Resonance Raman Spectroscopic And Solvent-dependent Study Of Pyrimidine-2-(1H)-thiones And Its Derivatives

In this study,we have investigated the 2-Mercaptopyrimidine?2-MPM?,4,6-Dimethyl-2-Mercaptopyrimidine?DMMP?and 4-Amino-2-Mercaptopyrimidine?AMP?geometries in solid and different solvents using resonance Raman spectra and Density Functional Theory?DFT?calculation.The study results indicate that strong hydrogen bonding between three mercaptopyrimidines and protic solvent,moreover,the protic solvent molecules can support the proton transfer from N atom to S atom in the mercaptopyrimidines molecules.The main results are as follows:?1?The absorption spectra of 2-MPM are obtained in acetonitrile,methanol,and water.And the B-band of the ultraviolet absorption spectra is assigned to ???* transition in acetonitrile,methanol,and water combing with the TD-DFT calculations.The distinguished differences of B-band conclude that the solvent-dependent effect can influence the electron transition of 2-MPM.The FT-Raman,FT-IR spectra and the DFT calculation could confirm the 2-MPM dimer in neat soild.However,2-MPM-solvent clusters could be confirmed in solvents by resonance Raman spectra.These results illustrate the solvent could break the dimer structure,forming cluster between the solute and solvent when 2-MPM is dissolved in polar solvents.?2?We obtain the absorption spectra of DMMP in cyclohexane,acetonitrile,methanol and water,indicating DMMP exists as thiol form in cyclohexane,thione form in polar or protic solvents.From the absorption spectra we can see the DMMP should strongly bond with solvent molecules in acetonitrile,methanol and water.The resonance Raman spectra is obtained using the wavelength of 245.9 nm,252.7 nm,266 nm,273.9 nm and 282.4 nm.The vibrational modes of C₅=C₆/N₃=C₄/N₁-C₆/S₁₆=C₂/N₃-C₂ stretch and H17N1C6/C5C6N1/C6N1C2/C4N3C2/N1C2N3 bend in plane reflect the multidimensional vibration modes of DMMP.According to DFT calculation,the DMMP can combine with water molecules in the side of SCNH to form the hydrogen bonding cluster,which is in agreement with the experimental results.?3?We obtain the absorption spectra of AMP in acetonitrile,methanol and water,and there are two strong bands observed.The A band differs nearly 15 nm shift in acetonitrile and water.The resonance Raman spectra is obtained using 266 nm,273.9 nm,282.4 nm and 299.1 nm excitation wavelength.The results indicate 13 fundamental vibrational modes,include six strong peaks at ?7,?8,?12,?14,?15 and ?24,and seven weak peaks at ?10,?13,?18,?19,?20,?27 and ?29 in water solution,which shows that the AMP on A-band is along N₉-C₄/N₃-C₄/N₁-C₆/N₃-C₂/N₁-C₂/S₁₂-C₂ stretch and C₅C₆N₁/H₁₃N₁C₂/N₃C₂N₁/C₂N₁C₆ bend in plane mainly.According to DFT calculation,AMP can bind with water molecules on the side of SCNH to form hydrogen bonding cluster.In the aspect of proton transfer,the proton transfer process is more likely to react with protic solvent-dependant cluster molecules in both ground state and excited state.For the ground state process,the proton transfer barrier energy greatly decreases for of solute-solvent cluster compared to solute molecule.Otherwise,for the excited state proton transfer,the T1 state makes an important role.