title: Theoretical study on the excitation energies of six tautomers of guanine: evidence for the assignment of the rare tautomers.
abstract: The CASPT2//CASSCF method with the 6-31G basis set and an active space up to (16,12) was used to calculate the excitation energies for six tautomers of guanine. Our calculations provide further support on the recent reassignment of the near-UV resonant two-photon ionization (R2PI) spectrum, in which two rare tautomers of the 7H-oxo-imino form were proposed to replace the previously assigned 7H/9H-oxo-amino tautomers. The adiabatic excitation energies of the 7H-oxo-imino tautomers are calculated to be 0.3-0.5 eV higher than those of the 7H/9H-oxo-amino tautomers. Our calculations also indicate that the missing most stable tautomers (7H/9H-oxo-amino tautomers) in the R2PI experiment is possibly due to the existence of an ultrafast nonradiative deactivation process in the excited-state of these two tautomers.
authors.affiliation: School of Chemistry and Chemical Engineering, Institute of Theoretical and Computational Chemistry, Lab of Mesoscopic Chemistry, Nanjing University, Nanjing 210093, P. R. China.
authors.forename: Hui
authors.initials: H
authors.lastname: Chen
authors.forename: Shuhua
authors.initials: S
authors.lastname: Li
mesh_terms: D006147:Guanine / Q000737:chemistry*; D008962:Models, Theoretical; D015394:Molecular Structure; D013237:Stereoisomerism
chemical_list: D006147:Guanine