Solvent-induced frequency shifts in the infrared spectrum of

Solvent-induced frequency shifts in the infrared spectrum of acetonitrile in organic solvents. W. Ronald Fawcett, Guojun Liu, and Tori E. Kessler. J. ...
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J. Phys. Chem. 1993,97, 9293-9298

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Solvent-Induced Frequency Shifts in the Infrared Spectrum of Acetonitrile in Organic Solvents W. Ronald Fawcett,' Guojun Liu, and Tori E. Kessler Department of Chemistry, University of California, Davis, California 9561 6 Received: March 22, 1993; In Final Form: June 21, 1993"

The solvent-induced frequency shifts (SIFS) for the C=N stretching vibration (v2) in acetonitrile and deuterated acetonitrile have been determined in a wide variety of organic solvents. It is shown that this band is shifted to higher energies in the presence of solvents which are more stronger Lewis acids than acetonitrile itself. On the other hand, in the presence of solvents which are stronger Lewis bases, the v2 band is shifted to lower energies. This is taken to be evidence that the electronegative end of the Lewis base interacts with the methyl group in acetonitrile. Supporting evidence for this conclusion is reported on the basis of shifts in the symmetrical ( V I ) and antisymmetrical (v5) CD3 stretching bands. The magnitude of the SIFS is shown to correlate either with the solvent's basicity as estimated by the Gutmann donor number or with the solvent acidity as estimated by its acceptor number. Further details of intermolecular interactions in both pure acetonitrile and in its dilute solutions in organic solvents are discussed in terms of the observed vibrational spectra.

Introduction It is well known that the vibrational frequencies for a molecule dissolved in a liquid depend on the nature of the molecular environment. The solvent-induced frequency shifts (SIFS) provide a means of investigatingintermolecularinteractionswhen the probe molecule is dipolar.' In early work,2,3 the KirkwoodBauer-Magat equation, which is based on dielectric continuum concepts, was proposed to estimate these shifts. However, this treatment met with limited success because the model did not consider the specific interactions in the immediate vicinity of the probe solute An alternate approach is to emphasize specific local interactions and to correlate observed SIFS with empirical solvent parameters measuring solvent acidity or basicity.68 In a recent paper, Nyquist* examined the effect of organic solvents, both polar and nonpolar, on the C=N stretch frequency for acetonitrile and benzonitrile as dilute solutes. A rough correlation was found between the SIFS for this band in acetonitrile and the Gutmann acceptor number AN of the solvent with the exception of dimethyl sulfoxide. Since the empirical parameter AN is meant to measure solvent acidity, this correlation wasmeant todemonstrate that themagnitudeof theSIFS reflects the degree to which the electronegative nitrogen atom in acetonitrileas a solute interacts with surroundingsolvent molecules through their ability to act as Lewis acids. However, the poor quality of the correlation and the fact that the CGN band is affected by a combination band at higher frequencies suggest that the cause of the SIFS is more complex in nature. In the present paper, we report a study of SIFS for both protonated and deuterated acetonitrile as solute in a wide range of solvents both polar and nonpolar. In normal acetonitrile, the combination band (u3 u4), where v3 is the C-C stretching frequency and u4 is the CH3 rocking frequency, is involved in Fermi resonance with the C=N stretching mode ( 4 . As a result, the frequency of the m N stretching mode depends on the strength of the coupling. In the deuterated solvent, the v3 and u4 bands are shifted to much lower frequenciesso that the v2 band is unperturbed by Fermi resonance. Moreover, many solvents contain methyl groups and therefore absorb in the CH3 stretch region. Thus, by using the deuterated solute, one can distinguish this group in acetonitrile and assess its interactions with the solvent. The C=N stretching band shape in pure acetonitrile is asymmetric with a shoulder on its low-frequency side. Since this mode is totally symmetrical, the asymmetric shape suggests that

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* Abstract published in Advance ACS Abstracts, August 15, 1993. 0022-3654/93/2091-9293$04.00/0

another species may absorb in this frequency region. Griffiths9 attributed this feature to an unspecified aggregate of acetonitrile molecules in equilibrium with the monomer. This aggregate was assumed to be different than a simple dimer proposed on the basis of matrix isolation studies.1° Loewenschuss and Yellinll resolved the spectra in the vicinity of the v2 mode into two components, one sharp component with an integrated intensity -26% of the broader low-frequency component. Since the intensity ratio did not change significantly with temperature these authors argued that the second band observed could not be due to an equilibrium between the acetonitrile monomer and dimer or higher aggregate. In the present paper, we return to thequestion of the absorption in this region of the spectrum by examining the behavior of acetonitrile as solute in a wide variety of solvents.

Experimental Section The solvents used were HPLC grade (Aldrich) and were at least 99.9% pure with residual water less than 0.005% (manufacturer's specifications). Solutions of CHJCN and CD3CN in other solvents were made in a constant molar ratio of 1:12. The IR cell (Spectra-Tech) was made of NaCl with a sealed path length of 0.053 mm. In the case of acetic acid and trifluoroacetic acid, a CaF2 cell was used. Spectra were collected using the Bruker FTIR 98 spectrometer with a resolution of 0.5 cm-l and no zero filling factor. The cell was cleaned with acetone and then with the test solution and then dried before the experiment. Each absorbancespectrumwasobtained byratioing to the empty optical bench. The spectra reported here are difference spectra obtained by subtracting the spectrum of the pure solvent from that of the acetonitrile solution. The subtrahend multiplying factor ranged from 0.8 to 1.1 and was chosen to ensure that a minimum of solvent features remained in the differencespectrum. All spectral manipulations were performed using Spectra Calc software (Galactic). This software was used to determine the number of bands in regions where overlap occurs and to fit the individual bands with Lorentzian line shapes after the band frequency had been determined. The temperature at which the experiments were performed was 22 OC. Results Spectra for pure CDJCN and CH3CN in the C=N stretching region are shown in Figure 1. In both cases, the observed band shape is asymmetric with a shoulder on the low-frequency side. In addition, the observed band can be fitted to two Lorenzian 0 1993 American Chemical Society

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(b)

(a)

;

N

N

I

7 Residual x 5 I

2280

Residual x 5 I

I

2240 2270

1

2230

2150

2390 Wavenumber / cm-’

Figure 1. FTIR spectra for CD3CN (a) and CH3CN (b) in the C 3 N stretching region. These bands have been resolved into one for monomers (I) and one for dimers (11) (see text).

bands, with a narrow component at higher frequencies and a broad component at lower frequencies. These frequencies are 2263 and 2258.5 cm-’ in the case of CDpCN and 2253.5 and 2248.5 cm-l in the case of CHpCN. Evans and Bernsteinlz reported the US asymmetric CD3 stretch frequency for the deuterated compound at 2258 cm-1 from Raman studies and emphasized that this band overlaps strongly with the C s N stretching band. However, they did not report this band in the IR spectrum. High-resolution data for CDpCN in the gas phase13 show that the us mode is active in the infrared. On the basis of the data which follow, we argue that this band is not IR active in the pure solvent and that the two bands resolved in the pure liquids are both due to the CEN stretching mode. The solvents considered in the study divide into three groups, namely, those solvents which are more acidic than acetonitrile (ACN), those which are more basic, and those which are essentially nonpolar. The acidic solvents are mainly protic in nature and interact with the solute by hydrogen bonding to the electron-rich nitrogen atom. They shift the u2 bands in the blue direction to an extent which increases with solvent acidity (Table I). However, there are significant differences in the spectra observed in this frequency region which merit close examination. Spectra in the region of the uz bands observed in the relatively strong acids acetic acid (AA) and trifluoroacetic acid (TFAA) are shown in Figure 2. In TFAA, there is one band for CDJCN at 2291.5 cm-I. If this is assigned to the mode at 2263 cm-l in the pure solvent, the rsulting SIFS is 28.5 cm-1, the largest shift for this band in any solvent. There is no trace of a band at 2258 cm-1 which should be present if the v5 mode were IR active. In the case of CHJCN two bands are observed, that at higher frequency being the combination band (u3 + uq) which is in Fermi resonance with the u2 band. In the case of AA, two bands are observed in this region close to 2260 cm-1 with quite different SIFS but approximately the same bandwidths. IR spectra obtained in the alcohols are shown in Figure 3. Except for tert-butyl alcohol (t-BuOH), there are clearly two C=N stretching bands for both CH3CNand CD3CN with SIFS in the range from 3 to 6.5 cm-l in the blue direction. In the case of the deuterated molecule, there is a shoulder appearing at about 2246 cm-*, except in benzyl alcohol. This band does not appear for CH3CN. Spectra obtained in the amides are shown in Figure 4. Two of these solvents, namely, dimethylformamide (DMF) and dimethylacetamide (DMA), are strongly basic with respect

Wavenumber / cm

-’

Flgure 2. FTIR difference spectra for CH$N and CD3CN in acetic acid (AA)and trifluoroaceticacid (TFAA)in the C 3 N stretching region. CD3CN

I

2330

CH$N

I

1

I

2200 2340 Wovenumber / cm

2200

-’

Figure 3. FTIR difference spectra for CDpCN and CH3CN in selected alcohols in the C=N stretching region. The alcohols are benzyl alcohol, methanol, ethanol, 2-propanol, and tert-butyl alcohol.

to acetonitrile and result in SIFS in the red direction (Table 11). Formamide (F) and N-methylformamide (NMF) are both protic solvents with higher acidities than AN. However, the SIFS for F is small, and that for N M F is a little larger but in the red direction. It should be noted that the band shape for CH3CN changes with solvent in this group. In the basic solvents, the bands have a shape similar to that observed in pure ACN with a shoulder on the low-frequency side. On the other hand, in F and N M F the bands are broader but symmetrical. In the case of CDpCN, an extra band appears at lower frequencies (22422247 cm-1) in all amides except F. When the data for CD&N in both alcohols and amides are examined together, it is clear that the extra band occurs only in solvents with methyl groups. Furthermore, its intensity increases and its frequency decreases

The Journal of Physical Chemistry, Vol. 97, No. 37, 1993 9295

IR Spectrum of Acetonitrile in Organic Solvents CD3CN

/

1

2300

ChCN

I \

\N

1

I

2200 2320

1

2220

Wavenumber / cm-’ Figure 4. FTIR difference spectra for CDaCN and CH3CN in selected amides in the m N stretchingregion. The amidesare dimethylacetamide,

dimethylformamide, N-methylformamide, and formamide. with the number of methyl substituents on the solvating molecule, and it is absent in solvents with no methyl groups (BzOH and F). On the basis of the data summarized in Table I, it is clear that the SIFS for the v2 bands in both CH3CN and CD3CN increase with solvent acceptor number AN except in the cases of F and NMF. This empirical parameter measuring solvent acidity16is based on the 3lP chemical shift of triethylphosphine oxide, a strong base, dissolved in the given solvent. An excellent correlation is obtained between these quantities, the slope in the case of the protonated molecule being less than that for thedeuterated (Figure 5 ) . This observation is attributed to the Fermi resonance with the v3 v4 combination band, which occurs close to the C s N stretching mode for CH3CN. The second band at lower frequency which is designated v1) is attributed to the C=N stretching frequency for an acetonitrile dimer,10J7in which the molecular dipoles are aligned in the antiparallel configuration of low energy. The dimer is still able to interact with more acidic solvent molecules via hydrogen bonding with a resulting SIFS which is equal to or less than that observed for the monomer. Furthermore, variation in the magnitude of the SIFS for the v1) band with solvent AN is absent. Apparently, the interaction between ACN and TFAA is so strong that dimers do not exist in this solution for the mole fraction studied (1/13). As pointed out earlier, the v2 band is much narrower than the v1) band in pure ACN. In alcohol solutions, the difference in bandwidth is less, that for the v2 band being between 7 and 8 cm-1 and that for the v1) band being 9 cm-l in the case of CH3CN. Bandwidths for the deuterated compound were slightly larger. F and N M F clearly do not follow the same trend as the other acidic solvents. They are unique in the group considered in that they are also relatively strong bases. As shown from the data presented in Table 11, the behavior of ACN in these solvents can be rationalized if they behave as both bases and acids to ACN. The behavior of ACN in dimethyl sulfoxide (DMSO) and hexamethylphosphoramide (HMPT), the two strongest bases considered, is shown in Figure 6 . In these solvents, the extra band in thedeuterated solvent (2233.Ocm-1 in HMPT and 2238.5 cm-1 in DMSO) is much stronger and shifted further in the red direction. On the other hand, there is only one symmetrical band for the v2 mode which has been shifted in the red direction. Thus, the extra band for the deuterated molecule is not associated with the C=N stretching mode. To identify the extra band, spectra for CH3CN were examined in the region of the CH3 stretching

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modes (Figure 7). The positive going bands in the solution spectra correspond to solute features and the negative going bands to solvent features. The marked bands correspond to the asymmetric stretch mode (v5 = 3002 cm-l in pure CH3CN) and the symmetric stretch mode (VI= 2943 cm-’ in pure CH3CN). SIFS for these bands in DMSO and HMPT are the same in both normal and deuterated ACN. This gives strong support to the assumption that the extra band observed at lower frequencies for CD3CN in basic solvents is due to the asymmetric v5 mode. The SIFS in the deuterated solvent are summarized in Table 111 for these solvents and other basic solvents. On the basis of the data reported in Table 11, it is clear that the SIFS for solvents more basic than ACN increases in the red direction with increase in solvent basicity as estimated by the Gutmann donor number DN6J4(Figure 8). Data for the solvents F and N M F fall somewhat off the correlation found for seven other basic solvents,which is otherwise quite good. These results are attributed to a strong interaction between the electronegative group on the given solvent and the positive end of the molecular dipole in ACN which is located on the methyl group. As a result, the two dipoles align in a head to tail configuration, which is the orientation of lowest energy for two isolated dipoles. Apparently, the molecular orbitals most affected in this interaction involve a weakening of the C=N bond so that the resulting SIFS is in the red direction. Since there is no evidence for the v1) band due to the solute dimers, the interaction of ACN with stronger bases is sufficiently strong to break up the dimers. The failure of the data for F and N M F to follow the trend for the other solvents is attributed to the fact that these systems are both strong acids and bases so that interactions of both types with ACN are present in solution. A related feature of the spectra are the SIFS observed for the symmetrical vl and asymmetrical v5 CH3 stretching modes. As pointed out above, evidence for the presence of a v5 band was obtained in the deuterated solvent in the presence of strong bases. In addition, a red SIFS was found in the same solvents for the v1 band, which occurs at 21 16.5 cm-’ in pure CDjCN. These data are summarized as a function of solvent donor number in Table 111. An excellent correlation exists between the SIFS for the v1 band and the donor number of the solvent (Figure 9). This provides further evidencethat the electronegativeend of the solvent molecules interacts with the methyl group in ACN. A similar correlation of somewhat less quality is seen between Av5 and the donor number. The fact that Av5 correlates with Avl and the DN provides further confirmation that this spectral feature has been correctly assigned. SIFS for the C=N stretching frequency are summarized in Table IV for the remaining solvents considered. All of these solvents are both weaker acids and bases than ACN so that the resulting SIFS are small. In the cases of solventswith polarizable phenyl rings, namely, benzene, nitrobenzene, and benzonitrile, the SIFS is negative. This may reflect a dominance of the effects of solvent polarization, the effects of acidity and basicity being weaker. On the other hand, in hexane and simple chlorinated solvents, the SIFS is small and positive. In conclusion, the number of solvents considered in this group is too small and their nature too varied to draw any general conclusions regarding the nature of their effects as solvents on the v2 band for ACN. Discussion

The Role of Acetonitrile in Intermolecular Interactions. Acetonitrile is a molecule with a strong dipole moment (3.5 D) consideringits size. Thus, dipole-dipole interactions are expected to involve the nonbonding electrons at the electronegative end of the dipole or the methyl group at the electropositive end. The A electronsin the C=N bond can also be involved in intermolecular interactions. It is well known from the spectroscopyof electrolyte solutions in ACNISZO that interaction of the nonbondingelectrons with cations results in a blue shift of both the -N and C-C

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TABLE I: Solvent-Induced Frequency Shifts (SIFS)for the e Solvents

Fawcett et al.

N Stretching Frequencies for CHjCN and C D $ N in Acidic ~~~

CH3CNb

CD9CNb solvent acceptor no.' Av2/cm-l Avz'lcm-I Au2lcm-l Avz'lcm-1 acetonitrile (ACN) 18.9 0 (2253.5) 0 (2248.5) 0 (2263.0) 0 (2258.5) methylene chloride (MeCI) 20.4 1.o 1.o nitromethane (NM) 20.5 0 0 chloroform (CIF) 23.1 2.0 2.0 tert-butyl alcohol (t-BuOH) 27.1 6.0 6.5 5.0 6.0 N-methylformamide(NMF) 32.1 -1.5 -2.0 2-propanol (i-PrOH) 33.8 6.0 6.0 6.0 5.0 benzyl alcohol (BzOH) 34.5 5.5 4.0 5.5 3.0 ethanol (EtOH) 37.9 6.5 6.0 6.0 4.5 formamide (F) 39.8 0.5 0 methanol (MeOH) 41.3 5.5 5.5 6.0 4.5 acetic acid (AA) 52.9 12.0 6.0 12.5 4.5 trifluoroacetic acid (TFAA) 105.3 25.5 28.5 From the compilation of Gutmann et al." except for benzyl alcohol.ls AUZis the SIFS for the C=N stretching frequency for ACN in solution with respect to that in the pure solvent; Avz' is the corresponding quantity for the acetonitrile dimer. I

icH3cNi"HM1

15

35

75

55

e5

,1\2

CD$N in HMPT

AN

Figure 5. Plot of the solvent-induced frequency shift for the v2 band in

CHJCN (0)and CDsCN ( 0 ) ,Auz, in more acidic solvents against the solvent's acceptor number, AN, using the data from Table I. The best straight lines estimated by least squares do not include the results for formamide (F) and N-methylformamide(NMF). The data for CD&N have been shifted vertically by 20 cm-I for the sake of clarity. TABLE 11: SIFS for the e N Stretching Frequency for CHJCN and CDjCN in Basic Solvents donor Au2lcm-l no." CHoCN CDsCN solvent 14.1 0 (2253.5) 0 (2263.0) acetonitrile (ACN) 15.1 -1 .o -2.0 propylene carbonate (PC) 17.0 -0.5 -1.0 acetone (AC) 19.2 0.5 0 diethyl ether (DEE) 24 0.5 0 formamide (F) dimethylformamide(DMF) 26.6 -2.5 -3.0 N-methylformamide(NMF) 27 -1.5 -2.0 27.8 -3.5 4.0 dimethylacetamide (DMA) dimethyl sulfoxide (DMSO) 29.8 4.5 -5.0 38.8 -5.5 -6.5 hexamethylphosphoramide (HMW a From the compilation of Gutmann et al.14 stretching frequencies. Several authors21s22have argued on the basis of molecular orbital calculations that the molecular orbital involved in charge donation to an electrophilic species a t the nitrogen end of the molecule has substantial nitrogen lone pair character as well as C=N and C-C antibonding contributions. Thus, as charge is removed from this orbital by coordination with a Lewis acid, the net bond order along the C-C=N axis is increased, and blue shift is observed in the vibrational spectrum for the v2 and v3 bands. Interaction of the ?r electrons in ACN with strong Lewis acids such as the alkali metal or alkaline earth metal cations has not been reported,la20 but examples exist in coordination chemistry.23 In this case, one expects the C=N stretching frequency to shift in the red direction. Specific interactions of Lewis bases with the electropositive end of the molecular dipole are not expected. The methyl group

I

CH~CNin DMSO

2i50

2 2

Wavenumber / cm-1

2

Figure 6. FTIR differencespectra for CH3CN and CD3CN in dimethyl

sulfoxide(DMSO) and hexamethylphosphoramide(HMPT) in the" stretching region. is not involved in hydrogen bonding, nor is it considered to have significant acidic properties. However, if strong Lewis bases interact with this end of the molecule as the result of dipole dipole interactions, they could affect the vibrational bands associated with the methyl group. In general, the charge density associated with the u bonds in this group is repelled and the bonds are slightly weakened. As a result, the frequencies associated with the CH3 bands should shift in the red direction. SIFS for the us Band. The present results clearly show two major effects acting in opposite directions in the presence of solvents which are strong Lewis acids or bases. In the case of acidic solvents, the C=N stretching frequency is blue-shifted by as much as 28.5 cm-1 in TFAA. The majority of the solvents considered are protic and therefore are expected to hydrogen bond with the lone pair electrons on the nitrogen atom of the ACN molecule. The fact that the C=N bond becomes stronger in the presence of this interaction is consistent with resultsobtained in the presence of other strong Lewis acids, namely, the alkali metal and alkaline earth metal cation^.^^^^^ An important observation for the acidic solvents is that the SIFS associated

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IR Spectrum of Acetonitrile in Organic Solvents

0

8

\

0

2

a-

C ~ C Nin DMSO

0 10

20

30

ii

40

DN

Q

Figure 9. Plot of the solvent-inducedfrequency shift for the v1 band in

CD3CN Avl, in more basic solvents against the solvent’s donor number DN using the data from Table 111. TABLE III: SIFS for the Symmetrical ( V I ) and Asymmetrical ( ~ 5 ) CD3 Stretching Frequency for CD&N in Basic Solvents SIFS solvent donor no. Avl fcm-l Avsfcm-l ACN 14.1 0 (21 16.5) 0 (2258) PC 15.1 -1 AC 17.0 4.5 MeOH 19.1 -2.0 -5 DEE 19.2 -2.0 -10 EtOH 19.2 -2.5 -10 i-PrOH 21.1 -3.0 -12 t-BuOH 21.9 -3.0 -14 DMF 26.6 -4.9 -13 NMF 27 -4.4 -1 1 DMA 27.8 -5.6 -16 DMSO 29.8 -8.1 -20 HMPT 38.8 -8.7 -25 ~~~~~

2750

31 50 Wovenumber /cm - 1

Figure 7. FTIR spectra for pure CHsCN, DMSO, and HMPT in the

CH3 stretching region together with difference spectra (with respect to the pure solvent) for CHpCN in DMSO and HMPT. I

10

I

30

20

40

DN

Figure 8. Plot of the solvent-inducedfrequency shift for the v2 band in

CH3CN (0)and CDpCN ( 0 ) ,Av2, in more basic solvents against the solvent’s donor number, DN, using the data from Table 11. The best straight lines estimated by least squares do not include the results for formamide (F) and N-methylformamide(NMF). The data for CD3CN have been shifted vertically by -10 cm-l for the sake of clarity. with the low-frequency band, A v i , is observed only for solvents of intermediate acidity with the exception of F and NMF, but not in the strongly acidicsolvent TFAA. The correlations between the SIFS in the blue direction and the solvent’s acceptor number (Figure 5) are very good with correlation coefficients of 0.985 for the CH3CN data and 0.993 for CD3CN data. The correlation between Av2 and the donor number for basic solventscan only be explained by interaction of the electronegative end of the base with the methyl group in ACN, which in turn affects the CEN stretching frequency. Strong support for this conclusion is obtained from the observation that the SIFS for the CD3 stretching frequencies, Avl and Av5, in CD3CN also correlate with the solvent’s donor number. This suggests that solvent and solute are aligned in a head-to-tail configuration as dipoles. Furthermore, the stronger base polarizes the ACN molecule so that the electrondensity associated with the W N bond is higher. This, in turn, leads to a red shift and weakening of this bond because the associated HOMO is antibonding. Although the

TABLE IV SIFS for the QN Stretching Frequency for C H F N and CD3CN in More Weakly Acidic and Basic Solvents Avzfcm-‘ solvent acceptor no. donor no. CH3CN CD3CN benzene 8.2 0 0 -0.5 15.5 11.9 -1.0 -1.5 benzonitrile 0 8.6 carbon tetrachloride 1.5 1.5 0 0 0 16.7 dichloroethane 0 hexane 0 2.5 2.5 14.8 4.4 -1.0 -1.5 nitrobenzene range of variation of solvent basicity is not as great as that for solvent acidity, good correlations are obtained between AVZand the donor number with values of r equal to 0.93. As pointed out above, the slope of the plots in Figures 5 and 8 are higher for deuterated ACN than for the protonated molecule. In the latter case, the v3 v4 combination band at higher frequencies is in Fermi resonance with the v2 band. The Fermi coupling which depends on the molecular anharmonicity constant and the separation between the v 2 and v3 + v4 bands results in a red shift of the v2 band. The separation of these bands decreases when solvent-solute interactions result in a blue shift and thus Fermi coupling increases. As a result, the SIFS for CH3CN are less than those for CD3CN. Similar arguments apply to the case of solvents which shift the C=N stretching band in the red direction. Another group of solvents which shift the v2 band in the red direction can be recognized among the weakly acidic solvents listed in Table IV. Benzene (BZ), nitrobenzene (NB), and benzonitrile (BzN) are all solvents which can interact with ACN through their ?r bonds. One can imagine this interaction taking place in a configuration in which the molecular dipoles are aligned in an antiparallel fashion. In fact, this type of interaction is

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Nevertheless, quite good correlations using the ACN point of proposed here to exist in pure ACN and to result in a red shift reference are obtained for the stronger acidic and basic solvents. of - 5 cm-1. Apparently, interaction between the T systems on Further consideration of this point could be made using spectra the two molecules results in a small weakening of the C=N bond obtained as a function of temperature. and a red shift of the YZ band. It is also noted that the extent of the shift is roughly correlated with the solvent molecules acceptor On the basis of the estimated interaction energies,” dimers are number which increases in the order Bz < NB < BzN C ACN. expected to predominate with respect to monomers in the headto-tail configuration. No information about the relative conSome comment should also be made regarding the results tributions of these two configurations can be obtained from the obtained in the protic amides, F and NMF. It was argued above that these solvents are moderately strong as both acids and bases. relative intensities of the u2 and v i bands in pure ACN because their relative extinction coefficients could be different. Since As a result, the SIFS observed for the vz band do not correlate more than two ACN molecules could be involved in a head-towell either with those for other acids or for other bases. It should tail chain in the pure solvent, the energy involved in breaking up also be kept in mind that the relative permittivity of these solvents such a chain could be approximately equal to the energy involved is very high. This may affect the nature of the intermolecular in dissociating an antiparallel dimer. This might explain the interactions observed and result in the absence of the dimeric relativetemperatureinsensitivityof the IRspectraofthese bands.” ACN species in these systems. Other solvents in which ACN dimers are absent are TFAA, a very strong acid, and the strong Conclusions bases DMSO and HMPA. In these cases, solute-solvent interactions are so strong that ACN dimers are broken up. The The present study has shown that the work of Nyquist* is only solvents in which the dimers can be seen spectroscopically somewhat incomplete. Thus, one needs to consider more than are the alcohols and acetic acid. Apparently, these solvents can just the Lewis acidity of the solvent in order to explain the SIFS hydrogen bond with the monomer and dimer without shifting the for the v2 band in ACN. By consideringa larger group of solvents monomer4imer equilibrium significantly. and other regions of the vibrational spectrum of the solute, a SIFS for the CDJ Bands. The interaction of basic solvents more compete picture of the solvationof ACN can be constructed with the electropositiveend of the ACN dipole is confirmed by in both protic and aprotic solvents with a wide range of solvating the data for the SIFS of the symmetrical v1 CD3 stretching band. properties. What is particularly striking about the present study is the usefulness of the acid-base parameters AN and DN in A very good correlation between Avl and the solvent’s donor number is obtained for solvents which are more basic than ACN. rationalizing the effects observed. Correlations of solvent effects It must be emphasized that this group includes some alcohols with these quantities have most often involved the properties of which were described as hydrogen bonding and which shifted the cations and ani0ns,2~*~’ that is, strong Lewis acids and bases. The fact that we have successfully explained intermolecular interv2 band in the bluedirection. Thedonor numbers for thesesolvents actionson the basis of these quantities lends much strongersupport cannot be determined directly on the basis of the experimental to their usefulness in understanding local solvation effects. definition of Gutmann;6however, indirect methods of estimating this parameter for the lower alcohols have been d e ~ c r i b e d ~ ~ , ~ ~ and the data compiled by Makitra et al.25 Thus, the alcohols Acknowledgment. This work was supported by the Office of must be considered as both stronger bases and stronger acids Naval Research. than ACN. By interacting as a base with the electropositiveend References and Notes of the ACN dipole, the base weakens the bonds in the CD3 group and causes a red shift of the v1 frequency. On the other hand, (1) Lutsky, A. E.; Prezhdo, V. V.; Detereva, L. I.; Gordienko, G. Usp. Khim. 1982,51,1398. an alcohol can also act as an acid with the electronegative end (2) (a) Kirkwood, J. G. J . Chem. Phys. 1936,4, 592. (b) Bauer, E.; of the ACN dipole via hydrogen bonding. The net result for the Magat, M. J. Phys. Radium 1938,8, 319. alcohol solvents is that three dipoles can be in a head-to-tail (3) (a) Pullin, A. D. E.Spectrochim. Acra 1960,16,12. (b) Buckingham, configuration with solvation at both ends of the solute dipole. A. D. Pure Appl. Chem. 1970,24,123. (4) (a) Allerhand, A.; Schleyer, P. R. J. Am. Chem. Soc. 1963,85,371. Similar results are obtained for the Y S asymmetrical CH3 (b) Lark, P. D.; Orr, B. J.; Rhoads, G. R. Ausr. J . Chem. 1975,28, 1417. stretching band but with much larger SIFS (Table 111). It should (5) Bellamy, L. J.; Nyquist, R. A. Spectrochim. Acta 1959,14, 192. be noted that the tabulated results are referred to the v5 band (6) Gutmann, V. The Donor-Acceptor Approach io Molecular Interobserved by Evans and BernsteinI2 in pure CD&N by Raman actions; Plenum: New York 1978, p 78. (7) Wohar, M. M.; Seera, J. K.; Jagodinski, P. W. Spectrochim. Acra spectroscopy. The v5 band was not discernible in the spectra 1988,44, 999. until solvent basicity reached that of methanol. The absence of (8) Nyquist, R. A. Appl. Specrrosc. 1990,44, 1405. the v5 band in pure CD3CN is difficult to explain. It may be (9) Griffiths, J. E. J. Chem. Phys. 1973,59, 751. related to the fact that the dimer configuration is preferred to the (10) Freedman, T. R.; Nixon, E. R. Spectrochim. Acra 1972,28A,1375. head-to-tail configuration in the pure solvent. Finally, the (11) Leowenschuss, A.; Yellin, N. Spectrochim. Acra 1975,3ZA, 207. (12) Evans, J. C.; Bernstein, H. J. Can. J. Chem. 1955,33, 1746. correlations between Av5 and Avl and between AVSand Avz for (13) Duncan, J. L.; McKean, D. C.; Tullini, F.; Nivellini, G. D. J. Mol. these basic solvents confirm that activation of the vs band results Spectrosc. 1978,69, 123. from interaction of the electronegativeend of the solvent molecule (14) Gutmann, V.; Resch, G.; Linert, W. Coord. Chem. Reo. 1982,43, with the CD3 group in ACN. 133. (15) Elias, H.; Dreher, M.;Neitzal, S.; Volz, H. Z . Narurforsch. 1982, Structure of Liquid Acetonitrile. On the basis of the observed 37, 684. SIFS, it has been argued that solvents can be divided into two (16) Mayer, U.;Gutmann, V.;Gerger, W. Monarsh. Chem. 1975,106, groups, namely, those which shift the v2 band in the bluedirection 1235. and those which shift it red. Pure acetonitrile is the point of (17) Saum, A. M. J. Polym. Sci. 1960, 42, 66. referencefor this division, and the question remains how it behaves (18) Pereligin, I. S.InlonicSoluarion; Krestov, G. A., Ed;Nauka: Moscow, 1987; Chapter 3. as a solvent itself with respect to the behavior of a hypothetically (19) Fawcett, W. R.; Liu, G. J . Phys. Chem. 1992,96,4231. inert solvent. According to the calculations of Saum,” the (20) Fawcett, W. R.; Liu, G.; Faguy, P. W.; Motheo, A. J. J. Chem. Soc., interaction energy of two ACN molecules in the dimer is -33 Faraday Trans. 1993,89, 8 11. kJ mol-’, whereas the interaction in the head-to-tail configuration (21) Purcell, K. F.; Drago, R. S . J. Am. Chem. Soc. 1%7,89, 247. involves - 4 kJ mol-1. The dimer interaction energy is not small (22) Pominov, I. S.;Serzhantova, V. N. Zh. Prikl. Spekrrosk. 1970,12, 1071. and can be compared to hydrogen-bonding interaction in the (23) Oloffson, G. Acra Chem. Scad. 1968,22, 1352. alcohols. As a result, the alkyl nitriles have comparable boiling (24) Kanevsky, E. A.; Zarubin, A. I. Zh. Org. Khim. 1975,45,130. points to alcohols of the same molecular weight. The existence (25) Makitra, R.G.; Pirig, Ya.N.; Kiveliuk, R. B. Privatecommunication. of both types of interactions in pure acetonitrile means that it is (26) Fawcett, W. R.; Krygowski, T. M. Can. J. Chem. 1976,54, 3283. not the perfect point of referencewith respect to measuring SIFS. (27) Fawcett, W. R. Langmuir 1989,5,661.