Intermolecular vibrations of the jet-cooled 2-pyridone center dot 2-hydroxypyridine mixed dimer, a model for tautomeric nucleic acid base pairs

The 2-pyridone.2-hydroxypyridine mixed dimer, 2PY.2HP, is linked by antipar allel N-H . . .N and O-H . . .O hydrogen bonds, closely analogous to tautom eric hydrogen-bonded DNA base pairs such as isoguanine.uracil. Mass-selecte d S-1<--S-0 vibronic spectra of supersonic jet-cooled 2PY.2HP and three N-D or O-D deuterated isotopomers were measured by two-color resonant two-phot on ionization and UV/UV-holeburning techniques. S-1-->S-0 fluorescence spec tra show intense hydrogen bond stretching vibrations nu (6)' = 158 cm(-1), observed for all four isotopomers and the weak nu (5)' "shear" vibration at 116 cm(-1). The "slanting" intermolecular vibration nu (4)' = 89 cm(-1) wa s tentatively identified via the 2 nu (4)' overtone. The observation of 2-p yridone intramolecular overtone and combination transitions for two out-of- plane modes indicate slight nonplanarity of the 2-pyridone moiety in the S- 1 state. Ab initio calculations using Hartree-Fock, configuration interacti on among single excitations (CIS), and B3LYP density functional methods wer e employed to calculate ground- and excited-state structures, rotational co nstants, harmonic intermolecular and intramolecular vibrational frequencies , binding energies D-e, and dissociation energies D-0. The B3LYP/6-311++(d, p) nu (4)' and nu (6)' frequencies are in very good agreement with the expe rimental ground-state intermolecular frequencies. Electronic excitation of the 2-pyridone moiety in 2PY.2HP leads to a decrease of dissociation energy by 2.36 kcal/mol and a concurrent increase of H bond lengths by approximat e to0.08 Angstrom, in qualitative agreement with the self-consistent field (SCF) and CIS ab initio calculations. (C) 2001 American Institute of Physic s.