Infrared spectra of the hydrogen bonded crystals of l-methylthymine and its deuterated form as well as spectra polarized in the parallel and perpendicular directions with respect to the crystal b axis were obtained by Kyogoku et al. [l]. The crystal symmetry is [?j C&and its unit cell contains 4 molecules joined by the 4 hydrogen bonds in the form of two dimers. The crystal IR spectra of the hydrogen bond show a distinct fine structure which is strongly influenced by the deuteration (figs. la,b).
A quantitative theoretical interpretation [3-51 of the IR spectra of the hydrogen bond has been recently obtained for a single isolated hydrogen bond in irnidazole [4] and for a number of hydrogen bonds of systems in the gas phase [4,6]. To the best of our knowledge no quantitative theoretical reconstruction of the IR spectra of the hydrogen bonds in an actual molecular crystal has been obtained yet -which task is attempted in the present note. We base our theoretical treatment on the same general assumption which was used previously [3,4]: the high frequency stretching vibration forms the potential energy for the hydrogen bond vibrations. This potential energy differs between the ground and excited states of the rapid motion by a term linear in the hydrogen bond coordinate and it can be shown that this term is diminished after deuterium substitution by a 21j2 factor. Consider now a unit cell of a crystal which con-Volume 20, number 6 CHEMICAL PHYSICS LETTERS