Abstract
The near ultraviolet spectroscopy and photodissociation dynamics of two families of asymmetrically substituted thiophenols (2- and 3-YPhSH, with Y = F and Me) have been investigated experimentally (by H (Rydberg) atom photofragment translational spectroscopy) and by ab initio electronic structure calculations. Photoexcitation in all cases populates the 11ππ* and/or 11πσ* excited states and results in S-H bond fission. Analyses of the experimentally obtained total kinetic energy release (TKER) spectra yield the respective parent S-H bond strengths, estimates of ΔE(A∼-X∼), the energy splitting between the ground (X∼) and first excited (A∼) states of the resulting 2-(3-)YPhS radicals, and reveal a clear propensity for excitation of the C-S in-plane bending vibration in the radical products. The companion theory highlights roles for both geometric (e.g., steric effects and intramolecular H-bonding) and electronic (i.e., π (resonance) and σ (inductive)) effects in determining the respective parent minimum energy geometries, and the observed substituent and position-dependent trends in S-H bond strength and ΔE(A∼-X∼). 2-FPhSH shows some clear spectroscopic and photophysical differences. Intramolecular H-bonding ensures that most 2-FPhSH molecules exist as the syn rotamer, for which the electronic structure calculations return a substantial barrier to tunnelling from the photoexcited 11ππ* state to the 11πσ* continuum. The 11ππ* ← S0 excitation spectrum of syn-2-FPhSH thus exhibits resolved vibronic structure, enabling photolysis studies with a greater parent state selectivity. Structure apparent in the TKER spectrum of the H + 2-FPhS products formed when exciting at the 11ππ* ← S0 origin is interpreted by assuming unintended photoexcitation of an overlapping resonance associated with syn-2-FPhSH(v33 = 1) molecules. The present data offer tantalising hints that such out-of-plane motion influences non-adiabatic coupling in the vicinity of a conical intersection (between the 11πσ* and ground state potentials at extended S-H bond lengths) and thus the electronic branching in the eventual radical products.
Highlights
Fragmentation on an nσ* or πσ* state potential energy surface (PES) is recognised as a competitive non-radiative decay path following near ultraviolet (UV) photoexcitation of many heteroaromatic and heteroatom containing aromatic molecules, e.g., azoles, phenols, thiophenols, and their alkylated analogues.1–7 Such states can be populated directly, when photoexcitation promotes an electron from the highest occupied n or π orbital to the σ* orbital
Using a combination of experiment and theory, we explore the extent to which the well-documented S–H bond fission dynamics following near UV photoexcitation of PhSH are influenced by the presence of a weakly perturbing (Me group) and an electron donating substituent in both the 2- and 3-positions
Room temperature UV absorption spectra were measured for each sample, and attempts were made to record jet-cooled, 1+1 resonance multiphoton ionisation (REMPI) spectra of the respective parent molecules in the gas phase at wavelengths around the long wavelength onset revealed by the absorption data
Summary
Fragmentation on an nσ* or πσ* state potential energy surface (PES) is recognised as a competitive non-radiative decay path following near ultraviolet (UV) photoexcitation of many heteroaromatic and heteroatom containing aromatic molecules, e.g., azoles, phenols, thiophenols, and their alkylated analogues. Such states can be populated directly, when photoexcitation promotes an electron from the highest occupied n (lone pair) or π orbital to the σ* orbital. Fragmentation on an nσ* or πσ* state potential energy surface (PES) is recognised as a competitive non-radiative decay path following near ultraviolet (UV) photoexcitation of many heteroaromatic and heteroatom containing aromatic molecules, e.g., azoles, phenols, thiophenols, and their alkylated analogues.. Fragmentation on an nσ* or πσ* state potential energy surface (PES) is recognised as a competitive non-radiative decay path following near ultraviolet (UV) photoexcitation of many heteroaromatic and heteroatom containing aromatic molecules, e.g., azoles, phenols, thiophenols, and their alkylated analogues.1–7 Such states can be populated directly, when photoexcitation promotes an electron from the highest occupied n (lone pair) or π orbital to the σ* orbital. The net outcome can still be similar, if there a)Present address: Department of Chemistry, University of Pennsylvania, Philadelphia, PA, USA. Whether the 1πσ* state is populated directly or indirectly, the typical outcome in a gas phase experiment is fission of the bond along which the σ*
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