Barrier of rotation about

To account for barriers of rotation about chemical bonds, i.e., the energetics of twisting the 1,4-atoms attached to the bonds formed by the atoms 2-3, a three-term torsion energy function like that in Eq. (24) is used, in the given form or slightly modified, in almost every force field. 

They found no evidence of (Z)/(E)-isomerism in the diazosulfones formed. This may be due to the lowering of the barrier of rotation about the NN double bond by the contributions of expanded octet structures such as 6.17 b. It is therefore likely that the observed diazosulfones are ( -compounds. 

Spivey and coworkers reported in 1999 the nse of axially chiral analogs of 4-DMAP 32 and 33, which rely on the high barrier of rotation about an aryl-aryl bond at the 3-position of 4-DMAP to produce atropisomers that are selective in the acylation of. yec-alcohols (Scheme 13) [117-127],... 

Irradiation of butadiene rubbers gives rise to a broad component, with the intensity increasing with the irradiation time. It is found that rubbers crosslinked with irradiation have more hindred motion than rubbers crosslinked with sulfur this is explained by the higher potential barrier of rotation about C—C bonds, as compared to C—S bonds ... 

The amide VII is a mixture of NH out-of-plane bend and CN torsion. It is related to the barrier of rotation about the CN bond. 

Barrier of rotation about carbon-heteroatom bonds (compilation published in [83])... 

The barrier of rotation about an sp -sp bond is related to ortho steiic... 

All these derivatives with varying R groups display a similar type of ordered structure in the solid state as well as in solution in aprotic solvents such as CDClj or CHjCN. This was derived by combining the results from NMR, IR and CD spectroscopic investigations [19, 20]. Ferrocene itself is an achiral compound. l,T-disubstituted derivatives, however, it may exist in two enantiomeric forms as shown in Scheme 5.4. In ferrocene, the barrier of rotation about the metal-Cp... 

Compare the energy barrier of conversion of c/j-l,2-dichloroethylene to // fl/zj -l,2-dichloroethylene to the barrier of rotation about the carbon-carbon single bond of ethane. 

The origin of a torsional barrier can be studied best in simple cases like ethane. Here, rotation about the central carbon-carbon bond results in three staggered and three eclipsed stationary points on the potential energy surface, at least when symmetry considerations are not taken into account. Quantum mechanically, the barrier of rotation is explained by anti-bonding interactions between the hydrogens attached to different carbon atoms. These interactions are small when the conformation of ethane is staggered, and reach a maximum value when the molecule approaches an eclipsed geometry. 

The barriers to rotation about the N-C bond have been determined b dynamic nuclear magnetic resonance for A -isopropyl (80. 81). propanoic acid (74). A -ethyl (82). N-benzyl. and A -neopentyl substituents (82). Selected values of these barriers are given in Tables VII-6 and VII-7. 

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

The small optical rotations of the alditols arise from the low energy barrier for rotation about C—C bonds, permitting easy iaterconversion and the existence of mixtures of rotational isomers (rotamers) ia solution (12). 

Before considering the special case of rotation about bonds in polymers it is useful to consider such rotations in simple molecules. Although reference is often made to the free rotation about a single bond, in fact rotational energies of the order of 2kcal/mole are required to overcome certain energy barriers in such simple hydrocarbons as ethane. During rotation of one part of a molecule about... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

Dynamic structural characteristics can also be interpreted in terms of resonance. There is a substantial barrier to rotation about the C—N single bonds in carboxamides. A frequently observed consequence is the nonidentity of NMR peaks due to the syn and anti... 

One of the frmdamental structural facets of organic chemistry, which has been explained most satisfactorily in MO terms, is the existence of a small barrier to rotation about single bonds. In ethane, for example, it is known that the staggered conformation is about 3kcal/mol more stable than the ecl sed conformation so that the eclipsed conformation represents a transition state for transformation of one staggered conformation into another by rotation. 

The torsional strain is a sinusoidal function of the torsion angle. Torsional strain results from the barrier to rotation about single bonds as described for ethane on p. 56. For molecules with a threefold barrier such as ethane, the form of the torsional barrier is... 

The 7t-electron delocalization requires proper orbital alignment. As a result, there is a significant barrier to rotation about the carbon-carbon bonds in the allyl cation. The results of 6-31G/MP2 calculations show the perpendicular allyl cation to be 37.8 kcal/mol higher than the planar ion. Related calculations indicate that rotation does not occur but that... 

Aldiough diese structures have a positive charge on a more electronegative atom, diey benefit from an additional bond which satisfies file octet requirement of the tricoordinate carbon. These carbocations are well represented by file doubly bonded resonance structures. One indication of file participation of adjacent oxygen substituents is file existence of a barrier to rotation about the C—O bonds in this type of carbocation. 

Such a structure implies that there would be a barrier to rotation about the C(2)—C(3) bond and would explain why the s-trans and s-cis conformers lead to different excited states. Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of [2 + 2] and [2 + 4] addition products on sensitizer energy. The s-Z geometry is suitable for cyclohexene formation, but the s-E is not. The excitation energy for the s-Z state is slightly lower than that for the s-E. With low-energy sensitizers, therefore, the s-Z excited state is formed preferentially, and the ratio of cyclohexene to cyclobutane product increases. ... 

Rotation about single bonds and conformational changes can be studied. Amides constitute a classic example. Because of the partial double bond character of the carbon-nitrogen bond as a consequence of the contribution of 2 to the electronic structure, there is an energy barrier to rotation about this bond. 

The C1 n.q.r. spectra of ClS02 N=PCl3 have been followed over a range of temperatures, thus enabling barriers to rotation about S-N and P-N bonds to be calculated as 0.940 and 6.3 kcal mol respectively. 

A quite different and complimentary approach is to assume that addition of a nucleophile to an acyl derivative (RCOX) would follow the linear free energy relationship for addition of the nucleophile to the corresponding ketone (RCOR, or aldehyde if R=H) if conjugation between X and the carbonyl could be turned off, while leaving its polar effects unchanged. This can be done if one knows or can estimate the barrier to rotation about the CO-X bond, because the transition state for this rotation is expected to be in a conformation with X rotated by 90° relative to RCO. In this conformation X is no longer conjugated, so one can treat it as a pure polar substituent. Various values determined by this approach are included in the tables in this chapter. 

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