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 of MESP 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 ( V min (D) or V n (D)) during complex formation is proportional to its electron donating ability while such a change on the acceptor molecule ( V min (A) or V n (A)) is proportional to its electron accepting ability. Further, the quantities V min = V min (D) − V min (A) and V n = V n (D) − V n (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.Keywords. Non-covalent complex; hydrogen bond; halogen bond; dihydrogen bond; pnicogen bond; tetrel bond; lithium bond; chalcogen bond; molecular electrostatic potential.

The value of the molecular electrostatic potential minimum (V ) and its topographical features (position, as well as the eigenvalues and eigenvectors of the corresponding Hessian matrix) are recently proposed as the criteria for characterizing a lone pair (Kumar A. et al., J. Phys. Chem. 2014, A118, 526). This electrostatic characterization of lone pairs is examined for a large number of small molecules employing MP4/6-311++G(d,p)//MP2/6-311++G(d,p) theory. The eigenvector of the Hessian matrix corresponding to its largest eigenvalue (λ ), is found to be directed toward the lone pair-bearing-atom, with λ showing a strong linear correlation with V . Large magnitudes of V and λ indicate a charge-dense lone pair. The topographical features of V are seen to provide insights into the interactive behavior of the molecules with model electrophiles, viz. HF, CO , and Li . In all the complexes of HF and majority of the other complexes, the interaction energy (E ) correlates well with the respective V value, but for some deviations occurring due to other competing secondary interactions. The electrostatic interactions are found to be highly directional in nature as the orientation of interacting atom correlates strongly to the position of lone pair. In summary, the present study on a large number of test molecules shows that electrostatics is able to probe lone pairs in molecules and offers a simple interpretation of chemical reactivity. © 2017 Wiley Periodicals, Inc.

Supramolecular self-assembly and molecular recognition processes are driven mainly by positive cooperativity in noncovalent interactions. Here, we report a large variety of hydrogen-, tetrel-, chalcogen-, pnicogen-, halogen-, aerogen-, and dihydrogen-bonded dimer and trimer complexes, computed using the MP2/6-311++G(d,p) level ab initio theory. The dimer to trimer change is associated with a positive cooperativity in all the complexes. Significant electron density reorganization occurs in monomers because of noncovalent bond formation which is quantified using the change in the molecular electrostatic potential (MESP) at bonded atoms. For a noncovalent dimer X D Y A , an electron density flow is observed from the donor molecule X D to acceptor molecule Y A . As a result, YA in dimer showed a tendency to form a stronger noncovalent bond with an electron-deficient center of a third molecule, whereas X D in the dimer showed a tendency to form a stronger noncovalent bond with an electron-rich center of a third molecule. The change in change-of-MESP at the donor and acceptor atoms involved in bond formation (ΔΔV n ) is used as a parameter to assess the extent of electron donor− acceptor (eDA) interaction in dimers and trimers and found that ΔΔV n is directly proportional to the total binding energy. A cooperativity rule has emerged from this study which states that the electron reorganization in the dimer due to the eDA interaction always enhances its donor−acceptor interactive behavior with a third molecule toward the formation of a noncovalent trimer complex.