Structural Considerations and Spectroscopic Properties

Structural Considerations and Spectroscopic Properties.—The geometry of three conformations of cyclopentane, planar half chair (C2), and envelope (CJ, has been investigated by means of a floating spherical Gaussian orbital model which allows for extensive optimization of geometry. Relative to the half-chair the and planar forms were calculated to be 2.6 and 9.8 kcal mol less stable, values which are larger than found previously. 

Torsion angles derived for (456) and (457) from R values have been reported as 40° and 45° respectively, and these angles are 13° and 10° greater than those derived from electron diffraction. The overestimation of torsion angles in five-membered rings may arise from a non-compliance of pseudo-three-fold symmetry which is implicit in the derivation of the torsion angle-R value relationship. A more general 

R value expression has been derived,although its utility is diminished by the inclusion of a parameter, the projection angle x in (463), the magnitude of which is difficult to assess. It is considered that the qualitative conclusions given for (456)—(462) are still valid. 

The C-satellite spectra have been obtained from the deuterium-decoupled n.m.r. spectra of [1,1,2,2,3,3- Hgjcyclopentane and [2,2,5,5- Hjcyclopentanone. From computer program-derived spectral parameters, an R value of 1.00 was determined, and with the assumption of a projection angle [c/ (463)] of 120° a ring torsion angle of 45° was determined (cf. ref. 130) and a half-chair conformation proposed. In cyclopentane the vicinal coupling constants are in accord with an unrestricted pseudo-rotational model. 

Two specifically deuteriated methylcyclopentanes (473) and (474) have been subjected to complete iterative H n.m.r. analysis between -1-17 and — 100°C It was shown that in the presence of a methyl substituent the cyclopentane ring no longer pseudorotates freely but is rather restricted to conformations where methyl is equatorial. A diagrammatic representation of the pseudorotational circuit is presented. 

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