Puckering motion

Microwave spectroscopy has been applied to the study of ring-puckering motions in small-ring compounds, and to the study of conformational isomers. See V. W. Laurie, Accounts Chem. Res., 3, 331 (1970). 

Conformational analysis of 1-chlorosilacyclobutane employing microwave spectroscopy data has shown that the equatorial conformer is more stable than the axial one by 185(40) cm-1. The potential energy function of the ring-puckering motion has been determined for that molecule (Figure 1). 

An Arrhenius-type plot of the correlation time of the overall motion (calculated with the use of Eq. 16) versus reciprocal temperature yields an activation energy of 20.4 kJ/mol. Among the various models examined to evaluate internal rotations, a two-state jump model56,143 was found satisfactory for interpreting the oscillatory and puckering motions of the flexible 5,6-O-isopropylidene ring. In this model the C—H vector jumps between two equivalent, stable states A and B characterized by correlation times ta and tb, respectively (Fig. 10). The angle (3 is defined by the... 

In the interpretation of the spectral data it is usually the constants A and B in Eq. (3.27) or some other set of reduced potential constants that are evaluated. The barrier height, AB2/4 for B < 0, is thus determined directly. However, if one wants to relate the value of the dimensionless parameter Z I at the minimum of a double-minimum potential function to the absolute geometry of the puckered ring, the reduced mass must be known in order to find x from IZI. One is then required to introduce assumptions about the dynamical model of the ring-puckering motion. 

Gwinn, W. D., Gaylord, A. S. Spectroscopic Studies of Ring-Puckering Motions, in MTP International Reviews of Science, Physical Chemistry Series 2, Vol. 3 Spectroscopy, Ramsay, D. A., (ed.) London and Baltimore Butterworths and University Park Press 1976... 

In some cases, the identity of paramagnetic "satellite ions" was established by ODMR [42,44,48]. For example, 9,10-octalin + was identified in decalins and their solutions [42]. ODMR spectra of "satellite ions" in cyclohexane were related to EPR spectra of matrix-isolated cyclohexene + (Note that in the liquid cyclohexane, cyclohexene + undergoes a fast ring-puckering motion that averages hyperfine coupling constants for equatorial and axial protons, so the the EPR spectra of cyclohexene + in liquid and solid matrices are different) [42,44,48]. In both of these cases, the olefin radical cations were formed in spurs rather than in a reaction of the solvent hole with the olefin in the solvent bulk [42] (octalins gradually accumulate as radiolytic products). Olefin "satellite ions" were also observed in squalane [24]. 

If this can be interpreted as implying that it is only the interconversion of two 7T bonds and two a bonds that determines the symmetry requirements of the reaction, it would appear that a composite bending cum puckering motion like that shown in (I) of Fig. 7.10 should suffice to allow concerted [ 2 + ]-cycloaddition, provided that the reacting olefins are suitably substituted for producing a czs-joined four-membered ring. 

Later workers contended that pseudo-rotation was an unnecessary concept and made vibrational assignments for cyclopentane in which the ring-puckering motion was treated as two genuine degrees of freedom. ... 

---