Ring-puckering determination

In the chapter on vibrational spectroscopy (Chapter 6) 1 have expanded the discussions of inversion, ring-puckering and torsional vibrations, including some model potential functions. These types of vibration are very important in the determination of molecular structure. 

The microwave spectmm of 3,6-dihydro-1,2-dioxin 2 was measured in the frequency range of 10-26 GHz at dry ice temperature <1994JST(323)79> the structures on the basis of the rotational constants are half-chair conformers which readily interconvert at ambient temperature by ring puckering. This interconversion process could be frozen out spectroscopically and its free energy of activation determined (AG = 9.82 kcal moF ) by low-temperature NMR. 

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). 

A time-resolved ion yield study of the adenine excited-state dynamics yielded an excited-state lifetime of 1 ps and seemed to support the model of internal conversion via the nn state along a coordinate involving six-membered ring puckering [187]. In order to determine the global importance of the tict channel, a comparison of the primary photophysics of adenine with 9-methyl adenine will be useful, as the latter lacks a tict channel at the excitation energies of concern here. The first study of this type revealed no apparent changes in excited-state lifetime upon methylation at the N9 position [188] a lifetime of 1 ps was observed for both adenine and 9-methyl adenine. This was interpreted as evidence that the tict is not involved in adenine electronic relaxation. 

We shall also not deal in our paper with the large amplitude ring puckering and pseudorotation in small ring molecules. Molecular dynamics of these motions and the determination of the barriers of pseudorotation from microwave and infrared spectra have been the subject of a great number of papers this problem has been recently reviewed in the monography ... 

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. 

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. 

Fig. 4.3. Potential function for the ring-puckering vibration of thietanone-3 determined from fitting the far infrared transitions.

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. 

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... 

C4H402 CH=CHOCH=CHO 1,4-Dioxadiene FIR Potential function determined. Planar. Evidence for interaction of ring puckering with ring twisting. (See Table 4.18D) 84)... 

FIR, MIR, R Double minimum potential 14, 2 0, 21,48) function determined. 49, 66, 78 82) Barrier = 232 cm"1. Evidence for interaction between ring- puckering and ring-twisting (See Table 4.IV)... 

Determination of Potential Functions and Barriers to Planarity for the Ring-Puckering Vibrations of Four-Membered Ring Molecules... 

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