Ring-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. 

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).

Fig. 2.3. Combination and difference band progressions involving the ring puckering vibration and a CH2 scissoring mode in the mid-infrared spectrum of cyclobutane.

Fig. 2.5. Variation of the B rotational constant (in MHz) with ring-puckering vibrational state for oxetanone-3. Similar variations are found for the A and C rotational constants. [Reproduced from Gibson, J. S., Harris, D. O. J. Chem. Phys. 57, 2318 (1972).]...

The variation of rotational constants with ring-puckering vibrational state is very sensitive to the presence of a barrier at the planar conformation. This is shown for cyclobutanone16 and methylenecyclobutane17) in Fig. 2.6. The presence of a very small barrier, ca. 7.6 cm-1 in the case of cyclobutanone, causes deviation from a smooth variation for the lower levels. In the case of methylenecyclobutane, a very pronounced zig-zag of the rotational constants is observed due to the presence of a 140 cm-1 barrier. The dependence of the rotational constants on vibrational state may be used quantitatively to determine the shape of the potential function as discussed in subsequent sections. 

Fig. 2.6. Variation of the A rotational constants (in MHz) with ring-puckering vibrational state for cyclobutanone and methylene-cyclobutane.

A recent review of ring-puckering vibrations with emphasis on the theory and applications of microwave spectroscopy has appeared. The reader is referred to this work for further details19. ... 

---