Of rotational barriers

The values of rotation barriers about the central E14-C bond are 11.6-13.0kcal/mol. 

TABLE 22. A comparison of rotational barriers and conformational energies (kcal mol-1) for several amino compounds and a single nitro compound between several commonly used force fields and experiment (all experimental and calculated data are taken from Reference 60 unless otherwise noted) die last entry provides the Average Absolute Error (kcal mol-1) between theory and experiment60. Reproduced by permission of John Wiley Sons, Inc. from Ref. 60... 

In the first of four chapters in this volume of Topics in Stereochemistry, Michinori Oki presents a comprehensive review of atropisomerism with special reference to the literature of the past two decades. The review summarizes restricted rotation about sp2-sp2, sp2-sp, and sp3-sp3 bonds and it concludes with an analysis of reactions of isolated rotational isomers. It places particular emphasis on the magnitude of rotation barriers as a function of structure (incidentally identifying some of the largest barriers yet measured to conformer interconversion) and on the isolation of stable single-bond rotational diastereomers. 

Several attempts have been made to analyse the captodative effect through rotational barriers in free radicals. This approach seems to be well suited as it is concerned directly with the radical, i.e. peculiarities associated with bond-breaking processes do not apply. However, in these cases also one has to be aware that any influence of a substituent on the barrier height for rotation is the result of its action in the ground state of the molecule and in the transition structure for rotation. Stabilization as well as destabilization of the two states could be involved. Each case has to be looked at individually and it is clear that this will provide a trend analysis rather than an absolute determination of the magnitude of substituent effects. In this respect the analysis of rotational barriers bears similar drawbacks to all of the other methods. 

Proton or deuteron relaxation is also useful in this respect as shown for NH4PO4H2(19e7 195 ana i96) an(j for e determination of rotational barriers in solid phosphonium halides.<1967-197 1968- 179andl8° iseo,210)... 

F. Leroux, Atropisomerism, biphenyls, and fluorine A comparison of rotational barriers and twist angles, ChemBioChem 5(5) (2004) 644-649. 

Clough Roberts J. Am. Chem. Soc. 1976, 95, 1018. For a study of rotational barriers in this system, see Cosmo Sternhcll Aust. J. Chem. 1987, 40, 1107. 

SCF MO calculations of rotational barriers are generally quite successful. For the NH3 inversion barrier, early SCF calculations yielded poor results. This was due to the inadequacy of the basis sets used. A calculation that included sufficient basis functions to reach near Hartree-Fock ip s gave a barrier of 5.1 kcal/mole, in error by only 0.7 kcal/mole. [A. Rauk, L. C. Allen, and E. Clementi, J. Chem. Phys., 52, 4133 (1970).]... 

Variable temperature NMR measurements have allowed a number of rotational barriers to be quantified.54 Similar measurements have been made on some phosphoranes.107... 

A semiempirical study of rotational barriers of various thioamides and thioureas was published by Feigel and Strassncr111" and a careful, high level ab-initio study of formamide and thioformamide has been carried out by Chu and coworkers1116. 

Simple yet important mechanistic cases concern the computation of rotational barriers around single bonds. A timely case in point is the rotational barrier of ethane, an old yet much debated subject [27, 28]. While the notion of hindered rotation in ethane is often... 

Wu, Y.-D. Houk, K. N. Theoretical studies of rotational barriers of heteroatom derivatives of methanol./. Phys. Chem. 1990, 94, 4856-4861. 

Biomimetic Efficiency A Structural and Electronic Investigation of Rotational Barriers Found in DFT-inspired Fe-hydrogenase Models... 

Molecular orbital calculations of rotation barriers around the carbon-nitrogen bond in thioamides, amidinium salts, amidines and enamines have been described by Sandstrom61. 

Templeton has carried out extensive studies of rotational barriers of alkynes and has calcnlated a nnmber of rotational energy profiles his papers provide leading references to earlier work. Pyrazolylborate analogues also show similar behavior and this work as well as earlier reviews provides leading references. 

Lanthanide shift reagents have been used to study dynamic NMR processes. One of the classic applications involves the study of rotational barriers of various amides " thioamides ", oximes " and amide oximes ". Lanthanide-induced shifts enabled the distinction of cis- and trans- or syn- and awri -isomers, and variable-temperature studies in the presence of the chelate were performed to determine the barrier to rotation. 

Musso, G.F. and Magnasco, V. (1982) Bond-orbital analysis of rotation barriers. Polarization and delocalization effects in ethane. J. Chem. Soc. Faraday Trans. II, 78, 1609-1616. 

The bulkiness of a CF3 group has been estimated on the basis of comparison of rotational barriers along the biphenyl axis of l,l -disubstituted biphenyls 8a to 8c [51] as well as 9a and 9b [52], These data clearly indicate that the bulkiness of a CF3 group is similar to that of a (CH3)2CH group (see Figure 1.5) [51-53],... 

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