Hydrogen bonding and tunneling in the 2-pyridone·2-hydroxypyridine dimer. Effect of electronic excitation

Abstract The 2-pyridone·2-hydroxypyridine (2PY·2HP) mixed dimer has been studied using high resolution ultraviolet spectroscopy in the region of the 2PY S1–S0 origin, and fluorescence-dip infrared spectroscopy in the region of the hydride stretch fundamentals. The dense rotational structure of the electronic spectrum is characteristic of a b-type transition with a transition moment at 8°±3° to the b-axis, consistent with excitation of the 2PY half of the dimer. A tunneling splitting of 520±10 MHz appears in the spectrum, due to a double proton transfer in 2PY·2HP. The double proton transfer exchanges the chemical identity of the two monomer units, thereby leading to a double tautomerization. Theoretical calculations suggest that the barrier to such motion is about 8 kcal/mol in the ground state; hence, the observed tautomerization apparently occurs in the excited state. An approximate fit of the high resolution spectrum gives rotational constants that are consistent with an excited state structure in which only the OH⋯O hydrogen bond in the dimer is lengthened substantially. The infrared spectrum out of the pair of ground state zero-point tunneling levels in the XH stretch region is reminiscent of that in the pure (2PY)2 dimer. Its peak absorption frequency is at 2700 cm −1 , but the infrared band is spread over about 500 cm −1 , with reproducible sub-structure due to strong, anharmonic coupling. The excited state spectrum, in contrast, is dominated by a transition at 3135 cm −1 . This band is assigned to the OH fundamental, which is shifted to higher frequency by the weakening of the OH⋯O hydrogen bond upon electronic excitation.