Ring inversion, rate

Pressure-dependent studies of ring inversion of the six-membered rings previously described in the temperature-dependent section (see Table 6 for their structures and code names) have also been reported. Table 8 lists threshold energies and state densities for these molecules. In all cases, the pressure-dependent ring inversion rate constants could be adequately modeled using RRKM theory. 

Measurements of ring inversion rates in a wide range of environments [29-31] and the consistent picture presented by these calculations lead to a... 

Stilbs and Forsen ascribes this behaviour to a reduction in the exchange rate of (33a) (33b), with rapid ring inversion being maintained. The previous explanation, which invoked a diminished ring-inversion rate,was considered deficient since geminal spin-spin coupling of the C-2 protons was not observed. 

Substitution on a cyclohexane ring does not greatly affect the rate of conformational inversion but does change the equilibrium distribution between alternative chair forms. All substituents that are axial in one chair conformation become equatorial on ring inversion, and vice versa. For methylcyclohexane, AG for the equilibrium... 

Energy differences between conformations of substituted cyclohexanes can be measured by several physical methods, as can the kinetics of the ring inversion processes. NMR spectroscopy has been especially valuable for both thermodynamic and kinetic studies. In NMR terminology, the transformation of an equatorial substituent to axial and vice versa is called a site exchange process. Depending on the rate of the process, the difference between the chemical shifts of the nucleus at the two sites, and the field strength... 

Subsequently, this interpretation has been commonly accepted however, it does not account for the factors responsible for a decrease in the rate of the fast classical ring inversion. Hence, in our opinion, it is more reasonable to consider the stereodynamics of isoxazolidines as a result of superposition of two processes, a fast ring inversion (IR) and slow nitrogen inversion (shown in the lower portion of Scheme 3.166) (418). 

Frozen conformational equilibria. Although the rate of the interconversion of the various conformers (chair, boat, twisted) of the oxane derivatives at ambient temperature is fast on the NMR timescale, this ring-inversion process is sufficiently slow at low temperature (T < -60°C) to permit the observation of individual signals for each conformer. Direct integration delivers K and -AG°, respectively [see Eqs. (1) and (2)]. 

At -40 °C the spectrum corresponds to an intermediate rate of interconversion. The outer lines and four of the six components of the central line retain the same spin assignment upon ring inversion and remain fixed and narrow, while the remaining lines broaden and disappear. 

The rate of ring inversion of pentafluoro-l,2-oxazetidine as determined by 19FNMR is very fast ( a=4.2 kJ mol-1), with the nitrogen inversion barrier much higher (70JOC927, 1607). 

Table IV. Measurement of Conformational Equilibrium and Rate of Ring Inversion...

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 rate of another dynamic process, a conformational ring inversion, depends in particular on the nature of the substituent R at the a-carbon. The geminal exo and endo methylene hydrogens are interchanged by opening of the bridging bond and planarization... 

It is obvious that independently of the rate of exchange the NMR spectrum of a system subject to a dynamic equilibrium depends on the relative concentrations of the components present. These may be expressed as mole fractions or, in the case of intramolecular exchange, as conformer populations. Sometimes the populations are governed by symmetry effects alone and are independent of the experimental conditions. This is always the case for mutual exchange when the process does not alter the chemical identity of a molecule. Examples of such exchange may be drawn from rotations of the t-butyl group in (CH3)3C-C(R1)(R2)(R3), and those about the CO-N bond in N,N-dimethyl-substituted amides (CH3)2N-C(=0)R. Other examples include ring inversions such as those in 1,2-ds-dimethylcyclohexane, 1,3-trans-... 

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