Gas-phase reactions between thiourea and Ca²⁺: New evidence for the formation of [Ca(NH₃)]²⁺ and other doubly charged species

The gas-phase reactions between Ca²⁺ and thiourea are investigated by means of electrospray ionization/mass spectrometry experiments. The MS/MS spectra of [Ca(thiourea)]²⁺ complexes show the appearance of new doubly charged species formed by the loss of NH₃ and HNCS. Other intense peaks at m/z 43, 56, 60, 73, 76 and 98 are also observed, and assigned to monocations produced in different coulomb-explosion processes. The structures and bonding characteristics of the different stationary points of the [Ca(thiourea)]²⁺ potential energy surface (PES) were theoretically studied by DFT calculations carried out at B3LYP/cc-pWCVTZ level. The analysis of the topology of this PES permits to propose different mechanisms for the loss of ammonia and HNCS, and to identify, the m/z 43, 56, 60, 73, 76 and 98 peaks as H₂NCNH⁺, CaNH₂⁺, H ₂NCS⁺, CaSH⁺, thiourea^+. and CaNCS⁺ ions respectively. There are significant dissimilarities between the reactivity of urea and thiourea, which are related to the lower ionization energy of the latter, and to the fact that thioenols are intrinsically more stable than enols with respect to the corresponding keto forms.