Dissociation pathways for the molecular cation of 3,4-dichloro-1,2,5-thiadiazole: A time-of-flight mass spectrometry and computational study

Rationale 1,2,5-Thiadiazoles are an important class of compounds mostly used in synthetic chemistry, and as herbicides, insecticides, drugs, organic conductors, etc. Recently, they have been used as a source for the generation and study of nitrile N-sulfides, RCNS, and its isomers. In this study, we monitor the fragmentation pattern of ionic halogenated 1,2,5-thiadiazoles, namely, 3,4-dichloro-1,2,5-thiadiazole, which generates the nitrile sulfides, to establish its various dissociation mechanisms. Methods The molecular cation of 3,4-dichloro-1,2,5-thiadiazole was prepared using multiphoton excitation using a laser at 235 nm. Various product ions upon fragmentation of the molecular ion were mass analyzed using time-of-flight mass spectrometry. Laser power dependence studies were conducted for various product ions to arrive at the dissociation mechanism. Theoretical calculations were performed for the estimation of the ΔH values for various reactions to support the experimental data. Results The most abundant product ion was observed to be the NS+ radical cation followed by the S+ ion and the SCl+ radical cation. The other product ions such as the CNS+ radical cation and the ClCNS+ and ClCN+ cations were also observed to a lesser extent in the fragmentation pattern of the parent molecular ion. Various dissociation channels were identified and supported with ab initio calculations. Conclusions In conclusion, we have studied the fragmentation pattern of the molecular cation of 3,4-dichloro-1,2,5-thiadiazole and the formation mechanisms of various product ions have been assigned. It has been also observed that most of the product ions are nitrile N-sulfides. Finally, it is inferred that there are two primary paths for the fragmentation of the parent molecular cation, namely, (1) Cl atom migration and subsequent ring opening by N–S bond cleavage and (2) direct ring opening by N–S bond cleavage. The ionization energies were accurately predicted for various species using ab initio calculations. Copyright © 2016 John Wiley & Sons, Ltd.