Gas phase rates

The gas-phase rate coefficient fcc is not affecded by the fact that a chemic reaction is taking place in the liquid phase. If the liquid-phase chemical reaction is extremely fast and irreversible, the rate of absorption may be governed completely by the resistance to diffusion in the gas phase. In this case the absorption rate may be estimated by knowing only the gas-phase rate coefficient fcc of else the height of one gas-phase transfer unit Hq =... 

For this derivation we use the gas-phase rate expression Na = J -ciy yi) and integrate over the tower to obtain... 

For example, the rate constant of the collinear reaction H -f- H2 has been calculated in the temperature interval 200-1000 K. The quantum correction factor, i.e., the ratio of the actual rate constant to that given by CLTST, has been found to reach 50 at T = 200 K. However, in the reactions that we regard as low-temperature ones, this factor may be as large as ten orders of magnitude (see introduction). That is why the present state of affairs in QTST, which is well suited for flnding quantum contributions to gas-phase rate constants, does not presently allow one to use it as a numerical tool to study complex low-temperature conversions, at least without further approximations such as the WKB one. ... 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

Gas phase rate constants are typically an order of magnitude higher than solution phase rate constants. Fischer and Radom4 have postulated that gas phase... 

The present paper tests the assumed original and enhancement mechanisms with rates and conversions for a broad range of contaminants measured under a fixed mass concentration (50 mg/m ) feed condition. The plots compared are reaction rates vs. (1) dark adsorption, Ot. (2) second order rate constant for (OH ) (TCE absent) or (Cl ) (TCE present), and (3) the product of these gas phase second order rate constant times the reactant dark coverage. Where a second order gas phase rate constant was not available, we estimated its value from correlations of kci vs. koH for tke same class of compounds. 

Figure 1 kci vs. koH- Second order gas phase rate constants for the reaction of Cl atoms vs. the corresponding OH radicals rate constants for the reactions with a. n-alkanes [11] b. n-alcohols [12] c. n-ethers [12] d. chloroethenes [13] and e. 1-chloroalkanes [14],... 

Oxidation rate constant k, for second order gas-phase rate constants koH, ko3 and kNQ3 for reactions with OH radicals, 03 and N03 radicals or as indicated, data at other temperatures and/or the Arrhenius expression see reference ... 

Atkinson, R., Aschmann, S.M., Carter, W.L., Pitts, Jr., J.N. (1983) Effects of ring strain on gas-phase rate constants. 1. Ozone reactions with cycloalkenes. Int. J. Chem. Kinet. 15, 721-731. 

Dilling, W.L., Gonsior, S.J., Boggs, G.U., Mendoza, C.G. (1988) Organic photochemistry. 20. A method for estimating gas-phase rate for reactions of hydroxyl radicals with organic compounds from their relative rates of reaction with hydrogen peroxide under photolysis in 1,1,2-trichlorotrifluoroethane solution. Environ. Sci. Technol. 22, 1447-1453. 

This simple view is clearly true for some reactions, e.g., the Diels-Alder dimerization of cyclopentadiene, where the rate constant in ethanol is the same as in hexane, and only a factor of three larger than in the gas phase. In contrast, for the example mentioned above of the 8 2 reaction (1), the reaction proceeds fifteen orders of magnitude faster in the gas phase than in methanol. For the Sfjl reaction of tert-butyl iodide, however, the gas phase rate constant can be estimated to be about 86 orders of magnitude slower than the solution phase rate constant. It is thus for ionic reactions that the tremendous changes in the rate constant upon solvation are seen. We are therefore specifically interested in those gas phase ion-molecule reactions that are the counterparts to the well-known solution phase reactions. 

Finally, the National Institute of Standards and Technology (NIST) in the United States has several chemical kinetics databases that are available for purchase from the Office of Standard Reference Data at NIST. The NIST Standard Reference Data Base 17 gives gas-phase rate constants through 1993 and Data Base 40 gives solution-phase data through 1992. In addition, aqueous-phase data are available through the Radiation Chemistry Data Center of the Notre Dame Radiation Laboratory (http //www.rcdc.nd.edu/). 

Kind, I., T. Berndt, O. Boge, and W. Rolle, Gas-Phase Rate Constants for the Reaction of N03 Radicals with Furan and Methyl-Substituted Furans, Chem. Phys. Lett., 256, 679-683 (1996). 

Rates of Gas-Phase Reactions. Reaction rates have been reported for only a few CVD gas-phase reactions, and most reports are primarily for the silane system. Because of the high temperatures and low pressures used in CVD, the direct use of reported gas-phase rate constants must be done with care. In addition to mass-transfer and wall effects, process pressure may be another factor affecting reaction rates. Process pressure affects major CVD processes, such as the deposition of Si from SiH4 and GaAs from Ga(CH3)3, reactions that involve unimolecular decomposition. The collisional activation, deactivation, and decomposition underlying these reactions can be summarized qualitatively by the following reactions (139, 140) ... 

Before we derive a relation between the rate constants in solution and gas phase, let us first verify that the expression for the rate constant in Eq. (10.40) simplifies to the well-known result for the gas-phase rate constant, when there is no solvent present. With no solvent, we have 14oi = 0. Also V t = 0, since this is the interaction energy between solvent and activated complex. The n-particle distribution function in Eq. (10.41) simplifies to... 

When this is introduced into the expression for the gas-phase rate constant, we get the familiar expression... 

An appropriate model for trickle-bed reactor performance for the case of a gas-phase, rate limiting reactant is developed. The use of the model for predictive calculations requires the knowledge of liquid-solid contacting efficiency, gas-liquid-solid mass transfer coefficients, rate constants and effectiveness factors of completely wetted catalysts, all of which are obtained by independent experiments. 

Gas-phase rate and equilibrium constants are generally not very different from solution-phase values. 

For most of the molecules discussed above, the experimentally determined pressure-dependent gas-phase rate constants can be modeled adequately with RRKM theory. SF4 and possibly aziridine are the exceptions. Due to uncertainties in the model parameters used in RRKM calculations, and the sensitivity of the calculations to these parameters, only qualitative conclusions can be drawn from the observed agreement. Since major departures are not observed, it can be... 

The actual reaction path for CH C. Unfortunately, no gas-phase rates have been accurately measured for... 

The results for reaetion no. 8 in Table 5-15 indieate that nueleophilie reaetivities of anions obtained in the gas phase are essentially in the same order as in molten salts and in dipolar non-HBD solvents [285, 290]. This again suggests that speeifie solvation of the anions is responsible for the reversed order obtained in protie solvents relative to dipolar non-HBD solvents. Whereas the relative nueleophilieities in aeetonitrile are similar to those found in the gas phase [282, 285, 290], the absolute gas-phase rates are some orders of magnitude greater than those in aeetonitrile. The speeifie rates of displacement reactions of anions with halomethanes exceed those in solution by factors of up to >10 [285, 290]. These large differences in absolute rates demonstrate the moderating influence of the solvent on all the reactivities [282]. See also Chapter 5.2. 

---