Rotational barriers barrier height

Calculated Values for Barrier of Rotation and Barrier Heights... 

In the meantime, other Tt-delocalized systems were discovered among open-chain systems, notably the three allylic systems, Scheme 4. All of these systems showed a behavior akin to the aromatic systems they are delocalized, have uniform geometries, and are more stable than saturated analogues.90-93 Furthermore, they all possess rotational barriers with heights related to the resonance stabilization of the species. 

CH Si Torsion in PDMS. The torsional potential curve has three degenerate minima because of the threefold symmetry of the methyl group. For this reason. Figure 6 shows only the behavior of one-third of the full rotation. The barrier height is 1.7 kcal/mol when the backbone is allowed to relax and 2.4 kcal/mol when the backbone is frozen. The experimental value is 2.2 kcal/mol (i), but this value is based on results determined at two temperatures above and below Tg. Our results indicate that at least one more experimental measurement, at Tg for the Si-CHa bond, is needed. These calculations predict that the new measured value will set the data... 

The functional form for dihedral angle (torsional) rotation is identical to that shown in equation (13) on page 175. The barrier heights are in kcal/mol and are in the file pointed to by the Fouri-erTorsion entry for the parameter set in the Registry or the chem. ini file, usually called tor.txt(dbf). If more than one term is... 

Fig. 55. The potential of hindered rotation of the CH3 group in nitromethane (CH3NO2) crystal, (a) calculated from INS data, Vi = 0.586 kcal/mol, V = 0.356 kcal/mol, S = 30°, and (b) calculated with the atom-atom potential method [Cavagnat and Pesquer 1986]. The barrier height is 0.768 kcal/mol.

The haloethanes all have similar rotational barriers of 3.2-3.7 kcal/mol. The increase in the barrier height relative to ethane is probably due to a van der Waals rqjulsive efiect. The heavier halogens have larger van der Waals radii, but this is ofiset by the longer bond lengths, so that the net efiect is a relatively constant rotational barrier for each of the ethyl halides. 

Absolute activity, 12, 13 Absolute intensity, 192 Acetaldehyde barrier height of internal rotation, 378, 382, 383, 388 Acetonitrile, in clathrate, 20... 

Bromoethane, barrier height of internal rotation, 382 hydrate, 10... 

In a real polymer chain, rotation around backbone bonds is likely to be hindered by a potential energy barrier of height AEr. If AEr < RT, the population of the... 

The ESR spectrum of methanesulfinyl radical (CH3SO), identified in a y-irradiated single crystal of dimethyl sulfoxide , indicates that the unpaired electron resides essentially (72%) on the sulfur 3p orbital with modest population on the sulfur 3s (0.65%) A detailed analysis of the temperature dependence leads to 2.6 kcal mol barrier height for the hindered internal rotation of the methyl group. At low temperature (88 K) the radical adopts a fixed conformation in which one proton lies in the nodal plane of the sulfur 3p orbital however, it was not possible to distinguish either experimentally or by ab initio SCF-MO calculations between the two possible conformations, that is, 2 and 3. 

With accurate calculated barriers in hand, we return to the question of the underlying causes of methyl barriers in substituted toluenes. For simpler acyclic cases such as ethane and methanol, ab initio quantum mechanics yields the correct ground state conformer and remarkably accurate barrier heights as well.34-36 Analysis of the wavefunctions in terms of natural bond orbitals (NBOs)33 explains barriers to internal rotation in terms of attractive donor-acceptor (hyperconjuga-tive) interactions between doubly occupied aCH-bond orbitals or lone pairs and unoccupied vicinal antibonding orbitals. 

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