Ab initio MP4/Aug-cc-pvDZ//MP2/6-311++g(d,p) level interaction energy (E_int) and molecular electrostatic potential analysis (MESP) of a large variety of non-covalent intermolecular complexes, viz. tetrel, chalcogen, pnicogen, halogen, hydrogen, dihydrogen and lithium bonded complexes have been reported. The electronic changes associated with the non-covalent complex formation is monitored in terms of MESP minimum (V_min) in the free and complexed states of the donor and acceptor molecules as well as in terms ofMESP at the donor and acceptor atoms (V_n) of the free monomers and complexes. The change in V_min or V_n on the donor molecule (ΔV_min(D) or ΔV_n(D)) during complex formation is proportional to its electron donating ability while such a change on the acceptor molecule (ΔV_min(A) or ΔV_n(A)) is proportional to its electron accepting ability. Further, the quantities ΔΔV_min = ΔV_min(D) −ΔV_min(A) and ΔΔVn = ΔVn(D) −ΔVn(A) have shown strong linear correlations with E_int of the complex (E_int values fall in the range 0.7 to 46.2 kcal/mol for 54 complexes) and suggest that the intermolecular non-covalent interactions in a wide variety of systems can be monitored and assessed in terms of change in MESP due to complex formation in the gas phase. With the incorporation of solvent effect in the calculation, charged systems showed significant deviations from the linear correlation. The MESP based analysis proposes that the large variety of intermolecular non-covalent complexes considered in this study can be grouped under the general category of electron donor-acceptor (eDA) complexes

The nature of dihydrogen bonding (DHB) in VI, VII, and VIII group octahedral metal hydride complexes with H₂O has been studied systematically using quantum theory of atoms-in-molecule (QTAIM) analysis. A dihydrogen bond (H···H) between hydride ligand and hydrogen of H₂O is revealed in QTAIM analysis with the identification of a bond critical point (bcp). The DHB is due to the donation of electron density from the hydride ligand to the hydrogen of H2O. A strong linear correlation is observed between intermolecular H···H distance (d_HH) and electron density (ρ) at the bcp. Structural parameters suggested the highly directional nature of DHB. Weak secondary interactions between oxygen of water and other ligands contribute significantly to the binding energy (E_int) of DHB complex (2.5 to 13.2 kcal/mol). Analysis of QTAIM parameters such askinetic- (G_c), potential- (V_c) and total electron energy density (H_c) revealed the partially covalent character of DHB in majority of the complexes while a few of them showed closed shell character typical of purely non-covalent interactions