Contrasting the excited state reaction pathways of phenol and para-methylthiophenol in the gas and liquid phases

Abstract

To explore how the solvent influences primary aspects of bond breaking, the gas and solution phase photochemistries of phenol and ofpara-methylthiophenol are directly compared using, respectively, H (Rydberg) atom photofragment translation spectroscopy and femtosecond transient absorption spectroscopy. Approaches are demonstrated that allow explicit comparisons of the nascent product energy disposals and dissociation mechanisms in the two phases. It is found, at least for the case of the weakly perturbing cyclohexane environment, that most aspects of the primary reaction dynamics of the isolated molecule are reproduced in solution. Specifically, in the gas phase, both molecules can undergo fast X-H (X = O, S) bond dissociation upon excitation with short wavelengths (193 < lambda(pump) < 216 nm), following population of the dissociative S2 (1 1(pi sigma*)) state. Product electronic branching, vibrational and translational energy disposals are determined. Photolysis of phenol and para-methylthiophenol in solution at 200 nm results in formation of vibrationally excited radicals on a timescale shorter than 200 fs. Excitation of para-methylthiophenol at 267 nm reaches close to the S1 (1 1(pipi*))/S2 (11(pi sigma*)) conical intersection (CI): ultrafast dissociation is observed in both the isolated and solution systems-again indicating direct dissociation on the S2 potential energy surface. Comparing results for this precursor at different excitation energies, the extent of geminate recombination and the derived H-atom ejection lengths in the condensed phase photolyses are in qualitative agreement with the translational energy release measured in the gas phase studies. Conversely, excitation of phenol at 267 nm prepares the system in its S1 state at an energy well below its S1/S2 CI; the slow O-H bond fission inferred in the gas phase experiments is observed directly in the time-resolved studies in cyclohexane solution via the appearance of phenoxyl radical absorption after -1 ns, with only S1 excited state absorption discernible at earlier delay times. The slow O-H bond fission in solution provides additional evidence for a tunnelling dissociation mechanism, where the H atom tunnels beneath the lower diabats of the S2/S1 CI. Finally, the photodissociation of phenol clusters in solution is considered, where evidence is presented that the O-H dissociation coordinate is impeded in H-bonded dimers.

Photochemical reaction dynamics of 2,2′-dithiobis(benzothiazole): direct observation of the addition product of an aromatic thiyl radical to an alkene with time-resolved vibrational and electronic absorption spectroscopy

“…Photodissociation studies in the gas phase, transient absorption spectroscopy in the liquid phase, and ab initio calculations for the p-methylthiophenoxy radical generated from photolysis of pmethylthiophenol all suggest that absorption from the à state may contribute to transient spectra of photochemically generated thiyl radicals. [19][20][21][22] Careful inspection of the inset in Figure 3(a) identifies an absorption feature located at around 400 -480 nm at early time delays (~ 100 fs) that may be a candidate. However, this feature has its maximum absorbance at time zero and can be observed only while the pump and probe beams are overlapped in time ( Figure 3(b) inset).…”

“…This difference, when compared to pmethylthiophenoxy, may be associated with the larger energy separation between the à and X states (2.39 eV) for the BS radical. 22 Table 3: Calculated excitation energies and transition dipole moments for the BS radical, obtained at the CASPT2(9/9)/aug-cc-pVTZ and CASSCF(9/9)/aug-cc-pVTZ levels of theory. Further inspection of Figure 3(a) indicates that there is a broad band centered at 440 nm which grows with time.…”

“…17 More recent investigations have concentrated on the generation of aromatic thiyl radicals by photolysis of the corresponding aromatic thiol compounds, using a combination of high level ab initio calculations and experiments. [19][20][21][22] Reactions of thiyl radicals with alkenes or alkynes, known as thiol-ene reactions, have been studied for a long time, 23 but have recently attracted greater attention from synthetic chemists interested in the "Click Chemistry" propounded by Sharpless and coworkers. 11 The objective of Click Chemistry is to establish a methodology for synthesis of functional molecules by simple, highly selective orthogonal reactions under mild conditions.…”

Non-equilibrium reaction and relaxation dynamics in a strongly interacting explicit solvent: F + CD3CN treated with a parallel multi-state EVB model

“…6,[34][35][36][37][38] The last decade or so has seen a number of attempts by a range of workers to efficiently and accurately simulate chemical reaction dynamics in condensed-phase environments using multistate molecular mechanics and multi-state empirical valence bond (MS-EVB) approaches. 19,[39][40][41][42][43][44][45][46][47][48][49] Such approaches represent important molecular configurations using diabatic valence bond states, which are coupled using any of a wide range of schemes.…”

Femtosecond to microsecond observation of the photochemical reaction of 1,2-di(quinolin-2-yl)disulfide with methyl methacrylate

“…5,6 Thiol-ene reactions proceed via reaction steps (i)  (iii), but also potentially include other reaction steps (iv)  (vii), which result in side-products: 6,7 Light irradiation or radical initiators can be used to produce thiyl radicals (step (i)), but the former method involving photodissociation of the corresponding thiol (as shown in (i)) or disulfide is preferred for the study of their mechanisms and dynamics because of the almost instantaneous (sub-picosecond) formation of thiyl radicals. [8][9][10] The photodissociation dynamics producing aromatic thiyl radicals have mostly been deduced from studies using transient electronic absorption spectroscopy (TEAS), [8][9][10][11][12][13][14][15][16][17] because aromatic thiyl radicals show strong absorption bands in the ultraviolet (UV) and visible regions. 6 The mechanisms of addition of thiyl radicals to alkenes (or alkynes), which yield carbon centered radicals (step (ii)), were extensively investigated by Ito et al in the 1980s using a microsecond flash photolysis method, and have been reviewed previously.…”