Conformational energies inversion barrier

Nitrogen inversion in a series of bicyclic hydrazines has been characterized from low temperature lineshapes obtained at the natural abundance level (53). Two sets of resonances separated by 1-2 ppm coalesced into singlets in the range 90-100 ppm over the temperature range 250-280 K. Activation free energy inversion barriers of 12.7-14.8 kcal/mol were consistent with those obtained from proton and experiments. Similarly, conformational equilibria in jpis-decahydroquinolines have been elucidated by comparison of shifts in rigid compounds with those in conformationally mobile ones (54). 

Rychnovsky et al. considered the formation of achiral conformers from chiral molecules and trapping the prochiral radical with a hydrogen atom donor based on memory of chirality (Scheme 12) [41], The photo-decarboxylation of optically active tetrahydropyran 40 leads to an intermediate 43, which now does not contain a stereocenter. If the intermediate 43 can be trapped by some hydrogen atom source before ring inversion takes place, then an optically active product 41 will be formed. This is an example of conformational memory effect in a radical reaction. It was reported that the radical inversion barrier is low (< 0.5 kcal/mol) while the energy for chair flip 43 44 is higher (5 to... 

Conformational inversion barriers of 5,10-substituted decalins (17) were determined by NMR, and MMI calculations were carried out to see whether the twist-chair (ct) or the boat-boat (bb), correspond to the higher barrier. The ct was found to be higher in cw-decalin (17, R = H), but the introduction of substituents at the ring junction is expected to raise the energy of the bb. The observed AG agreed with the calculated value for ct, which is about 5 kcal/ mol higher than that of bb when R = CH3 or OH (124). 

This chapter assesses the ability of molecular mechanics and quantum chemical models to properly assign preferred conformation, and to account quantitatively for differences in conformer energy as well as for barriers to rotation and inversion. The chapter ends with a discussion of ring inversion in cyclohexane. 

Closely related to conformational energy differences are barriers to single-bond rotation and to pyramidal inversion. Here the experimental data are restricted to very small systems and derive primarily from microwave spectroscopy, from vibrational spectroscopy in the far infrared and from NMR, but are generally of high quality. Comparisons with calculated quantities are provided in Table 8-3 for single-bond rotation barriers and Table 8-4 for inversion barriers. The same models considered for conformational energy differences have been surveyed here. 

As with conformational energy differences, SYBYL and MMFF molecular mechanics show marked differences in performance for rotation/inversion barriers. MMFF provides a good account of singlebond rotation barriers. Except for hydrogen peroxide and hydrogen disulfide, all barriers are well within 1 kcal/mol of their respective experimental values. Inversion barriers are more problematic. While the inversion barrier in ammonia is close to the experimental value, barriers in trimethylamine and in aziridine are much too large, and inversion barriers in phosphine and (presumably) trimethylphosphine are smaller than their respective experimental quantities. Overall,... 

Compilations of experimental data relating to conformational energy differences and rotation/inversion barriers may be found in (a) T. A. Halgren andR.B. Nachbar,/. ComputationalChem., 17, 587 (1996) (b) T.A. Halgren,... 

The conformational equilibria of 2,5,6-tri- and 2,5,6,6-tetramethyltetrahy-dro-l,2,5-oxadiazine were also studied.354 These compounds show a high-energy barrier at 14 kcal mol -1 together with 5-N—Me inversion barriers of 8.2 and 7.7 kcal mol-1. The insertion of a 2-CMe into the l-oxa-3-aza system displaces the 3-NMe equilibrium toward the 3-NMe equatorial con-former (cf. AG° 0.10 kcal mol-1 favoring 453 for 2,5,6-trimethyltetrahydro-1,2,5-oxadiazine). 

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