Isomerization model

Van der Zwan G and Hynes J T 1984 A simple dipole isomerization model for non-equilibrium solvation dynamics in reactions in polar solvents Chem. Phys. 90 21-35... 

The observed low Tg s of most polyphosphazenes are consistent with the low barrier to internal rotation predicted for them and indicate the potential these polymers have for elastomeric applications, Theoretical calculations, based on rotational isomeric models assuming localized it bonding, predict the lowest ( 100 cal per mol of repeating unit) known polymer barrier to rotation for the skeletal bonds of polydifluorophosphazene,... 

For random coils, is directly proportional to the contour length. If n is the number of main chain atoms in the chain, = an. The parameter a is relatively insensitive to environment (21), and has been calculated for a number of polymers from strictly intramolecular considerations using the rotational isomeric model (22). The root-mean-square distance of segments from the center of gravity of the coil is called the radius of gyration S. The quantity S3 is an approximate measure of the pervaded volume of the coil. For Gaussian coils,... 

Necklace models represent the chain as a connected sequence ctf segments, preserving in some sense the correlation between the spatial relationships among segments and their positions along the chain contour. Simplified versions laid the basis for the kinetic theory of rubber elasticity and were used to evaluate configurational entropy in concentrated polymer solutions. A refined version, the rotational isomeric model, is used to calculate the equilibrium configurational... 

Attempts have been made to identify primitive motions from measurements of mechanical and dielectric relaxation (89) and to model the short time end of the relaxation spectrum (90). Methods have been developed recently for calculating the complete dynamical behavior of chains with idealized local structure (91,92). An apparent internal chain viscosity has been observed at high frequencies in dilute polymer solutions which is proportional to solvent viscosity (93) and which presumably appears when the external driving frequency is comparable to the frequency of the primitive rotations (94,95). The beginnings of an analysis of dynamics in the rotational isomeric model have been made (96). However, no general solution applicable for all frequency ranges has been found for chains with realistic local structure. 

Guenza M, Freed KF (1996) Extended rotational isomeric model for describing the long time dynamics of polymers. J Chem Phys 105(9) 3823-3837 Hansen JP, McDonald JR (1986) Theory of simple liquids, 2nd edn. Academic Press, London... 

The majority of models proposed for the mechanism of the primary event suggest that cis-trans isomerization of the chromophore occurs during formation of bathorhodopsin. The strongest evidence favoring the isomerization models comes from the original work of Hubbard and Kropf [94] and Yoshizawa and Wald [96], who demonstrated that at low temperature it is possible to establish a photoequilibrium between rhodopsin and isorhodopsin via bathorhodopsin as a common intermediate. It is now well established that this common intermediate has a transoid chromophore. The isomerization models differ in the manner in which this isomerization can be accomplished so as to be consistent with the following experimental data (1) bathorhodopsin is still a protonated Schiff base and it is in a perturbed... 

An isomerization model in which arguments were presented to show that rhodopsin and bathorhodopsin are interconvertible via a common, barrierless, thermally re-... 

Figure 10. Schematic potential energy curves accounting for the primary event in vision. I. Excited state of rhodopsin and bathorhodopsin. II. Ground state of rhodopsin and bathorhodop-sin. III. Ground state of the isolated PRSB chromophore. E] and E2 are estimated from the equilibrium constants IC = [rhodopsin]/ [11-cis retinal][opsin] < 10 and K2 + [bathorhodopsin]/[all-trans retinal][opsin] < 10"2, respectively. With E3 = 1 kcal, this leads to E4 = E2 + Ei - E3 > 13 kcal. The value estimated from thermal noise data in photoreceptors is E4 = 30 kcal. Note that according to the isomerization model (Section III-B-l-c),...

An extension of the isomerization model has been proposed by Schulten and co-workers in which the primary act in BRt involves a concomitant rotation around both C(single) and 013-14 (double) bonds (437,343). The postulated change is BRt (14s-trans, 13-trans) -+ (14s-cis, 13-cis), or BRt (14s-cis,... 

The isomerization models discussed above differ from that described by Warshel, where J625 and PBAT are partially isomer-ized chromophores (90°) and their decay to Kfc g and BAT, respectively, is described by a motion on a potential surface involving both protein relaxation (proton translocation) and additional chromophore isomerization. This model implies that in K ig and BAT, proton translocation has taken place in the opsin, but it is not discriminative concerning whether a chromophore isomerization has taken place at this stage. 

The existence of two independent photocycles for BRt and BRi3 cis and the claim that trans - 13-cis isomerization takes place in the former, lead to a mechanistic complexity that is absent in the case of visual rhodopsins, where only a single photocycle is observed. As suggested by Lozier et al. (388), adoption of the isomerization model for BR 3 cis must lead to schemes for photocycles involving isomerizations around at least two bonds for example, BRt (13-trans, X-trans) intermediates... 

Clearly, the A and B isomer states should be inside the separatrix, and the state C should be in the phase-space region outside of the separatrix but inside the energy boundary. A schematic diagram of this three-state isomerization model is presented in Fig. 20. From the results of previous analyses of predissociation we expect that within the A and B domains there are, in general, intramolecular bottlenecks to energy transfer. However, these bottlenecks are... 

Figure 24. Reactive island structure for a two-well potential isomerization model, generated from the stable and unstable branches of the transition state hxed point, (a) Stable branch structure, (b) Unstable branch structure. [From A. M. O. De Almeida et al., Physica D 46, 265 (1990).]...

Flgare 20 Graphs of dielectric saturation correlation factors, (i) B Z) calculated from formula (304) for the rotational isomerism model with Z = U/kT Oi) R Z) calculate from formula (307) for internal rotation, with Z = UJkT. In ather case an invasion in sign of is found to occur very rapUily with... 

Presented polymer mixtures are composed of amorphous macromolecules with different molecular architecture homopolymers and random copolymers, with different segments distributed statistically along the chain, form partly miscible isotopic and isomeric model binary blends. The mixing of incompatible polymers is enforced by two different polymers covalently bonded forming diblock copolymers. Here only homopolymers admixed by copolymers are considered. The diblock copolymer melts have been described recently in a separate review by Krausch [17]. 

NMR chemical shift prediction by quantum chemical calculation and the 7-gauche effect method has become a useful approach for stereochemical assignment of vinyl polymers. Recently C NMR assignments based on 7-effect and the rotational isomeric model were demonstrated successfully for a series of polyolefins, poly(l-pentene) to poly(l-octene).188... 

The first challenge to the isomerization model was made in 1972 by picosecond laser photolysis [34], The first picosecond laser photolysis observed formation of bathorhodopsin within 6 ps after excitation of bovine rhodopsin at room tempera-... 

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