Vibronic structures and dynamics of the predissociating dimethyl sulfide and its isotopomers (CH3SCH3, CD3SCD3, CH3SCD3) at the conical intersection

Abstract

Conical intersection seam comprised of crossing surfaces of two lowest excited states of dimethyl sulfide (DMS) has been directly accessed by the one-photon excitation from the ground equilibrium state. Since the S-C bond rupture takes place promptly, the molecular structure on the excited state effectively belongs to C(S) symmetry. Namely, excited states of 1(1)B1 and 1(1)A2 in C(2)V become 1(1)A'' and 2(1)A'' states in C(S), respectively, and the optical transition from the ground equilibrium state to the dissociating molecule at the conical intersection seam is symmetry-allowed to facilitate the nonadiabatic transition on the 2(1)A'' state, leading eventually to the CH3S + CH3 products. The dynamic study of DMS, in this sense, gives the great opportunity to unravel the vibronic structure of the conical intersection seam by the conventional one-photon excitation method. In this work, utilizing the photofragment excitation (PHOFEX) spectroscopic method, the vibronic structures of DMS and its isotope analogs (CD3SCD3, CH3SCD3) at the conical intersection seam have been revealed, providing accurate lifetimes and detailed dynamics associated with individual vibronic transitions. The lifetime of the excited DMS is estimated to be ~100 fs, indicating that the dissociation is complete within one single oscillation in the conical intersection region. It is also found that the symmetric CSC stretching mode is strongly coupled to the reaction coordinate, as manifested by our experimental finding that the fragmentation yield of the S-CD3 bond is enhanced compared to that of the S-CH3 bond in the CH3SCD3 dissociation reaction only when the CSC symmetric stretching vibrational mode is excited at the conical intersection region. This work demonstrates that the better understanding of the excited state could make the bond-selective chemistry into reality.

Quantifying rival bond fission probabilities following photoexcitation: C–S bond fission in t-butylmethylsulfide

“…Spin–orbit coupling, which introduces a small (tens of cm –1 ) energy splitting in the ground state MS radical,55 has not been included in the present calculations. As in the cases of H 2 S27,28 and MSM,19,29 the present calculations find two close-lying and interacting excited states of 1 A′′ symmetry, the lower energy of which correlates to the B–SM and BS–M dissociation limits of current interest. As Table 1 showed, the calculated C–S bond strengths are very similar.…”

“…The right half of each panel shows the corresponding central slice through the (symmetrized) reconstructed 3-D velocity distribution. As in the UV photodissociation of MSM,17–19 the M + image shows one anisotropic ring with large radius. Similar (albeit smaller) anisotropic rings are also evident in the MS + , B + and BS + images.…”

“…Thioethers17–19 and thioanisoles20–24 are popular test-beds for exploring and illustrating the role of non-adiabatic couplings between potential energy surfaces (PESs) in the photodissociation of polyatomic molecules. The UV absorption spectrum of MSM, the prototypical thioether, shows a long wavelength onset at λ ∼ 240 nm, an obvious peak at λ ∼ 228 nm and undulatory structure to shorter wavelengths 25,26.…”

“…en route to S–C bond fission), and excitations to both the 1 1 A′′ and 2 1 A′′ states contribute to the observed absorption. The PESs for these two states show a seam of conical intersection in the region accessed by vertical (or Franck–Condon (FC)) excitation from the X[combining tilde] state,29 which accounts for the prompt S–C bond fission observed following UV photolysis of MSM 16–19. The present work reveals similar fragmentation dynamics following UV excitation of BSM, quantifies the deduced preference for B–SM bond fission, and reports cuts through ab initio PESs for the excited states that offer a (far from complete) rationale for the observed fragmentation dynamics.…”

Combined Quantum Trajectory Mean‐Field and Molecular Mechanical (QTMF/MM) Nonadiabatic Dynamics Simulations on the Photoinduced Ring‐Opening Reaction of 2(5H)‐Thiophenone

“…Photo‐fragmentation dynamics of thio‐substituted heterocyclic compounds have been investigated experimentally, which has been summarized by Ashfold and co‐workers in a recent publication . UV‐induced ring‐opening of 2TP isolated in low temperature argon matrices has been studied by Breda and Fausto, and the photoproducts were identified with the FTIR spectroscopy technique, together with electronic structure calculation.…”