Deviation from planarity

Calculations show that the deviation from planarity leads to greater conformational stability for the phenylthiazoles (143, 145). In particular, the potential energy minimum is achieved at a twist angle of about 30° for 4-phenylthiazole, 40° for 2-phenylthiazole, and 45° for 5-phenylthiazole. 

The deviation from planarity that is present in a structure such as 1 raises the question of how severely a conjugated system can be distorted from the ideal coplanar alignment of p orbitals and still retain aromaticity. This problem has been analyzed by determining the degree of rehybridization necessary to maximize p orbital overlap in 1. It is found that rehybridization to incorporate fractional amounts of s character can improve orbital alignment substantially. Orbitals with about 6% s character are suggested to be involved... 

Fig. 9.3. (A) Carbon framework from X-ray crystal stmcture of iyn-tricyclo[8.4.1.1]hexadeca-1,3,5,7,9,11,13-heptaene. (B) Side view showing deviation from planarity of aimulene ring. (Reproduced from Ref 60 by permission from the International Union of Crystallography.)...

Precise description of the pyramidal structures would also require that the bond angles be specified. The EPR spectrum of the methyl radical leads to the conclusion that its structure could be either planar or a veiy shallow pyramid. The IR spectrum of the methyl radical has been recorded at very low tempertures in frozen argon. This IR study puts a maximum of 5° on the deviation from planarity. A microwave study has also indicated... 

Three-membered saturated rings are usually planar, but other three-membered rings can have some flexibility. Cyclobutane is not planar but exists as in 96, with an angle between the planes of The deviation from planarity is presumably... 

Almost all evidence indicates that the transition state must be planar. Deviations from planarity as in 17-3 (see p. 1322) are not found here, and indeed this is why six-membered heterocyclic nitrogen compounds do not react. Because of the stereoselectivity of this reaction and the lack of rearrangement of the products, it is useful for the formation of trans cycloalkenes (eight-membered and higher). 

Isomers are, of course, possible when the equatorial ligand lacks a plane of symmetry, as in the corrinoids [see (HI)]. All the acetamide side chains project to one side of the corrin ring, which we shall call the upper side, and all the propionamide side chains and the nucleotide side chain to the lower side. Isomers are then theoretically possible whenever the two axial ligands are different, and their existence has been shown experimentally for corrinoids where one axial ligand is CN , Me, or Et and the other is H2O or is absent [for further details see Section 8.2 of ref. (136)]. Salen and BAE also show minor deviations from planarity due to the bending of the two halves (mentioned above in Section II,B,2) and to torsion about the C—C bonds in the ethylene bridge (see references in Table I), but these are not expected to give rise to separable isomers. 

The compound exists normally as the trans or ( )-isomer 21a. This molecule is essentially planar both in the solid state and in solution, although in the gas phase there is evidence that it deviates from planarity. When irradiated with UY light, the ( )-isomer undergoes conversion substantially into the cis or (Z)-isomer 21b which may be isolated as a pure compound. In darkness, the (Z)-isomer reverts thermally to the (F)-isomer which is thermodynamically more stable because of reduced steric congestion. Some early disperse dyes, which were relatively simple azobenzene derivatives introduced commercially initially for application to cellulose acetate fibres, were found to be prone to photochromism (formerly referred to as phototropy), a reversible light-induced colour change. C. I. Disperse Red 1 (22) is an example of a dye which has been observed, under certain circumstances, to give rise to this phenomenon. 

During the last decade, a number of reports regarding X-ray structure determination of [l,3,2]diazaphosphenium ions have appeared [50-52, 55, 58, 59, 63, 64], It is found interesting to note that while in P-hydro diazaphospholenes, there is considerable deviation from planarity [54, 55], the diazaphosphenium ring (P-N, 1.66-1.69 A C-N, 1.37-1.39 A C-C, 1.34-1.39 A) is strictly planar indicating aromatic stabilization. 

In order to understand the rates of racemization of biphenyls and bihetero-cyclics, an accurate knowledge of their geometric structure is essential. Such knowledge makes it possible to estimate the amount of interference caused by substituents in the vicinity of the pivot bond in an assumed coplanar transition state for rotation. A study of the crystal structure of 1 (X = Se) found that the selenophene rings have a small but significant deviation from planarity and are nearly perpendicular to each other.17 The deviation from 9(T is such that the carboxyl groups are in transoid positions. 

Five-membered cyclopalladated rings of type 137 (Scheme 58) were synthesized and studied by spectroscopic methods. Their X-ray structures were analyzed in terms of deviations from planarity, and data were used for the calculation of two aromaticity indices, V and HOMA,179 which are in agreement with an aromatic structure. 

There is evidence for very small deviations from planarity, of the order of 2-3° at the carbonyl centre, in response to antiperiplanar substituents on the a-carbon (Laube, 1987 Lenoir el at., 1988). These are unrelated to rotation about the single bond. 

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