parameter fit leads to the equation

νr|(CD3) ) 816.8 + 0.210AN

(8)

with R ) 0.911 and SD ) 1.2 cm-1. Inclusion of the second parameter in importance (solvent DN) in the fit improves it significantly, the resulting equation being

νr|(CD3) ) 815.8 + 0.185AN + 0.108DN

(9)

with R ) 0.976 and SD ) 0.7 cm-1. In this case ∼69% of the explained correlation is due to the solvent AN and ∼31% to the solvent DN. No significant improvement is obtained by adding more parameters. No reliable data for the frequency of the r|(CD3) A′ vibration were obtained in trifluoroacetic acid due to band overlaps. For this reason the data point corresponding to this solvent could not be included in the table. Unlike the case of the SO stretch, the values of ν0 calculated by the LSER method for these vibrations are exactly those obtained from gas phase measurements. This result can be attributed to the less pronounced effect of the associative interactions between DMSO molecules on the respective frequencies. This also results in the smaller values of SD obtained for these vibrations than those for the case of the SO stretch. The high sensitivity of the SO stretching vibration of DMSO to its molecular environment observed in the present study implies that the association with the solvent molecules takes place through the SO moiety of DMSO (DMSO-d6). The key to the explanation of these phenomena and of the general nature of the solvent dependence of the observed SIFS appears to be the description of the SO bond as a resonance hybrid of a semipolar double bond and a (pfd)π double bond

(CH3)2S+sO- T (CH3)2SdO

(10)

The sulfur atom in the DMSO molecule is sp3 hybridized and

2024 J. Phys. Chem., Vol. 100, No. 6, 1996 the bond is formed by the (sp3 - px ) σ overlap along a π bond of the type dxz - pz (or dxy - py). Such (pfd)π overlaps are not very strong24 and a strongly polar SO bond results, where the oxygen atom is electronegative with respect to the sulfur atom.17,20,24 These considerations explain the large dipole moment, high electrodonicity of DMSO and the SO bond length corresponding to a bond order of 1.55.17 Interaction of the electron rich oxygen terminal of the DMSO molecule with the electron accepting site of the solvent molecule polarizes the SO bond of DMSO and shifts equilibrium 10 to the left, which causes a decrease in the SO bond order. The association of the oxygen atom of the DMSO molecule with an electron acceptor also decreases the partial negative charge on it, which weakens the electrostatic attraction of the sulfur and oxygen atoms. This weakening of the SO bond causes a decrease in the ν(SO) frequency. At the same time, the increase of the partial positive charge on the sulfur atom strengthens its bond with the electron-donating CH3 groups, which is accompanied by a stabilization of the C-H bonds. This explains the blue shifts of the CSC and CH vibrations. This mechanism allows one to elucidate the strong correlation of the SIFS of all the bands analyzed above to the solvent electrophilicity (AN). The small observed contributions of the solvent DN in SIFS of the CSC and CH3 vibrations can be attributed to the associative interactions of the sulfur terminal of the SO bond with the electron-donating site of the solvent molecule. Although the sulfur atom in DMSO does have some electron-accepting abilities, this is expected to be weak because of the lone pair on this atom. Acknowledgment. This research was supported by the Office of Naval Research, Washington.

Fawcett and Kloss References and Notes (1) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 2nd ed.; VCH Publishers: New York, 1988. (2) Nyquist, R. A. Appl. Spectrosc. 1989, 43, 1208. (3) Nyquist, R. A. Appl. Spectrosc. 1990, 44, 426. (4) Nyquist, R. A. Appl. Spectrosc. 1990, 44, 433. (5) Nyquist, R. A. Appl. Spectrosc. 1992, 46, 306. (6) Fawcett, W. R.; Liu, G.; Kessler, T. E. J. Phys. Chem. 1993, 97, 9293. (7) Kirkwood, J. G. J. Chem. Phys. 1937, 5, 14. (8) Bauer, B.; Magat, M. J. Phys. Radium 1983, 9, 319. (9) Onsager, L. J. Am. Chem. Soc. 1936, 58, 1486. (10) Fawcett, W. R. In Theoretical and Computational Chemistry: QuantitatiVe Treatments of Solute/SolVent Interactions; Politzer, P., Murray, J. S., Eds.; Elsevier: Amsterdam, 1994; Vol. 1, p 183. (11) Mayer, U.; Gutmann, V.; Greger, W. Monatsh. Chem. 1975, 106, 1235. (12) Gutmann, V.; Wychera, E. Inorg. Nucl. Chem. Lett. 1966, 2, 257. (13) Gutmann, V. Coord. Chem. ReV. 1976, 19, 225. (14) Koppel, I. A.; Palm, V. A. In AdVances in Linear Free Energy Relationship; Chapman, N. B., Shorter, J., Eds.; Plenum: London, 1972; Chapter 5. (15) Koppel, I. A.; Palm, V. A. Reakt. Sposobn. Org. Chem. Soedin. 1971, 8, 291. (16) Luzar, A. J. Mol. Liquids 1990, 46, 221. (17) Vaisman, I. I.; Berkowitz, M. L. J. Am. Chem. Soc. 1992, 114, 7889. (18) Jacob, S. W.; Rosenbaum, E. E.; Wood, D. C. Dimethyl Sulfoxide; Marcell Dekker, Inc.: New York, 1971; Vol. 1. (19) Forel, M.-T.; Tranquille, M. Spectrochim. Act 1970, 26A, 1023. (20) Thomas, R.; Shoemaker, C. B.; Eriks, K. Acta Crystallogr. 1966, 21, 12. (21) Wiberg, K. B. Physical Organic Chemistry; Wiley: New York, 1964; p 136. (22) Marcus, Y. Ion SolVation; Wiley-Interscience: New York, 1985. (23) Lorentz, H. Ann. Phys. 1980, 9, 640. Lorenz, L. Ibid. 1880, 11, 70. (24) Martin, D.; Hauthal, H. G. Dimethyl Sulphoxide; Wiley: New York, 1975.

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