Resolving Discrepancies between Theory and Experiment: IR Spectrum of the Proton-Shared HBr:Pyridine Complex

The computed MP2/6-31+G(d,p) structure of the HBr:pyridine complex and the computed harmonic and experimental spectra of this complex agree that HBr:pyridine is stabilized by a proton-shared hydrogen bond. However, differences are evident between the computed and experimental spectral patterns with respect to the frequencies and intensities of strong bands. Solution of a one-dimensional nuclear vibrational equation along the normal coordinate mode allows for an estimation of an anharmonicity of the proton-stretching motion. When an anharmonic stretching force constant is used to recompute the spectrum, excellent agreement is found between theory and experiment.