Interactions through-bond

Let us now look at the details of this through-bond interaction mechanism. We consider the four-orbital system shown in Fig. 32. 

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

Through-bond interactions, 27, 47 Through-space interactions, 26 Trimethylene (edge-to-edge), 161... 

Fig. 5a-c Through-bond interactions in the triplet state of 1,3-diradical, a Mechanism of electron delocalization and polarization of a-spin electrons, b Cyclic orbital interaction, c Orbital phase continuity... 

All the above-mentioned acyclic 1,3-diradicals are less stable than the a-bonded isomers. Therefore, in addition to using various substituents, other factors should be further considered in our design of persistent singlet 1,3-diradicals. In Sect. 5.2, ring structure is taken into account. Strain prevents the ring closure in the singlet state. Two linkers between the radical centers multiply the through-bond interactions. 

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-bond interaction . Because of the molecule s lack of planarity, each of the two 7r-orbitals 7Ta and 7Tb will interact with those of the cr-frame s cr-orbitals which exhibit the appropriate symmetry behaviour. 

It can be seen from display 51 and Figure 16 that pure through-bond interaction (B = 0) places the symmetric orbital (p on top of the antisymmetric orbital through-space interaction ( /i > 0), the energy gap 3 — 2 between normal order of cross-over occurs when the parameters obey the condition... 

A more detailed analysis of through-space and through-bond interactions... 

The heuristic success of the concepts of through-space and through-bond interactions introduced by Hoffmann200 made it desirable to link it to many electron procedures, at least on the level of semiempirical or ab initio SCF calculations. Such a treatment, which had been proposed some time ago204, has been reviewed in detail elsewhere173. For this reason we shall only sketch the essential steps involved, using norbornadiene 75 as an example. 

FIGURE 18. The symmetry-adapted, orthogonal linear combinations of the localized a-orbitals of norbornadiene 75 belonging to the irreducible representations / and 62 of the point group C2v- The A] and B2 combinations are the relay orbitals for through-bond interaction between jra and 7Tb which define, according to equation 56, the orbitals jt and jt... 

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