Resonance Raman Spectroscopic and Theoretical Study of Geometry Distortion of Thiourea in 21A State.

The A-band resonance Raman spectra of thiourea were obtained in water and acetonitrile solution. B3LYP/6-311++G(3df,3pd) and RCIS/6-311++G(3df,3pd) calculations were done to elucidate the ultraviolet electronic transitions, the distorted geometry structure and the saddle point of thiourea in 2¹A excited state, respectively. The resonance Raman spectra were assigned. The absorption spectrum and resonance Raman intensities were modeled using Heller's time-dependent wave packet approach to resonance Raman scattering. The results indicate that largest change in the displacement takes place with the C=S stretch mode v₆ (ǀΔǀ=0.95) and noticeable changes appear in the H₅N₃H₆+H₈N₄H₇ wag v₅ (ǀΔǀ=0.19), NCN symmetric stretch+C=S stretch+N₃H₆+H₈N₄ wag v₄ (ǀΔǀ=0.18), while the moderate intensities of 2v₁₅ and 4v₁₅ are mostly due to the large excited state frequency changes of v₁₅, but not due to its significant change in the normal mode displacement. The mechanism of the appearance of even overtones of the S=CN₂ out of plane deformation is explored. The results indicate that a Franck-Condon region saddle point is the driving force for the quadric phonon mechanism within the standard A-term of resonance Raman scattering, which leads to the pyramidalization of the carbon center and the geometry distortion of thiourea molecule in 2¹A excited state. [ABSTRACT FROM AUTHOR]