Laser-Induced and Spontaneous Conformational Transformations in Monomers of Isoguanine Isolated in Low-Temperature Ar...

Conformational transformations were selectively induced in molecules of isoguanine isolated in Ar and Ne matrices by excitation with narrowband near-infrared (NIR) light from a tunable diode laser. This allowed reliable identification of the infrared spectra of two OH rotameric forms of the most stable amino-hydroxy N(9)H form of the compound. Rotamerizations of the OH group, in both directions, were achieved by excitations with narrowband NIR light tuned precisely to the frequency of the first OH stretching overtone, as well as by excitations with narrowband NIR light tuned to precise frequencies of the stretching overtones of the remote NH₂ or N(9)H group. Conformational conversions were also induced by the irradiation of matrix-isolated monomers of isoguanine with broadband near-infrared and infrared light. The same photostationary state was achieved as the final stage of broadband irradiation regardless of the initial population ratio of the two conformers. Finally, a spontaneous conversion of the higher-energy form to the lowest-energy structure occurred when the low-temperature Ar or Ne matrices were kept at a low temperature and in the dark. The time constants of these H-atom-tunneling transformations were experimentally established as 2.3 or 4.1 h in different trapping sites of the Ar matrix and 2.0 h for the Ne matrix.