Bond rotation, barrier

From such comparisons one can judge that the syn/anti difference is sensitive to antibond polarity (which shifts the nodal plane relative to the adjacent donor NBO) and to the bond angles at which donor and acceptor NBOs are canted toward one another. Compared with C—H bonds, rotation barriers involving polar A—X bonds... 

Wilcox and co-workers (145) reported that the stereoselectivity of 1,3-dipolar cycloaddition reactions can be controlled in a predictable manner when ion pairs are located at a proper position near the reaction site (Scheme 11.40). He has employed an A-phenylmaleimide substrate having a chiral center in the substituent at ortho position of the phenyl group. Due to serious steiic hindrance, this phenyl group suffers hindered rotation around the aryl-nitrogen bond (rotation barrier 22 kcal/mol). Four diastereomeric cycloadducts are possible in the cycloaddition step with a nitrile oxide. When the cycloaddition reaction is carried out in... 

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. 

Semi-empirical models do not provide good descriptions of the energy barrier to ring inversion in cyclohexane. The MNDO model underestimates the barrier by a factor of three, and the AMI and PM3 models by almost a factor of two. This behavior is consistent with previous experience in dealing with single-bond rotation barriers. 

Betaines, with 1,2-vinylene linking group double bond rotation barriers, 60, 226 formation, 60, 205, 2( -9. 213-4. 216 nmr spectra. 60, 224-6, 227 theoretical calculations, 60, 240-1 Bicyclo(2.2.0]hexa-2,5-dienes, see Dewar benzenes... 

Fig. 3. Important valence orbitals of some metal fragments. The energy scale markings are eV. (Reprinted, with permission, from Ethylene Complexes, Bonding, Rotational Barriers, and Conformational Preferences, Albright, T. A. et al. J. Am. Chem. Soc. 101, 3802, Fig. 1, copyright, 1979, by the American Chemical Society)...

The structural and chemical similarities prevalent between C and (9-glycosides are illustrated in Table 7.1 and are summarized as follows. Given that the bond lengths, van der Waals radii, electronegativities and bond rotational barriers are very similar between (9- and C-glycosides,... 

The C-N bond rotation barriers in amides are generally lower in the gas phase than in solution. For example, the barrier in dimethylformamide is on average 1.5 kcal / mol lower in the gas phase than in the solution phase. There are at least two p>os-sible explanations. What are these ... 

The olefin bond rotation barriers have also been determined for the vinylaniline complexes 2 using several NMR techniques, namely selective inversion, total line shape analysis, and Tip measurements (i.e., spin-lattice relaxation rate in the rotating frame). 

Otani, Y, Nagae, O., Naruse, Y. et al. (2003) An evaluation of amide group planarity in 7-azabicyclo[2.2.1]heptane amides. Low amide, bond rotation barrier in solution. Journal of the American Chemical Society, 125,15191 15199. 

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