Visualization of Shapes of Molecular Anions *

One could expect to find a simple yet rigorous treatment of anisotropics of polyatomic ions in treatises on structural and inorganic chemistry. However, despite many discussions [1-4] of sizes, shapes and anisotropics of interactions, a unified approach to these anisotropics is, to the authors ' knowledge, conspicuous by its absence from the chemical literature. In fact, desirability of such an approach is apparent from many diverse areas in chemistry. The classic book due to Wells [2] depicts the 'pictures' of C N " and O H " anions in the crystals of N a C N and Ca(OH) 2 . Marcus [3] has discussed the size and shape of a planar nitrate anion and inferred (from neutron diffraction studies) that the axial and equatorial 'radii' of N O J are 1.26 and 2.01 Ä, respectively. The subject of anisotropy in gas-phase ionic interactions [4] is receiving considerable attention in experimental thermochemistry. Thus, the need of a simple practical visualization of polyatomic ions can hardly be overemphasised and this is what is at tempted in this note. Recent investigations due to Gadre et al. [5] provide a clue to such three-dimensional visualization of polyatomic anions. They showed that the Molecular Electrostatic Potential (MESP) of a negative ion, X~ (owing to its asymptotic decay as — q/r) must exhibit a negative-valued (V) minimum along any arbitrary ray starting from the nuclear skeleton. They also demonstrated the existence of a surface (2 obeying