Dissociative photoionization of 2-Thiouracil and 4-Thiouracil: A molecular dynamics study

Thionated nucleobases have drawn considerable attention due to their role in medical treatment and biology, which triggered studies of their photophysics and photochemistry with various experimental and theoretical techniques. In particular, vacuum ultraviolet (VUV) induced dissociative photoionization of 2-thiouracil (2-TU) has been recently studied using synchrotron radiation [Robinson et al., Molecules 2023, 28, 2354]. Here, using molecular dynamics simulations at the semiempirical OM2 level, we study fragmentation dynamics of the cations of 2-TU and its isomer, 4-thiouracil (4-TU). Considering various VUV photon energies, we calculate energy-resolved mass spectra and breakdown diagrams for 2-TU and 4-TU. Remarkably, we find that the major fragments are different among the two compounds — 69 amu (C3NH3O+) for 2-TU, and 85 amu (C3NH3S+) for 4-TU. Our simulations provide direct mechanistic insight into this observation and in fragmentation processes of the thionated uracils in general.