On the pyrazine and pyrazine–pyrimidine dimers

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Abstract
Spectra of the pyrazine-d4, pyrazine-h4–pyrazine-d4, and pyrazine-d4–pyrimidine dimer are obtained and analyzed with the help of Lennard-Jones–hydrogen-bonding (LJ–HB) potential energy calculations. The pyrazine isotopic hetero and homo dimers possess nearly identical spectra with the exception that the perpendicular dimer features are displaced to the red by ∼11 cm−1. Exchange or exciton interactions in this system are vanishingly small (less than 1 cm−1). The geometries suggested by the isotopically substituted pyrazine dimer spectra are the same as those found for the pyrazine-h4 homo dimer: a parallel planar hydrogen bonded and a perpendicular dimer. The pyrazine-d4– and pyrazine-h4–pyrimidine dimer spectra are quite complicated. These spectra can be assigned as arising from one parallel stacked head-to-tail displaced dimer, one parallel planar dimer, and three perpendicular dimers based on comparisons with spectra of the pyrazine and pyrimidine dimers.