Towards developing a criterion to characterize non-covalent bonds: a quantum mechanical study†

Chemical bonds are central to chemistry, biology, and allied fields, but still, the criterion to characterize an interaction as a non-covalent bond has not been studied rigorously. Therefore, in this study, we have attempted to characterize the non-covalent bonds by considering a total of 85 model systems depicting different chemical bonds comprising 43 non-covalent and 42 other bonds such as covalent, ionic, and coordinate bonds. The characterization has been done based on interaction energy, energy decomposition analysis (EDA), the NCI plot, and the analysis of topological properties of electron density. The interaction energy values, energy decomposition analysis, and NCI plot give insights into the full understanding of bond strength and its nature, but they fail to distinctively characterize the interaction as a non-covalent bond. Herein, a special criterion has been developed based on the topological parameters to characterize an interaction as a non-covalent bond. Topological parameters illustrate that the values of both backward difference (2)rho and H(r) are positive with a value of rho < 0.03 a.u. and [-G(r)/V(r)] >= 1.00 for the non-covalent bonds. The value of rho increases up to 0.06 a.u. with a positive value of backward difference (2)rho and a negative value of H(r) if the non-covalent bond is partially covalent in nature. The analysis of G(r) suggests that it dominates the H(r) of non-covalent bonds with greater than 50% contribution, whereas the contribution of G(r) varies between 35 and 50% in the case of bonds which are partially covalent in nature. The criterion based on topological parameters is likely to be very helpful to comprehend and ascertain the non-covalent bonds in the chemical as well as complex biological systems.

Automated summary

Chemical bonds are crucial in chemistry, biology, and related fields. A new criterion has been developed based on topological parameters to accurately characterize non-covalent bonds. The analysis of backward difference (2)rho and H(r) values in the topological parameters is effective in distinguishing non-covalent bonds and those with partially covalent nature.