Electron Communications and Chemical Bonds

Abstract

Recent developments in Orbital Communication Theory (OCT) of the chemical bond are summarized. Conditional probabilities defining molecular information networks are generated using the bond-projected Superposition Principle (SP) of quantum mechanics. The communications between atomic orbitals (AO) are proportional to squares of elements in the Charge-and-Bond-Order (CBO) matrix, thus being related to Wiberg’s quadratic indices of the chemical bond-order. Molecular propagation of information exhibits the communication-noise due to electron delocalization via the system chemical bonds, the entropic bond-covalency, measured by the channel conditional entropy. Information bond-ionicity is reflected by the channel mutual information (information flow) descriptor. The amplitude and probability information systems are distinguished, with the former being capable of the communication interference. This phase-aspect is lost in the probabilistic treatment of classical Information Theory (IT). The OCT perspective identifies the “indirect” chemical bonds in the “cascade” propagation realized via AO intermediates. These through-bridge “bonds” extend range of chemical interactions and supplement the “direct” (through-space) bonds realized by the constructive AO interference or direct communications between them. The flexible-input extension of OCT provides a continuous description of the fragment dissociation, giving a fair agreement between chemical expectations, OCT bond-multiplicities, and Wiberg bond-orders. The occupational orbital decoupling in OCT is properly represented, when separate communication systems for each occupied MO are used and their occupation-weighted entropy/information contributions are classified as bonding (positive) or antibonding (negative), in accordance with the signs of CBO matrix elements reflecting AO phases. These developments are illustrated in two-orbital model and representative π-electron systems. Resultant amplitude communications in multiconfiguration theory are explored, AO communications in Valence-Bond (VB) structures of H2 are examined, and intrinsic nonorthogonality problem of VB description is discussed. The covalent structure generates noiseless channel involving solely the offdiagonal (inter-orbital) propagations, while the ionic state corresponds to deterministic channel of the diagonal (intra-atomic) communications.