Investigation of C-H...O=C and N-H...OC hydrogen-bonding interactions in crystalline thymine by DFT calculations of O-17, N-14 and H-2 NQR parameters.

A computational study at the level of density functional theory (DFT) was carried out to investigate C-H...O=C and N-H...O=C hydrogen-bonding interactions (HBs) in the real crystalline cluster of thymine by O-17, N-14 and H-2 calculated nuclear quadrupole resonance (NQR) parameters. To perform the calculations, a hydrogen-bonded pentameric cluster of thymine was created using X-ray coordinates where the hydrogen atoms positions are optimized and the electric field gradient (EFG) tensors were calculated for the target molecule. Additional EFG calculations were also performed for crystalline monomer and an optimized isolated gas-phase thymine. The calculated EFG tensors at the level of B3LYP and B3PW91 DFT methods and 6-311++G**and CC-pVTZ basis sets were converted to those experimentally measurable NQR parameters, quadrupole coupling constants and asymmetry parameters. The results reveal that because of strong contribution to N-H...O=C HBs, NQR parameters of O2, N1 and N3 undergo significant changes from monomer to the target molecule in cluster. Furthermore, the NQR parameters of O2 also undergo some changes because of non-classical C-H...O=C HBs.