Experimental infrared spectra of matrix isolated complexes of HCl with 4-substituted pyridines. Evaluation of anharmonicity and matrix effects using data from ab initio calculations

Infrared spectra have been measured for HCl complexes with 4-cyanopyridine, 4-chloropyridine, pyridine and 4-methylpyridine isolated in argon and nitrogen matrices at about 12 K. The experimental spectra are dramatically different from computed MP2/6-31 + G(d,p) harmonic spectra, a consequence of the anharmonicity of the potential energy surface in the hydrogen-bonding region. Comparisons of computed and experimental data suggest that the experimental spectra correspond to omplexes with HCl distances that are much longer than the computed equilibrium distances. These longer distances, R-cor(HCl), are related to the average HCl distance in the ground vibrational state of the proton-stretching mode. The value of R-cor(HCl) determines values of three effective anharmonic force constants for the HCl stretch, the NH stretch and the coupling between them for each complex. The simulated anharmonic spectra obtained when these anharmonic force constants are used in place of the corresponding harmonic constants show spectral patterns with respect to both frequencies and intensities that are very similar to those observed in the experimental spectra obtained in Ar and N-2 matrices. 1D anharmonic potential curves related to the experimental spectra are presented. They provide insight into anharmonicity of the hydrogen-bonded proton stretch for these systems, and into the sensitivity of the potential energy surface to the environment. (C) 2000 Elsevier Science B.V. All rights reserved.