The Electron Pair in Chemistry

Abstract

The reality of the electron pair as a fundamental unit in the electronic structure of molecular systems is evidenced by calculations which show that the most probable partitioning of a system is the one which localizes pairs of electrons in well-defined spatial regions or loges. The loges in turn, correspond to those regions of space generally associated with core, bonded, and non-bonded electrons. In terms of information theory, they yield the maximum amount of information concerning the localizability of the electrons. The most probable three-loge partitioning of the six-electron BH(X1∑+) system, for example, is dominated by the event which places two electrons in each of three loges, the location and shape of the loges being such as to justify the labelling of the electron pairs they localize as core, bonded and nonbonded. Since the loges are defined in real space and are totally nonoverlapping, one may define the volume of space occupied by pairs of electrons. In BH, for example, the volume of space required to contain 95% of the nonbonded pair of electrons is over two times larger than that required to contain 95% of the bonded pair. It is possible to define core loges which exhibit pair occupation probabilities ranging in value from 95% in LiH+ to 85% in BH. Corresponding probabilities ranging in value from 75% to 90% are obtained for bonded and nonbonded loges. In the set of molecules studied here, the occurrence of events with such high probabilities is found only for loges which maximize the probability of a pair occupation.