Rotation barrier, also

The back-donation interactions towards the carbenes decrease, so the rotation barrier also decreases. 

For M = Rh, the H and H can be observed separately in the NMR spectrum at -20 C, but the peaks collapse to one at 57°C. The analogous C2F4 compound shows no collapse of the F spectrum up to 100°C. The rotational barrier also is increased by CH3 groups on the cyclopentadiene ring and by changing from Rh to Ir. These differences can be explained by steric and better tc back bonding factors, but they do not determine whether rotation is about the metal-olefin or C=C bond. 

These structural effects are also found by MO calculations. Calculations at die MP4/6-311++G level have been performed on the ally cation and indicate a rotation barrier of 36-38 kcal /mol. ... 

One way of reducing the number of parameters is to reduce the dependence on atom types. Torsional parameters, for example, can be taken to depend only on the types of the two central atoms. All C-C single bonds would then have the same set of torsional parameters. This does not mean that the rotational barriers for all C-C bonds are identical, since van der Waals and/or electrostatic tenns also contribute. Such a reduction replaces all tetra-atomic parameters with diatomic constants, i.e. 

Methylenecyclopropane, 48, 194 bond lengths, 38 rotational barrier, 38 Methylenimine, 83 MINDO A 54 Molecular geometries, 287 Molecular orbitals, 57, see also individual molecules... 

For 6, the activation energy for rotation about the MSi bond has been measured as AG = 40.3 (+ 5) kJ/mol [143]. According to MO calculations, a genuine Cr = Si double bond has no rotational barrier worth mentioning. This applies also, with some restrictions, to the discussed base adducts. 

The theoretical approach used above to elucidate the conformational preferences of CH3-CO-X molecules can also be applied to a discussion of the methyl rotational barrier in these systems. The methyl rotational barrier corresponds to the energy difference between the eclipsed and staggered conformations with the eclipsed conformation being an energy minimum and the staggered conformation being an energy... 

The above analysis can also be used in connection with the problem of the methyl rotational barrier in double rotor molecules, e. g. dimethyl ether, relative to... 

Finally, experimental measurement as well as ab initio computation show that the methyl rotational barrier is also higher in the cis than the tram conformation. These results are shown in Table 17. 

A. Greenberg and T. A. Stevenson, Molecular Structure and Energetics Studies of Organic Molecules (Eds. J. F. Liebman and A. Greenberg), VCH, Deerfield Beach, 1986. See also the discussion in J. F. Liebman and R. M. Pollack, in The Chemistry ofEnones, Part 1 (Eds. S. Patai and Z. Rappoport), Wiley, New York, 1989 wherein the resonance energy of crotonaldehyde was shown to be less than that of piperylene while the rotational barriers are in the reverse order. 

Most of the data in Table 12 come from the work of Shvo et al. (78). Careful band-shape analysis and solvent-effect studies permitted evaluation of the rate constants and AG values at 298 K, which renders the discussion of substituent effects more meaningful than usual. The authors obtained reasonably linear Hammett plots when correlating log km with Or (79) for X and Y, holding one of these substituents constant. They also found that the dihydropyridine system may act as an unusually efficient donor, giving a AG of 17.6 kcal/mol with X, Y = H, CN, the only barrier below 25 kcal/mol reported for any donor-substituted cyanoethylene. However, with other acceptor combinations the dihydropyridine moiety is not so outstanding, and this illustrates the difficulty of measuring donor and/or acceptor effects by rotational barriers alone (vide infra). 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

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