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Ab initio molecular orbital calculations on complexes of chloride ion with acetonitrile and vibrational spectroscopic studies
Journal of
MOLECULAR STRUCTURE
ELSEVIER
Journal of Molecular Structure 327 (1994) 107- 112
Ab initio molecular orbital calculations on complexes of chloride ion with acetonitrile and vibrational spectroscopic studies* A.F. Jayaraj, Surjit Singh* Department
of Chemistry,
Indian Institute of Technology, Madras-600
036. India
Received 7 October 1993
Abstract 3-21G and 6-31 + G ab initio molecular orbital calculations have been performed on the (CHsCN)Cl- complex with three different configurations. A comparison of calculated harmonic frequencies and IR intensities of acetonitrile in the CH stretching region thus obtained in the gas phase with the experimental results for (CH$N)X- systems in the liquid phase suggests that the halide ions interact with acetonitrile linearly through its CH bonds in the C, configuration.
1. Introduction Vibrational spectroscopic studies of interactions of acetonitrile in general [l-5] and ion molecular interactions in particular [ 1,6-141 have been reported by several authors. It is, in general, found that interaction of halide ions with acetonitrile leads to the development of a low frequency component in the C-H stretching band whereas interaction with cations shows high frequency components in C-C and C z N stretching bands in the fundamental region. Interaction of perchlorate ions does not lead to the formation of any new bands although it produces an intensity increase and shows some broadening of the C-H bands on interaction with anions. Coetzee and Sharpe [ 121, Roche and Huong [ 131 and Ramana and Singh [14,15] have carried out IR and Raman spectral studies of solutions of electrolytes in *Dedicated to Professor C.N.R. Rao on the occasion of his 60th birthday. * Corresponding author.
acetonitrile and have shown the appearance of low frequency bands in the CH(D) stretching region due to anion molecular interactions. Ramana and Singh [15] have also reported detailed Raman spectral studies on solutions of tetrabutylammonium bromide in acetonitrile (Ds) in the C-D stretching region and reported an equilibrium constant of 0.4 lmol-’ for the Br- . . . CH$ZN complex in nitromethane. Theoretical calculations have been performed on (CHsCN)M”+ complexes, and the anomalous shift of C-C and C = N bands to higher frequencies on interaction with cations is attributed to a rehybridisation of the nitrogen atom thus increasing the s character of the sigma bond between the carbon and the nitrogen atom [ 12,16,17]. Yamabe and Hirao [ 181have carried out 4-3 1G ab initio calculations on X-(CHsCN) and have concluded that Cl-(CHsCN) is more stable in the Csv configuration where the interaction is purely electrostatic in nature than in the C, configuration where partial covalent bonding can be envisaged. Since vibrational spectral studies in the C-H stretching
0022-2860/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-2860(94)08 15 l-7
A.F. Jayaraj. S. SinghlJournal of Molecular Structure 327 (1994) 107-112
108
0.245032
0.368872
-0.456335
C
N
Fig. 1. Atomic charges as obtained by 6-3 1+ G calculations for acetonitrile.
region for systems exhibiting X- . . . CH$N interaction are available in the literature as outlined above, we feel it may be of interest to compare these experimental observations with the theoretical results to rationalise the mode of interaction of halide ions with acetonitrile. To the authors knowledge no MO calculations on the evaluation of vibrational frequencies or force constants have been reported in the literature on interactions of acetonitrile with halide ions. We have carried out ab initio calculations on the force constants, vibrational frequencies and relative IR intensities of acetonitrile and its complexes with the Cl- ion. Calculations have been performed both on the 3-21G and 6-31 +G levels. Diffused functions 6-31+ G are preferred to 6-31G functions because of their superiority [19,20] in determining interspecies distances as well as energies of stabilisation while considering complexes with halide ions. 2. Experimental Lithium bromide (Loba), after recrystallisation from acetonitrile, was dried under vacuum for 2 days. Tetrabutylammonium iodide (Sigma) was also used without further purification. Acetonitrile (Merck) was kept on 4A molecular sieves and used without further purification. IR spectra were recorded on a PE 1760 spectrometer and Raman spectra were recorded on a Dilor 224 with an argon ion laser. 3. Computational
details
Ab initio MO calculations were carried out using
the GAUSSIAN m/90 package [21] at the 3-21G and 6-3 1 + G levels for both geometry optimisation and vibrational analysis. Calculations were carried out using the gradient algorithm contained within the program. Optimisation in each case was carried out using the supermolecular approach where all geometrical parameters of complexes were allowed to relax. The calculations were carried out on a VAX/ MS version V5.3-1 computer.
4. Results and discussion
Figure 1 shows the preliminary results of Mulliken population analysis for the optimised geometry of acetonitrile. The results clearly indicate two distinct sites for interactions with anions. The localised C z N dipole has the possibility of interacting with a carbon atom (C,) whereas the extended dipole over the entire CHsCN molecule has the possibility of interacting with the hydrogen atoms of the CH bonds. Keeping this in view, three different possibilities of interaction of Cl- with acetonitrile are considered. The structural parameters for the species considered are defined in Fig. 2. Geometrical parameters and stabilisation energies for the systems considered are given in Table 1. It is found that the stabilisation energy is much lower when interaction takes place through the C2 atom (structure IV) of acetonitrile in comparison with interaction taking place through the CHs group (structures II and III). These observations are in line with the 4-31G results reported earlier [18]. The ion molecular interactions do not affect the bond distances and bond
109
A.F. Jayaraj, S. SinghlJournal of Molecular Structure 327 (1994) 107-l 12
r3
N
:
: r4
. : e
r3
N
IV
III
Fig. 2. Geometrical parameters for acetonitrile and (CHsCN)Cl- complex in various configurations.
angles significantly; changes are found only in the third decimal place in all the cases in the bond distances and one or two degrees in the bond
angles. The CH bond distance increases by a maximum of 0.005A when the interaction takes place through the CH bond (structure II).
Table 1 Geometrical parameters and stabilisation energies of CH,CN and Cl-. ..CH,CN calculations Structure I
II III IV
r2 1.460b 1.451 1.461b 1.451 1.465b 1.461 1.459b 1.456
1.082b 1.083 1.083b 1.084
1.082b 1.084
r;”
r3
1.087b 1.093 1.080b 1.080 1.085b 1.088
1.146b 1.139 l.148b 1.141 1.149b 1.141 1.147b 1.140
“r; and a; represent associated C-H bond distance and C-C-H b Values obtained by 6-3 1 + G calculations. Bond distances in A, angles in degrees and A in kcal.
as obtained by 6-31 +G and 3-21G ab initio
r4
2.534b 2.411 3.432b 3.281 3.874b 3.641
1l0.20b 110.14 109.74b 109.75
110.69b 110.75
aia
a2
-AE
112.04b 111.75 111.39b 111.67 107.27b 105.96
180.00b 180.00 68.67b 68.26 79.44b 79.94
10.4793b 13.1775 11.8598b 13.3658 2.8865b 4.7063
angle, and r2 and a2 represent the corresponding free values.
110
A.F. Jayaraj, S. Singh/Journal of Molecular Structure 327 (1994) 107-112
Table 2 Harmonic CH stretching force constants, 6-31+ G and 3-21G ab initio calculations Structure
frequencies
C-H force constant (mdyn A-‘)
and IR activities
C-H
(frequency
(in parantheses)
for CH$N
Cl- as
and CH,CN..
obtained
by
in cm-‘) E
Free I
5.91a 5.89 5.85= 5.82 6.02” 6.02 5.85” 5.83
II III IV
a Values obtained
A’
Bonded
3215.5526’ 3218.5020 3162.2774a 3106.4179 3243.9566= 3256.2203 3189.9068= 3174.7293
5.71a 5.50
5.80a 5.67
A’ (5.2058) (2.1375) (165.0349) (333.9299) (0.0410) (0.6466) (42.7905) (65.7886)
3298.1802a 3285.8591 3254.9700a 3233.2537 3330.2403a 3323.3886 3274.1477” 3260.0637
A” (3.1355) (1.7501) (71.9354) (34.4997) (0.0134) (0.0260) (18.8454) (18.6085)
3298.1802’ 3285.8591 3278.6061a 3266.8800 3330.2723a 3323.4240 3282.7654” 3263.5615
(3.1355) (1.7501) (14.8481) (15.4765) (0.0118) (0.0294) (15.2269) (19.4923)
by 6-3 1 + G calculations.
Table 2 summarises the results of harmonic CH stretching force constants and frequencies as well as relative IR intensities for the various species considered. The CH stretching force constant is found to decrease on interaction with the Cl- ion for structures II and IV whereas it increases in the case of structure III. Similar trends are reflected in
3060
2900
2980 WAVE
NUMBER
Icm-11
Fig. 3. IR spectra of LiBr in acetonitrile in the CH stretching region. Concentration of salt: (a) O.OM, (b) 0.53 M and (c) 0.76 M.
the CH stretching harmonic frequencies also. As mentioned above both IR and Raman spectra of solutions of halide ions in acetonitrile show a low frequency component in the CH stretching band due to (CH&N)Xcomplexes. For the sake of demonstration of these observatiolis IR and Raman spectra for two systems comprising solutions of Br- and I- ions in CH3CN (CD3CN) are shown in Figs. 3 and 4. The develbpment of low frequency components in CH(D) stretching .bands in these cases can clearly be observed with increasing concentration of halide ions. Similar’obbervations are also made for Cl- . . . CH3CN $&terns where the low frequency bands are not, as well developed because of the lower solubility of chlorides in acetonitrile. These observations rule out the possibility of the existence of the complex in the form of structure III where the CH stretching frequency is predicted to be higher than that of the free acetonitrile, even though it is found to have a higher stabilisation energy than strtictures II and IV, as reported in the previous calculations [ 181as well as in the present studies. Out of the two remaining structures (II and IV), the latter shows a much smaller shift to the low frequency side for the CH bands on complexation and also, as mentioned above, it has a much lower stabilisation energy than the former structiri. On the basis of these arguments, it seetis to be clear that structure II with .Cs symmetry represents the right type of
A.F. Jayaraj, S. Singh/Journal of Molecular Structure 327 (1994) 107-112
111
parison of the relative intensities of the antisymmetric to symmetric CH stretching bands for the (CHsCN)Cll complex in structure II and free CHxCN in structure I also shows, within the orders of magnitude, trends similar to those observed experimentally [13]. The two high frequency bands at about 3255cm-’ and about 3279cm-’ together have an intensity nearly one-half of that of the low frequency band at about 3 162 cm-‘, whereas the corresponding bands for the free acetonitrile seem to be of similar intensities. 5. Conclusions
4
,
2110
2130 WAVE
NUMBER
I
2090 (cm-‘1
Fig. 4. Raman spectra of solutions of tetrabutylammonium iodide in CD3CN in the CD stretching region. Concentration of salt: (a) 0.0 M, (b) 0.65 M and (c) saturated.
configuration representing the interaction of acetonitrile with halide ions. Another interesting observation which has been reported in the literature concerns the relative intensities of symmetric and anti-symmetric stretching bands of acetonitrile under different environments. It is observed [1,22] that the peak heights of the fundamental for the CHs antisymmetric and symmetric CH stretching modes of pure acetonitrile are comparable in their IR spectrum but as the concentration of acetonitrile (in CC14) increases, the relative intensity of the anti-symmetric stretching bands with respect to the symmetric stretching bands reduces. In the IR spectra of solutions of alkali metal halides in acetonitrile [13] it is observed that the anti-symmetric CH stretching band of associated acetonitrile has a lower intensity than the symmetric CHs stretching band and the overall intensity of both the bands is much higher for associated acetonitrile than for the bulk acetonitrile. The calculated intensities for structures I and II also show that the CH stretching intensity is much higher for the (CHsCN)Cl- complex in comparison with free CHsCN. A com-
MO calculations on various configurations considered for the (CH,CN)Cl- complex show that interaction of the Cl- ion with acetonitrile through the carbon atom of the C-N dipole is too weak in comparison with its interaction with the CH3 groups. The vibrational frequency data give evidence of the possibility of the existence of the (CHsCN)Cl- complex as structure II and not as structure III even though the latter is slightly more stable than the former with respect to its calculated stabilisation energy in the gas phase. The trends in the relative intensities of the antisymmetric to symmetric CHs stretching bands for the X- (CH$N) complex in the gas phase as obtained in the present studies are comparable with the reported experimental observations in the liquid phase.
Acknowledgements SS is thankful to Professor E. Knoezinger of the University of Siegen for offering a position and computer facilities for carrying out a part of this work. AFJ is thankful to IIT Madras for the research fellowship.
References [l] H. Michel and E. Lippert, in A.D. Buckingham, E. Lippert and S. Bratos (Eds.), Organic Liquids: Dynamics and Chemical Properties, Wiley, New York, 1978, p. 293.
112
A.F. Jayaraj. S. Singh/Journal of Molecular Structure 327 (1994) 107-112
[2] J. Yarwood and R. Amdt, in R. Foster (Ed.), Molecular
Association, Vol. 2, Academic Press, London, 1979 p. 267. [3] J. Yarwood, Annu. Rep. Prog. Chem., Sect. C, 76 (1979) 99; 79 (1983) 157. [4] A.D. Buckingham and R.E. Raab, J. Chem. Sot., (1961) 5511. [5] M.I.S. Sastry and S. Singh, Curr. Sci., 55 (1986) 1157. [6] J. Sadlej, Spectrochim. Acta, Part A, 35 (1979) 681. [7] B.G. Oliver and G.J. Janz, J. Phys. Chem., 74 (1970) 3819. [8] K. Miyaji and K. Morinaga, Bull. Chem. Sot. Jpn., 56 (1986) 695. [9] IS. Pominow and A.Z. Gadzhiev, Izv. Vyssh. Uchebn. Zaved. Fiz., 5 (1965) 19. [lo] IS. Perelygin, Opt. Spectrosc., 13 (1962) 198. [ll] G.J. Janz, M.J. Tait and J. Meier, J. Phys. Chem., 71 (1967) 963. [12] J.F. Coetzee and W.R. Sharpe, J. Solution Chem., 1 (1972) 1. [ 131 J.P. Roche and P.V. Huong, J. Chim. Phys. PhysChem. Biol., 67 (1970) 211. [14] K.V. Ramana and S. Singh, Spectrochim. Acta, Part A, 44 (1988) 277.
[15] K.V. Ramana and S. Singh, J. Mol. Struct., 194 (1989) 73. [16] J.C. Evans and G.Y.S. Lo, Spectrochim. Acta, 21 (1965) 1033. K.F. Purcell and R.S. Drago, J. Am. Chem. Sot., 88 (1966) 919. [17] J. Sadlej and Z. Kecki, Rocz. Chem., Ann. Sot. Chim. Polon., 43 (1969) 2131. [18] S. Yamabe and K. Hirao, Chem. Phys. Lett., 84 (1981) 598. [19] S. Singh and E. Knoezinger, Spectrochim. Acta, Part A, 48 (1992) 1767. P. Mohandas, MC. Shivaglal, S. Singh and J. Chandrasekar, J. Mol. Struct. (Theochem), 284 (1993) 147. [20] T. Clark, Handbook of Computational Chemistry, Wiley, New York, 1985. [21] M.J. Frisch, J.S. Binkley, H.B. Schlegel, K. Raghavachary, C.F. Melius, J.L. Martin, J.J.P. Stewart, F.W. Bobrowicz, C.M. Rohlfing, L.R. Kahn, D.J. DeFrees, P. Seeger, R.A. Whiteside, D.J. Fox, E.M. Fluder and J.A. Pople, GAUSSIAN 88, 90, Carnegie-Mellon Quantum Publishing Unit, Pittsburgh, PA, 1984. [22] E. Knoezinger, personal communication, 199 1.
MOLECULAR STRUCTURE
ELSEVIER
Journal of Molecular Structure 327 (1994) 107- 112
Ab initio molecular orbital calculations on complexes of chloride ion with acetonitrile and vibrational spectroscopic studies* A.F. Jayaraj, Surjit Singh* Department
of Chemistry,
Indian Institute of Technology, Madras-600
036. India
Received 7 October 1993
Abstract 3-21G and 6-31 + G ab initio molecular orbital calculations have been performed on the (CHsCN)Cl- complex with three different configurations. A comparison of calculated harmonic frequencies and IR intensities of acetonitrile in the CH stretching region thus obtained in the gas phase with the experimental results for (CH$N)X- systems in the liquid phase suggests that the halide ions interact with acetonitrile linearly through its CH bonds in the C, configuration.
1. Introduction Vibrational spectroscopic studies of interactions of acetonitrile in general [l-5] and ion molecular interactions in particular [ 1,6-141 have been reported by several authors. It is, in general, found that interaction of halide ions with acetonitrile leads to the development of a low frequency component in the C-H stretching band whereas interaction with cations shows high frequency components in C-C and C z N stretching bands in the fundamental region. Interaction of perchlorate ions does not lead to the formation of any new bands although it produces an intensity increase and shows some broadening of the C-H bands on interaction with anions. Coetzee and Sharpe [ 121, Roche and Huong [ 131 and Ramana and Singh [14,15] have carried out IR and Raman spectral studies of solutions of electrolytes in *Dedicated to Professor C.N.R. Rao on the occasion of his 60th birthday. * Corresponding author.
acetonitrile and have shown the appearance of low frequency bands in the CH(D) stretching region due to anion molecular interactions. Ramana and Singh [15] have also reported detailed Raman spectral studies on solutions of tetrabutylammonium bromide in acetonitrile (Ds) in the C-D stretching region and reported an equilibrium constant of 0.4 lmol-’ for the Br- . . . CH$ZN complex in nitromethane. Theoretical calculations have been performed on (CHsCN)M”+ complexes, and the anomalous shift of C-C and C = N bands to higher frequencies on interaction with cations is attributed to a rehybridisation of the nitrogen atom thus increasing the s character of the sigma bond between the carbon and the nitrogen atom [ 12,16,17]. Yamabe and Hirao [ 181have carried out 4-3 1G ab initio calculations on X-(CHsCN) and have concluded that Cl-(CHsCN) is more stable in the Csv configuration where the interaction is purely electrostatic in nature than in the C, configuration where partial covalent bonding can be envisaged. Since vibrational spectral studies in the C-H stretching
0022-2860/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZ 0022-2860(94)08 15 l-7
A.F. Jayaraj. S. SinghlJournal of Molecular Structure 327 (1994) 107-112
108
0.245032
0.368872
-0.456335
C
N
Fig. 1. Atomic charges as obtained by 6-3 1+ G calculations for acetonitrile.
region for systems exhibiting X- . . . CH$N interaction are available in the literature as outlined above, we feel it may be of interest to compare these experimental observations with the theoretical results to rationalise the mode of interaction of halide ions with acetonitrile. To the authors knowledge no MO calculations on the evaluation of vibrational frequencies or force constants have been reported in the literature on interactions of acetonitrile with halide ions. We have carried out ab initio calculations on the force constants, vibrational frequencies and relative IR intensities of acetonitrile and its complexes with the Cl- ion. Calculations have been performed both on the 3-21G and 6-31 +G levels. Diffused functions 6-31+ G are preferred to 6-31G functions because of their superiority [19,20] in determining interspecies distances as well as energies of stabilisation while considering complexes with halide ions. 2. Experimental Lithium bromide (Loba), after recrystallisation from acetonitrile, was dried under vacuum for 2 days. Tetrabutylammonium iodide (Sigma) was also used without further purification. Acetonitrile (Merck) was kept on 4A molecular sieves and used without further purification. IR spectra were recorded on a PE 1760 spectrometer and Raman spectra were recorded on a Dilor 224 with an argon ion laser. 3. Computational
details
Ab initio MO calculations were carried out using
the GAUSSIAN m/90 package [21] at the 3-21G and 6-3 1 + G levels for both geometry optimisation and vibrational analysis. Calculations were carried out using the gradient algorithm contained within the program. Optimisation in each case was carried out using the supermolecular approach where all geometrical parameters of complexes were allowed to relax. The calculations were carried out on a VAX/ MS version V5.3-1 computer.
4. Results and discussion
Figure 1 shows the preliminary results of Mulliken population analysis for the optimised geometry of acetonitrile. The results clearly indicate two distinct sites for interactions with anions. The localised C z N dipole has the possibility of interacting with a carbon atom (C,) whereas the extended dipole over the entire CHsCN molecule has the possibility of interacting with the hydrogen atoms of the CH bonds. Keeping this in view, three different possibilities of interaction of Cl- with acetonitrile are considered. The structural parameters for the species considered are defined in Fig. 2. Geometrical parameters and stabilisation energies for the systems considered are given in Table 1. It is found that the stabilisation energy is much lower when interaction takes place through the C2 atom (structure IV) of acetonitrile in comparison with interaction taking place through the CHs group (structures II and III). These observations are in line with the 4-31G results reported earlier [18]. The ion molecular interactions do not affect the bond distances and bond
109
A.F. Jayaraj, S. SinghlJournal of Molecular Structure 327 (1994) 107-l 12
r3
N
:
: r4
. : e
r3
N
IV
III
Fig. 2. Geometrical parameters for acetonitrile and (CHsCN)Cl- complex in various configurations.
angles significantly; changes are found only in the third decimal place in all the cases in the bond distances and one or two degrees in the bond
angles. The CH bond distance increases by a maximum of 0.005A when the interaction takes place through the CH bond (structure II).
Table 1 Geometrical parameters and stabilisation energies of CH,CN and Cl-. ..CH,CN calculations Structure I
II III IV
r2 1.460b 1.451 1.461b 1.451 1.465b 1.461 1.459b 1.456
1.082b 1.083 1.083b 1.084
1.082b 1.084
r;”
r3
1.087b 1.093 1.080b 1.080 1.085b 1.088
1.146b 1.139 l.148b 1.141 1.149b 1.141 1.147b 1.140
“r; and a; represent associated C-H bond distance and C-C-H b Values obtained by 6-3 1 + G calculations. Bond distances in A, angles in degrees and A in kcal.
as obtained by 6-31 +G and 3-21G ab initio
r4
2.534b 2.411 3.432b 3.281 3.874b 3.641
1l0.20b 110.14 109.74b 109.75
110.69b 110.75
aia
a2
-AE
112.04b 111.75 111.39b 111.67 107.27b 105.96
180.00b 180.00 68.67b 68.26 79.44b 79.94
10.4793b 13.1775 11.8598b 13.3658 2.8865b 4.7063
angle, and r2 and a2 represent the corresponding free values.
110
A.F. Jayaraj, S. Singh/Journal of Molecular Structure 327 (1994) 107-112
Table 2 Harmonic CH stretching force constants, 6-31+ G and 3-21G ab initio calculations Structure
frequencies
C-H force constant (mdyn A-‘)
and IR activities
C-H
(frequency
(in parantheses)
for CH$N
Cl- as
and CH,CN..
obtained
by
in cm-‘) E
Free I
5.91a 5.89 5.85= 5.82 6.02” 6.02 5.85” 5.83
II III IV
a Values obtained
A’
Bonded
3215.5526’ 3218.5020 3162.2774a 3106.4179 3243.9566= 3256.2203 3189.9068= 3174.7293
5.71a 5.50
5.80a 5.67
A’ (5.2058) (2.1375) (165.0349) (333.9299) (0.0410) (0.6466) (42.7905) (65.7886)
3298.1802a 3285.8591 3254.9700a 3233.2537 3330.2403a 3323.3886 3274.1477” 3260.0637
A” (3.1355) (1.7501) (71.9354) (34.4997) (0.0134) (0.0260) (18.8454) (18.6085)
3298.1802’ 3285.8591 3278.6061a 3266.8800 3330.2723a 3323.4240 3282.7654” 3263.5615
(3.1355) (1.7501) (14.8481) (15.4765) (0.0118) (0.0294) (15.2269) (19.4923)
by 6-3 1 + G calculations.
Table 2 summarises the results of harmonic CH stretching force constants and frequencies as well as relative IR intensities for the various species considered. The CH stretching force constant is found to decrease on interaction with the Cl- ion for structures II and IV whereas it increases in the case of structure III. Similar trends are reflected in
3060
2900
2980 WAVE
NUMBER
Icm-11
Fig. 3. IR spectra of LiBr in acetonitrile in the CH stretching region. Concentration of salt: (a) O.OM, (b) 0.53 M and (c) 0.76 M.
the CH stretching harmonic frequencies also. As mentioned above both IR and Raman spectra of solutions of halide ions in acetonitrile show a low frequency component in the CH stretching band due to (CH&N)Xcomplexes. For the sake of demonstration of these observatiolis IR and Raman spectra for two systems comprising solutions of Br- and I- ions in CH3CN (CD3CN) are shown in Figs. 3 and 4. The develbpment of low frequency components in CH(D) stretching .bands in these cases can clearly be observed with increasing concentration of halide ions. Similar’obbervations are also made for Cl- . . . CH3CN $&terns where the low frequency bands are not, as well developed because of the lower solubility of chlorides in acetonitrile. These observations rule out the possibility of the existence of the complex in the form of structure III where the CH stretching frequency is predicted to be higher than that of the free acetonitrile, even though it is found to have a higher stabilisation energy than strtictures II and IV, as reported in the previous calculations [ 181as well as in the present studies. Out of the two remaining structures (II and IV), the latter shows a much smaller shift to the low frequency side for the CH bands on complexation and also, as mentioned above, it has a much lower stabilisation energy than the former structiri. On the basis of these arguments, it seetis to be clear that structure II with .Cs symmetry represents the right type of
A.F. Jayaraj, S. Singh/Journal of Molecular Structure 327 (1994) 107-112
111
parison of the relative intensities of the antisymmetric to symmetric CH stretching bands for the (CHsCN)Cll complex in structure II and free CHxCN in structure I also shows, within the orders of magnitude, trends similar to those observed experimentally [13]. The two high frequency bands at about 3255cm-’ and about 3279cm-’ together have an intensity nearly one-half of that of the low frequency band at about 3 162 cm-‘, whereas the corresponding bands for the free acetonitrile seem to be of similar intensities. 5. Conclusions
4
,
2110
2130 WAVE
NUMBER
I
2090 (cm-‘1
Fig. 4. Raman spectra of solutions of tetrabutylammonium iodide in CD3CN in the CD stretching region. Concentration of salt: (a) 0.0 M, (b) 0.65 M and (c) saturated.
configuration representing the interaction of acetonitrile with halide ions. Another interesting observation which has been reported in the literature concerns the relative intensities of symmetric and anti-symmetric stretching bands of acetonitrile under different environments. It is observed [1,22] that the peak heights of the fundamental for the CHs antisymmetric and symmetric CH stretching modes of pure acetonitrile are comparable in their IR spectrum but as the concentration of acetonitrile (in CC14) increases, the relative intensity of the anti-symmetric stretching bands with respect to the symmetric stretching bands reduces. In the IR spectra of solutions of alkali metal halides in acetonitrile [13] it is observed that the anti-symmetric CH stretching band of associated acetonitrile has a lower intensity than the symmetric CHs stretching band and the overall intensity of both the bands is much higher for associated acetonitrile than for the bulk acetonitrile. The calculated intensities for structures I and II also show that the CH stretching intensity is much higher for the (CHsCN)Cl- complex in comparison with free CHsCN. A com-
MO calculations on various configurations considered for the (CH,CN)Cl- complex show that interaction of the Cl- ion with acetonitrile through the carbon atom of the C-N dipole is too weak in comparison with its interaction with the CH3 groups. The vibrational frequency data give evidence of the possibility of the existence of the (CHsCN)Cl- complex as structure II and not as structure III even though the latter is slightly more stable than the former with respect to its calculated stabilisation energy in the gas phase. The trends in the relative intensities of the antisymmetric to symmetric CHs stretching bands for the X- (CH$N) complex in the gas phase as obtained in the present studies are comparable with the reported experimental observations in the liquid phase.
Acknowledgements SS is thankful to Professor E. Knoezinger of the University of Siegen for offering a position and computer facilities for carrying out a part of this work. AFJ is thankful to IIT Madras for the research fellowship.
References [l] H. Michel and E. Lippert, in A.D. Buckingham, E. Lippert and S. Bratos (Eds.), Organic Liquids: Dynamics and Chemical Properties, Wiley, New York, 1978, p. 293.
112
A.F. Jayaraj. S. Singh/Journal of Molecular Structure 327 (1994) 107-112
[2] J. Yarwood and R. Amdt, in R. Foster (Ed.), Molecular
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