Trimethylene oxide , ring-puckering

Historically, trimethylene oxide was the first ring molecule for which a ring-puckering potential was determined from spectroscopic data. It has been the most extensively studied ring molecule, having been investigated by far infrared, micro-wave, Raman and mid infrared techniques5,9 4S-S9). Several isotopic species have been synthesized and studied. 

During the late 1950 s and early 1960 s when the initial work on the ring puckering in trimethylene oxide was done, data were much harder to obtain. High-resolution far infrared spectroscopy was in its infancy and Raman spectra of puckering vibrations had not yet been obtained. Today there is a wealth of data available on trimethylene oxide that strikingly demonstrates the success of the simple onedimensional quartic-quadratic Hamiltonian [Eqs. (3.22), (3.27)]. At the same time, since the data are so extensive, the limitations of the simple one-dimensional potential function can be examined. 

Fig. 4.5. Variation of the rotational constants (in MHz) with ring-puckering vibrational state for trimethylene oxide. The effect of the small barrier is quite dramatic when compared to the smooth variation in Fig. 2.5 for oxetanone-3.

Fig. 4.6. Ring-puckering potential function for trimethylene oxide. The height of the barrier ( 15 cm ) is less than the zero-point energy.

In principle, it should be possible to obtain enough data to correct the effective potential function for trimethylene oxide to a vibrationless state. This potential function should then be isotopically invariant. This may require determination of the ring-puckering intervals in the excited states of the other 3N - 7 modes and... 

This is not meant to be a historical review. It should be mentioned, however, that ring-puckering transitions were first directly observed in the far-infrared spectrum of trimethylene oxide (TMO) by Lord and his co-workers at MIT.4 Microwave studies on TMO were concurrently carried out by Gwinn and co-workers at Berkeley.5-8 Theoretical methods were principally developed there, particularly by Chan.6 8 An excellent review of the field has been written by Blackwell and Lord.9... 

Trimethylene oxide, the simplest oxetane, was first prepared by Reboul in 1878. ° Subsequent analysis of the stracture of this heterocycle revealed some interesting features. Thus, in contrast to what is observed with cyclobutanes, the oxetane ring is less puckered (Figure 13.1). The replacement of a methylene unit by an oxygen reduces the otherwise unfavorable eclipsing interactions that are minimized by out-of-plane distortion in the cyclobutanes. - As small saturated heterocycles, oxetanes display chemical as well as physical characteristics whose origins can be traced to their... 

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