(ππ*/πσ*) Conical Intersection Seam Experimentally Observed in the S–D Bond Dissociation Reaction of Thiophenol-d₁

Abstract

Surface crossing of bound (S 1 , ππ*) and continuum (S 2 , πσ*) states has been observed in the ultrafast S−D bond dissociation reaction of thiophenol-d 1 . It is manifested by an unanticipated variation of fragment angular distribution as a function of the excitation energy. The anisotropy parameter (β) of +0.25 at the S 1 origin decreases to −0.60 at ∼600 cm −1 above the S 1 zero-point level, giving a broad peak in β with a bandwidth of ∼200 cm −1 . The peak in β is ascribed to the in-plane S-D bending mode excitation by which the nuclear configuration in the proximity of the S 1 /S 2 conical intersection seam is directly accessed, showing a mixed character of parallel (S 1 −S 0 ) and perpendicular (S 2 −S 0 ) transition dipole moments at the same time. As a result, the dynamic aspect of the conical intersection is experimentally revealed here through direct access to the nuclear configuration on the multidimensional conical intersection seam. C hemical reactions are often described as nuclear movements of which directions and speeds are determined by shapes of multidimensional electronic potential energy surfaces. This adiabatic picture is based on the seminal Born− Oppenheimer approximation that assumes that the nuclear motion is separable in time from the much faster electronic motion. 1 The Born−Oppenheimer approximation has been enormously successful in describing chemical reactions on the ground electronic state. For chemical reactions on the electronically excited states, however, coupling of electronic and nuclear motions becomes nontrivial, and significant nonadiabatic transitions where nuclear motions induce surface-hopping (and vice versa) occur in many circumstances. 2 Nonadiabatic transitions are ubiquitous in nature and even essential in a number of important chemical and biological processes. 3−6 Notably, for polyatomic molecules, the noncrossing rule does not strictly apply and a conical intersection point is generated by intersection of two potential curves on the 2D branching plane. In general, the conical intersection lies on the (3N−8) dimensional seam, where (3N−6) is the number of internal degrees of freedom of N atomic molecular system. 7−9 As the energy gap between two adiabatic potential surfaces becomes infinitesimally small in the proximity of conical intersections, nonadiabatic transitions are facilitated when the reactive flux or wavepacket reaches the conical intersection region during the chemical reaction process. Most dynamic outputs of nonadiabatic transitions are then dictated by the nature of the conical intersections, and it would be extremely valuable if one could figure out the structure and dynamic role of conical intersections.In this regard, there have been numerous theoretical and experimental studies on conical intersection dynamics in recent decades. Theoretical descriptions of conical intersections have been very successful in explaining many important nonadiabatic chemical processes. These include photochemical organic reactions, 10 photoisomerization, 11,12 the ra...

Born–Oppenheimer approximation in optical cavities: from success to breakdown

“…[2][3][4][5][6][7] Nonadiabatic phenomena are ubiquitous in photochemistry, photophysics, particularly in molecular fragmentation, proton transfer, isomerization or radiationless deactivation processes of excited states as the CI can provide a very efficient channel for ultrafast interstate crossing on the femtosecond timescale. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] Conical intersections and avoided crossings (ACs) can be created both by classical and quantum light, as well. To form a CI, the molecule must have at least two independent nuclear dofs.…”

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

“…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.…”

A comparative multi‐reference configuration interaction study of the low‐lying states of two thione isomers of thiophenol

“…Although there is some contribution of the n orbital (perpendicular to the π system of the ring) to the σ SH and σ SC orbitals of thiophenol, [24] configurations containing excitations from these orbitals are absent in the lowest four excited states of thiophenol. [23][24][25][26][27] On the other hand, in 25 (and also in 24, as discussed later) the n orbital is very well localized (as shown in the supporting information). While in thiophenol S 1 is a ππ* state, in 25 it is an nπ* state (see Table 3).…”

“…An important difference between the results obtained for thione isomers 24 and 25 and those obtained for the thiol isomer (thiophenol) is the absence of configurations formed by excitations to the σ* orbital, a feature observed for the S 2 state of thiophenol. [23][24][25][26][27] However, such difference is expected, as in thiophenol this orbital is localized on a considerably weaker (S-H) bond.…”