Rotatable torsions

Translation and rotation Translation, rotation, torsion Translation, rotation, torsion, vibration... 

Rad, n. wheel, -achse, /. axle or shaft of a wheel, -arm, m. spoke (of a wheel), -be-wegung, /. rotary motion, rotation, -dre-hnng, /. rotation torsion. 

This measures the free energy penalty associated with the freezing of rotatable torsions. 

Although some of the speculation about non-thermal microwave effects appears to emanate from a misconception that microwave radiation can excite rotational transitions, the frequencies at which these occur are much higher than 2.45 GHz. For example, the first absorption lines of OCS, CO, HF and MeF occur at 12.2, 115, 1230 and 51 GHz, respectively. Internal bond rotations (torsional vibrations) also require higher frequencies, in the order of 100-400 cm-1 or 3000-12 000 GHz, for excitation61,62. 

One could completely describe the three-dimensional structure of a protein by placing it in a Cartesian coordinate system and listing (x, y, z) coordinates for each atom in the protein. Indeed, this is how experimentally determined protein structures are stored in the Protein Data Bank at the Brookhaven National Laboratory in the United States of America. However, the structure of a protein can be more succinctly described by listing the angles of rotation (torsion angles) of each of the bonds in the protein (see Fig. 4-4). For example, the backbone conformation of an amino acid residue can be specified by listing the torsion angles (rotation around the N—Ca bond), (rotation around the Ca—C bond), and ca (rotation around the N—C bond). 

Scheme 1. Comparison of whole molecules to determine isomeric relationships. The question marks signify Superposition yes or no . The three tests are Syi —comparison by symmetry operations of the first kind (rotation, torsion) BG—comparison of bonding (connectivity) graphs vertex by vertex Syn—comparison by symmetry operations of the second kind (reflection).

Fig. 7. Time range of processes relevant to the free electron transfer. Rotation (torsion) stands also for other similar motions such as necking, wagging, bending, etc.

A vibrational degree of freedom may be replaced by internal rotation (torsion) around a a bond. In this case the microwave spectrum of the molecule is modified by torsion-rotation interaction. By studying this effect on the rotational spectrum, the internal rotation potential barrier can be determined. The hindering potential of CH3N3 was found to be V3 = 695 20 cal/mole (the subscript 3 stands for the 3-fold axis of the hindering potential). The potential is rather small but is not smaller than the value expected from a hyperconjugation effect . 

Two specific aspects of this data are especially noteworthy. The central bonds of vitamin D2-yne 3 are 2-3 times more flexible than its other rotational torsions. This flexibility can also be seen in Figure 5b—large amplitude rotations. The vitamin D2-yne 3 data shows how Hex quantitates a well known phenomena—the lack of steric interactions around a triple bond. The acceptance rate for the third bond of vitamin D3-oxy 4 is only 19%, relatively low compared to the others and suggestive of conformational restriction. 

Figure 4 Rigid internal rotation (torsional angle 0) involving the methyl group as a unit, and the dihedral motions (t), which describe a single H —C —C —H deformation, in ethane. Although there is a single definition of , there are 9 independent...

Topic 1.1. The Origin of the Rotational (Torsional) Barrier in Ethane and Other Small Molecules... 

So far we have discussed statistical and dynamical theories that have largely been based on a quadratic expansion of the PES about the reaction path (albeit with a limited account of anharmonicity in some vibrational modes). Still within that same spirit, very anharmonic vibrations such as intramolecular hindered rotations (torsional motion) have been treated more realistically [56,110,138]. However, in many... 

The rotational spectram of a molecule with internal rotation (torsion) is modified due to torsion-rotation interactioa The interpretation of this modification allows the determination of the internal rotation potential barrier [59Lin, 68Dre, 84Gor]. The molecule is generally taken to be rigid except for internal rotation. However, special methods have been developed to include the interaction with other vibrational degrees of freedom. 

RTV Rotation-Torsion-Vibration [72Dre, 76Bau] Flexible model [79Mey]... 

Zhang and coworkers (237, 238) studied aqueous solutions of polyacrylamide (PAAM) mixed with three different smfactants at the hexadecane/water interface, using a rotational torsion viscometer. The structure of the interfacial films were shown to be dependent on the shear rate. Based on the experimental results, a mechanism for PAAM-sm-... 

Using a rotational-torsional surface viscosimeter, the surface shear viscosity of the C15-C20 straight-chain fatty acids have been determined (Moo-Young et aL, 1981). The even fatty acids were found to show a surface-Newtonian behaviour, with s-values of 1.5, 2 and 3 mN s/m for the Ci, Cis and C20 members respectively. The odd adds, however, showed a surface-pseudoplastic behaviour. Thus C17 gave decreasing surface shear viscosity up to a sharp rate of 8 s , and beyond that value the viscosity remained constant at about 0.6 mN s/m. These data indicate different monolayer structures of the even and odd members. 

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