πσ*-Mediated Nonadiabatic Tunneling Dynamics of Thiophenols in S₁: The Semiclassical Approaches

Abstract

The S–H bond tunneling predissociation dynamics of thiophenol and its ortho-substituted derivatives (2-fluorothiophenol, 2-methoxythiophenol, and 2-chlorothiphenol) in S1 (ππ*) where the H atom tunneling is mediated by the nearby S2 (πσ*) state (which is repulsive along the S–H bond extension coordinate) have been investigated in a state-specific way using the picosecond time-resolved pump–probe spectroscopy for the jet-cooled molecules. The effects of the specific vibrational mode excitations and the SH/SD substitutions on the S–H­(D) bond rupture tunneling dynamics have been interrogated, giving deep insights into the multidimensional aspects of the S1/S2 conical intersection, which also shapes the underlying adiabatic tunneling potential energy surfaces (PESs). The semiclassical tunneling rate calculations based on the Wentzel–Kramers–Brillouin (WKB) approximation or Zhu–Nakamura (ZN) theory have been carried out based on the ab initio PESs calculated in the (one, two, or three) reduced dimensions to be compared with the experiment. Though the quantitative experimental results could not be reproduced satisfactorily by the present calculations, the qualitative trends among different molecules in terms of the behavior of the tunneling rate versus the (adiabatic) barrier height or the number of PES dimensions could be rationalized. Most interestingly, the H/D kinetic isotope effect observed in the tunneling rate could be much better explained by the ZN theory compared to the WKB approximation, indicating that the nonadiabatic coupling matrix elements should be invoked for understanding the tunneling dynamics taking place in the proximity of the conical intersection.

Dynamic Role of the Intramolecular Hydrogen Bonding in the S1 State Relaxation Dynamics Revealed by the Direct Measurement of the Mode-Dependent Internal Conversion Rate of 2-Chlorophenol and 2-Chlorothiophenol

“…The abrupt increase of the tunneling rate had also been observed for 2-fluorothiophenol or 2-methoxythiophenol, 49,50 and it was nicely rationalized by the significant lowering of the adiabatic H atom tunneling barrier with respect to the C−C−S−H dihedral out-of-plane torsional coordinate. 50,65,66 Remarkably, the S 0 −S 1 internal conversion rate is also found to have the substantial mode-dependent behavior, giving k IC ∼ 4.5 × 10 9 s −1 at the 10b 2 mode excitation which is ∼3-fold larger than k IC ∼ 1.6 × 10 9 s −1 measured at ZPL. This indicates that the delocalization of the reactive flux along the nonplanar geometries should facilitate the internal conversion process of which the initial path involves the C−Cl bond extension (vide infra).…”

“…Namely, it is far less than the kinetic isotope effect of ∼23 or ∼14 which had been found for 2fluorothiophenol or 2-methoxythiophenol, respectively. 50 The tiny kinetic isotope effect of 2-CTP and 2-CTP-d 1 then strongly suggests that the contribution of the H atom tunneling dissociation to the S 1 state relaxation may not be large enough to account for the whole relaxation process. Instead, the concomitant S 0 −S 1 internal conversion may kinetically compete even at the ZPL.…”

“…71−75 The similar situation has also been found for other heteroaromatic systems with no intramolecular hydrogen bonding such as trans-2-chlorophenol or 3-chlorophenol. Even for 2-fluorophenol 37,54,76,77 or 2fluorothiophenol 47,49,50 where the relatively weak intramolecular hydrogen bonding exists, 78−83 the ultrafast internal conversion such as found in the low S 1 internal energy region of 2-CP and 2-CTP could not be observed. It is intriguing to note, on the other hand, that the S 1 vibronic states of 2chlororesorcinol (Figure S3 of the Supporting Information), 2bromophenol, 53 or 2-bromothiophenol 84,85 are reported to be quite diffuse in the S 1 −S 0 transition spectra due to the homogeneous lifetime broadening effect.…”

“…In particular, the experimental finding that the apparent kinetic isotope effect of the S 1 lifetime ( k H / k D ∼ 2.8) is unusually small may suggest that the internal conversion rate is comparable with the tunneling rate in the low S 1 internal energy region. Namely, if the kinetic isotope effect of the pure tunneling reaction ( k H T / k D T ) is arbitrarily chosen to be ∼20 according to our previous study, then k IC should be ∼1.4 × 10 9 s –1 (τ ∼ 710 ps) according to the above equation. This in turn gives the estimation of k H T ∼ 3.1 × 10 9 s –1 (τ ∼ 330 ps) and k D T ∼ 0.15 × 10 9 s –1 (τ ∼ 6.51 ns) at the ZPL.…”

“…In the predissociation event (e.g., the S–CH 3 bond dissociation of the S 1 thioanisole), − the reactive flux placed in the proximity of the conical intersection either nonadiabatically funnels through the narrowly defined conical intersection or sticks to the adiabatic potential energy surfaces to explore the phase space for riding on the minimum energy reaction path. For the tunneling case, the light atom (H or D) escapes from the S 1 potential well via tunneling through the barrier which is dynamically shaped by the upper-lying conical intersection. − Notably, the tunneling dynamics of the S 1 state has recently been thoroughly investigated in a state-specific way for a number of heteroaromatic molecular systems such as phenol, ,− substituted phenols, − o -cresol, , thiophenol, , 2-fluorothiophenol, ,, 2-methoxythiophenol, − or 2-chlorothiophenol. ,, It has been found that the tunneling dynamics in terms of the tunneling rate (also the energy disposal dynamics or nonadiabatic transition probability in some cases) are quite mode-dependent, and their qualitative behaviors could be rationalized from the dynamic shaping of the tunneling barrier by the upper-lying S 1 (ππ*)/S 2 (πσ*) conical intersection in many cases. ,,, …”