Flash pyrolysis vacuum ultraviolet photoionization mass spectrometry of cycloheptane: A study of the initial decomposition mechanism

Thermal decomposition of cycloheptane was studied using flash pyrolysis coupled with vacuum ultraviolet (118.2 nm) single photon ionization time-of-flight mass spectrometry at temperatures ranging from 295 K to 1380 K. C-C bond breaking of cycloheptane leading to the 1,7-heptyl diradical was considered as the initiation step. The 1,7-heptyl diradical could readily isomerize to 1-heptene and decompose into several fragments, with dissociation to •C₄H₉ and •C₃H₅ as the predominant product channel. The 1,7-heptyl diradical could undergo direct dissociation, as evidenced by the production of the C₅H₁₀ species. Quantum chemistry calculations at UCCSD(T)/cc-pVDZ//UB3LYP/cc-pVDZ level of theory on the initial reaction pathways of cycloheptane were also carried out to support the experimental observations. Other possible initiation channels, as well as some secondary reaction products, were also identified.