Electronic Spectra, Excited-State Geometries, and Molecular Electrostatic Potentials of Hypoxanthine: A Theoretical Investigation

The ground-state geometry of hypoxanthine was optimized at the MP2, B3LYP, and HF levels by employing the 6-311++G(d,p) basis set. The vertical singlet transition energies were calculated at the CASSCF/ 6-31+G(d), TD-B3LYP/6-311++G(d,p), and CIS/6-311++G(d,p) levels by using the MP2-, B3LYP-, and Hartree−Fock-optimized geometries, respectively. In the case of the CASSCF calculations, the active space consisted of the 2σ, 6π, and 4π* orbitals. The σ orbitals were used to compute the nπ* transitions. The effects of dynamic correlation on the CASSCF energies were considered at the second-order multiconfigurational quasi-degenerate perturbation (MCQDPT2) theory. The effect of hydration was considered by including three water molecules in the first solvation shell of hypoxanthine. The geometry of the molecule was also optimized in the lowest singlet ππ* and nπ* excited states at the CIS/6-311++G(d,p) level. The characteristics of the ground and excited-state potential energy surfaces were ascertained from a harmonic vibrational analysis in the respective states. The molecular electrostatic potentials were computed in the ground and vertical singlet ππ* and nπ* excited states. The computed vertical singlet transition energies are found to be in good agreement with the corresponding experimental data. It has been suggested that hypoxanthine has a weak ππ* transition near 225 nm. The geometry of the molecule is found to be highly nonplanar in the lowest singlet ππ* excited state and approximately planar in the lowest singlet nπ* state, except for the CO group which is displaced appreciably out-of-plane from the ring plane. Hydration does not have a significant effect on the geometry of the isolated molecule. The molecular electrostatic potentials are found to be altered in going from the ground to different vertical singlet excited states. Significant reduction in the electrostatic potential magnitude near the carbonyl oxygen site of the molecule is found in the lowest singlet nπ* excited state.