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 resonant two-photon ionization and UV/UV-holeburning techniques. The latter allows the identification of the spectrum of the 2-pyridone.2-hydroxypyridine mixed dimer, present at approximate to 5% relative concentration. S-2--> S-0 fluorescence emission spectra show dominantly the hydrogen bond shearing vibration nu(4)" at 98.5 cm(-1) and the stretching vibration nu(6)" at 163.5 cm(-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 identified. Ground state structures, rotational constants, harmonic intermolecular and intramolecular vibrational frequencies, interaction, and dissociation energies were calculated using Hartree-Fock and density functional (B3LYP) methods. The B3LYP/6-311++(2d,2p) results are in excellent agreement with all experimental observations. (C) 2000 American Institute of Physics. [S0021-9606(00)00607-3].