Study Of The Photoinduced Dynamics Of Thiourea And Its Derivatives

The short-time photo-induced dynamics of the thiourea, ETU and thioacetamide have been investigated in the Franck-Condon region by the resonance Raman spectra in combination with density functional theoretical calculations. The innovation of this paper is that there are a series of new vibrational modes2v and4v of C=S-N2out of plane deformation in the resonance Raman spectra for the thiourea. The appearance of asymmetric-out of plane deformation vibration mode in the excited state due to changes of the geometries at the saddle points. To explore this variation, the derivatives of thiourea, ethylene thiourea (ETU) and thioacetamide (thioacetamide) were also explored in water and acetonitrile solvent in order to find the effects of the substituent effects and solvent effect on the excited state saddle point geometry. The major findings are as follows:(1) We obtained the absorption spectrum of thiourea in water and acetonitrile solutions. There are two charge-transfer bands, A-band near245nm and B-band band near220nm. According to our calculating electronic absorption spectra of thiourea, the A-band were assigned to Ï€â†'Ï€*transition, and the B-band were assigned to nâ†'Ryd transition. Our research resulults indicate solvent effects had a great impact on the the B-band. The Resonance Raman spectra were acquired for thiourea in water and acetonitrile solvent with228.7,239.5,245.9,252.7and266nm excitation wavelengths. It is interesting that a new vibration mode appeared in the resonance Raman spectra:the S=C-N2out of plane deformation2v15(near1260cm-1), which belongs to " B" irreducible representative. This shows that the excited state of thiourea undergoes geometry structure deformation in Fanck-Condon region. The21A (S2) state was optimized at the RCIS/6-311++G(3df,3pd) level. The results indicated that thiourea had C2point group in the ground state, while Ci point group in the second singlet excited state, S2, which is due to a pyramidalization of the carbon center upon photoexcitation.Saddle point calculations were done for S2excited state of thiourea by partial optimization using RCIS/6-311++G(3df,3pd) keeping the C2symmetry and compare with its ground state So. Results indicated that excited state and the saddle point geometries were similar to the ground state, and one imaginary frequency for the saddle point was out-of-plane deformation vibration mode. This was in good agreement with the resonance Raman observation that the S2excited state undergoes considerably large geometry deformation along S=C-N2out of plane vibrations.The depolarization ratio p of the Raman fundamentals was measured. Intensity analysis was also done in acetonitrile solvent. The results indicate that in the Franck-Condon the photochemical reaction coordinates region has multidimensional region character with motion predominantly along the C=S stretch (v6,744cm-1,|â–³|=0.95), H5N3H6+H8N4H7wag(v5,1063cm-1,|â–³|=0.18), N-C-N symmetric stretch+C=S stretch+N3H6+H8N4wag (v4,1405cm-1,|â–³|=0.16), which contributes to the geometry changes. Intensity analysis also indicates that the moderate intensities of2v15and4v15are mostly due to the large excited state frequency changes of v15, but not due to its significant change in the normal mode displacement.(2) We obtained the absorption spectrum of thioacetamide and ethylene thiourea, which are two charge-transfer absorption bands in water and acetonitrile solution. For ethylene thiourea, the A-band (233nm) was assigned to Ï€â†'Ï€*transition, the B-band was assigned to nâ†'Ryd and Ï€â†'Ryd transition; For thioacetamide, the A-band (266nm) was assigned to Ï€â†'Ï€*transition, the B-band were assigned to nâ†'Ryd transition. Solvent effect had a great impact on the the B-band both for ethylene thiourea and thiourea.(3) According to electronic absorption spectrum, the resonance Raman spectra of thiourea and thiourea were acquired for thioacetamide and ethylene thiourea in water and acetonitrile solvent with228.7,239.5,245.9,252.7and266nm excitation wavelengths. For ethylene thiourea, the reaction coordinates mainly along the v12(C10C7stretch), v11(N-C-N stretch), v10(N3C7+N5C10stretch), v8(C=S stretch+N3H4+N5H6rock in plane) and v7(C=S stretch+N3H4, N5H6rock in plane); while for thioacetamide the reaction coordinates mainly along the C=S stretch (v15,718cm-1), NH2rock in plane (v14,973cm-1) and CH3-C stretch (v11,1302cm-1). The saddle point structures were investigated for ethylene thiourea and thioacetamide By resonance Raman spectra in combination with RCIS theoretical calculations, For ethylene thiourea, the overtone of S=C-N2(v27) out of plane deformation vibration,2v27and4v27, appeared on the resonance Raman spectra. For thioacetamide, the overtone of S=CN-C (v17) out of plane deformation vibration,2v17and4v17, disappeared on the resonance Raman spectra.