Raman and infrared spectra, normal coordinate analysis and ab initio calculations of 4-Amino-2-chloropyrimidine-5-carbonitrile

The present work reports an experimental and theoretical study of molecular structure of 4-Amino-2-chloropyrimidine-5-carbonitrile (ACPC) in solid phase. Raman and infrared (IR) spectra of ACPC have been recorded in the region of 3600-100 cm^-1 and 4000-200 cm^-1, respectively. Calculations with the methods of B3LYP and Møller-Plesset second perturbation (MP2) were carried out for structural and vibrational predictions. The computational approaches were tested and adapted by comparing the predicted spectra to results obtained experimentally. In order to assign the calculated frequencies for the recorded ones, a normal coordinate analysis has been performed for vibrations with respect to the nuclei displacements for all the fundamental frequencies. By computing and analyzing, in detail, the infrared and Raman spectra of ACPC, the changes in the vibrational features were probed by identifying two tautomers of ACPC; (i) amino ACPC which could exist in two conformational structures, and (ii) imino ACPC which could exist in four conformational structures. Both computational and spectral results were in favor of the amino tautomer with no evidence for the existence of interconversion between amino and imino forms. Moreover, only one of the amino ACPC conformers was found to be the most stable one where the hybridization of amino group was found to be sp³. The theoretical predictions agree well with the available experimental data, accounting for the interconversion process in the amino/imino functional groups. We demonstrate that vibrational spectroscopy constitutes a powerful tool for studying molecular structure due to its high sensitivity to changes in molecular geometry.