Torsional distortions

Molecular Mechanics Models. Methods for structure, conformation and strain energy calculation based on bond stretching, angle bending and torsional distortions, together with Non-Bonded Interactions, and parameterized to fit experimental data. 

Two mechanisms (i.e. direct hydrolysis and alternatively a path via an unstable acyl phosphate intermediate) are involved in the hydrolysis in phosphate buffer of N-arylsulfonyl / -lactams such as (130).107 The acyl phosphate intermediate can be trapped with hydrazine. The alkaline hydrolysis of some torsionally distorted lactams, i.e. the bridged benz[rfe]isoquinolin-l-ones (131), in 70% (v/v) DMSO-water has been compared under the same conditions with the hydrolysis of AvA-dimethyl-1 -naphthamide (132). The relative rates of reaction and activation parameters indicate the effect of torsional distortion.108 The reaction of the tricyclic azetidinones (133) with trifluoroacetic acid gives the bicyclic thioesters (135). The mechanism may involve acid-catalysed elimination of methanethiol to give an azetinone intermediate (134) which, after nucleophilic attack of the thiol, is converted into (135).109... 

Thus, the collapse of the tetrahedral adduct to the mixed anhydride intermediate is determined purely by rotation around a single C-0 bond of the tetrahedral species formed after substrate binding. The above authors have suggested that the torsional distortion of the substrate by the sites of secondary recognition provides the mechanical driving force that causes the required bond rotation to convert 50 into 51. This interpretation is supported by inspection of molecular models. 

The light-induced reversible trans-cis photoisomerisation of franv-stilbene molecule in condensed media includes at least four macroscopic stages excitation of the stilbene chromophore, radiative deactivation of the excited state with the rate constant kr, medium relaxation around the excited stilbene molecule with the rate constant km (to provide space for torsional distortion during the photoisomerisation process) and eventually, twisting transition with the rate constant kt c. 

An electronic balance based on a parallelogram flexure support and an electromagnetic force cell. The flexure points allow limited movement about the null position and provide resistance to torsional distortion by off-center loading of the weighing pan. Drawing taken from Ref. 3 with permission. 

Shea, K. J., Kim, J. S. Influence of strain on chemical reactivity. Relative reactivity of torsionally distorted double bonds in MCPBA epoxidations. J. Am. Chem. Soc. 1992, 114, 3044-3051. 

Strained and sterically hindered alkenes are of special interest in physical organic chemistry. By their very nature these structures are difficult to synthesize and the McMurry reaction represents the only viable route to many such compounds (see Table 2). Torsionally distorted alkenes, capable of exhibiting optical activity without a chiral center, have also been prepared. If the reaction is carried out in a chiral solvent the ( ) isomer is formed selectively (equation 82). 

Figure 1.14. Schematic views of alkali-metal-dopcd poly-acetylenc showing a possible sequence of polymer chain torsional distortions about an axis displaced from the chain centres (indicated by the small dots). (Reproduced from ref 104 with kind permission. Copyright (1992) American Physical Society.)...

Needham et al. [32] also used X-ray diffraction to show that, in the case of Mn(II), Cd(II) and Cu(II)-based C12- and Ci4-ammonium tetrachlorometallate salts, two mechanistic pathways were present on melting to the mesophase. Each pathway was shown to have a minor and major structural transformation. The minor change was thought to be a torsional distortion of the alkyl chains and the major change the melting of the chains to form a disordered layer. The order in which the... 

Fig. 4. Three-dimensional representation of the topography of the excited- and ground-state potential-energy surfaces in the framed region (see dashed rectangle) of Fig. 3. Although one of the two coordinates is referred to as torsion , it does not really correspond to the change of a dihedral angle, but to a local torsional distortion of the central segment of the molecule. Indeed, it can be described as a coupled pyramidalization at the Cs and Ce centers of the carbon skeleton.

Derivative work, based on this analysis, erroneously assumes degeneracy of the linear combinations when defining a set of orthogonal real orbitals . There is no theoretical basis in the seminal paper to justify this assumption. However, what is clearly implied is that the orbital moment on each carbon atom is directed perpendicular to the molecular plane. The molecular angular momentum is therefore quenched vectorially only for this planar arrangement of the molecule. Torsional distortion which creates residual angular momentum therefore requires work, the true basis of a barrier to rotation. 

Results for biaxial smectics and columnar phases have additional compressional terms, but tilted smectic phases can support additional torsional distortions. Such phases are conveniently described in terms of two directors, one along the tilt direction, corresponding to the nematic director n, and the projection of n on the layer plane, known as the c-director (see Fig. 6). 

Torsional distortions can now be written in terms of derivatives of a and c, and it is found [10] that nine torsional elastic constants are required for the smectic C phase. Mention should be made of the biaxial smectic C phase, which has a twist axis along the normal to the smectic layers. This helix is associated with a twist in the c-di-rector, and so elastic strain energy associated with this can be described by terms similar to those evaluated for the chiral nematic phase. 

This is at the base, for example, of the MARTINI force field, which is to date one of the most popular and broadly used CG models in the literature." In particular, this model uses strictly the same harmonic or periodic analytical form for the stretching, bending and torsional distortion associated to bonds, angles and dihedral structural parameters, and Coulomb and Lennard-Jones like potentials for the non-bonded parts. 

The measured elastic constants for 4,4 octylcyanobiphenyl (8CB) are also plotted as a fimction of temperature in FIGURE 2, where it can be seen that the elastic constants for twist and bend increase as the underlying smectic A phase is approached on cooling in the nematic phase. This is a general result, since the smectic A phase cannot support either a twist or bend torsional distortion. 

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