Intermolecular vibrations of jet-cooled (2-pyridone)(2): A model for the uracil dimer

The 2-pyridone dimer, (2PY)(2), which is linked by two antiparallel N-H ... O hydrogen bonds, is a model for hydrogen bonded nucleotide base pairs, e. g., the uracil dimer. Mass- and isomer-selected S-2<-- S-0 vibronic spectra of supersonically cooled (2PY)(2) were measured by laser two-color resonan t two-photon ionization and UV/UV-holeburning techniques. The latter allows the identification of the spectrum of the 2-pyridone.2-hydroxypyridine mix ed dimer, present at approximate to 5% relative concentration. S-2--> S-0 f luorescence emission spectra show dominantly the hydrogen bond shearing vib ration nu(4)" at 98.5 cm(-1) and the stretching vibration nu(6)" at 163.5 c m(-1). The hydrogen bond stretching vibration force constant was determined to be 75.4 N/m, or 37.7 N/m per hydrogen bond, a very high value. The nu(2 )" (a(u)) torsional and the nu(3)" (b(u)) slanting vibrations were also ide ntified. Ground state structures, rotational constants, harmonic intermolec ular and intramolecular vibrational frequencies, interaction, and dissociat ion energies were calculated using Hartree-Fock and density functional (B3L YP) methods. The B3LYP/6-311++(2d,2p) results are in excellent agreement wi th all experimental observations. (C) 2000 American Institute of Physics. [ S0021-9606(00)00607-3].