Pre-effect

Cp. Heat capacity at constant pres- Effective thermal conductivity... 

PRE effects can also be seen for nuclear spins belonging to the solvent or other molecules present in solution, that essentially do not enter the first coordination sphere of the paramagnetic metal ion. This mechanism is... 

The main difficulty in theoretical predictions of the PRE effects is caused by the description of electron spin relaxation. The transient ZFS interaction is usually considered as present in every complex with S> 1 and its modulation is assumed to provide the dominant mechanism for the electron spin relaxation. The commonly used pseudorotation model 27,85,86), described earlier, has two advantages it captures the essential physics of electron spin relaxation (i.e., the fact that relaxation can be caused by motions faster than the overall rotation of the paramagnetic complex, provided that these motions displace the principal axis of the ZFS), and it leads to relatively straightforward mathematical formulation. The assumption that the... 

Values are taken from ref, 39. Polymerization temperature 295.5 K, photochemical initiation. Rate constants were determined from the stationary polymerization rate and from the non-stationary pre-effect. The necessary conditions for the calculation were not quite fulfilled the rate constant values are not exact but they adequately illustrate a characteristic situation. 

Values are taken from ref. 40. Polymerization temperature 298 K. Rate constants were determined from the stationary polymerization rate and from the non-stationary pre-effect. 

It is a gross error to apply stationary state theory to non-stationary polymerizations, of which many exist. In a great majority of cases, the stationarity conditions are only fulfilled after a certain non-stationary phase in which the concentration of active centres increases. When this occurs in a kinetically pure medium, i.e. in the absence of inhibitors or other intervening compounds, it usually signifies slow initiation and is called the pre-effect. The general shape and meaning of the pre-effect is represented graphically in Fig. 5. For very small conversions ([M] [M]0) we can write [57]... 

Fig. 5. Non-stationary phase caused by slow initiation pre-effect.

THE PRE-EFFECT OF RADICAL POLYMERIZATION ACCOMPANIED BY MUTUAL RADICAL TERMINATION... 

The total pre-effect is the difference between the conversion actually observed during the nonstationary state and the conversion which would have been obtained, if the reaction rate of the new stationary state had been set up immediately. The value of the total pre-effect is given by... 

The only measured quantities in this equation are the ratio of the initiation rates and the monomer concentration. By measuring these values and the total pre-effect as well, it is possible to evaluate the ratio kp/kf. The aftereffect can be treated analogously. In the beginning there is a stationary state corresponding to /i + /2J at time = 0, the initiation rate is lowered to /j. The total aftereffect is given by ... 

Figure 15. Determination of total pre-effect by extrapolation Results and Discussion...

PRE effects. Schilder and co-workers investigated encounter complexes between cytochrome c peroxidase and cytochrome c carrying a paramagnetic spin label. Burmann et reported a related study of conformation and dynamics of a periplasmic chaperone and its complexes with outer-membrane proteins. 

A couple of papers have dealt with dendrimers. Wang and co-workers reported a study of host-guest behaviour of poly(amidoamine) dendrimers with various functionalities interacting with paramagnetic TEMPO derivatives. They measured the PRE effects on the polymer protons. Neelov et investigated second- and fourth-generation lysine dendrimers by MD simulations, combined with relaxation rates measurements. 

Fig. 7 Trimer model of S26CR1 Anabaena sensory rhodopsin (ASR). The individual monomers are shown in different colors. R1 side chains are shown in blue. Side chains of residues experiencing large intermolecular paramagnetic relaxation enhancement (PRE) effects and therefore spatially close (< 15 A) to the nitroxide of a neighboring monomer are shown in magenta. Adapted with permission from [189]. Copyright (2012) American Chemical Society...

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