(ππ*/πσ*) Conical Intersection Seam Experimentally Observed in the S–D Bond Dissociation Reaction of Thiophenol‑<i>d</i>₁

Surface crossing of bound (S₁, ππ*) and continuum (S₂, πσ*) states has been observed in the ultrafast S–D bond dissociation reaction of thiophenol-<i>d</i>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 S₁ origin decreases to −0.60 at ∼600 cm^–1 above the S₁ 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 S₁/S₂ conical intersection seam is directly accessed, showing a mixed character of parallel (S₁–S₀) and perpendicular (S₂–S₀) 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

Dynamic Role of the Intramolecular Hydrogen Bonding in Nonadiabatic Chemistry Revealed in the UV Photodissociation Reactions of 2‑Fluorothiophenol and 2‑Chlorothiophenol

The dynamic interplay between the intramolecular hydrogen bonding and intramolecular vibrational redistribution is found to be critical in nonadiabatic reaction dynamics. Herein, it has been demonstrated that the molecular planarity, directed by the intramolecular hydrogen bonding, plays an important role in the nonadiabatic passage of the reactive flux at the conical intersection in the photodissociation reactions of 2-fluorothiophenol and 2-chlorothiophenol. As the internal energy increases in the excited state, the intramolecular hydrogen bonding of 2-fluorothiophenol loosens. The floppiness brought into the molecular structure then modifies the dynamic path of the reactive flux, leading to the diminishment of the nonadiabatic transition probability at the conical intersection. On the contrary, for 2-chlorothiophenol having the relatively stronger intramolecular hydrogen bonding, the reactive flux seems to retain the molecular planarity even with the increase of the internal energy as manifested by the constant nonadiabatic transition probability over the wide range of the S₁ internal energy. The effect of the intramolecular hydrogen bonding on the molecular structure and its relation to the nonadiabatic dynamics along the tunneling path has been experimentally demonstrated

Spatial Isolation of Conformational Isomers of Hydroquinone and Its Water Cluster Using the Stark Deflector

Conformational isomers of hydroquinone and their 1:1 clusters with water have been spatially separated using a Stark deflector in a supersonic jet. <i>trans</i>-Hydroquinone (HyQ) conformer with zero dipole moment is little influenced by inhomogeneous electric fields, whereas <i>cis</i> conformer with nonzero dipole moment (2.38 D) is significantly deflected from the molecular beam axis into the direction along which the strong field gradient is applied. Resonant two photon ionization carried out by shifting the laser position perpendicular to the molecular beam axis after the Stark deflector then gives an exclusive S₁–S₀ excitation spectrum of the <i>cis</i> conformer only, making possible immaculate conformer-specific spectroscopy and dynamics. As the spatial separation is apparently proportional to the effective dipole moment strength, conformational assignment could be absolute in the Stark deflector, which contrasts with the hole-burning spectroscopic technique where identification of a conformational isomer is intrinsically not unambiguous. <i>trans</i>- and <i>cis</i>-HyQ–H₂O clusters have also been spatially separated according to their distinct effective dipole moment strengths to give absolute spectroscopic identification of each cluster isomer, nailing down the otherwise disputable conformational assignment. This is the first report for the spatial separation of conformational cluster isomers