Reaction enthalpy and reaction volume changes upon photoenolization: 2-methylbenzophenone

The transient grating (TG) signal after photoexcitation of 2-methylbenzophenone is fully analyzed in detail in terms of the thermal contribution by the nonradiative transition from the excited states, enthalpy change (Delta H) by the photoisomerization from the cis-and trans-enol forms to the keto form, molecular volume change (Delta V) by the reaction, and population grating contribution due to the absorption change. From a quantitative measurement of the TG intensity combined with that of the photoacoustic intensity, Delta H and Delta V are determined simultaneously without changing any external properties such as solvents, pressure or temperature. The intermolecular hydrogen bonding species (trans-enol) diffuses more slowly than the keto form, which provides clear evidence of the enol formation. The molecular volume of the enol form is smaller than that of the keto form, and the contraction is explained by the intermolecular hydrogen bonding. The obtained physical quantities, Delta H, Delta V, and diffusion constant of the keto and enol forms, are discussed relating to the molecular structure.