Investigation of H-bonding and halogen-bonding effects in dichloroacetic acid: DFT calculations of NQR parameters and QTAIM analysis.

A theoretical study was performed to examine hydrogen and halogen bonds properties in gas phase and crystalline dichloroacetic acid (DCAA). The specific pattern of O-H∙∙∙O, C-H∙∙∙O, HCl, Cl∙∙∙O and Cl∙∙∙Cl interactions in DCAA dimers is described within the quantum theory of atoms in molecules (QTAIM) formalism. Based on QTAIM results, a partial covalent character is attributed to the O-H∙∙∙O hydrogen bonds in DCAA, whereas all the C-H∙∙∙O, Cl···O and Cl∙∙∙Cl intermolecular interactions are weak and basically electrostatic in nature. MP2/6-311++G** calculations indicate that the interaction energies for DCAA dimers lie in the range between -0.40 and -14.58 kcal mol(-1). One of the most important results of this study is that, according to energy decomposition analyses, halogen bonds are largely dependent on both electrostatic and dispersion interactions. For those nuclei participating in the hydrogen-bonding and halogen-bonding interactions, nuclear quadrupole coupling constants exhibit significant changes on going from the isolated molecule model to the crystalline DCAA. Of course, the magnitude of these changes at each nucleus depends directly on its amount of contribution to the interactions.