Through-space terms

An analysis of the mechanism governing the couphng between the vicinal protons in ethane and fluoroethane has been studied by Contreras and co-workers by the developed by them method of natural bond orbital interactions between bonds and antibonds, cTin->a n. The authors have concluded that the main contribution to Jhh comes from the through space term while the introduction of a fluorine changes this term and yields a direct contribution to the coupling. 

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. 

What does an atom experience in an encounter with another atom The nucleus, which contains most of the atom s mass, is confined to a tiny volume. Electrons, on the other hand, are spread out through space. Therefore, a collision between two atoms is a collision of their electron clouds. The electron clouds repel each other but are attracted by the nuclei. Chemists describe molecular structure, properties of materials, and chemical reactions in terms of how electrons respond to these electrical forces. 

Coupling between fluorine and a hydrogen, a carbon or another fluorine that may be separated by many bonds (four, five, six or more) can result from overlap of electronic orbitals occupied by lone pair electrons which are unshared and therefore not involved in normal covalent bonding. The term applied to this effect, through space is somewhat misleading, since all isotropic coupling must be transmitted in some way by electrons, either in bonds or in unshared pairs. 

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 ... 

Through-space interaction. Although the two TT-orbitals 7ra and 7Tb are not in conjugation, there exists a small but finite cross term B between them which, to a first approximation, will be proportional to their overlap integral Sab = (7ra 7Tb). 

Radiation The emission and propagation of energy through space or through a material medium in the form of waves. The term also includes subatomic particles, such as a, P, and cosmic rays and electromagnetic radiation. 

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