Adjacent DIM isoelectronic molecules and chemical similarity among triatomics

Isoelectronic molecules and isovalent molecules commonly have chemical similarity. This paper adds a third distribution of molecules and demonstrates that it exhibits chemical similarity in the case of linear/bent triatomics: the similarities of same-period triatomics which have equal C-1 + 2C(2) + C-3 = g(C), where C-i is the number of valence electrons of atom i and where i = 2 is the middle atom. It is shown, by graphical and statistical analyses of critically analyzed data for several observables of main-group molecules, that molecules with constant g(C) are at least as similar as isoelectronic molecules and are vastly more similar than are molecules chosen at random. It is suggested that since g(C) can be written additively as (C-1 + C-2) + (C-2 + C-3), a theoretical basis for the new tool might be constructed using a ''diatomics-in-molecules'' (DIM) or similar theory in which the 1-3 bond is ignored. The generalization to molecules with more than three atoms is presented.