Dissociation dynamics of CH3SH at 222, 248, and 193 nm: An analog for probing nonadiabaticity in the transition state region of bimolecular reactions

Abstract

CH3SH ultraviolet absorption cross sections in the region 192.5-309.5 nm and photodecomposition at 222 and 193 nm and 296 K A crossed lasermolecular beam study of the one and two photon dissociation dynamics of ferrocene at 193 and 248 nm These experiments use molecular photodissociation of CH 3 SH to probe the dynamics and the influence of nonadiabatic coupling in the transition state region of the CH 3 +SH--+CH 3 S+H reaction. Photoexcitation at 222 and 248 nm in the first of two absorption bands accesses the lower of the two coupled potential energy surfaces near the saddle point of the excited state reaction coordinate. Measurement of the resulting photofragments' velocities and angular distributions determine the branching between the CH 3 +SH and the CH 3 S+H exit channels. At all wavelengths within the first absorption band, we observe preferential fission of the stronger S-H bond over the weaker C-S bond. Fission of the C-S bond occurs only to a small degree at 222 nm and is not observable at 248 nm. Comparison with our earlier data at 193 nm, corresponding to excitation to the upper bound adiabat which is nonadiabatically coupled to the lower dissociative surface reached at 222 nm, shows that the branching ratio between C-S bond fission and S-H bond fission is a factor of eight larger at 193 nm. To probe the forces in the Franck-Condon region, we also measure the photoemission spectrum from dissociating CH 3 SH excited at 222 nm and compare it to the previous measurement at 193 nm. The 222 nm spectrum evidences emission into the S-H stretch and methyl stretch vibrations but not into C-S stretching modes, consistent with the dominance of S-H fission on the lower adiabat, while the 193 nm emission spectrum, reassigned here, has only a progression in the C-S stretch. The comparison of the spectra suggests a model in which stretching along the C-S coordinate on the bound upper state occurs as the amplitude couples nonadiabatically to the lower dissociative surface, allowing the molecule to access the region near the saddle point on the lower surface at extended C-S bond lengths. This results in better overlap with the C-S fission exit channel and thus an increased branching to C-S bond fission over that observed upon direct excitation to the lower dissociative surface at 222 nm. To further advance the experimental conclusions, we present collaborative calculations of the potential energy surfaces using the effective valence-shell Hamiltonian method developed by Freed and co-workers.

Chemical Reaction Dynamics Beyond the Born-Oppenheimer Approximation

“…Alternatively, when the molecule is excited at 193 nm in the higher energy absorption band, it is promoted to the upper bound adiabat-the 2 1 A surface-where it dissociates via nonadiabatic coupling to the transition state region of the lower adiabat. Photofragment velocity and angular distribution measurements on CH 3 -S-H at 193 nm show that the nonadiabatic decay to the transition-state region of the lower surface occurs in less than a picosecond and results in a factor of eight larger branching for decay of the transition state complex to the CH 3 + SH exit channel than direct excitation to the lower adiabat at 222 nm (90)(91)(92). The larger branching ratio results from the stretching of the C-S bond on the upper adiabat, evidenced in emission spectroscopy experiments (93).…”

“…One can do just that with the excited state reaction CH 3 S + H → CH 3 + SH (90). As depicted schematically in Figure 13, excitation of CH 3 SH at 222 nm offers Franck-Condon access to a region near the transition state on the lower adiabatic potential energy surface-the 1 1 A surface-for the excited state bimolecular reaction, since the geometry of CH 3 SH in the ground electronic state is close to that of the transition state on the 1 1 A surface.…”

Photochemistry of XCH2CN (X = −Cl, −SH) in Argon Matrices

“…Kawaguchi et al identified HCCNC 44 and later the same year HNCCC 45 in Taurus Molecular Cloud 1 (TMC-1). Gensheimer et al 46 Photolysis of thiol species of the form R-SH results mainly in scission of C-S and S-H bonds 52,53,54 with the main products being radicals and, depending on photolysis conditions, various radical recombination products. In 195 nm flash photolysis experiments on gaseous CH 3 SH Callear et al 52 Hydrogen atom production and escape of H 2 from the matrix cage has been reported as well, producing even more complex, dehydrogenated species, where photolysis of methanethiol at 121nm in a nitrogen matrix at 14 K leads to formation of CH 4 , CS, CS 2 , CH 3 and H 2 CS as the main products 58 .…”

A velocity map ion-imaging study on C2H5SH and C3H7SH photodissociation at 243 nm

“…The UV photodissociation dynamics of CH 3 SH has been extensively investigated previously [2][3][4][5][6][7][8][9]. Experimental and theoretical results indicate that photodissociation of CH 3 SH in the UV region occurs primarily through the direct fission of the S-H bond via a perpendicular electronic excitation.…”

“…Experimental and theoretical results indicate that photodissociation of CH 3 SH in the UV region occurs primarily through the direct fission of the S-H bond via a perpendicular electronic excitation. Even though the UV photodissociation dynamics of CH 3 SH has been well understood, photodissociation of CH 3 CH 2 SH and C 3 H 7 SH (both CH 3 CH 2 CH 3 SH and CH 3 CHðSHÞ CH 3 ) have received little attention. In this work, the velocity map ion-imaging technique was employed to study the H-atom elimination from the above three compounds at the photolysis wavelength of 243 nm, at which the radiation in the low atmospheric region from the Sun is still significant.…”