Inertial axis reorientation in the S1 <-- S0 electronic transition of 2-pyridone. A rotational Duschinsky effect. Structural and dynamical consequences
Rotationally resolved fluorescence excitation spectra of two vibronic bands in the S1←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. Likely atomic displacements that are responsible for this ``rotational'' Duschinsky effect, which may have significant dynamical consequences in 2PY and other molecules, are discussed.