Bonding isologous

Isologous bonding is very important in oligomeric enzymes, and it has been suggested that isologous interactions evolved early. Initially there may not have been much complementarity in the bonding but two... 

Figure 7-11 (A) Isologous bonding between pairs of subunits (B) an "isologous square" arrangement of subunits (C) an apparently "tetrahedral" arrangement of subunits. Note the three twofold axes.

The same group has investigated 10-phenylphenoxazine (263) and -phenothiazine (264) cation-radicals and the influence of substituents in the phenyl group upon the spin distribution. The interest here lay in the effect of the conformation on electronic effects. Earlier studies on similarly shaped radicals had indicated that a twist of about 65° must exist in the V-phenyl bond (cf. phenylxanthenyl and isologs Sections II,B,3,a III,C,1 IV,B,1). This value may be calculated trigonometrically for a model of such radicals which has planar aromatic components, each C—C and C—H bond equal to those in benzene, and in which the phenyl ortho and heterocyclic 1- and 9-protons are at van der Waals separation. A rather similar angle has to be allowed in MO calculations in order correctly to reproduce experimental hyperfine splittings. 

A homodesmotic reaction is a reaction in which not only are the number of bonds of each type conserved, but the number of carbon atoms with zero, one, two, or three hydrogen atoms is also conserved. For details, see George, P. Trachtman, M. Bock, C. W. Brett, A. M. Tetrahedron 1976, 32, 317. Isomers interconverted by homodesmotic reactions are termed isologous (cf. Engler, E. M. Andose, J. D. Schleyer, P. v. R. /. Am. Chem. Soc. 1973, 95, 8005). 

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