UV-Induced Isomerization Dynamics of <i>N</i>‑Methyl-2-pyridone in Solution

The photoisomerization dynamics of <i>N</i>-methyl-2-pyridone (NMP) dissolved in CH<sub>3</sub>CN have been interrogated by time-resolved electronic and vibrational absorption spectroscopy. Irradiation at two different wavelengths (330 or 267 nm) prepares NMP­(S<sub>1</sub>) molecules with very different levels of vibrational excitation, which rapidly relax to low vibrational levels of the S<sub>1</sub> state. Internal conversion with an associated time constant of 110(4) ps, leading to reformation of NMP­(S<sub>0</sub>) molecules, is identified as the dominant (>90%) decay pathway. Much of the remaining fraction undergoes a photoinitiated rearrangement to yield two ketenes (revealed by their characteristic antisymmetric CCO stretching modes at 2110 and 2120 cm<sup>–1</sup>), which are in equilibrium. The rate of ketene formation is found to be pump-wavelength dependent, consistent with ab initio electronic structure calculations which predict a barrier on the S<sub>1</sub> potential energy surface en route to a prefulvenic conical intersection, by which isomerization is deduced to occur. Two kinetic modelsdifferentiated by whether product branching occurs in the S<sub>1</sub> or S<sub>0</sub> electronic statesare presented and used with equal success in the analysis of the experimental data, highlighting the difficulties associated with deducing unambiguous mechanistic information from kinetic data alone.