Dynamics of H-atom loss in adenine

Abstract

Time- and frequency resolved detection of H-atoms was used to investigate the excited-state photophysics and photochemistry of isolated adenine and 9-methyladenine at two excitation wavelengths, 266 nm and 239.5 nm. By comparison of the two molecules it is shown that dissociation of the N₉–H bond is an important deactivation pathway in adenine. The appearance time of the H-atoms was shorter than a few nanoseconds, indicating a rapid non-statistical dissociation. From the Doppler profiles a dissociation energy of 393 ± 20 kJ mol⁻¹ was determined for adenine. Typically 40–50% of the excess energy is released as translation. The Doppler profile of the H-atom photofragments from adenine excited at 239.5 nm shows an anisotropic angular distribution and suggests that dissociation occurs in less than a rotational period. An anisotropy parameter β ≈ −0.9 was derived, indicating an almost pure perpendicular transition. Our data are consistent with a H-atom loss in adenine that can be adequately described by a coupling with πσ* excited states as recently suggested in the literature. The Doppler profiles of 9-methyladenine on the other hand are more isotropic. From the width of the profile it is assumed that not only one of the N–H bonds in the amino group is cleaved, but some H-loss also occurs on the methyl group.