Active average lifetime

Thus, in order to reproduce the effect of an experimentally existing activation barrier for the scission/recombination process, one may introduce into the MC simulation the notion of frequency , lo, with which, every so many MC steps, an attempt for scission and/or recombination is undertaken. Clearly, as uj is reduced to zero, the average lifetime of the chains, which is proportional by detailed balance to Tbreak) will grow to infinity until the limit of conventional dead polymers is reached. In a computer experiment Lo can be easily controlled and various transport properties such as mean-square displacements (MSQ) and diffusion constants, which essentially depend on Tbreak) can be studied. 

The second reason has to do with the nature of the screens run at BMS. The development of new screens is an active enterprise at BMS, as at all major pharmaceutical companies, and the average lifetime of a screen is between 3 and 6 months. It thus is impossible to compare hit rates of samples from two sources, because almost no two sets of extracts will have been tested in the exact same set of screens. 

This metabolic activity is achieved by a turnover of amino acids and proteins that is as rapid as that of lipids and carbohydrates. In an adult human male, 400 g of body proteins is turned over each day. Of this, 50g is used to replace digestive enzymes (Sec. 15.2), and 6g to replace hemoglobin (Sec. 15.8). The concentration of free amino acids in plasma is small (total 3.2mmol L l, of which 25 percent is glutamine), but the turnover of 400g per day of protein is equivalent to the uptake, and release back into the plasma, of 4.6 moles of a-amino-N, so that the average lifetime of an amino... 

From the largest CAS calculation ( CAS-2 in ref. [26]) a lifetime of 2.58 s was predicted for one of the 0 = 1 spin-sublevels (Ms = 1) of the lowest triplet vibrational level. In a rotationless molecule the 0 = 0 sublevel does not radiate, so the observed spin-averaged radiative lifetime of the Vegard-Kaplan emission is equal to (3/2)rn=i = 3.87 s. It should be compared with Piper s value, 2.37 s [89]. We can see from Fig. 7 that for the lower v" vibronic quantum numbers the agreement is much better than for the most intensive transitions to the upper vibronic levels. The underestimation of the transition moment in the range 1.3-1.4 A is more serious for the small active spaces. In the valence type CAS ( CAS-1 in ref. [26]) an averaged lifetime equal to 5.48 s was obtained. 

The deoxyribose residues that are generated at sites of base loss in DNA exist in equilibrium between the closed furanose form and the open aldehyde form (13) (Fig. 3). The 3 phospho-diester bonds associated with the latter are labile and are readily hydrolyzed by -elimination during which the pentose carbon tothe aldehyde is activated at alkaline pH and at increased temperatures (13). The same reaction proceeds at a reduced rate at neutral pH. At physiologic pH and temperature, a site of base loss in DNA has an estimated average lifetime of 400h (14). 

In a further study of the active nitrogen-iodine reaction in which intense emission from electronically excited I2 is observed, Phillips " measured the decay of the emission at low iodine pressures. By extrapolation, the first-order rate coefficient for the decay at zero pressure and the average lifetime of the precursor to the excited species are obtained as 0.89 + 0.41 sec" and 1.1 sec. respectively at room temperature. Phillips believes the precursor to be N2(v4 E ) although no direct measurements on this state were made. For the excitation reaction... 

More detailed information on the activation pathway, which distinguishes the polymeric photoinitiators from the corresponding low-molecular-weight structural models, has been obtained by photophysical measurements [22,27,28] in terms of quantum yield and average lifetime of the triplet excited state of benzophenone moieties in the above systems. However, in order to get a better comprehension of this point, it is necessary to introduce some basic concepts about the kinetic treatment of a photoinitiated chain polymerization. 

To calculate the average lifetime (f) in systems where the catalyst species are unstable it is necessary to know at any specific time the polymerization rate, the active site concentration and the number average degree of polymerization of the polymer being formed, since... 

The precise experimental conditions for the measurements of chain lifetimes of polyethylene with the TiCl4/Al(i-Bu2 )H catalyst are not explicitly stated, but there is clear evidence for a steady increase in lifetime with polymerization time. For an average lifetime of 4 min after 40 min polymerization time, the instantaneous values were 4 min after 18 min polymerization and 10 min after 40 min polymerization. As the concentration of active centres remains almost steady after a sharp initial fall, the increase cannot be accounted for wholly by changes in the monomer/active sites ratio. The explanation may lie in a reduced rate of chain transfer with increase in conversion, as has been found for propene with a-TiClj/AlEt2 Cl [121]. In accord with this view average chain lifetimes of polypropene have been calculated to increase with conversion [123]. 

E.E.Nikitin, Average lifetime of activated molecules, Kinetika i Kataliz, 6, 17 (1965)... 

Free radical polymerization is the most widely used process in PVDF synthesis. It involves the reaction of VDF and other comonomers with active center followed by successive addition of monomer(s) under the condition in which monomers cannot react with each other without intervention of the active center. Active centers are generated by thermal decomposition of initiator and in some cases by photoinitiation of the catalyst. The average lifetime of each active center (free radical) is approximately few seconds depending on the degree of polymerization and the initiator concentration. For successful polymerization, the sequence of reaction must take place. 

Radioactive slope factors calculated by EPA s Office of Radiation and Indoor Air (ORIA). Slope factors are central estimates in a linear model of the age-averaged, lifetime attributable radiation cancer incidence (fatal and nonfatal cancer) risk per unit of activity ingested, expressed as risk per picocurie (pCi). 

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