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

The 2-pyridone⋅2-hydroxypyridine mixed dimer, 2PY⋅2HP, is linked by antiparallel N–H⋯N and O–H⋯O hydrogen bonds, closely analogous to tautomeric hydrogen-bonded DNA base pairs such as isoguanine⋅uracil. Mass-selected S1←S0 vibronic spectra of supersonic jet-cooled 2PY⋅2HP and three N–D or O–D deuterated isotopomers were measured by two-color resonant two-photon ionization and UV/UV-holeburning techniques. S1→S0 fluorescence spectra show intense hydrogen bond stretching vibrations ν6″=158 cm−1, observed for all four isotopomers and the weak ν5″ “shear” vibration at 116 cm−1. The “slanting” intermolecular vibration ν4″=89 cm−1 was tentatively identified via the 2ν4″ overtone. The observation of 2-pyridone intramolecular overtone and combination transitions for two out-of-plane modes indicate slight nonplanarity of the 2-pyridone moiety in the S1 state. Ab initio calculations using Hartree–Fock, configuration interaction among single excitations (CIS), and B3LYP density functional methods were employed to calculate ground- and excited-state structures, rotational constants, harmonic intermolecular and intramolecular vibrational frequencies, binding energies De, and dissociation energies D0. The B3LYP/6-311++(d,p) ν4″ and ν6″ frequencies are in very good agreement with the experimental 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 ≈0.08 Å, in qualitative agreement with the self-consistent field (SCF) and CIS ab initio calculations.