The DFT and MP2 based computational scrutiny on blue-shifted H-F stretching vibrational frequencies in hydrogen-fluoride complexes with nitriles: Insights into the decisive role of intermolecular hydrogen bonding (IMHB) in ground and electronic excited states

In the current study, the RCN center dot center dot center dot HF (R = H, F, Cl, Br, CN, OH, SH, NH2, NO2 and CH3) systems are studied as the interest sample for the consideration of different measures of H-bond strength. The calculations are conducted by means of DFT (B3LYP) and MP2 methods in conjunction with the 6-311++G** and aug-cc-pVTZ basis sets for geometry optimization of complexes and monomers and also evaluation of the intermolecular interaction energies. It is found that a blue-shifted hydrogen bonding (HB) in RCN center dot center dot center dot HF complexes in that N atom of the nitrile molecules acts as a proton-acceptor. The nature of intermolecular hydrogen bond has been investigated by means of the Bader theory of Atoms In Molecules (AIM) and Natural Bond Orbital (NBO) analysis. The calculated highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO, respectively) energies show that charge transfer occurs within the studied complexes. The results obtained from molecular orbital interactions play an important role toward characterization of the chemical reactivity and kinetic stability of the H-bonded systems. Likewise, the strengthening of the intermolecular hydrogen bond and moving the H-1 chemical shift of participating H atom in the hydrogen bridge to up fields are associated with each other. The obtained results indicate a strong influence of the R substituent on the hydrogen bond strength. Moreover, the excited-state properties of intermolecular hydrogen bonding in the RCN center dot center dot center dot HF systems have been investigated theoretically using the time-dependent density functional theory (TD-DFT) method. Besides, numerous correlations between topological, geometrical and energetic parameters are also found. (C) 2015 The Authors. Published by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).