Shapes and sizes of molecular anions via topographical analysis of electrostatic potential

The theorem proposed by Pathak and Gadre J. Chem. Phys. 93, 1770 (1990), that the electrostatic potential (ESP) of negative ions must exhibit a directional negative valued minimum along any arbitrary direction has been verified for some small negative molecular ions, viz., OH−, CN−, N−3, NO−3, and NH−2. Also, as predicted by Gadre and Pathak Proc. Ind. Acad. Sci. (Chem. Sci.) 102, 18 (1989), the molecular ESP (MESP) maps are found to be devoid of local maxima. As a consequence, these maps reveal rich topographical details in the form of several saddle points as well as point minima. From the location of these critical points, estimates of the sizes and shapes of the negatively charged molecular ions are obtained. For anions, there exists a surface on which ■V⋅dS=0 and which passes through all the negative valued critical points (∇V=0). The ionic size estimates from the location of the critical points of the MESP are found to be in good agreement with the corresponding (spherically averaged) literature values.