Pressure Effects on Rates

Picosecond kinetics, 266 Pre-equilibria, 133-135 Pre-steady-state region, 116 Pressure, effect on rate constants, 166-167... 

Temperature and pressure effects on rate constants for [Fe(phen)3] +/[Fe(phen)3] + electron transfer in water and in acetonitrile have yielded activation parameters AF was discussed in relation to possible nonadiabaticity and solvation contributions. Solvation effects on AF° for [Fe(diimine)3] " " " " half-cells, related diimine/cyanide ternary systems (diimine = phen, bipy), and also [Fe(CN)6] and Fe aq/Fe aq, have been assessed. Initial state-transition state analyses for base hydrolysis and for peroxodisulfate oxidation for [Fe(diimine)3] +, [Fe(tsb)2] ", [Fe(cage)] " " in DMSO-water mixtures suggest that base hydrolysis is generally controlled by hydroxide (de)hydration, but that in peroxodisulfate oxidation solvation changes for both reactants are significant in determining the overall reactivity pattern. ... 

In this communication we report some results of pressure effects on rates and equilibria for the dissociation of tetra-butylammoniumpicrate (TBAP) in benzene-chlorobenzene mixtures. 

Figure 9.6 Pressure-effect on rates of some self-exchange electron-transfer reactions between metal ions comparison of observed volumes of activation with values calculated from classical Marcus theory for adiabatic reactions. The plot shows calculated and observed AP values (cm mol ) at mid-range of pressure (100 MPa, except 70 MPa for Fe(H20)g ) for adiabatic (filled symbols) and nonadiabatic (open circles) self-exchange in couples with rigid ligands. Solvents (o, ) water ( ) CD3CN (A) (CD3)2CO (V) CD3OD. Key (A,B) (C,D) Cu(dmp)2 (E-G) Ru(hfac)j (H) Fe(C5H5)2 (I-K) Mn(CN-t-Bu)g ...

Because the flame thickness in this equation is the same parameter discussed and analyzed by Summerfield (S7), Penner (P3), and Nachbar (Nl), this equation suggests that the pressure effect on burning rate should approximate... 

Flammable or toxic vapors can be piped to a flare after separation of liquid is obtained. An important design problem in flare use is the very high vent rate experienced for a relatively short time, if an existing flare is used. Also back-pressure effects on the liquid separator vessel must be considered, especially if choked flow of vapor occurs downstream of the separator. 

Pressure in the range of 1-20 kbar (units of pressure 1 kbar = 100 MPa = 0.1 GPa = 1013.25 atm) has a strong effect on rate and position of equilibrium of many chemical reactions. Processes accompanied by a decrease of volume are accelerated by pressure and the equilibria are shifted toward the side of products while those accompanied by an increase of volume are retarded and the equilibria are shifted toward the side of... 

It is customary to justify the study of pressure effects on reaction rates on the grounds that it can elucidate reaction mechanisms. A somewhat different and, I think, more... 

In this spirit, an attempt will be made to account for the magnitude of pressure effects on ligand substitution reaction rates. Attention will necessarily be confined to a few simple model systems two recent reviews (1, 2) of pressure effects on reactions of transition metal complexes in solution may be consulted for more comprehensive surveys of the field. 

It is important to point out here, in an early chapter, that the Born-Oppenheimer approximation leads to several of the major applications of isotope effect theory. For example the measurement of isotope effects on vapor pressures of isotopomers leads to an understanding of the differences in the isotope independent force fields of liquids (or solids) and the corresponding vapor molecules with which they are in equilibrium through use of statistical mechanical theories which involve vibrational motions on isotope independent potential functions. Similarly, when one goes on to the consideration of isotope effects on rate constants, one can obtain information about the isotope independent force constants which characterize the transition state, and how they compare with those of the reactants. 

The usual starting point in enzyme kinetics is the Michaelis-Menten equation for the reaction rate v. This also seems a convenient starting point for interpretation of pressure effects on enzyme mechanisms. It will be shown that this formalism may be deceptive if the definitions and interpretations have not been made clear from the beginning. For the mechanism... 

A study of the temperature and pressure effects on the rate constant k must take Eq. (6) as a point of departure. A plot of In k versus T yields an experimental activation energy (which is not to be interpreted as the internal activation energy) through the relation... 

From such measurements one then obtains AV21 and can use this in Eq. (172) to find AV. Then one is able to decide the different contributions to the pressure effect on the rate from the two cooperating volumes. 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

The hexacyanoferrate(II)/(III) electron exchange reaction is strongly catalyzed by cations such as K" ". However if the K" " is complexed by, e.g., 18-crown-6 or the cryptand [2.2.2] then the rate constant for the uncatalyzed reaction can be determined. Carbon-13 NMR spectroscopy has established that is 240 s (at 298 K), with AVyyJ = —11.3 cm mol Pressure effects on... 

Temperature and Pressure Effect on Reaction Rate Coefficients and Diffusivities... 

Recently Lee et al (Ref 3) re-examined the behavior of PETN under 10 to 50 kbars of external pressure. They also find a reduction in decomposition rate with increasing applied pressure. HMX behaves similarly to PETN. TNT whose explosion products contain a high proportion of solid carbon, as expected from LeChatelier s Principle, shows little pressure effect on its thermal decomposition. Nitro-methane, however, appears to decompose more rapidly under an external pressure of 50 kbars than 10 kbars. This effect is not completely understood but Lee et al suggest that high pressure may favor the formation of the thermally less stable aci form of Nitromethane ... 

Chemical reactions at supercritical conditions are good examples of solvation effects on rate constants. While the most compelling reason to carry out reactions at (near) supercritical conditions is the abihty to tune the solvation conditions of the medium (chemical potentials) and attenuate transport limitations by adjustment of the system pressure and/or temperature, there has been considerable speculation on explanations for the unusual behavior (occasionally referred to as anomalies) in reaction kinetics at near and supercritical conditions. True near-critical anomalies in reaction equilibrium, if any, will only appear within an extremely small neighborhood of the system s critical point, which is unattainable for all practical purposes. This is because the near-critical anomaly in the equilibrium extent of the reaction has the same near-critical behavior as the internal energy. However, it is not as clear that the kinetics of reactions should be free of anomalies in the near-critical region. Therefore, a more accurate description of solvent effect on the kinetic rate constant of reactions conducted in or near supercritical media is desirable (Chialvo et al., 1998). 

The pressure effect on the bimolecular rate constants for the esterification of phthalic anhydride with methanol in supercritical carbon dioxide (Ellington et al., 1994). 

Besides being of considerable commercial interest, Diels-Alder reactions are clean, well-characterized reactions that generally proceed in a single step through a pseudoaromatic transition state. There have been studies on the pressure effect on ionic reactions in SCFs by Zhang et al. (1996) who measured the rates of aryhnethyl cation ion-neutral reactivity in SCF. 

In a thermochemical study of the reaction (73), it is concluded that >(Pt—H) = D(Pt—Cl) and that the reaction occurs primarily because of the formation of a strong H—Cl bond rather than because of the ease of rupture of the Pt—H bond.171 Pressure effects on the insertion of alkynes into frans-PtHCl(PEt3)2 causes a large rate acceleration. The influence is large when ionic intermediates are involved, and in some cases a change of reaction mechanism is claimed.172... 

---