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

Abstract

The πσ*‐mediated photochemistry of heteroaromatic molecules has provoked the investigation of the conical intersection dynamics. The Born–Oppenheimer approximation fails at the conical intersection where the S1 (ππ*) and S2 (πσ*) states cross. The nonadiabatic transitions are much influenced by the nuclear configuration of the reactive flux particularly in the curve‐crossing region encountered along the reaction pathway. In this article, we focus on the tunneling dynamics of phenols and thiophenols. The O (S)H bond cleavage occurs via tunneling through the barrier which is dynamically shaped by the upper‐lying S1/S2 conical intersection in terms of the couplings at the individual branching planes as well as along the (3N‐8) dimensional seam coordinates. State‐specific tunneling rates and their interpretation are given for phenol, substituted phenols, thiophenol, ortho‐substituted thiophenols, and benzenediols including their 1:1 water clusters. The completely orthogonal modes to the tunneling coordinate are very critical in the dynamic shaping of the reaction barrier.

Mode-dependent H atom tunneling dynamics of the S1 phenol is resolved by the simple topographic view of the potential energy surfaces along the conical intersection seam

“…[1][2][3][4][5] Accordingly, a number of model systems including a variety of chemical derivatives of phenols, thiophenols, anisoles, pyrroles, or thioanisoles have been tackled for interrogating the nonadiabatic dynamics taking place in the vicinity of the conical intersections encountered along the relaxation and/or reaction pathways. [6][7][8][9][10] Among those, the H atom tunneling dynamics of the S 1 phenol is one of the most-studied systems. Rydberg-tagging H atom translational spectroscopy on the O-H bond predissociation of the S 1 phenol by the Ashfold et al has provided the very detailed energy disposal dynamics resulting from the nonadiabatic couplings among the close-lying S 0 , S 1 (ππ * ) and S 2 (πσ * ) states.…”

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

“…Thereafter, the nonadiabatic transition especially in the proximity of the conical intersection occurs with the significantly high probability to govern the overall dynamic outputs such as reaction rates, product yields, branching ratios, or energy disposals. − The nonadiabatic transition probability is extremely sensitive to the nature of the reactive flux with respect to the electronic/nuclear configurations at the conical intersection, and thus its theoretical prediction (or the explanation of the experiment) has been quite challenging especially for polyatomic molecular systems. In this aspect, the πσ*-mediated photochemistry of the heteroaromatic molecular system − has provided the nice platform for many recent years not only for elucidating the mechanism of the ultrafast nonradiative transitions frequently found in biological building blocks but also for the thorough understanding of the conical intersection dynamics. − Among the systems of interest, predissociation dynamics of thioanisole is particularly notable. − Therein, the S 1 /S 2 and S 0 /S 2 conical intersections are encountered along the S–CH 3 bond extension coordinate. The former is close to the vertical transition region whereas the latter is located at the later stage of the reaction.…”

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 πσ*-mediated photochemistry of the heteroaromatic molecular system − has long been spotlighted not only because it gives the mechanistic clue for the ultrafast excited state relaxation of biological building blocks carrying the genetic code but also because it is extremely helpful to elucidate the nonadiabatic transition dynamics in the vicinity of the conical intersections. − The Born–Oppenheimer approximation breaks down when the conical intersections are encountered along the passage of the reactive flux, giving the nonstraightforward dynamic outputs of reaction rates, product yields, or branching ratios. − From the dynamic point of view, the πσ*-mediated chemistry of the heteroaromatic system has provided the ideal model for investigating the predissociation dynamics (Herzberg type-I or -II) − and/or tunneling dynamics. 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.…”