A study of the dissociation of CH₃CH₂SH⁺ by collisional activation : evidence of nonstatistical behavior

Abstract

The absolute total cross sections for CH₃CH₂⁺, C₂H₄⁺,C₂H₃⁺,CH₃⁺,CH₂SH⁺(CH₃S⁺), CH₂S⁺(HCSH⁺), CHS⁺(CSH⁺), and H₂S⁺ produced by the collision-induced dissociation (CID) reaction of CH₃CH₂SH⁺+Ar have been measured in the center-of-mass collision energy (E[sub c.m.]) range of 1–42 eV. Using the charge transfer probing technique, we found that the mass 47 product ions have overwhelmingly the CH₂SH⁺ structure. The onsets for CH₃CH₂⁺, C₂H₄⁺, C₂H₃⁺, CH₂SH⁺, H₂S⁺, and CH₃⁺ are consistent with their corresponding thermochemical thresholds. The formation of the higher energy channels CH₃CH₂⁺+sH and CH₃+CH₂SH⁺, which involve the C–S and C–C bond scissions, are found to dominate in the entire E[sub c.m.]) range. The lower energy channel corresponding to the formation of CH₃CHSH⁺+H is not found. The strong preference observed for the formation of the higher energy channels is in accord with the conclusion obtained in the recent CID study of CH₃SH⁺, providing evidence that the CID of CH₃CH₂SH⁺ is also nonstatistical. The high yields of CH₃CH₂⁺+SH and CH₂SH⁺+CH₃are attributed to the more efficient translational to vibrational energy transfer for the low frequencies C–S and C–C stretching modes than for the high frequencies C–H and S–H stretching modes, along with the weak couplings between these low and high frequencies vibrational modes of CH₃CH₂SH⁺. The relative abundances of product ions formed by the single-photon ionization of CH₃CH₂SH were also measured for comparison with the CID results. The CH₃CHSH⁺+H channel is observed in the photoionization of CH₃CH₂SH. Similar to the finding in the photoionization of CH₃SH, the relative abundances of fragment ions formed in the photoionization of CH₃CH₂SH are in qualitative accord with statistical predictions. To rationalize the dissociation mechanisms of CH₃CH₂SH⁺, we have also performed ab initio calculations to locate the possible transition structures for the observed dissociation channels.