Factors Affecting the Branching Ratio of Photodissociation: Thiophenol Studied through Quantum Wavepacket Dynamics

Abstract

The photodissociation dynamics of thiophenol (PhSH) excited to the 1(1) ππ* state was investigated by time-dependent quantum wavepacket propagation within two-dimensional (2D) space consisting of the S-H bond and -SH torsion. We systematically studied the dependence of the branching ratio (Ã/X(~)) between the two electronic states of the phenylthiyl radical (PhS(.) ) on several factors of the 2D potential energy surfaces (PESs). The effect of a reduced initial barrier to the first ππ*/πσ* conical intersection (CI) was found to be marginal, whereas the effects of a reduced torsional barrier of -SH on the excited ππ* state and the mitigated slope of the πσ* PES between the first (ππ*/πσ*) and the second (πσ*/S0 ) CIs were noticeable. The effect of the slope on the branching ratio has never been previously noticed. It was shown that the branching ratio can be sufficiently above unity without pre-excitation of the torsion mode of -SH, which has been assumed so far.

The near ultraviolet photodissociation dynamics of 2- and 3-substituted thiophenols: Geometric vs. electronic structure effects

“…The spectrum is most logically interpreted by assuming unintended photoexcitation of an overlapping resonance associated with syn-2-FPhSH(v 33 = 1) molecules. Parent mode ν 33 is a low frequency, out-of-plane torsional motion, and the present data provide tantalising hints in support of recent theoretical predictions 23,28,29 that such outof-plane motions in molecules of this type should influence the non-adiabatic coupling in the vicinity of CI-2 and the eventual electronic branching in the radical products.…”

“…Such fragmentation dynamics have been demonstrated in phenol [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] and thiophenol, [24][25][26][27][28][29][30] in a range of substituted phenols [31][32][33][34][35][36] and thiophenols, [37][38][39] and in the methylated analogues anisole 40,41 and thioanisole, [42][43][44][45][46] in both the gas and condensed [47][48][49][50] phase. The present study focusses on thiophenols and, particularly, how the much-studied S-H bond fission process is affected by asymmetric substitution (i.e., in the 2-and 3-positions) of the aromatic ring.…”

Tunneling dynamics dictated by the multidimensional conical intersection seam in the πσ*‐mediated photochemistry of heteroaromatic molecules

“…This indicates that the quite large portion of the reactive flux funnels through the conical intersection, disobeying the Born–Oppenheimer approximation. On the two‐dimensional branching plane of the conical intersection (for both S 1 /S 2 and S 0 /S 2 ), the gradient vector is along the S  H(D) extension coordinate whereas the most influential coupling vector is parallel to the C 2  C 1  S  H(D) dihedral torsional angle ( ϕ ) 81–83,87–90 . In other words, when ϕ ≈ 0° (planar), the probability of the reactive flux to funnel through the conical intersection is expected to be high whereas it should be significantly reduced as the molecule becomes nonplanar with the increase of the torsional angle ( ϕ ≠ 0°).…”

“…The πσ*‐mediated photochemistry, in this aspect, is quite unique as it gives the rational explanation for the ultrafast nonradiative transitions of many heteroaromatic molecular systems upon the electronic excitations, which are often relevant to the photochemical or photobiological activities 6,19–38 . As a matter of fact, spectroscopic and dynamic studies on heteroaromatic systems including phenols, 9,12,39–75 thiophenols, 10–12,76–97 anisoles, 98–102 thioanisoles, 5,103–115 or many others are vast in terms of their diversities and detailed dynamic features. Herein, we focus on the H‐atom detachment reactions of phenols and thiophenols especially in terms of the state‐specific tunneling rate constants.…”

“… 4,12,59,76 Nonadiabatic transition at the second S 0 /S 2 conical intersection gives rise to the ground state of the (thio‐)phenoxyl radical whereas the excited state of the (thio‐)phenoxyl radical is generated by the adiabatic pathway. The nonadiabatic transition probability at the S 0 /S 2 conical intersection could be then precisely estimated experimentally by measuring the branching ratio of these pathways from translational energy distribution of the H‐fragment, 10–12,76–78,83 providing the great opportunity to understand and control the conical‐intersection dynamics in the state and structure specific ways 11,77,80–83,86,93–95 . The rate of the whole reaction though is governed by that of the tunneling process through the barrier which is dynamically shaped by the S 1 /S 2 conical intersection 54,96,116–121 .…”