Electron interactions through space

Outer-sphere reactions occur with minimal electronic interaction through space. Their rates can be predicted by a classical statistical-mechanical approximation (see 12.2.3). Insuring a minimum of electronic interaction between the reagents often re-... 

In general, induced shifts Ad may arise by contact interaction (through-bond interaction) or by dipolar (pseudo-contact) interaction (through-space interaction)87. In rare earth ions the inner/electrons are well shielded and do not participate in covalent bonding (contact interaction). Thus induced shifts originate mainly from pseudo-contact interactions. 

The spectroscopic and chemical properties of l,4-diazabicyclo[2.2.2]octane (DAB-CO) are consistent with a strong interaction of the in-phase combination of the non-bonded electron pairs of the nitrogen atoms with the symmetric combination of the C—C a bonds, one consequence of which is that the in-phase combination is the HOMO. The two lowest IPs are 7.6 and 9.7 eV [89]. Compare these to the IPs of trimethyl amine (8.44 eV) and ammonia (10.5 eV) [90]. The relative importance of intramolecular orbital interactions through space and through bonds has been reviewed by Hoffmann [6]. 

Investigation of environmental effects. As has been stressed in this chapter, homoaromaticity is just a matter of a few kcal mol-1 stabilization energy in most cases, and therefore environmental effects may have a large impact on structure, stability and other properties of a homoaromatic compound. Future work in theory (as well as in experiment) has to clarify how environmental effects can influence electron delocalization, through-space interactions and bonding in homoaromatic molecules. The theoretical methods are now available to calculate solvent and counter ion effects (for homoaromatic ions in solution) or to study intermolecular and crystal packing forces in the solid state. 

Azo compounds also have two lone pairs of electrons that can interact through space or through bond. Azomethane 33 (R = CH,) has been studied in detail. In the cis form 33a the n, orbital is below the n orbital and the transition at 353 nm is allowed, e = 240. In contrast, the energy order of the n orbitals is reversed in the trans compound 33b, as is to be expected from the negative overlap of the sp -hybridized n orbitals. The n,—transition at 343 nm is therefore symmetry forbidden, as is seen from the value e = 25 (Hasel-bach and Heilbronner, 1970). For azobenzene 33 (R = Ph) the corresponding... 

H ) interaction through space with nearby nuclear magnetic moments, chemical shielding (77 ), modification of the magnetic field at the nucleus due to the surrounding electrons and indirect spin-spin... 

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