Puckering vibrations

Figure 6.42 (a) Cyclobutane, its ring-puckering vibration and dihedral angle, (b) Cyclopentene... 

The structures of four-membered rings are of considerable interest, owing in part to the low-frequency ring puckering vibration . The comparison of the structures and conformational preferences of thietane oxides and dioxides discussed above with those of dithietane oxides and dioxides is therefore appropriate and will follow. 

Substitution of alkyl and aryl groups on the oxetane ring, on the other hand, increases the barrier of the puckering vibration. 2-Methyloxetane and 3-methyloxetane are non-planar, existing as mixtures of stable equatorial and axial conformers at room temperature (79JST(56)157>. 

Recent studies in vibrational spectroscopy have centered on the far-IR and Raman region, in order to study the ring puckering vibration. The results of this work are taken up in Section 5.03.1.4. 

Table 5 Potential Barrier for Ring Puckering Vibration in Oxetanes...

Table 5 lists values of the potential barrier for the ring puckering vibration in various oxetanes (see also Section 5.13.2.3.2). 

Figure 4-13 (a) Ring-puckering vibration of cyclopentene. (b) Definition of the ring-puckering coordinates for a four-membered ring molecule. (Reproduced with permission from Ref. 30.)... 

Figures 4-14 and 4-15 show the gas-phase IR and Raman spectra, respectively, of this compound obtained by Irwin et al. (31). Using the foregoing procedures, these workers obtained the potential energy curve shown in Fjg. 4-16. The values of a, b and the barrier were found to be 2.3 x 105 cm 1/A4, -9.0 x 103cm 1/A2 and 87cm-1, respectively. For more details on ring puckering vibrations, see a review by Laane (32).

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