Low-pressure rate constant

This approximate treatment of termolecular reactions can also be used to examine how the third-order, low-pressure rate constant A111 relates to the rate constants k.d, kh, and Ac. for the elementary reactions assumed to be involved. As [M] approaches zero, Aft approaches AaAc[M]/Ab, so that Aft1 is given by... 

An alternate explanation can be seen by treating termolecular reactions as the sum of bimolecular reactions, as was illustrated in Section A.2 for the OH + SOz + M reaction. Recall that the third-order, low-pressure rate constant A 111 can be expressed as the product of the three rate constants A., Ab, and Ac for the three individual reaction steps (12), ( — 12), and (13) ... 

The low-pressure rate constant Mmol.o can be specified with a three-parameter Arrhenius expression... 

Recently, some additional low-pressure rate constants for the syn-anti process in MeONO have been obtained [770]. Figure 5 shows pressure-dependent rate constants (k/k J for syne anti exchange in pure MeONO at 262.2 K. The solid line corresponds to RRKM calculated values using E0 = 49.8 kJ mol-1 and a = 5.4 A. 

The second method permits analytical solutions to be obtained for a limited dass of transition probabilities, such as the exponential models (8) and (9). The nonequilibrium factors HE) approach 1 at energies < Eo, marked deletion HE) < 1 extends from Eo down to Eo minus a few RT. From tte solution for HE), the low-pressure rate constant (ito) follows directly from equatitm (15), and... 

Figure 7 Collision efficiencies Ac of the low-pressure rate constants kofor the expmwntial model of equation (8) (full Ibie) and the shifted exponential model of equatum (9) (dotted line, combined with statistical theory, as Figure 3, for a, ft, and y )...

Fall-off Curves of Themu Activation. As shown above, the low-pressure rate constants depend on as long as 5 ET. An influence of the functional form of the transition probability, however, is only small (see Figures... 

The two limiting cases of low ([M] -> 0) and high ([M] co) collider concentrations lead to expressions for the low-pressure rate constant ka and the high-pressure rate constant koo. ... 

For [M]->0 we find a linear dependence of the low-pressure rate constant on [M]... 

Termolecular rate constants ( ) limiting low-pressure rate constants) ... 

Representation of the low-pressure rate constant in the form pkg, where kg is given in terms of the strong-collision mechanism and p by Eq. (17.15), provides in principle the possibility of obtaining the mean transferred energy by comparing the theoretical and experimental rate constants. It has been found that p is relatively frequently substantially lower than unity, i.e. (AE) is appreciably smaller than kT [487]. Strictly speaking, this means that the calculation of the rate constant at intermediate pressures is to be based on the diffusion equation, rather than on equation (17.4). However, the expected difference in the fall-off curves is small [487]. 

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