Supersonic molecular jet studies of the pyrazine and pyrimidine dimers

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Abstract
Mass selected optical spectra for the first excited singlet nπ* states of the pyrazine and pyrimidine dimers are presented. The species are created in a pulsed supersonic jet expansion. The spectra are analyzed based on ionization energy, vibronic structure, and relative energy with respect to the isolated monomer (cluster spectroscopic shift). Calculations of binding energy and geometry for these dimers are carried out employing a Lennard-Jones (6-12-1) and hydrogen bonding (10-12-1) potential. In the case of pyrazine, calculations and experiments agree that both parallel planar hydrogen bonded and perpendicular dimers are present in the expansion. The calculations also predict a parallel stacked and 90° rotated pyrazine dimer which is not observed. This latter species most likely forms an excimer in the excited state with a short lifetime and a highly red shifted broad spectrum. In the case of pyrimidine, calculations yield four planar hydrogen bonded species and a parallel stacked and displaced species. The spectra for the pyrimidine dimer are consistent with these configurations, in agreement with the calculations. No perpendicular configuration is calculated for the pyrimidine dimer and no spectroscopic features require postulating the existence of such a configuration. To explore further the agreement between calculated and experimental results for aromatic dimers, calculations are also presented for the tetrazine dimer. Three calculated geometries are obtained for the tetrazine dimer: a parallel stacked and 90° rotated species, a planar hydrogen bonded species, and a perpendicular species. Experimental spectra and calculations are in basic agreement for all dimers studied and, in general, support one another.