A Study of the Reactions of Molecular Fluorine with CH3SCH3 and CH3SSCH3 with UV Photoelectron Spectroscopy: First Observation of the HFCS Molecule

Abstract

The title reactions have been investigated in the gas phase using UV photoelectron spectroscopy. By recording spectra at different reaction times, it was possible to detect reaction intermediates and hence identify pathways by which each reaction proceeds. The F2 + CH3SCH3 reaction appears to proceed via the intermediate CH3-SFCH3 whereas the F2 + CH3SSCH3 reaction gives H2CS and HF as the first observed products. For both reactions at longer reaction times, a new spectrum associated with a reaction product, consisting of three photoelectron bands with adiabatic ionization energies of 10.15 f 0.01, 11.49 f 0.02, and 13.71 0.01 eV, has been observed. This spectrum is assigned to the HFCS molecule. The first two adiabatic ionization energies of HFCS have been calculated using the Gaussian-2 (G2) theoretical procedure as 10.09 and 1 1.5 1 eV, respectively, supporting this assignment. This work represents the first observation and spectroscopic study of this molecule. A broad band with a vertical ionization energy of 10.58 f 0.06 eV, assigned to the first ionization energy of HCF, has also been observed as a secondary product from both reactions under conditions of excess F2 partial pressure. This assignment is supported by first adiabatic and vertical ionization energies of HCF computed at the G2 level of theory. For each reaction, a reaction mechanism is proposed on the basis of the results obtained.

Dynamics of the F2+CH3SCH3 reaction: A molecule-molecule reaction without entrance barrier

“…A structure corresponding to CH 3 -S͑F͒ -CH 3 is the most reasonable assignment for this mass with the vertical IE calculated to be 7.89 eV. This finding is consistent with an intermediate species observed in a previous flow-tube experiment, 11 in which the vertical IE was measured to be 8.03 eV with photoelectron spectroscopy and tentatively assigned to the above structure.…”

“…10 It has been speculated 10 that this atypical reactivity correlates with the stability of the ion pair ͓CH 3 SCH 3 ͔ + ͓F 2 ͔ − . Although several products have been investigated in bulk by mass spectrometry, infrared emission spectroscopy, 10 and photoelectron spectroscopy, 11 it is hard to deduce the reaction mechanism due to difficulty of identifying the primary products in the multiple-collision experiments. Moreover, in the absence of high level quantum calculations, the nature of the unusual reactivity remains elusive.…”

Barrierless reactions between two closed-shell molecules. II. Dynamics of F2+CH3SSCH3 reaction

“…Other products such as H 2 CS, H 2 CCS, CH 3 SH, HFCS, and HCF were observed for longer reaction times. Although many products were observed in those bulk experiments, 12,13 most of them are actually secondary-or even tertiaryproducts formed in multiple steps of reactions. As mentioned in Ref.…”

“…Turnipseed and Birks also detected a product with a mass-to-charge ratio of 80 amu/ e, which is likely to be the CH 2 SFCH 3 product ͑channel I͒ observed in our study. 2 Baker et al 13 studied the same reaction by vacuum UV photoelectron spectroscopy under similar flow tube conditions, in which an intermediate species was detected in early reaction times with a vertical ionization energy of 8.03Ϯ 0.02 eV and tentatively assigned to CH 3 SFCH 3 . This observation is consistent with our results for channel II.…”

Dynamics of reactions between two closed-shell molecules

“…Quite a few secondary products have been observed in the flow tube experiments. 37,39 With the crossed molecular beam technique, two primary product channels have been unambiguously identified: 26,27…”