Through-bond terms

The through-bond terms (Ea) are comparable with the terms used in vibrational spectroscopy, and consist of two-body interactions (bonding energy Eb) three-body... 

Thus, the through-space term makes a positive contribution to the z component of the atomic vector and the two through-bond terms make... 

Independent molecules and atoms interact through non-bonded forces, which also play an important role in determining the structure of individual molecular species. The non-bonded interactions do not depend upon a specific bonding relationship between atoms, they are through-space interactions and are usually modelled as a function of some inverse power of the distance. The non-bonded terms in a force field are usually considered in two groups, one comprising electrostatic interactions and the other van der Waals interactions. 

Distilled or deionised water contains small amounts of organic impurities which can cause problems in long term use with bonded phase columns in the reverse phase mode. The non-polar stationary phase will collect these organics, which can alter the nature of the stationary phase or sometimes produce spurious peaks (Fig. 4.3c is an example of this). Water purification can be done by distillation from permanganate, by passage of the water through bonded phase columns, or by means of commercial systems, eg the Milli-... 

The first such experiment (Fig. 21a), introduced independently by Gan [258] and Bodenhausen et al. [259], used 13C as the so-called spy nuclei in a 2D experiment similar to the HMQC scheme used in solution NMR [260], The transfer of magnetization occurred by a combination of weak ]JCN (through-bond) couplings and second-order quadrupolar-dipolar (through-space) cross-terms between 14N and 13C, which contain the isotropic and anisotropic terms l = 0, 2, and 4 and are referred to as residual dipolar splittings (RDS) [261-263], Under MAS, the 1 = 0 term results in isotropic coupling, which for NH3 groups is expected to be... 

Although ab initio or semiempirical SCF calculations account rather well for this observation, it has proved extremely enlightening, especially from a qualitative point of view, to discuss the interactions between non-conjugated 7r-orbitals in terms of the concepts of through-space and through-bond interactions introduced by Roald Hoffmann200-203. With reference to the schematic diagram 43 of a non-planar diene, these interactions are defined as follows ... 

Although the exchange mechanism was originally formulated in terms of direct orbital overlap between the donor and acceptor chromophores, it is clear that it can be extended to cover the case of through-bond mediated EET. The reason is because TB coupling provides a mechanism for spatially extending the active orbitals of the two chromo-... 

In broad definition, regio-control over reactivity (regiochemical control) involves the demonstration of locale preferences for a chemical reaction. Generally, regio-control is thought of in terms of through bond separation of reactive sites, as in the resonance forms for the cyclohexanedionyl anion of Figure 6.1. 

Bond terms Ei the work index, and expresses it as the amount of energy required to reduce unit mass of material from an infinite particle size to a size L2 of 100. im, that is q = 00. The size of material is taken as the size of the square hole through which 80 per cent of the material will pass. Expressions for the work index are given in the original papers 8,9) for various types of materials and various forms of size reduction equipment. 

This type of treatment is particularly interesting as it allows one to understand, in a molecule containing two redox sites, whether or not there is an electronic interaction (independently from its through-bond or through-space nature) between the sites, or, with a more attracting term, an electronic communication between the sites. 

---