Infrared depletion spectra of 2-aminopyridine2-pyridone, a Watson-Crick mimic of adenine.uracil

The 2-aminopyridine2-pyridone (2AP2PY) dimer is linked by 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-specific infrared spectra of 2AP2PY and its seven N-H deuterated isotopomers have been measured between 2550 and 3650 cm(-1) by IR laser depletion combined with UV two-color resonant two-photon ionization. The 2PY amide N-H stretch is a very intense band spread over the range 2700-3000 cm(-1) due to large anharmonic couplings. It is shifted to lower frequency by 710 cm(-1) or approximately 20% upon H bonding to 2AP. On the 2AP moiety, the "bound" amino N-H stretch gives rise to a sharp band at 3140 cm(-1), which is downshifted by 354 cm(-1) or approximately 10% upon H bonding to 2PY. The amino group "free" N-H stretch and the H-N-H bend overtone are sharp bands at approximately 3530 cm(-1) and 3320 cm(-1). Ab initio structures and harmonic vibrations were calculated at the Hartree-Fock level and with the PW91 and B3LYP density functionals. The PW91/6-311++G(d,p) method provides excellent predictions for the frequencies and IR intensities of all the isotopomers.