Dissociation of CH₃SH⁺by Collisional Activation:  Evidence of Nonstatistical Behavior

Abstract

We have measured the absolute total cross sections for CH2SH+(CH3S+), CH2S+, HCS+, HS+, CH3 +, and CH2 + produced by the collision-induced dissociation (CID) reaction of CH3SH+(12A‘‘) + Ar in the center-of-mass collision energy range of 1−36 eV. While the onset for CH3 + is consistent with the thermochemical threshold for the formation of CH3 + + SH, the onsets for other product ions are higher than their corresponding thermochemical thresholds. Using a charge transfer probing technique, we conclude that the m/e = 47 amu ions observed in the CID reaction have mostly the CH2SH+ structure. The relative yields for CH2SH+, CH2S+, HCS+, HS+, CH3 +, and CH2 + formed in the CID reaction, which strongly favor the C−S bond scission process leading to the formation of CH3 + + SH, are significantly different from those measured in previous photoionization and charge exchange studies. Since the CH3 + + SH channel is not among the most stable product channels, this observation suggests that the collision-activated dissociation of CH3SH+ is nonstatistical. The high yield for CH3 + + SH observed in CID is attributed to the more efficient translational to vibrational energy transfer for the C−S stretch than for the C−H stretches of CH3SH+, and to weak couplings between the low-frequency C−S and the high-frequency C−H stretching vibrational modes of CH3SH+. The differences in excitation mechanisms for CH3SH+ via collision activation, photoionization, and charge exchange are responsible for the different fragment ion distributions from CH3SH+ observed in these experiments.

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

“…These assumptions lead to the conclusion that the most favorable product corresponds to the most stable product channel provided that a tight transition structure is not involved. As pointed out above, the formation of CH 2 In the E c.m. range of this CID experiment, the collisions are most efficient for the promotion of translation-vibration and translation-rotation energy exchanges.…”

“…is increased from 6 eV to 42 eV. The decrease of the relative abundance for C 2 H 5 ϩ appears to coincide with the increase in the relative abundance for C 2 are significantly higher than the endothermicities of the dissociation reactions ͑3͒-͑12͒.…”

“…2 The latter experiment can be taken as a validation of the charge transfer probing technique used here for the structural identification of the mass 47 ions ͑CH 2 SH ϩ or CH 3 S ϩ ͒ formed in the PI and CID reactions. The data acquisition for the TQDO apparatus has recently been upgraded to be controlled by a Pentium PC system.…”

“…First, the CH 3 CHSH ϩ ion observed in the PI of CH 3 CH 2 SH is not found in the CID study. The other difference is that the relative abundance curves for C 2 2 SH, which involve the respective cleavage of a single C-H, C-S, and C-C bond, are predicted to have no potential barriers. These theoretical predictions are consistent with the experimental observation that the AE͑CID͒ and ⌬͑PI͒ for these product channels agree with their corresponding thermochemical thresholds.…”

“…In the previous CID study of the CH 3 ϩ resulting from the low energy collision with Ar is predominantly deposited in the C-S stretching mode instead of the C-H or S-H stretching modes of CH 3 SH ϩ . 2 Owing to the large difference in vibrational frequencies between the C-S and C-H ͑S-H͒ stretching modes of CH 3 SH ϩ , the C-S and C-H ͑or S-H͒ stretching modes are only weakly coupled, i.e., the energy flow between the C-S and C-H ͑or S-H͒ vibrational modes of CH 3 SH ϩ is inefficient. As a consequence, the product CH 3 ϩ ion, which results from the C-S bond cleavage, is favored over the product CH 2 SH ϩ ion associated with the cleavage of the H-CH 2 SH ϩ bond.…”