Ring-flip energy barrier

Such a change would cause a substituent in an axial position to go to an equatorial position and vice versa. This process is called ring inversion and its rate often is called the inversion frequency. With cyclohexane, inversion is so fast at room temperature that, on the average, the molecules flip about 100,000 times per second, over an energy barrier of about 11 kcal mole-1. 

The mean value of the energy barrier for the 38 resulting phenyl ring flips was 45 d= 28 kj mol-1. The peak energy barriers showed a broad distribution, which could be fitted to a Williams-Watts distribution function with a between 0.1 and 0.2. By applying the transition state theory, the distribution of ring flip frequencies could be derived, as shown in Fig. 59. 

The mean value of the energy barrier for carbonate group rearrangements is 43 28kJmol-1. The energy barrier distribution is quite similar to that obtained for ring flips. 

When the energy barrier for phenyl ring it-flips found in bulk BPA-PC (45 28kJmol x) is compared to the value calculated for the BPA-PC repeat unit (42 kj mol-1), there is not too much difference. However, this energy barrier... 

Quantitatively, line-shape analysis was used to determine rate data for these stereoisomerizations in terms of the two-ring flip mechanism. 32> The associated free energies of activation for the various exchange processes at 20 °C are shown schematically in Fig. 10. For the equilibrium BB AA, AG°2o is 0.3 kcal/mol. For the conversion of BB to AA, the calculation yielded AG 0 16.2 kcal/mol. and for the reverse reaction (AA- -BB), AGt0 15.9 kcal/mol. The barrier to enantiomerization of B and B is AG%B 14.6 kcal/mol. Thus, at 20 °C the enantiomerization of B and B is energetically more favorable by 1.6 kcal/mol than that of A and A. 

The stereochemically correspondent triarylcarbenium ions show similar behavior, and isomerizations in these systems can also be explained in terms of two-ring flips. 9>33> However, carbenium ions which have only hydrogen atoms in the ortho positions have activation energies of ca. 10—14 kcal/mol 34> (Table 3). It follows that in systems with comparable substituent patterns, i.e., in systems whose ortho ligands offer comparable steric encumbrance, triarylcarbenium ions have substantially higher barriers than the corresponding triarylboranes. No... 

We turn to empirical force field calculations in order to choose between these two mechanisms. 44 Such calculations indicate that the two-ring flip mechanism is the lowest-energy pathway, and yield a barrier of 20 kcal/mol for the two-ring flip of 7. 44> The experimental free energy of activation for stereoisomerization derived from the temperature-dependent 1H-nmr spectrum, is AG 67 21.9 kcal/mol 43, in excellent agreement with the calculated value. This high barrier admits of the possibility that 7 is separable into its optical antipodes at moderately low temperatures. 

A chair cyclohexane can be ring-flipped by keeping the middle four carbon atoms in place while folding the two ends in opposite directions. An axial. substituent in one chair form becomes an equatorial substituent in the ring-flipped chair form, and vice versa. For example, axial bromocyclohexane becomes equatorial bromocydohexane after ring-flip. Since the energy barrier to chair-chair interconversion is only about 45 kJ/mol (10.8 kcal/mol), the process is extremely rapid at room temperature. We therefore see only what appears to be a single structure, rather than distinct axial and equatorial isomers. 

The spectrum of the cyclohexyl radical at — 80°C shows that the four jS-protons couple as two non-equivalent pairs, indicating that the chair-chair interconversion is relatively slow. As the temperature is raised, certain lines broaden and ultimately disappear, corresponding to an increase in the rate of ring-flipping the activation energy barrier has been calculated to be 4 9 0-5 kcalmole" (Ogawa and Fessenden, 1964), which may be compared with the barrier of 11 kcalmole" for cyclohexane itself (Jensen et al., 1962). 

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