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Molecular conductance junctions are structures in which single molecules or small groups of molecules conduct electrical current between two electrodes. In such junctions, the connection between the molecule and the electrodes greatly affects the current-voltage characteristics. Despite several experimental and theoretical advances, including the understanding of simple systems, there is still limited correspondence between experimental and theoretical studies of these systems.

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This is an overview of some of the important, challenging, and problematic issues in contemporary electron transfer research. After a qualitative discussion of electron transfer, its time and distance scales, energy curves, and basic parabolic energy models are introduced to define the electron transfer process. Application of transition state theory leads to the standard Marcus formulation of electron transfer rate constants. Electron transfer in solution is coupled to solvent polarization effects, and relaxation processes can contribute to and even control electron transfer. The inverted region, in which electron transfer rate constants decrease with increasing exoergicity, is one of the most striking phenomena in electron transfer chemistry. It is predicted by both semiclassical and quantum mechanical models, with the latter appropriate if there are coupled high- or medium-frequency vibrations. The intramolecular reorganizational energy has different contributions from different vibrational modes, which, in favorable cases, can be measured on a mode-by-mode basis by resonance Raman spectroscopy. Alternatively, mode-averaging procedures are available for including multimode contributions based on absorption or emission spectra. Rate constants for intramolecular electron transfer depend on electronic coupling and orbital overlap and, therefore, on distance. Mixed-valence systems have provided an important experimental platform for investigating solvent and structural effects and the transition between localized and delocalized behavior. One of the important developments in electron transfer is the use of absorption and emission measurements to calculate electron transfer rate constants. Ultrafast electron transfer measurements have been used to uncover nonequilibrium relaxation effects, an area that presents special challenges to the understanding of the dynamics and relaxation of the coupled processes. Electron transfer in the gas phase offers substantial insights into the nature of the electron transfer process. Similarly, electron transport in conductive polymers and synthetic metals depends on the basic principles of electron transfer, with some special nuances of their own.

Medium basis sets based upon contractions of Gaussian primitives are developed for the third-row elements K through Zn. The basis functions generalize the 6-31G and 6-31G* sets commonly used for atoms up to Ar. They use six primitive Gaussians for 1s, 2s, 2p, 3s, and 3p orbitals, and a split-valence pair of three and one primitives for valence orbitals, which are 4s and 5p for atoms K and Ca, and 4s, 4p, and 3d for atoms Sc through Zn. A 6-31G* set is formed by adding a single set of Gaussian polarization functions to the 6-31G set. They are Cartesian d-functions for atoms K and Ca, and Cartesian f-functions for atoms Sc through Zn. Comparison with experimental data shows relatively good agreement with bond lengths and angles for representative vapor-phase metal complexes.