Selective bond fission in methyl mercaptan at 193 nm via radial derivative coupling between the 2 1A″ and 1 1A″ adiabatic electronic states

Abstract

We investigate the origin of the observed fission of the stronger S–H bond over the weaker C–S bond in CH3SH excited at 193 nm using the complementary techniques of mass-resolved photofragment time-of-flight spectroscopy and emission spectroscopy. The velocities and angular distributions of the CH3S and SH photofragments show that both C–S and S–H bond fission occur on a subpicosecond timescale and impart considerable energy to relative product translation. The dispersed emission from photoexcited CH3SH molecules in a flow cell evidences a progression in the CH3 umbrella mode and combination bands with one quantum in the C–S stretch, but no progression with S–H stretch. Examination of the results with reference to previous ab initio calculations of the excited state surfaces reveals the importance of nonadiabatic coupling in the dissociation dynamics. This is a clear example of selective bond fission upon excitation of an electronic state that is not repulsive in the bond that breaks. We discuss the implication of the work with respect to using the Born–Oppenheimer approximation in reactive collisions near a saddle point along the reaction coordinate.

Imaging CH3SH photodissociation at 204 nm: the SH + CH3 channel

“…The result is consistent with previous theoretical and experimental work. 4,5,8,[15][16][17] For the inner part of the v 2 = 0 image, a set of finely structured rings were observed as shown in Fig. 2A and 1A.…”

“…The highly anisotropic photofragment angular distributions measured in crossed laser-molecular beam experiments 4,5 show that both C-S and S-H fission processes occur on a sub-picosecond time scale upon excitation in both the first and second absorption bands. Using H Rydberg atom photofragment translational spectroscopy, Wittig and co-workers 7 confirmed that the partner CH 3 S fragments formed in the 248 nm photolysis of CH 3 SH are vibrationally cold, while those formed as a result of photolysis at 193 nm carry high (up to v = 17) levels of C-S stretching excitation.…”

“…The strong enhancement of internal energy excitation observed to occur between 213.8 and 184.9 nm may indicate that another electronic excited state (2 1 A 0 0 ) becomes important and leads to highly excited methylthio (CH 3 S) radical. Using a conventional photofragment translational spectrometer and mass spectrometric techniques, Butler et al [4][5][6] have investigated methyl mercaptan photodissociation at several standard excimer laser wavelengths (248, 222, and 193 nm). Their results reveal that the C-S rupture starts to become significant at photolysis wavelengths below B220 nm, assigned to excitation to the second excited singlet state (the 2 1 A 0 0 state) of CH 3 SH.…”

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).…”

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.…”