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 present interpretation of water structure is that water molecules are connected by uninterrupted H bond paths running in every direction, spanning the whole sample. The participation of each water molecule in an average state of H bonding to its neighbors means that each molecule is connected to every other in a fluid network of H bonds. The average lifetime of an H-bonded connection between two HgO molecules in water is 9.5 psec (picoseconds, where 1 psec =10 sec). Thus, about every 10 psec, the average HgO molecule... 

FIgO. Consequently, a water molecule is trapped only several hundred times longer by the electrostatic force field of an ion than it is by the FI-bonded network of water. (Recall that the average lifetime of FI bonds between water molecules is about 10 psec.)... 

Standard life is described as the average lifetime that is acceptable to any plant failure analyst or troubleshooter. Therefore, if we arrive at defect limits in machinery within the maintenance program, we have also reached the standard life of all the failure modes in the plant. Do we now ... 

The preceding models permit an evaluation of the ultimate strength of some simple polymers under mechanical stress. Under specific circumstances, it is important to predict the average lifetime of a bond stressed at a level which is below its... 

Thus the average lifetime, x, of O on the catalyst surface (x=TOF ) equals 770 s or 13 min. This then should be the time needed for the rate, r, to decay to its unpromoted value upon current interruption. This is in excellent agreement with experiment (Fig. 4.13) and nicely confirms the sacrificial promoter concept of NEMCA The promoter (O5 ) is sacrificed by eventually reacting with the oxidizable species (C2H4). But before being sacrificed , i.e. consumed, it has caused on the average the reaction of A extra oxygen atoms with the oxidizable species. 

The inverse of these numbers express roughly the average lifetimes of oxygen at the two adsorption states at steady state, i.e. 

Trace-gas Lifetimes. The time scales for tropospheric chemical reactivity depend upon the hydroxyl radical concentration [HO ] and upon the rate of the HO/trace gas reaction, which generally represents the slowest or rate-determining chemical step in the removal of an individual, insoluble, molecular species. These rates are determined by the rate constant, e,g. k2s for the fundamental reaction with HO, a quantity that in general must be determined experimentally. The average lifetime of a trace gas T removed solely by its reaction with HO,... 

The diffusion constant of a primary radical must be of the order of 10 cm.2 sec.- the radius r is about 5X10 cm., and as we have seen 1 10 " per second. Hence ]ag l0 radicals per cc. But the radicals are being generated at a rate of 10 cc. sec. hence the average lifetime of a radical from generation to capture by a polymer particle will be only 10 sec. " The rate of termination by reaction between two radicals in the aqueous phase at the calculated equilibrium concentration, 10 radicals per cc., will be given by... 

The termination constants kt found previously (see Table XVII, p. 158) are of the order of 3 X10 1. mole sec. Conversion to the specific reaction rate constant expressed in units of cc. molecule" sec. yields A f=5X10". At the radical concentration calculated above, 10 per cc., the rate of termination should therefore be only 10 radicals cc. sec., which is many orders of magnitude less than the rate of generation of radicals. Hence termination in the aqueous phase is utterly negligible, and it may be assumed with confidence that virtually every primary radical enters a polymer particle (or micelle). Moreover the average lifetime of a chain radical in the aqueous phase (i.e., 10 sec.) is too short for an appreciable expectation of addition of a dissolved monomer molecule by the primary radical prior to its entrance into a polymer particle. 

NS (general population) Developmental Moderate deficit in Wechsler Performance IQ in children 6.5 years old (Cincinnati cohort) 220 (average lifetime) Dietrich et al. 1993a... 

However, results obtained by Koo et al. (1991) indicate that low to moderate lead exposure (average lifetime PbB level range of 4.9-23.6 pg/dL, geometric mean of 9.8 pg/dL, n=105) in young children with adequate nutritional status, particularly with respect to calcium, phosphorus, and vitamin D, has no effect on vitamin D metabolism, calcium and phosphorus homeostasis, or bone mineral content. The authors attribute the difference in results from those other studies to the fact that the children in their study had lower PbB levels (only 5 children had PbB levels >60 pg/dL and all 105 children had average lifetime PbB levels <45 pg/dL at the time of assessment) and had adequate dietary intakes of calcium, phosphorus, and vitamin D. They concluded that the effects of lead on vitamin D metabolism observed in previous studies may, therefore, only be apparent in children with chronic nutritional deficiency and chronically elevated PbB levels. Similar conclusions were reached by IPCS (1995) after review of the epidemiological data. 

Another concept that may cause confusion is that of stability. Here, the term thermodynamic stability is used in relation to populations, while the term kinetic stability is used in relation to average lifetimes. Thus, the thermodynamically most stable conformer is the most populated one, while the kinetically most stable conformer is the one with the longest average lifetime. Whether these are necessarily the same is discussed on the basis of the examples. 

Fig. 5. Correlations between variables Number of residues, average lifetime of the most populated conformer, and average time of folding to the most populated conformer (see Table I).

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