Density functional theoretical (DFT) and surface-enhanced Raman spectroscopic study of guanine and its alkylated derivatives: Part 1. DFT calculations on neutral, protonated and deprotonated guanine

Density functional theory (DFT) at the B3LYP/6-31++G(d,p) level has been used to study the geometries, energies, and Raman spectra of guanine in its neutral, protonated, and deprotonated forms. The calculated proton affinity for protonation at N7 is in good agreement with its experimental value. Deprotonation at N9 is predicted to be favoured over deprotonation at N1 in the gas phase, but the latter is stabilised when solvation is taken into account in the context of the Onsager dielectric continuum model. The influences of hydrogen bonding, protonation and deprotonation on the geometric parameters of the amino group are discussed. The normal Raman scattering (NRS) spectra of polycrystalline guanine, guanine hydrochloride and guanine in alkaline aqueous solution are assigned by comparison with the respective DFT predicted Raman spectra. In the assignment of polycrystalline guanine, predicted and observed wavenumber shifts caused by the isotope exchange in four isotopomers of guanine are also considered. The consideration of hydrogen bonding effects by the explicit addition of seven water molecules in the DFT calculations leads to a re-assignment of several NRS bands, particularly in the 1050 cm^-1 to 1450 cm^-1 wavenumber region, where the normal modes have strong contributions of NH bending motions. This study represents the highest level and most comprehensive assignment of the NRS spectrum of polycrystalline guanine published to date.