Inertial axis reorientation in the S1←S electronic transition of 2-pyridone. A rotational Duschinsky effect. Structural and dynamical consequences

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Abstract
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.

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