Barriers of rotation

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. 

Vn is often called the barrier of rotation. This is intuitive but misleading, because the exact energetic barrier of a particular rotation is the sum of all V components and other non-bonding interactions with the atoms under consideration. The multiplicity n gives the number of minima of the function during a 360° rotation of the dihedral angle o). The phase y defines the exact position of the minima. 

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. 

Semiempirical calculations (MNDO//MNDO) have been performed for sesquifulvalene 6 and its aza analogs 11 (R = Me), 12 (R = Me), 72a/72b, and 73a/73b (91JOC4223). In all cases the most stable conformation is the planar one (S AH = 0).Tlie calculated barrier of rotation around the cen-... 

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. 

A conformational analysis of the methanesulfonyl radical indicates the staggered structure 1 as the most stable, with a barrier of rotation of ca. 3.8 kcal mol"17. The... 

Table 1. Double bond lengths X=X [A] p-p(x) overlap integrals S ionization potentials IP [eV] of the dissociation products XH2 T-bond strengths Erot [kcal mol 1] from the barrier of rotation, calculated at the CASSCF(2,2)/6-31G + ZPE (zero point energy) level of theory.

The AG values for the rotation around the C3-N bond were obtained by the coalescence method and kinetic measurements of the equilibration of isolated and configurationally established cis-trans isomers of type 232. The barrier of rotation is considerably lower than in ordinary immonium cations, e.g. 233 AG is decreased by electron-withdrawing substituents at the nitrogen atom, whilst at the three-ring the opposite effect is observed. 

It is also worthwhile to compare the ferrocenyl ethylene (vinylferrocene) anion-and cation-radicals. For the cyano vinylferrocene anion-radical, the strong delocalization of an unpaired electron was observed (see Section 1.2.2). This is accompanied with effective cis trans conversion (the barrier of rotation around the -C=C- bond is lowered). As for the cation-radicals of the vinylferrocene series, a single electron remains in the highest MO formerly occupied by two electrons. According to photoelectron spectroscopy and quantum mechanical calculations, the HOMO is mostly or even exclusively the orbital of iron (Todres et al. 1992). This orbital is formed without the participation of the ethylenic fragment. The situation is quite different from arylethylene radical cations in which all n orbitals overlap. After one-electron oxidation of ferrocenyl ethylene, an unpaired electron and a positive charge are centered on iron. The —C=C— bond does not share the n-electron cloud with the Fe center. As a result, no cis trans conversion occurs (Todres 2001). 

On mixed ligated complexes of palladium(II) and platinum(II), the cis coordination of the NHC and a phosphine ligand is thermodynamically favored. i27,261,262 Thermal isomerization was reported for tra i -[(NHC)2Cr(CO)4] (M = Cr, Mo) to the ds-complex. For [(NHC)2Mo(CO)4] this proceeds even in the solid state. A barrier of rotation due to a double-bond character of the metal-NHC bond could not be determined so far. This is in agreement with the single-bond character of the metal-carbon bond. Barriers of rotation determined so far are due to steric hindrance. 

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],... 

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

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 ... 

Alkylaryl ethers and diaryl ethers undergo protonation on either oxygen or carbon, depending upon the acidity of the medium87 (Scheme 4.3). Both the C-protonated species 28 and the (9-protonated species 29 have been observed. The evidence mainly comes from NMR and UV data. Sommer et al.88 have used para-anisaldehyde as an indicator in acidity measurements in the superacidity range. The barrier of rotation around the Cipso—0 bond upon (9-protonation has been used as a criterion in such studies. The torsional barrier around the phenyl-alkoxy bond in the C-protonated forms of aromatic ethers has also been measured by spin-saturation transfer measurements.89... 

Although all proteinogenic amino acids form predominantly anti peptide bonds, a search in the Brookhaven Protein Database revealed that approximately 6-7% of all X-prolyl peptide bonds are found in the syn conformation in the native state of proteins [8]. The reason for this relatively frequent occurrence of syn-prolyl peptide bonds lies in steric repulsion of the proline 3 protons and the adjacent N-terminal amino acid in the anti conformation, resulting in a low barrier of rotation and energetically similar syn and anti isomers (Figure 1.2.3). 

In the first expression, the summation is extended to the quadruples of sequentially bonded atoms ABCD and the energy constants Vn are specific for the quadruples of the types of the atoms involved. The torsion angle is that between the planes ABC and BCD. The n = 1 term describes a rotation which is periodic by 360°, the n = 2 term is periodic by 180°, the n = 3 term is periodic by 120° and so on. The Vn constants determine the contribution of atoms A and D to the barrier of rotation around the... 

Appendix) provide an example of frozen rotation with three different substituents. Without the bridge, however, the barrier of rotation for bent bis(tetrahydroindenyl)metaUocenes is Ukely to be too small to justify classification as a chiral system. 

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 (racemization) for biphenyls (diagram XXXIII) (from [42,43])... 

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