Torsional conformation

Steric parameters include bond length, bond angle, bond torsion, conformation, topology or connection among atoms in the molecule, symmetry, moment of inertia, molecular diameter, molecular surface area, molecular volume, and molecular shape. 

S ince no torsional conformers are expected for each of these rather rigid isomers, the contributions to entropy are only caused by the distinct molecular rotational degrees of freedom of each isomer considered as a rigid unit which can be evaluated by symmetry considerations. The symmetry contribution to entropy is -R In a, where a is the symmetry number i.e., the number of undistinguishable positions adopted by the isomer (considered rigid) by simple rotations. Symmetry numbers are o s 2 for the Q isomer and a = 6 for the Dj one. 

Figure 12.14 Calculated structures of the [C2Cjlm]+ cation, showing the two different torsion conformers obtainable by rotation of the ethyl group around the C-N bond relative to the imidazolium ring. Planar (left) and nonplanar (right) rotamers are viewed perpendicular to and along the ring plane. The nonplanar form is known from x-ray structures 69. (Adapted from Umebayashi, Y., Fujimori, T, Sukizaki, T., Asada, M., Fujii, K., Kanzaki, R., and Ishiguro, S.-i., /. Phys. Chem. A, 109, 8976-8982, 2005. With permission.)...

In an earlier paper (S 044], anomalies in the angular moments of the monomer-monomer distribution function for flexible polymers are established. It is shown here how these anomalies arise from the tetrahedral symmetry of the three-state RIS model and how they disappear in the continuum limit of torsional conformations. It is concluded that the eighth and higher radial moments contain spurious contributions when calculated within the usual three-state model. 

Influence of Flexible Bond Angle on Torsional Conformations, Hexamethyldisiloxane was used as a test compound for several procedures applied in conformational analysis. This compound is relatively simple, and structural data are available in the literature. The experimental value for the Si-O-Si bond angle in hexamethyldisiloxane is 148 3°, as measured by electron diffraction (12). The electron diffraction data indicate a staggered conformation 2 symmetry), although a model with twist angles around Si-O bonds of about 30° cannot be excluded. 

The side chain at C-3 undergoes profound redox-dependent alterations in its chemical properties. The electron orbitals of the carboxamide in NAD are not delocalized into the pyridine ring. On reduction, however, the side chain becomes effectively a vinylogous urea. It shows strong resonance delocalization into the double bonds, with the carbonyl coplanar [although skewed conformations at 150° also represent local minima in the torsional conformation space... 

Kcal/mol at 25°C. For flexible molecules, binding research clearly requires a facility for the study of distorted conformations to effect an experimental approach to the minimum energy conformation of the receptor-inhibitor complex. An excellent example of this approach is provided in the conformational analysis of some flexible antidepressants by Andrews and co-workers (60). The overall mobility of two tricyclic antidepressants (imiprine, amitriptyline) was explored in a step-wise manner to describe a multitude of torsional conformations at and just above the ground state. 

Allinger NL, Fermann JT, Allen WD, Shaefer III HF (1997) The torsional conformations of butane definitive energetics from ab initio methods. J Chem Phys 106 5143-5250... 

When motion effects are expected to be due to the existence ofdifierent conformations (multiple rotameric states significantly populated), rather to local fluctuations, they can also be accounted for with staggered-rotamer models [29], in which discrete probabilities for the different conformers are introduced. They are often used when describing amino acid side-chain torsion conformations. See, for instance. Ref [30] for a recent critical analysis of several models. 

Two torsional conformers were found for CF3OOCI, with twist angles cpi(66%) and (34%). The nozzle temperature was 27 °C. 

In neat or dilute aqueous solutions the vicinal coupling between hydroxyl and methylene protons is also measurable (J[OH,CH2] = 5 Hz), but additional H-O-C—H torsional conformers would be needed to obtain the appropriate Boltzmann-weighted theoretical estimate. ... 

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