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

Alexander R Davies,Timothy G Wright,David J Kemp

doi:10.1063/5.0032146

Alexander R Davies, Timothy G Wright + Show 1 more

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https://doi.org/10.1063/5.0032146

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Journal: AIP Advances	Publication Date: Dec 1, 2020
Citations: 6	License type: cc-by

Affiliation: University of Nottingham

Abstract

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.

Highlights

Anharmonic coupling in molecules leads to delocalization, and so dispersal, of internal energy within a molecule—an important aspect to enhanced photostability.1–4 Which of the vibrations in a molecule can couple, and to what extent, depends on a number of factors, but having the same symmetry and being close in energy are two key considerations,1 along with the relative motions of the atoms in those vibrations
We have extended the examination of levels, where we see limited interactions are present for levels below 950 cm−1, but significant intramolecular vibrational redistribution (IVR) occurs above this, moving toward the statistical IVR regime; the latter is demonstrated by the presence of a largely unstructured underlying background in both the 2D-laser-induced fluorescence (LIF) and zero-electron-kinetic-energy scitation.org/journal/adv (ZEKE) spectra recorded at ∼960 cm−1 and ∼1260 cm−1
In the case of pXyl, at these energies, most structure was lost in the ZEKE spectra recorded, suggesting almost complete delocalization of energy; these observations were discussed in terms of symmetry and the density of states (DOS)

Summary

Introduction

Anharmonic coupling in molecules leads to delocalization, and so dispersal, of internal energy within a molecule—an important aspect to enhanced photostability. Which of the vibrations in a molecule can couple, and to what extent, depends on a number of factors, but having the same symmetry and being close in energy are two key considerations, along with the relative motions of the atoms in those vibrations. Which of the vibrations in a molecule can couple, and to what extent, depends on a number of factors, but having the same symmetry and being close in energy are two key considerations, along with the relative motions of the atoms in those vibrations Such coupling leads to actual vibrational motions having mixed character, and this can be discerned through an analysis of the vibrational activity in electronic and photoelectron spectra, where vibrational eigenfunctions of one electronic state are projected onto those of another. It was concluded that m-fluorotoluene (mFT) undergoes intramolecular vibrational redistribution (IVR) more than an order of magnitude faster than p-fluorotoluene (pFT) and, that the location of substituents is important in internal energy delocalization Parmenter and coworkers have published numerous studies on the effect of methylation on intramolecular vibrational redistribution (IVR), with the most pertinent study here being that of Timbers et al. In that work, it was concluded that m-fluorotoluene (mFT) undergoes intramolecular vibrational redistribution (IVR) more than an order of magnitude faster than p-fluorotoluene (pFT) and, that the location of substituents is important in internal energy delocalization

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