Density functional theory (DFT) prediction of structural and spectroscopic parameters of cytosine using harmonic and anharmonic approximations

Abstract

Abstract The applicability of popular and efficient B3LYP hybrid density functional and medium-size Pople-type basis set in combination with computationally expensive anharmonic model to obtain more accurate theoretical structure, vibrational frequencies and GIAO NMR parameters of cytosine was tested. We report on prediction of cytosine equilibrium (R e) and rovibrationally averaged (R v) structures and vibrational frequencies in the gas phase and DMSO solution using density functional theory combined with 6-311++G∗∗basis set. The harmonic and anharmonic vibrational frequencies (using second-order vibrational perturbation theory, VPT2) were critically discussed. In comparison with initial harmonic data, a significantly better agreement between scaled and anharmonic frequencies and experiment was observed. Proton and carbon nuclear magnetic shieldings were calculated at R e and R v structures of cytosine in the gas phase and DMSO solution using BHandH and B3LYP density functionals combined with 6-311++G∗∗, aug-cc-pVTZ-J and STO-3G mag basis sets. The obtained NMR results were compared with available experimental data and discussed at length.