Methyl-torsion-facilitated internal energy delocalization following electronic excitation in m-fluorotoluene: Can meta and para substitution be directly compared?

Coupling between vibrations, and between vibrations and torsions—a generalization of intramolecular vibrational redistribution (IVR)—provides routes to internal energy delocalization, which can stabilize molecules following photoexcitation. Following earlier work on p-fluorotoluene (pFT), this study focuses on m-fluorotoluene (mFT) as probed via the S1 ↔ S0 electronic transitions and the D0+ ← S1 ionization, using two-dimensional laser-induced fluorescence and zero-electron-kinetic energy spectroscopy, respectively. Wavenumbers are reported for a number of vibrations in the S0, S1, and D0+ states and found to compare well to those calculated. In addition, features are seen in the mFT spectra, not commented on in previous studies, which can be assigned to transitions involving vibration–torsion (“vibtor”) levels. Comparisons to the previous work on both m-difluorobenzene and mFT are also made, and some earlier assignments are revised. At lower wavenumbers, well-defined interactions between vibrational and vibtor levels are deduced—termed “restricted IVR,” while at higher wavenumbers, such interactions evolve into more-complicated interactions, moving toward the “statistical IVR” regime. It is then concluded that a comparison between mFT and pFT is less straightforward than implied in earlier studies.