Knowledge Bank

Rotationally resolved fluorescence excitation spectra of two vibronic bands in the $S1\leftarrow S0$ electronic transition of 2-hydroxypyridine (2HP), and of the corresponding bands in the hydroxy-deuterated molecule, have been obtained. A comparison of the rotational constants of the two molecules shows that the two bands both originate in the zero-point vibrational level of the planar keto tautomer of 2HP, 2-pyridone (2PY), and terminate in different zero-point levels of 2PY that have different out-of-plane equilibrium geometries at nitrogen. Additionally, all four bands exhibit ""anomalous"" rotational line intensities that are shown to result from an in-plane inertial axis reorientation which occurs on absorption of the photon. The rotationally resolved $S2\leftarrow$ $S0$ electronic spectrum of the dimer of 2PY, $(2PY)2$, was recorded $-945cm-1$ to the blue of the first monomer origin. Structural information derived from the spectrum shows that the protonated dimer has a doubly hydrogen bonded planar $C2h$ structure in its $S0(1Ag)$ and $S2(1Bu)$ states. Analogous studies of $(2PY)2-d1$ and $(2PY)2-d2$ show that $(2PY)2$ and $(2PY)2-d2$ are energy delocalized dimers in their $S2$ states. A comparison of the effective and substitution structures of $(2PY)2$ shows that there is a distortion of the hydrogen bonded geometry when hydrogen is replaced by deuterium. These structural changes will be discussed.