Inversion barriers, force fields

Trioxocane exists as a 1 1 mixture (i.e, AG° = 0) of a boat-chair and a crown family conformation with an interconversion barrier of 36 kJ mol-1 (72JA1390,1389). The ring inversion barrier in the boat-chair is 28 kJ mol-1 but the barrier to pseudorotation has not been determined. One set of force-field calculations shows that the BC-1, 3, 6 (435) is of... 

The same authors, in a different paper (93JA3494), have shown that the N-R substituent prefers to be equatorial and that the conformation about the exocyclic N-R bond is due to a rapid equilibrium between two nearly eclipsed conformations (cf. Scheme 37b). The second ground state of the dynamic process, the own-conformation (after 180° of rotation), could not be detected experimentally but was calculated by molecular dynamics to be less stable by 5.5 kcal/mol than the preferred, vy -conformers, e.g., 99h (cf. Scheme 37). The transition state in 99b-e and 99g-l occurs when the exocyclic substituent has its maximum interaction with an equatorial methyl group about 120° on either side of the ground state. The dynamic behavior of 99h,i and 99o (R = Bu) was also studied by the MM3 force field (99CEJ449) the authors concluded that the experimental barriers of 99h,i and 99o belong to a complex combination of ring inversion, N-inversion and rotation about the exocyclic bond. 

Cyclo-octatetraene presents an interesting calculational problem. The molecule exists in a tub conformation, the structure of which is well established experimentally (Traetteberg, 1966). Two barriers to inversion have also been determined. The molecule can invert (or reach a planar conformation) with or without a bond shift, so that the double bonds become single, and the single bonds become double. The barriers to both processes are known. The simple mechanical inversion can be treated adequately by force-field methods, and that barrier has been calculated to be 15-1 kcal mole-1 (Allinger et al., 1973) [experimental (Anet, 1962 Anet et al., 1964) 13 7 kcal mole-1 for AG+]. Because of quantum mechanical complications, the force-field method is not suitable in its present form for calculation of the barrier to bond shifting. 

Aped et al. (214) used the MM3 force field to calculate the minimum energy conformations of propellane 120 and the corresponding transition states on the inversion path (Scheme 36). A comparison of the inversion barriers calculated for the overall process (86 kj/mol) and found experimentally by NMR measurements (49 kJ/mol) led the authors to the conclusion that it is difficult to obtain accurate results, since MM3 is currently not well parametrized for the gauche effect also the solvent and entropy effects could not be properly assessed (214). 

Aarset, K., Csaszar, A.G., Sibert 111, E.L., AUen, W.D., Schaefer 111, H.F., Klopper, W., Noga, J. Anharmonic force field, vibrational energies, and barrier to inversion of SiH3, J. Chem. Phys. 

Another important property of thiepine is the equilibrium between thiepine (1) and its valence isomer, benzene episulflde (9). Benzene episuliides are found to be so unstable that they can not be detected hence theoretical treatment should provide helpful information. MNDO molecule force field calculations of the zero point energy, potential energy, and thermodynamic parameters for the valence tautomerization of benzene episulflde (9) and thiepine (1) show that (9) is more stable than (1) both in the gas phase and in polar media (Equation (1)). The equilibrium is shifted towards (1) in polar media. The vapor phase inversion barrier for (1) is 3-5 kJ mol , and is reduced to zero by polar solvents <83MI 903-01 >. 

In the study by Anet and Anet the abilities of nine different force fields were tested to reproduce geometry and energy data on a single molecule cis-sy -cis-perhydroanthracene. The aim of this study was to test the performance of force fields with experimental data not available at the time of force field parameterization. Experimental conformational energy data (inversion energy barrier) was best reproduced by the Boyd force field, followed by MM2 and the White-Bovill force field. Except for MM2, the force fields included in this study are not extensively used today. A discussion of these earlier force fields has been published. ... 

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