Infrared spectra of pyrazine, pyrimidine and pyridazine in solid argon

The vibrational spectra of monomeric diazines (pyrazine, pyrimidine and pyridazine) isolated in solid argon and of the neat crystalline phase of these compounds, at 10 K, are reported and discussed. Full assignment of the spectra is presented, providing evidence that the assignments of several bands previously undertaken for the compounds under other experimental conditions (e.g., gas phase, neat liquid or solution) shall be reconsidered. The interpretation of the experimental data is supported by extensive DFT calculations performed with the B3LYP functional and the 6-311++G(d,p) basis set and by comparison with the anharmonic vibrational calculations reported by Boese and Martin [J.Phys.Chem. A, 108 (2004) 3085] and Berezin et al. [Russian J.Phys.Chem., 79 (2005) 425; Opt.Spectrosc., 97 (2004) 201]. Spectra/structure correlations were extracted from the data, enabling to conclude that, while the π-electron systems in both pyrazine and pyrimidine rings are strongly delocalized over all heavy-atoms, in pyridazine the canonical form with one CC and two CN double bonds strongly predominates. Finally, the UV-induced photoisomerization of matrix isolated monomeric pyrazine to pyrimidine is reported.