5387
J. Phys. Chem. 1991,95, 5387-5388
Structure, Binding Energy, and Vibrational Frequencies of CHsCNmHCl Janet E. Del Bene,* Howard D. Mettee, Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555
and Isaiah Shavitt Department of Chemistry, The Ohio State University, Columbus, Ohio 43210 (Received: May 2, 1991: In Final Form: May 23, 1991)
The structure and vibrational frequencies of the CH,CN--HCl complex have been obtained at MP2/6-31+G(d,p), and the binding energy of this complex has been computed at MP4SDTQ with a large basis set including two sets of first polarization functions and one set of second polarization functions on all atoms. The computed results are compared with the experimental structure and vibrational frequencies and with the binding energy AH3'' and the dissociation energy De estimated from experimental data.
In a recent paper, Ballard and Henderson reported a study of the hydrogen bond energy of CH,CN.-HCl based on FTIR photometry of the H-Cl stretching mode in the complex.' They compared their experimental hydrogen bond energy with some previous theoretical calculations of the Hartree-Fock binding energy.2, They also characterized the low-frequency vibrations of the complex and used them to estimate De, the binding energy at absolute zero measured from the minimum of the potential well. Recently, high-level calculations have been carried out in this laboratory to determine the correlated structure, stabilization energy, and vibrational frequenciesof the CH3CN--HCl complex. In this Letter a comparison will be made between these computed results and available experimental data. The structures of the complex CH3CN-HC1 and of the corresponding isolated monomers HCl and CH3CN were fully optimized by using many-body second-order Maller-Plesset perturbation theory (MP2)C9 with the 6-31+G(d,p) basis which is a split-valence plus polarization basis augmented with diffuse functions on non-hydrogen atoms. Vibrational frequencies were computed to confirm that the monomers and the complex are equilibrium structures (no imaginary frequencies) and to evaluate both zero-point and thermal vibrational energies. Single-point calculations in which the valence electrons were correlated at fourth-order Maller-Plesset perturbation theory (MP4SDTQ = MP4) were carried out on all species to evaluate the binding energy of the complex. For the MP4 calculations, the Dunning correlation-consistent polarized triple-split valence basis set'' was used for N, C, and H atoms. This triple-split basis set, which has two sets of first polarization functions and one set of second polarization functions on all atoms, was augmented with diffuse s and p functions on C and N, with exponents of 0.04 and 0.06, respectively. The chlorine atom was described by the Ballard, L.; Henderson, G. J . Phys. Chem. 1991, 95,660. Boyd, R. J.; Choi, S . C. Chem. Phys. krr.1986,129, 62. (3) Hinchliffe, A. Ado. Mol. Relax. Interact. Processes 1981, 19, 227. (4) Bartlctt, R. J.; Silver, D.M.J. Chem. Phys. 1975,62,3258; 1976,64, (1) (2)
1260,4578. ( 5 ) Binkley. J. S.; Pople, J. A. Inr. J. Quanrum Chem. 1975, 9, 229. (6) Pople, J. A.; Binkley, J. S.;Seeger, R. Inr. J. Quanrum Chem., Quanrum Chem. Symp. 1976,10, I . (7) Krilnan, R.; Pople, J. A. Inr. J . Quanrum Chem. 1978, 14, 91. (8) Purvis, G. D.; Bartlett, R. J. J . Chem. Phys. 1978, 68, 2114. (9) Bartlett, R. J.; Purvis, G. D. Inr. J . Quanrum Chem. 1978, 14, 561. (IO) Hariharan, P. C.; Pople, J. A. J. Chem. Phys. 1975, 62, 2921. (1 1) Dill, J. D.; Pople, J. A. J. Chem. Phys. 1975, 62, 2921. (12) Spitznagcl,0.W.; Clark, T.; Chandrawkhar,J.; Schleyer. P. v. R. J . Compur. Chem. 1983, 4, 294. (13) Clark, 7.; Chandrawkhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J . Compur. Chem. 1983,4, 294. (14) Dunning, T.H., Jr. J. Chem. Phys. 1989,90, 1007.
0022-3654/91/2095-5387$02,50/0
TABLE I: Vibrational Modes (in coil) of the CH3CN-HCI Complex
this work normal-wordinate mode description ub out-of-phase bending of 40 20 35 bending of rigid rigid monomers monomers Y, hydrogen bond stretching 97 3' 117 hydrogen bond stretching vB in-phase bending of rigid 350 IO@ 417 excursion of proton monomers from linearity a Reference 17. Estimated from the infrared measurement of CH,CN-HF in ref 19. exptl data from ref 1
*
McLean-Chandler (12,9) basis set contracted to (6,5).15 This basis was also augmented with a set of diffuse s and p functions, two sets of d polarization functions, and a single set off functions, with exponents taken from the 6-31+G(2df,2pd) basis set.16 The complex CH,CN.-HCl, shown in Figure 1, has C , symmetry, with a computed MP2/6-3 l+G(d,p) intermolecular Cl-N distance of 3.316 A. This distance is much shorter than the HF/6-31G(d,p) distance of 3.445 A of ref 2 and in agreement with the experimental distance of 3.291 A reported by Legon, Millen, and North." Formation of the hydrogen-bonded complex CH3CN-HC1 gives rise to five new vibrational modes. These have been reported and described by Ballard and Henderson' as ) 40 f 20 cm-', a hydrogen a degenerate out-of-phase bend ( v ~ at bond stretch (v,) at 97 f 3 cm-', and a degenerate in-phase bend (vB)at 350 f 100 cm-1 (see Table I). The degenerate bending modes were described as motions in which the monomer subunits effectively librate about their centers of mass. The computed MP2 vibrational frequencies, also reported in Table I, are in good agreement with the values quoted by Ballard and Henderson.' A normal-coordinate analysishdicates that the lowest frequency vibration, which is computed at 35 cm-', is a bending vibration of the complex in which one monomer moves in a clockwise direction while the other moves counterclockwise. The hydrogen bond stretching mode appears at 117 cm-'. The higher energy degenerate vibration computed at 417 cm-l corresponds to an excursion of the hydrogen-bonded hydrogen away from its equilibrium position on the Cl-N line. It is interesting to note that not all of the five new dimer vibrations have the lowest frequencies in the complex, since the doubly degenerate rocking ~
~
(15) McLean, A. D.;Chandler, G. S.J. Chem. Phys. 1980, 72, 5639. (16) Frisch, M. J.; Pople, J. A.; Binkley, J. S.J . Chem. Phys. 1984,80,
3265.
(17)
5210.
Legon, A. C.; Millen, D. J.; North, H.M.J . Phys. Chem. 1987,91,
Q 1991 American Chemical Society
5388
M:
J. Phys. Chem. 1991, 95,5388-5390
l.u)la
1.1nA
I.&
reaction enthalpy are determined solely from the five new modes of the complex. The effect of this assumption on the derived De value can be estimated by using the MP2/6-31ffi(d,p) vibrational frequencies in the harmonic approximation. Considering just the five new modes of the complex, the computed zero-point and thermal vibrational contributions to CLH3I4 are 2.4 and 1.7 kcal/mol, respectively, compared to 2.0 and 1.6 kcal/mol, respectively, if all the modes of the complex and monomers are taken into account. If the combined difference of 0.5 kcal/mol were to be applied as a correction to the determination of the experimental value of D, the mult would be 4.0 f 0.4 kcal/mol. (The 0.4 kcal/mol overestimation of the zero-point vibrational energy results mainly from the neglect of the decrease in the H-CI stretching frequency in the complex, which lowers the zero-point energy of this mode by 0.5 kcal/mol. Although the H-CI stretching frequency is overestimated at MP2/6-31 +G(d,p), the computed red shift of the H-CI band in the complex is 168 cm-', which is in satisfactory agreement with the experimental value of 155 c~n-'.'~)The computed value of De at MP4 obtained in the present work is 5.9 kcal/mol, which makes the complex much more stable than the estimated experimental value. Some of the limitations on the experimental determination were noted in ref 1. On the theoretical side, the effect of the basis set superposition error (BSSE)20 should be considered. This error should disappear as the basis set approaches completeness. To obtain an estimate of the upper bound on the BSSE, we have carried out a counterpoise The counterpoise correction at MP4 is 0.7 kcal/mol. If the total value of this correction were applied to the complex, the theoretical binding energy would be lowered to 5.2 kcal/mol. Part of the discrepancy between the experimental and the theoretical De values can be ascribed to the neglect of anharmonicity in the low-frequency modes used to obtain De from the experimental e l 4 . The magnitude of the thermal vibrational energy contribution would increase if these modes were treated anharmonically, leading to a greater binding energy De.
1. MPZ/C3I+G(d,p)optimized structures of CH,CN-HCI (C) and of the comsponding monomen HCI and CH3CN (M).The C-H bond lengths and CC-H angles are 1.087 A and 1 0 9 . 9 O , respectively, in CH,CN and 1.086 A and 109.8O, respectively, in CH,CN--HCI.
vibration of the methyl group is computed to occur at 352 cm-'. On the basis of their temperature dependence measurements, Ballard and Henderson' calculated a MI4 value of -3.3 f 0.3 kcal/mol for the formation of the complex. (It is not clear how these measurements were corrected for the temperature dependence of pure HCl background. There appears to be a residual absorbance present in the HCl rovibrational windows chosen in ref 1 for the integrated absorption measurements of the complex.) From this value, with the assumption that the vibrational modes in the "crsdo not change in the complex, and with a classical evaluation of the heat capacities of monomers and complex, they estimated a 0, value of 5.3 f 0.4 kcal/mol. In this approximation, the thermal vibrational energy computed classically from the heat However, the capacity as C,T contributes 2.9 kcal/mol to @I4. classical formula should not be used for vibrations at room temperature except for frequencies much below 200 cm-l, and its use for this complex leads to a significant error in the thermal vibrational energy contribution from the degenerate vibration at 350 cm-'. When evaluated in the harmonic approximation as1*
hE,T = RTCx,/(exp(x,) I
- 1)
where x, = hu,/kT, the thermal vibrational energy contribution from the fne new modes of the complex demases to 2.1 kcal/mol. This correction leads to an experimental De value of 4.5 f 0.4 kcal/mol. In addition, the assumption was also made in ref 1 that the zero-point and thermal vibrational energy contributions to the
Acknowledgment. These calculations were carried out on the Cray Y-MP8/864 at the Ohio Supercomputer Center. (19) Thomas, R. K.; Thompson,H. Proc. R. Soc. London, A 1970,316, 303. (20) Boys, S. F.; Bemardi, F. Mol. Phys. 1970, 19, 553.
(18) Pitzer, K. S. Quunrum Chemistry; Prmtia-Hall: Englewood Cliffs, NJ, 1961.
Infrared Evldence for Two Isolated Silanol Species on Activated Slllcas A. J. McFarlan and B. A. Morrow* Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada Kl N 6N5 (Received: March 27, 1991; In Final Form: May 29, 1991) Infrared spectros()8py has been used to study the OH stretching vibration of isolated silanol group on an aerosil and a precipitated silica which have been activated under vacuum at 450,600, or 800 OC. When the 450 OC activated samples are cooled to -191 O C , the normally asymmetric OH peak splits into two components having a peak maximum near 3750 or 3746 cm-' for the aerosil and precipitated silica, respectively, and a shoulder near 3738 em-'. The main peak is attributed to truly isolated SiOH groups and the low-wavenumber shoulder to pairs of isolated SiOH groups on adjacent silicon atoms which are sufficiently close to slightly perturb each other. The latter are preferentially eliminated as the temperature of activation is increased.
~~~
(1)
Zhuravlev, L. T. hngmuir 1987, 3, 316. 0022-3654/91/2095-5388$02SO/O
(2) Morrow, B. A.; McFarlan, A. J. J . Non-Crysr. Solids 1990, 120,61. (8
1991 American Chemical Society
