Spin theory, complex molecules

The first quantitative theory of chemical bonding was developed for the hydrogen molecule by Heitler and London in 1927, and was based on the Lewis theory of valence in which two atoms shared electrons in such a way that each achieved a noble gas structure. The theory was later extended to other, more complex molecules, and became known as valence bond theory. In this approach, the overlap of atomic orbitals on neighbouring atoms is considered to lead to the formation of localized bonds, each of which can accommodate two electrons with paired spins. The theory has been responsible for introducing such important concepts as hybridization and resonance into the theory of the chemical bond, but applications of the theory have been limited by difficulties in generating computer programs that can deal efficiently with anything other than the simplest of molecules. 

XIll. Resonance and the Structure op Complex Molecules Spin Theory and Bond Eigenfunctions, 232. Evaluation of the Integrals, 240. The Two-Electron Problem, 244. The Four-Electron Problem, 245. The Concept of Resonance, 248. The Resonance Energy of Benzene, 249. The Resonance Energy of Benzene by the Molecular Orbitals Method, 254. 

One way to calculate the ZFS parameters is through ligand field theory (52,53) but for quantitative calculations it is nowadays possible to use accurate correlated ab initio methods. It is known that the direct spin—spin contribution (SSC) is dominant for organic molecules, and for a long time it was assumed that for inorganic complexes exclusively the SOC dominates. Nevertheless, exceptions to this rule have been identified (54,55). 

Molecular motions in low molecular weight molecules are rather complex, involving different types of motion such as rotational diffusion (isotropic or anisotropic torsional oscillations or reorientations), translational diffusion and random Brownian motion. The basic NMR theory concerning relaxation phenomena (spin-spin and spin-lattice relaxation times) and molecular dynamics, was derived assuming Brownian motion by Bloembergen, Purcell and Pound (BPP theory) 46). This theory was later modified by Solomon 46) and Kubo and Tomita48 an additional theory for spin-lattice relaxation times in the rotating frame was also developed 49>. 

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