Vibrational spectroscopy of dimer radical cation of thioureas in water

Time-resolved resonance-enhanced Raman spectra of thiourea dimer radical cation, (Tu)₂^•+, and its methyl substituted analogues prepared by pulse radiolysis, have been obtained and interpreted in conjunction with theoretical calculations to provide detailed information on the molecular geometry and bond properties of these class of species. Seven Raman bands of (Tu)₂^•+ observed in the 80-1600cm^-1 region are assigned in terms of the strongly 212cm^-1 and weakly enhanced 719cm^-1 fundamentals, their overtones and combinations. Upon substitution of labile protons of (Tu)₂^•+ with deuterons in D₂O solutions frequencies of fundamental vibrations observed in H₂O decrease slightly to 208 and 668cm^-1, respectively, and two additional fundamentals become apparent at 380 and 410cm^-1 overlapping overtone of 208cm^-1 band.  Calculations by range-separated hybrid density functionals support the spectroscopic assignments of the 212(208)cm^-1 vibration to a predominantly symmetric SS stretching mode and the feature at 718.6(668)cm^-1 to symmetric SC stretching mode, respectively. Our findings are compared to analogous symmetric SS hemibonded methyl substituted thiourea dimer radical cations to provide insights about relation between their structure and properties.