Four tautomers of isolated guanine from infrared laser spectroscopy in helium nanodroplets.

Infrared laser spectroscopy is used to study the four lowest energy tautomers of guanine, isolated in helium nanodroplets. The large number of vibrational bands observed in the infrared spectrum are assigned by comparing the corresponding experimental vibrational transition moment angles with those obtained from ab initio theory. The result is the conclusive assignment of the spectrum to the N9H-Keto, N7H-Keto, N9Ha-Enol(trans), and N9Hb-Enol(cis) tautomers. The dipole moments of these tautomers are also experimentally determined and compared with ab initio theory.