Couplings torsional characteristics

Coupling selection is highly dependent on the mode of operation, which includes torsional characteristics, speed, and the operating envelope. 

Softening and cure is examined with the help of a torsional pendulum modified with a braid (65), which supports thermosets such as phenoHcs and epoxies that change from a Hquid to a soHd on curing. Another method uses vibrating arms coupled to a scrim-supported sample to measure storage and loss moduH as a function of time and temperature. An isothermal analytical method for phenoHc resins provides data regarding rate constants and activation energies and allows prediction of cure characteristics under conditions of commercial use (47). 

Most characteristic for the overall conformation of the cinchonan carbamates in solution is the dihedral angle of the torsion H8-C8-C9-H9, which can be inferred from the vicinal coupling constant of the NMR proton signal of H9. The experimentally measured coupling constant /hsh9 represents actually the average over the populations P(i) of the different conformers in solution according to [100]... 

Karplus analyses of the NH-a-CH coupling constants which correlate with the characteristic torsion angle d, offer valuable information about the conformation of the peptide [17]. We compared the iJ values of the free peptides with those in the complex with their optimized binders. Usually signals became much sharper and the coupling constants markedly increased, approaching the calculated values (MacroModel). 

The characteristic features of the CT formation dynamics in AA proved similar to those of ADMA [83]. The detailed analysis of the vibrational structure revealed the presence of a new low-lying CT state in addition to the LE state [84]. Taking account of a coupling between this CT state and the LE state, we successfully reproduced the spectra and evaluated the torsional potentials of the ground, the LE and the CT states, as shown in Eigure 10. In an isolated AA molecule, the CT state is non-perpendicular and charge separation is incomplete. The presence of the NH2 substituent is responsible for the low-lying CT state while the flexibility in torsional... 

Two characteristics determine the shape of molecular aggregates. The first is the shape of the constituent molecules, which sets the curvature of the aggregate. The second is coupled to the chirality of the molecules, which also determines the curvature of the aggregate, via the geodesic torsion. The bulk of this chapter is devoted to an exploration of the effect of molecular shape on aggregation geometry. An account of the theory of self-assembly of chiral molecules is briefly discussed at the end of this chapter. 

To update this work a new calculation has been performed on 5 -dCMP. The molecule was optimized at HF/6-31 G(d,p) with only the glycosidic torsion angle frozen to keep the phosphate group away from the N3 protonated cytosine. Spin densities were calculated on the optimized structure using UB3LYP/6-311 G(2df,p). These calculations show the hole localized at the P-OH oxygen (p(019) = 0.832). The phosphorous isotropic hyperfine couplings is calculated to be Aj o (P) = -102.2 MHz. These features are characteristics of a P04 radical. For a pure P04 radical A o (P) = - 81 MHz, and can increase to >100 MHz as the hole be-... 

In addition to ROM data, other characteristics of the natural joint that should be reflected in the reconstructed joint are the center of rotation (if the motion is two-dimensional), or the helical axis of motion (Kinzel, Hall, and Hillberry 1972) (if the motion is three-dimensional), and the extent and nature of the coupled motions that will accompany a specific motion. For example, because of the shape of the facet joints in the lower cervical spine (C3-C7), axial torsion... 

Multiple Conformations Ensembles. When multiple conformations are present, the problem rapidly becomes underdetermined. For example, if a nucleotide can exist in two major conformational states, with x(N)P(N) and x(S)P(S) where the glycosidic torsion angles in the N and S sugar states are not equal, each conformation will have its own characteristic NOEs and coupling constants. For n bases, there are 2" conformations, and the contribution of each one to the ensemble will depend on its population. Even for a dinucleotide, there are at least 4 conformations (viz. SS, SN, NS, NN), which multiplies the number of structural parameters to be determined fourfold, plus four equilibrium constants. There are not enough independent NMR data to determine all of these parameters and the problem is underdetermined. In this situation, the best that can be hoped for is to derive a set of structures that in some way represent the ensemble of structures that is present in solution. 

The characteristic ratio Coo characterizes chain flexibility. It depends on the 6 and torsional potential and is determined by the chemical structure of the monomers [20]. The rotational isomeric state (RIS) model, introduced by P.J. Flory [20] is essentially an adaptation of the one-dimensional Ising model of statistical physics to chain conformations. This model restricts each torsion angle to a discrete set of states (e.g., trans, gauche , gauche"), usually defined around the minima of the torsional potential of a single bond, V((f>) (see Fig. 2d). This discretization, coupled with the locality of interactions, permits calculations of the conformational partition... 

Since flexible shafting is relatively stiff in torsion and compliant in bending, flexible couplings inherently possess a imique combination of characteristics that permit them to accommodate both lateral and angular misalignment while being subjected to torsional loading. 

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