Molecular Similarity for Small Species:  Refining the Isoelectronic Index

Abstract

This paper tests a chemical-similarity model against properties of gas-phase neutral triatomic and four-atom molecules. The model is a variant of the Diatomics-in-Molecules (DIM) picture, which considers a molecule to be the superposition of all diatomic molecules that could be formed from adjacent atoms in the molecule. The variant is that adjacent atoms are counted as a diatomic molecule only if they are bonded. The tests consist of investigating whether molecules with the same number of electrons, computed by the adjacent-DIM model, have data more similar than do molecules selected at random. The tests vindicate the model for the heat of atomization and for the equilibrium constant for formation, they agree with the model with lesser confidence for the entropy and the partition function, and they show that the model fails for the ionization potential. The model applies with most confidence to molecules with the more electronegative atoms from rows 2 or 3 (those of greatest interest in organic chemistry) and with lesser confidence otherwise. For these properties and these molecules, the model passes graphical and statistical tests at least as well as does the traditional isoelectronic model. Thus, this work refines what may be called the isoelectronic index.

An Atlas of Forecasted Molecular Data. 1. Internuclear Separations of Main-Group and Transition-Metal Neutral Gas-Phase Diatomic Molecules in the Ground State

“…Other atlases of globally predicted data for vibration frequencies and ionization potentials of diatomic molecules are in preparation. These efforts have already yielded an interesting phenomenon of possible theoretical interest: certain series of triatomic molecules have, for at least one property and for certain regions of chemical data space, chemical similarity at least as pronounced as do isoelectronic series in many regions of the space. , …”

Kronecker-Product Periodic Systems of Small Gas-Phase Molecules and the Search for Order in Atomic Ensembles of Any Phase

“…This model is somewhat akin to the "Diatomics in Molecules" (DIM) model except that there is no term for the (nonexistent) bond AC. The model shows statistical significance for various properties of isomers of four-atom molecules [47]. Fig.…”

“…Our group also graphed several properties on Kong's "molecular number" axis, but with more resolution: the heat of atomization ( H a ) [46,47]; IP [46]; the internal entropy at 1000K [S o 1000 (internal)] [46]; the total entropy (S o 298.15 ); the partition function Q(1000); and log p K 1000 . One such plot is shown in Fig.…”

“…(11). Just as the next step for diatomics was printing main-group molecular configurations and terms, or data, on 8 x 8 chessboards, so the next step (after item 5.3.e) for triatomics entailed plotting H a [47]; IP; S o 298.15 [48]; S o (internal) 1000 ; Q(1000) and the bond angle in 8 x 8x 8 cubes, as shown by one example in Fig. (12).…”