Hydrogen bond vibrations of 2-aminopyridine.2-pyridone, a Watson-Crick analogue of adenine.uracil.

The 2-aminopyridine.2-pyridone complex (2AP.2PY) is linked by antiparallel N-H...O=C and N-H...N hydrogen bonds, providing a model for the Watson-Crick hydrogen bond configuration of the adenine-thymine and adenine-uracil nucleobase pairs. Mass-selected S-1 <----> S-0 vibronic spectra of the supersonically cooled 2AP.2PY base pair analogue were measured by laser resonant two-photon ionization and emission spectroscopies. The hydrogen bond vibrations nu(2) (buckle, out-of-plane) and the three in-plane vibrations nu(3) (opening), nu(5) (shear), and nu(6) (stretch) were observed in the S-0 and S-1 states, giving detailed information on the stretching and deformation force constants of the (amide)N-H...N(pyridine) and the (amino)N-H...O=C hydrogen bonds. Density functional calculations with the B3LYP functional and the 6-311 ++ G(d,p) and 6-311 ++ G(2d,2p) basis sets yield ground-state hydrogen bond frequencies in close agreement with experiment.