Solvation of LiClO4 and NaClO4 in deuterated acetonitrile studied by means of infrared and Raman spectroscopy

Spectrochimica Acta Part A 58 (2002) 1747– 1756 www.elsevier.com/locate/saa

Solvation of LiClO4 and NaClO4 in deuterated acetonitrile studied by means of infrared and Raman spectroscopy Jun-Sik Seo, Byeong-Seo Cheong, Han-Gook Cho * Department of Chemistry, Uni6ersity of Incheon, 177 Dohwa-dong, Nam-ku, Incheon 402 -749, South Korea Received 27 August 2001; accepted 18 September 2001

Abstract Vibrational characteristics of CD3CN solutions of LiClO4 and NaClO4 have been studied by means of infrared and Raman spectroscopy. Blue shifts of 22 and 11 cm − 1 of the w2 CN stretch are observed resulting from interaction of CD3CN with Li+ and Na+, respectively. The number of primary solvation sites of both Li+ and Na+ in acetonitrile is believed to be four from the comparison of the Raman intensities of the CN stretch for free CD3CN and those coordinated to Li+ and Na+. Evidently formation of contact ion pairs of the cation (Li+ or Na+) and anion (ClO− 4 ) is more probable at a higher concentration of the salt. The characteristics of the w2 CN stretch, w4 CC stretch, and w8 CCN deformation bands vary substantially upon coordination, while other vibrational bands are relatively immune to the donor-acceptor interaction. DFT calculations have also been performed at the BLYP/6-31+G(2d,p) level to examine the structures and vibrational characteristics of CD3CN coordinated to Li+ and Na+. The calculated results are in good agreement with the observed vibrational characteristics. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Acetonitrile; Li+; Na+; Solvation; Coordination number; Frequency shift

1. Introduction Acetonitrile has long been a model compound in the process of development of vibrational as well as rotational spectroscopy and is still an interesting compound [1 – 3]. Recently the donor – acceptor complexes of acetonitrile have drawn much attention [4– 7]. Especially, the unusually large blue shifts of the w2 (the CN stretch) and w4 (the CC stretch) bands observed in aqueous solu* Corresponding author. Tel.: +82-32-770-8236; fax: + 8232-770-8238. E-mail address: [email protected] (H.-G. Cho).

tion containing inorganic salts, along with the frequency shifts of other vibrational bands, have been a focus of spectroscopic as well as theoretical studies [5,6,8 –12]. Complexation of acetonitrile with other Lewis acids also leads to similar blue shifts [13 –16]. The distinctive blue shifts, in contrast to the general trend of bond weakening in donor-acceptor complexation, [17] are generally thought to be due to alleviation of the antibonding character of the CN and CC bond as a result of electron donation to the electron acceptor. Utilizing the large variation of vibrational characteristics of acetonitrile upon coordination, the compound is often used as a probe for determination of electrophilicities of Lewis acids [18].

1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 0 1 ) 0 0 6 3 6 - 9

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Recently acetonitrile solutions of metal salts have been the subject of various spectroscopic studies. Fawcett et al. used attenuated total reflection FT-IR spectroscopy for the improved quality of the IR spectra and analyzed the effects of the electrolytes on the vibrations of the hydrogen stretches and deformations along with the CN and CC stretches [19]. From the variation of the IR intensities of perchlorate bands with concentration, they estimated that 30 and 15% of the Li+ and Na+ ions actually form contact ion pairs with the ClO− 4 ions, respectively. More recently Fawcett and Liu also studied the possibility of ion pairing for Mg(ClO4)2 dissolved in acetonitrile, [4] and argued that 70% of the Mg2 + ions actually form contact ion pairs with ClO− 4 ions. Cho et al. investigated the effects of solvation of Mg(ClO4)2 in deuterated acetonitrile [20]. Large changes in the frequencies and IR intensities were observed along with effects of residual water. The number of primary coordination sites of Mg2 + in acetonitrile is determined to be six. Sadlej presented evidences for existence of the acetonitrile dimer in the study on variation of IR and Raman intensities with addition of a Li salt [21]. Solvation of metal salts in binary mixtures has also been studied. Evans and Lo showed that the complexed nitrile retains its original C3v symmetry, the point of attachment to the metal cation being through the nitrogen lone-pair electrons [22]. The coordination number around the Ag+ ion is determined to be four in a Raman study of silver nitrate in water – acetonitrile mixtures by Oliver and Janz [23]. Wo´jcik et al. studied mixtures of water and acetonitrile with trivalent cations by means of infrared spectroscopy and estimated the composition of the solvation spheres around the metal ions [5,6]. Solvation of Li+ ion in binary mixtures of acetonitrile and dimethyl formamide was studied by Sajeevkumar and Singh [24]. The large frequency shifts of the CN stretch and NCO deformation were monitored and used to investigate the composition in the solvation shell. Ramada and Singh performed Raman spectral studies on solution of lithium bromide in binary mixtures of water and acetonitrile in the CH and CN stretching regions [25]. They observed the dramatic evolution of the

lineshape in the CH and CN stretching regions with concentration of LiBr. In this study we dissolved anhydrous LiClO4 and NaClO4 in CD3CN and examined the variation of the vibrational characteristics of CD3CN as results of solvation by means of infrared and Raman spectroscopy. Remarkable changes in the frequencies and intensities of the vibrational bands occur, and the primary solvation numbers of Li+ and Na+ in CD3CN are determined as a function of concentration from the Raman intensities of the CN stretch for free CD3CN and those coordinated to Li+ and Na+ in the solutions. DFT calculations were also carried out to examine the structure and vibrational characteristics of the donor –acceptor complexes and the results were compared with the observed values.

2. Experimental and computational details CD3CN was used for this study, which allows to avoid the interference originated from the strong Fermi resonance between the w2 band and the w3 + w4 combination band about 40 cm − 1 toward higher frequency observed from CH3CN [4]. CD3CN (Aldrich, 99.6%) was purchased in ampule from Aldrich and used without further treatment. Anhydrous lithium and sodium perchlorates were also used as purchased from Aldrich. The concentrations (molality) of LiClO4 and NaClO4 in CD3CN range from 0 to 1.4 m and from 0 to 2.5 m, close to the saturation points, respectively. The solutions were prepared and stored under argon atmosphere. Spectra were collected using an FT-IR spectrometer (Nicolet Magna 560) with a resolution of 1.0 cm − 1 and a Raman module incorporated into another FT-IR spectrometer (Bio-Rad FTS 175C) with a resolution of 4.0 cm − 1, which is accompanied with a YAG laser, a Ge detector, and a dielectric filter as the light source, detector, and Rayleigh filter, respectively. The FT-IR spectrometer along with the sample chamber was purged with air free of moisture and CO2. However, the bench was not completely free of CO2 or water vapor. The IR sample cell was equipped with a pair of KBr windows and a spacer. The Raman cell provided

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by Bio-Rad was a bulb-type one with reflective coating on one side for higher signal intensity. For determination of the center frequency, width, and intensity of each band, the line profile was fitted to a combination of Lorentzian and Gaussian lineshapes, whose ratio of composition was adjusted for a best fit. In the process of deconvolution of congested regions, such as the w4 region, the center frequencies of one or two of the bands were, if needed, fixed at predetermined values in order to break the strong correlation among the lineshape parameters resulting from serious overlapping of the absorption bands. All the experiments were performed at the room temperature. DFT calculations were carried out with the Gaussian 98 [26] package at the BLYP/6-31 + G(2d,p) level. The molecular geometries of + CD3CN, Li[CD3CN]+ were 4 , and Na[CD3CN]4

Fig. 1. The IR spectra in the w2 region for pure CD3CN and CD3CN solutions of LiClO4 and NaClO4 at concentrations of 1.2 and 2.5 m, respectively. The frequency of the w2 band of free CD3CN at 2263 cm − 1 remains virtually the same even in LiClO4 and NaClO4 solutions. The new absorption features at 2285 and 2275 cm − 1 emerge in the spectra of LiClO4 and NaClO4 solutions, which are attributed to the CN stretches of CD3CN coordinated to Li+ and ClO− 4 , respectively.

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fully optimized with no geometrical constraints, and at the converged geometry, the vibrational frequencies were calculated. For estimation of the binding energies of the complexes, the zero-point energy corrections were included.

3. Results and discussion Fig. 1 shows the infrared spectra in the w2 CN stretch region for pure CD3CN and CD3CN solutions of LiClO4 and NaClO4 at concentrations of 1.2 and 2.5 m, respectively. There is a strong band in the spectrum of pure CD3CN peaked at 2263 cm − 1, which is asymmetric with a shoulder on the low-frequency side. Studies indicate that the band actually consists of the w2 band of the monomer at 2263 cm − 1 and that of associated CD3CN including the dimer at 2258 cm − 1 [27]. Fig. 1 also shows that new bands emerge at 2285 and 2275 cm − 1 in the IR spectra of LiClO4 and NaClO4 solutions, respectively. The bands are attributed to the w2 CN stretching bands of CD3CN complexed with Li+ and Na+ [4]. It is well known from the spectroscopy of acetonitrile in electrolyte solution that interaction of the nonbonding electrons on the nitrogen end with cations results in blue shifts of both the CN (w2) and CC (w4) stretching frequencies [28–31]. The electrophilic coordination of the cation reduces the antibonding contributions of nitrogen lone pair to the CN and CC bonds, [16] and, therefore, the net bond order along the CCN axis increases. The larger force constants of the bonds along the CCN axis result in the blue shifts of the w2 and w4 bands observed in the vibrational spectrum. The shoulder at 2258 cm − 1, which arises from associated CD3CN including the dimer and multimers, weakens slightly, as the new band emerges. It is expected that the concentration of associated acetonitrile is lower in solution, since the chances for free acetonitrile molecules to collide each other should be lower than in pure acetonitrile, as more acetonitrile molecules being coordinated to the solute. Fig. 1 also shows that the center frequency of the w2 band of free CD3CN (2263 cm − 1) remains essentially unchanged in the LiClO4 and NaClO4 solutions. The w2 band is

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Fig. 2. The Raman spectra in the w2 region for pure CD3CN and CD3CN solutions of LiClO4 and NaClO4 at concentrations of 1.2 and 2.5 m, respectively. The Raman intensities of the n2 bands of CD3CN coordinated to Li+ and Na+ at 2285 and 2275 cm − 1 relative to those of free CD3CN at 2263 cm − 1 are much lower than the IR intensities shown in Fig. 1.

often used as a probe for measurement of the electrophilicity of a Lewis acid in solution based upon the large variations in the vibrational characteristics, [6] particularly the frequency of the band. If CD3CN that is not in the primary solvation shell interacts inductively with solvated Li+ or Na+, the w2 band with a different degree of blue shift is expected to emerge. Absence of such w2 band indicates that only the directly coordinated CD3CN molecules to Li+ or Na+ are affected by the electrophilicity of the cation, while the electronic structure of other CD3CN molecules in the solution remains virtually unaltered. The w2 frequencies of the acetonitrile complexes with the cations stay virtually the same regardless of concentrations of the salts, while the intensities increase linearly. It is noticeable in Fig. 1 that the w2 intensity of the Li+ complex is comparable

with that of the Na+ complex, despite that the concentration of LiClO4 solution is less than half that of NaClO4 solution. One possible reason for the disparity is that the molar extinction constant of the w2 band of CD3CN coordinated to Li+ is about twice that of CD3CN coordinated to Na+. Another possible reason is that the primary coordination number of the Li+ ion is about twice that of the Na+ ion. Also shown in Fig. 1 is the larger blue shift (22 cm − 1) of the w2 band observed from LiClO4 solution, which is twice the blue shift (11 cm − 1) from NaClO4 solution. This indicates that Li+ is more electrophilic in the organic solvent than Na+ is, mainly due to the smaller ionic radius. Fig. 2 presents are the Raman spectra in the w2 region of pure CD3CN and CD3CN solutions of LiClO4 and NaClO4. The w2 intensities in the Raman spectra of CD3CN coordinated to Li+ and Na+ (2285 and 2275 cm − 1) are much lower than the IR intensities shown in Fig. 1. Intermolecular interactions often result in a large variation of the IR intensity, whereas the Raman intensity virtually remains unaffected. The ratio between the w2 intensities in the Raman spectra of the coordinated and free CD3CN is proportional to the concentration, regardless of the type of cation. Previous studies showed that the molar Raman intensity for the w2 CN stretching band is numerically the same for the solvated acetonitrile and bulk, and as a result, there is a direct proportionality between the intensity and the species concentration [23]. Accordingly, Raman intensity measurements have been used to determine the primary solvation numbers for solutions of salts in acetonitrile using the relation Ib csalt =N Ib + If cacetonitrile where Ib and If are the intensities of the band shifted by solvation and the band of free acetonitrile, respectively, csalt and cacetonitrile are the concentrations of the salt and acetonitrile, respectively, and N is the average number of solvent molecules coordinated to the solute in the primary layer [32]. Fig. 3 shows the primary solvation numbers of CD3CN for Li+ and Na+ as a function of con-

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centration, which are determined from the integrated w2 intensities in the Raman spectra of CD3CN directly coordinated to Li+ and Na+ at 2285 and 2275 cm − 1 and that of free CD3CN at 2263 cm − 1. The primary solvation number varies between 2.31 and 3.23 for Li+ depending on the concentration, whereas the number varies between 1.74 and 2.37 for Na+. Due to the smaller frequency shift, the w2 band of CD3CN coordinated to Na+ is overlapped by the much stronger w2 band of free CD3CN, causing higher uncertainty in measurement of the integrated w2 intensity of coordinated CD3CN, particularly at low concentration. Consequently, determination of the primary coordination number at low concentration below 0.6 m was not attempted for Na+. As shown in Fig. 3, there is a general trend that the primary solvation number increases with decreasing concentration. Variation of the solvation number as a function of concentration has also been reported in recent studies [20,23]. At a very low concentration, the cations are probably coor-

Fig. 3. The primary solvation numbers of Li+ and Na+ in CD3CN as a function of concentration. The concentration is in molality. The uncertainty in determination of the coordination number increases at a low concentration due to the low intensity of the w2 band of CD3CN coordinated to the cation.

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dinated only with CD3CN molecules, leading to a large solvation number, whereas at a high concentration close to the saturation point, the cations have more chances to form contact ion pairs with ClO− 4 ions, leading to a smaller solvation number of CD3CN. The present result indicates that the number of primary solvation sites around the Li+ ion in the organic solvent is probably four, consistent with previous results [19,33]. Moreover, one or more of the sites are most likely filled with the counter ion (ClO− 4 ) unless the concentration of the salt is very low. Since a single ClO− 4 ion can occupy more than one site, it is not possible at this point to determine exactly how much portion of the cations are actually paired with the anions. Fig. 3 also suggests that even at a high concentration near the saturation point, at least two of the primary coordination sites of Li+ are on the average filled with CD3CN. On the other hand, the number of primary solvation sites around the Na+ ion in acetonitrile is still not clearly determined, mainly due to lack of data at low concentration. Since the primary solvation number of Na+ is about 2 at high concentration even above 2 m, the number is certainly well over 2 at a lower concentration. At a concentration of 2 m, there are only eleven CD3CN molecules per each Na+ ion. It should be remembered at this point that both Li+ and Na+ are generally regarded as having a primary solvation shell containing four molecules in a variety of nonaqueous solvents [19,33]. The number of primary solvation sites of the Na+ ion in acetonitrile is probably also four. Shown in Fig. 4 are the IR spectra in the n4 region of pure CD3CN and CD3CN solutions of LiClO4 and NaClO4. Under the line profiles for the CD3CN solutions, there are three major absorption bands overlapped each other for each solution, the w4 CC stretching band of free CD3CN at 832 cm − 1, the w4 bands of CD3CN coordinated to Li+ and Na+ at 845 and 836 cm − 1, respectively, and the w7 CD3 rocking band at 849 cm − 1. A small band is also noticeable at 837 cm − 1, which is previously attributed to the w4 band of CD3CN interacting with water residue in solution [4,19].

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Fig. 4. The IR spectra in the w4 region of pure CD3CN and CD3CN solutions of LiClO4 and NaClO4. The w4 and w7 bands of free CD3CN are located at 832 and 849 cm − 1, and the w4 bands of CD3CN coordinated to Li+ and Na+ at 845 and 836 cm − 1, respectively. A small band observed at 837 cm − 1 is attributed to the w4 band of CD3CN coordinated to the water residue [4,19].

Fig. 5 shows the w4 CC stretch (a) and w8 CCN deformation (b) regions in the Raman spectra of pure CD3CN and CD3CN solutions of LiClO4 and NaClO4 at 1.2 and 2.5 m, respectively. The w8 band of acetonitrile, which is normally not observed in the IR spectrum because of its low frequency, also shows a blue shift upon complexation. Unlike in the IR spectra, the w7 CD3 rocking band expected at 849 cm − 1 is too weak to be observed clearly in the Raman spectra. As observed from the w2 band, the vibrational characteristics of the w4 and w8 bands of free CD3CN in the solutions remain the same, also indicating that CD3CN molecules other than the ones in the primary solvation shell are virtually unaffected by the solvated cations. The Raman intensities of the w4 band of the complexes are much lower than the IR intensities, the same as in the w2 band. While due to the strengthened bonds in the CCN axis by

nucleophilic coordination to Li+ and Na+, the frequency shifts of the w4 and w8 bands are all in the blue direction, Li+ again causes much larger frequency shifts, 13 versus 4 cm − 1 for the w4 band, and 9 versus 6 cm − 1 for the w8 band. As shown in Figs. 1, 2, 4 and 5, the w2 and w4 bands of CD3CN coordinated to Li+ and Na+ in the IR spectra are apparently much stronger than those in the Raman spectra. Based upon the ratios of the integrated band intensities of free and coordinated CD3CN in the IR and Raman spectra, the increases in the IR extinction constants of the n2 and n4 bands by coordination to Li+ are about 3.5 and 12 folds, respectively, and those by coordination to Na+ are about 1.4 and 11 folds, respectively, assuming that the primary solvation numbers of Li+ and Na+ are the same. The large increases in the IR intensities of the CN and CC stretches are traced to the charge redistribution along the CCN axis due to the interaction of the electrophilic coordination of the cation to the electron-rich nitrogen end of CD3CN. The frequencies and frequency shifts observed in this study are summarized in Table 1. While the w2 CN stretch, w4 CC stretch, and w8 CCN deformation bands show large variations in the frequencies and intensities, the vibrational characteristics of other vibrational modes including the CD3 stretches remain virtually the same. This reaffirms the conclusion that the interaction between acetonitrile and the cation does not occur through the methyl group of acetonitrile and the point of attachment to the cation is indeed through the nitrogen lone-pair electrons [22]. As mentioned above, while the chemical bonds in the CCN axis of the CD3CN molecules directly coordinated to Li+ or Na+ are greatly affected by the electrophilicity of the cation, the induction effects to the remaining CD3CN molecules through the primary shell of coordination are very small. There is essentially only a single solvation shell of CD3CN around the monovalent cation in which the electronic or vibrational characteristics of acetonitrile are noticeably affected by the electrophilicity. Similar results were observed from acetonitrile solution of Mg(ClO4)2 [20]. This is contrast to the common idea that the metal cation

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Fig. 5. The Raman spectra in the w4 (a) and w8 (b) regions of pure CD3CN and CD3CN solutions of LiClO4 and NaClO4 at concentration of 1.2 and 2.5 m. (a) The w4 CC stretch band of free CD3CN is located at 832 cm − 1, and the w7 CD3 rocking band expected at 849 cm − 1 is too weak to be observed clearly. The bands found at 845 and 836 cm − 1 are attributed to the w4 CC stretches of CD3CN coordinated to Li+ and Na+, respectively. (b) The w8 CCN deformation band of free CD3CN is located at 349 cm − 1, and those of CD3CN coordinated to Li+ and Na+ at 358 and 355 cm − 1, respectively. Similar to the case of the w2 band, the w4 intensities in the Raman spectra (a) of CD3CN coordinated to Li+ and Na+ at 845 and 836 cm − 1 are much smaller than the IR intensities shown in Fig. 4. Table 1 Frequencies observed from pure CD3CN and CD3CN solutions of LiClO4 and NaClO4 Mode

Description

Pure CD3CN

LiClO4 solutiona L[CD3CN]+b 4

A1 w1 w2 w3 w4 E w5d w6 w7 w8e

CD3 symmetric stretch CN stretch CD3 symmetric deformation CC stretch CD3 antisymmetric stretch CD3 antisymmetric deformation CD3 rock CCN deformation

2116.6 2263.1 1101.9 832.0

2285.1 (22.0) 844.5 (12.5)

NaClO4 solutiona Freec

2115.3 2262.6 1101.9 831.9

Na[CD3CN]+b 4

(−1.3) (−0.5) (0.0) (−0.1)

1038.1

1038.8 (0.7)

848.7 349.3

848.7 (0.0)f 348.7 (−0.6)

358.1 (8.8)

2274.5 (11.4) 836.4 (4.4)

Freec

2115.3 2262.5 1101.8 831.9

(−1.3) (−0.6) (−0.1) (−0.1)

1040.0 (1.9)

354.8 (5.5)

848.4 (−0.4) 349.3 (0.0)f

All frequencies are in cm−1. a Numbers in parantheses are the frequency shifts relative to that of pure CD3CN. b Newly emerging bands from the solution. The bands are believed to arise from the coordinated CD3CN to Li+ or Na+ in the solution. c Free CD3CN in the solution. d Frequencies could not be determined because of the sever overlap by the w2 band of CD3CN. e Determined from the Raman spectrum. Other frequencies are determined from the IR spectrum. f Fixed for fitting heavily overlapped bands.

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+ Fig. 6. The optimized geometries of Li[CD3CN]+ 4 and Na[CD3CN]4 at BLYP/6-31+ G(2d,p). The complex have T symmetry, and the CCD plane is tilted about 40° from the NM+N plane.

in inorganic solvents such as water and ammonia carries multiple sheaths of solvation. Unlike water or ammonia, acetonitrile has a considerably larger molecular size, and the molecular dipole moment is primarily originated from the nitrile group. As a result, a single solvation shell of acetonitrile may be able to block effectively the induction of the cation. DFT calculations at the BLYP/6-31 + G(2d,p) level were carried out for the CD3CN complexes with Li+ and Na+. The coordination numbers were assumed to be four. The optimized geometries are shown in Fig. 6, and the geometrical parameters of CD3CN coordinated to Li+ and Na+ are compared with those of free CD3CN in Table 2. The variation of Mulliken charge on the nitrogen atom and the binding energy are also listed in Table 2. Since the optimized geometries have T symmetry, the four coordinated CD3CN molecules to a cation carry the same geometrical parameters. In the optimized geometry, the CCD plane is tilted about 40° against the NM+N plane for both complexes. The shortening of the CN and CC bond lengths, consequent to complex formation by electrophilic coordination of cations, is consistent with the previous results observed in other solution systems of acetonitrile [5,8].

The calculated frequencies and intensities of free CD3CN and those coordinated to the cations are listed in Table 3. The frequency shifts and large increases in the IR intensity by solvation of LiClO4, particularly for the w2, w4, and w8 bands, are well reproduced by calculation. The calculation results, particularly the w2, w4, and w8 frequencies and the N···M+ and CN bond lengths, also show that Li+ is indeed a stronger Lewis acid than Na+ is.

4. Conclusions Vibrational studies were carried out for CD3CN solutions of LiClO4 and NaClO4. The blue shifts of the w2 CN stretch result from solvation of Li+ (22 cm − 1) and Na+ (11 cm − 1). By measuring the w2 intensities of free and coordinated CD3CN in the Raman spectra, the primary solvation numbers are determined as a function of concentration. The number of primary solvation sites for Li+ in acetonitrile is believed to be four, and the number for Na+ is probably also four, despite lack of data at low concentration. The decrease of solvation number at a high concentration indicates that the chances for the cation to form a contact ion pair with the ClO− 4 ion increase with concentration. The IR extinction constants of the

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Table 2 Geometrical parameters of free CD3CN and those coordinated to Li+ and Na+ Geometrical parameters

r(N···M+) r(CH) r(CC) r(CN) ÚHCH ÚCCH ÚNCC ÚNM+CDc vd qf DE g

CH3CN Experimentalb

Calculated

1.094 1.460 1.157 109.0

1.101 1.467 1.170 108.6 110.3 180.0

180.0 3.9252e

4.05 −0.24

Li[CD3CN]+a 4

Na[CD3CN]+a 4

2.050 1.101 1.463 1.166 108.9 110.0 180.0 39.0 0.00 0.25 −486

2.402 1.101 1.463 1.167 108.9 110.0 180.0 38.4 0.00 0.084 −388

Bond lengths are in A, , angles in degree. a Calculated values. b Reference [16]. c Angle between the planes of NM+N and CCD. d Dipole moment in Debye. e Reference [34]. f Mulliken charge on nitrogen. g + Binding energy of M[CD3CN]+ 4 in kJ/mol relative to the separate CD3CN and M . Table 3 Calculated frequencies of free CD3CN and those coordinated to Li+ and Na+ Mode

CD3CNa

Li[CD3CN]+ 4

w1 w2 w3 w4 w5 w6 w7 w8

2142 2257 1092 819 2258 1036 836 337

2146(0.030) 2283 (16) 1091 (0.050) 834 (1.9) 2266 (0.40) 1029 (3) 839 (0.55) 352 (1.4)

CD3 symmetric stretch CN stretch CD3 symmetric deformation CC stretch CD3 antisymmetric stretch CD3 antisymmetric deformation CD3 rock CCN deformation

(2.7) (9.5) (0.22) (0.80) (2.0) (9.9) (0.77) (1.3)

a

Dw b

Na[CD3CN]+a 4

Dw b

4 26 −1 15 8 −7 3 15

2146 2273 1091 829 2266 1030 839 356

4 16 −1 10 8 −6 3 19

(0.012) (14) (0.095) (0.98) (0.021) (3) (0.53) (1.3)

All frequencies are IR frequencies in cm−1, and the Raman frequencies are essentially the same within 1 cm−1. a Numbers in parentheses are the calculated molar IR intensities for CD3CN in km/mol. b Frequency difference between the vibrational bands of pure and complexed CD3CN.

w2 CN and w4 CC stretch bands increase about 3.5 and 12 folds by coordination of Li+ and 1.4 and 11 folds by coordination of Na+, respectively. The blue shifts of the w8 CCN deformation band by coordination of Li+ and Na+ are also observed in the Raman spectra. The solvation effects for the vibrational characteristics of CD3CN are summarized. It turns out that the vibrational characteristics of free CD3CN in the solutions remain essentially the same as those of

pure CD3CN, indicating that the induction effect through the primary solvation shell of acetonitrile is virtually negligible. The structures and vibrational characteristics of CD3CN coordinated to Li+ and Na+ were also examined by means of DFT methods at the BLYP/6-31 +G(2d,p) level, assuming the primary solvation number of four. The CN and CC bonds are shortened and the force constants increase upon solvation, consistent with the previ-

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ous results observed in studies of interaction of acetonitrile with other Lewis acids. The variations in the vibrational characteristics, particularly the blue shifts and the increases in intensities of the w2 CN and w4 CC stretch and w7 CCN deformation bands, are well reproduced. The variation of the structure and vibrational characteristics is originated from the rearrangement in the electronic structure including alleviation of the antibonding character along the CCN moiety as a result of solvation.

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