Ultraviolet Photodissociation Dynamics of 2-Methyl, 3-Furanthiol: Tuning π-Conjugation in Sulfur Substituted Heterocycles

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2010-01, Vol.114 (3), p.1338-1346
• Main Authors: Oliver, Thomas A. A, King, Graeme A, Nix, Michael G. D, Ashfold, Michael N. R
• Format: Article
• Language: English
• Subjects: Computer Simulation; Furans - chemistry; Heterocyclic Compounds - chemistry; Models, Chemical; Photochemistry; Sulfhydryl Compounds - chemistry; Ultraviolet Rays
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp905784f

H atom loss following ultraviolet photoexcitation of 2-methyl, 3-furanthiol (2M,3FT) at many wavelengths in the range 269 nm ≥ λphot ≥ 210 nm and at 193 nm has been investigated by H (Rydberg) atom photofragment translational spectroscopy. The photodissociation dynamics of this SH decorated aromatic ring system are contrasted with that of thiophenol (Devine et al. J. Phys. Chem. A 2008, 112, 9563), the excited electronic states of which show a different energetic ordering. Ab initio theory and experiment find that the first excited state of 2M,3FT is formed by electron promotion from an orbital comprised of an admixture of the S lone pair and the furan π system (n/π) to a σ* orbital centered on the S−H bond. Photoexcitation at long wavelengths results in population of the 1(n/π)σ* excited state, prompt S−H bond fission, H atoms displaying a (nonlimiting) perpendicular recoil velocity distribution, and partner radicals formed in selected low vibrational levels of the ground state. This energy disposal can be rationalized by considering the forces acting as the excited molecules evolve on the 1(n/π)σ* potential energy surface (PES). Energy conservation arguments, together with the product vibrational state analysis, yield a value of 31320 ± 100 cm−1 for the S−H bond strength in 2M,3FT. Excitation at shorter wavelengths (λphot ≤ 230 nm) is deduced to populate one or more (diabatically bound) 1(n/π)π* excited states which decay by coupling to the 1(n/π)σ* PES and/or to high vibrational levels of the electronic ground state.