Energetics and dissociation pathways of dimethyl disulfide and dimethyl diselenide using photoelectron photoion coincidence spectroscopy

Internal energy selected dimethyl disulfide and dimethyl diselenide cations were prepared by vacuum ultraviolet threshold photoionization in Imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy experiments. XH-, CH3- and CHnX-loss reactions (n = 2-4, X = S, Se) were observed in both samples with varying branching ratios. SH loss from dimethyl disulfide, DMDS, and SeH loss from dimethyl diselenide were both found to be slow at threshold, and proceed through a tight transition state. By modeling the breakdown diagram and the ion time-of-flight distributions to extract unimolecular dissociation rates to account for kinetic shifts, we obtained a new, significantly revised 0 K SH-loss CH3SCH2+ appearance energy. At slightly higher energies, CHnX+ (n = 2-4) fragments are observed, still in the metastable energy range of the parent ion. Later, CH3-loss outcompetes the lower energy channels and becomes dominant. At yet higher energies, the CH3-loss fragment ion, probably CH3X2+, forms CHX+ by H2X abstraction. The newly obtained 0 K appearance energies are used in the ion cycle to discuss the heats of formation of CH3SCH2+, CH3S2+, CH2S+, C2H5Se+, and CH3Se2+.