Infrared electroabsorption spectroscopic study of a nucleobase model compound 4(3H)-pyrimidinone in solution
In this thesis, 4(3H)-Pyrimidinone, an important model compound of pyrimidine base, in p-dioxane solution was studied with infrared (IR) electroabsoprtion spectroscopy. A previous study using FT-IR spectroscopy showed that the monomer and dimer of 4(3H)-pyrimidinone, coexist in the solution phase. Although it is crucial to understand the effects of an electric field on this molecule with respect to molecular structure and monomer–dimer equilibrium, direct experimental studies are yet to be done. IR electroabsorption spectroscopy can detect the molecular responses to an externally applied electric field with high sensitivity. In particular, it has been proven a powerful technique for studying solution sample in which different molecular species coexist. We measured IR electroabsorption (DA) spectrum in the C=O stretching region of 4(3H)-pyrimidinone in p-dioxane in which both monomer and dimer appear. In a series of c-dependent DA spectra, we did not observe marked c dependence of the DA signals of both monomer and dimer. Here, angle c is the angle between the direction of the applied field and the electric field vector of the IR light. To examine the c-dependence of the DA spectra quantitatively, we performed singular value decomposition (SVD) analysis, in which the whole set of the spectra were decomposed into the c-dependent and c-independent spectral components. The c-independent component, shows the prominent electronic polarization signal in contrast to other molecular liquids studied before. In addition, the DA signal arising from equilibrium change between the monomer and dimer was not observed in the c-independent component, which implies that the dipolar stabilization with the applied electric field (~10MVm-1) may not be strong enough to dissociate the dimer. The absence of a large c dependent component is rather unusual for a polar molecule having a permanent dipole moment. We attribute this observation to the angle between the vibration transition moment and the permanent dipole moment which is very close to the magic angle 54.7°, at which the orientational polarization signal vanishes. Results of our least-squares fitting iii analysis of the c-dependent spectral component are consistent with our interpretation. The present IR electroaborption study of 4(3H)-pyrimidinone has revealed experimentally for the first time the electric-field effects of a nucleobase model in a biologically relevant environment (i.e., ambient solution).