Comparison of Various Means of Evaluating Molecular Electrostatic Potentials for Noncovalent Interactions

The various heterodimers formed by a series of Lewis acids with NH₃ as Lewis base are identified. Lewis acids include those that can form chalcogen (HSF and HSBr), pnicogen (H₂ PF and H₂ PBr), and tetrel (H₃ SiF and H₃ SiBr) bonds, as well as H-bonds and halogen bonds. The molecular electrostatic potential (MEP) of each Lewis acid is considered in a number of ways. Pictorial versions show broad regions of positive and negative MEP, on surfaces that vary with respect to either the value of the chosen isopotential, or their distance from the nuclei. Specific points are identified where the MEP reaches a maximum on a particular isodensity surface (V_s,max ). The locations and values of V_s,max were evaluated on different isodensity surfaces, and compared to the stabilities of the various equilibrium geometries. As the chosen isodensity is decreased, and the MEP maxima drift away from the molecule, some points maintain their angular positions with respect to the molecule, whereas others undergo a reorientation. The lowering isodensity also causes some of the maxima to disappear. In general, there is a fairly good correlation between the energetic ordering of the equilibrium structures and the values of V_s,max . A number of possible Lewis acid sites on the heteroaromatic imidazole ring were also considered and presents some cautions about application of V_s,max as the principal criterion for predicting equilibrium geometries. © 2017 Wiley Periodicals, Inc.