Transient Raman studies of hemibonded thiopyrimidine dimer radicals in water

Structures of dimeric thiopyrimidine intermediates were examined by means of transient Raman spectroscopy and DFT calculations. Neutral and anionic dimer radicals were prepared by pulse radiolysis of aqueous 2-thiopyrimidine (TP) solutions via one electron oxidation at pH 4 and 10, respectively. Raman spectra of these radicals were obtained in resonance with their pronounced electronic transitions using 415 and 500 nm as excitation wavelengths. Fifteen Raman bands of neutral dimer radical (TP)₂^• and twelve Raman bands of dimer radical anion (TP)₂^•- were observed in the 80-1800cm^-1 region and interpreted in conjunction with our previous studies of thiourea dimeric intermediate models as well as theoretical calculations. The highest enhancement of bands at 212 or ~214 cm^-1 in (TP)₂^• or (TP)₂^•-, respectively, indicates that S-S hemibond stretch undergoes greatest geometrical change upon electronic excitation. This confirms supposition that excited states of these intermediates have mostly SS antibonding character. The pattern of relative intensities of higher frequency bands to SS stretch frequency provides insight into coupling of other molecular vibrations to SS displacement coordinate, by which we gain information upon change in relative structural arrangement of pyrimidine rings in solution upon deprotonation at higher pH.