Intra-red absorption spectra of the single crystals of 1-methyl-thymine, 9-methyladenine and their 1:1 complex

Abstract Infra-red absorption spectra have been observed of thin sections of the single crystals of 1-methylthymine, 9-methyladenine, and their hydrogen-bonded 1:1 complex (AT dimer) by using the polarized radiation in the spectral region of 4000-400 cm −1 . The absorption bands due to the in-plane and out-of-plane vibrations are distinguished in the spectra of the sections perpendicular to the molecular plane. The directions of the transition moments were determined of some of the absorption bands in the spectra of the sections parallel to the molecular plane. Assignments of the absorption bands due to the NH and NH 2 deformation were made on the basis of the observed N-deuteration effects. A marked difference was observed between the spectra of the AT dimer and an equimolar mechanical mixture of 1-methylthymine and 9-methyladenine. The frequencies assignable to the out-of-plane vibrations of the NH and NH 2 groups were found to be sensitive to the manner of their hydrogen bonding.