Theoretical studies of hydrogen bonding in water–cyanides and in the base pair Gu–Cy

Abstract

Density-functional (DFT) and many-body-perturbation theories (MBPT/CC) are used to study the hydrogen bonding in the water–cyanide complexes H–CN⋯H 2O, H 3C–CN⋯H 2O and (CH 3) 3C–CN⋯H 2O. Structures, binding energies and changes in vibrational frequencies are analyzed. The calculated CN stretching frequency is found to shift to the blue upon complexation in H–CN⋯H 2O and H 3C–CN⋯H 2O. To investigate electron correlation effects on the binding energies of these complexes, single-point calculations are performed at the MBPT/CC (MP2, MP3, MP4, CCSD and CCSD(T)) levels using the optimized MP2 geometries. Binding energies are also obtained at different levels of DFT (B3LYP and PW91) and compared with the MBPT/CC results. All calculations include corrections for basis set superposition error (BSSE) and zero-point vibrational energies. Additionally, the triple hydrogen-bonded guanine–cytosine (Gu–Cy) base pair is analyzed. The binding energy of the Watson–Crick model for Gu–Cy is calculated using the Hartree–Fock calculations and DFT (B3LYP and BP86) methods. The results for the hydrogen bonding distances and binding energies are in good agreement with experimental and recent theoretical values. The calculated dipole moment of the Gu–Cy complex is compared with the direct vector sum of the isolated bases. After taking into account the BSSE effects we find that the electron polarization due to the hydrogen binding leads to an increase of ∼20% of the calculated dipole moment of the complex.