Rate constant of ionization

Dynamic NMR can be used to determine the kinetics of ionization if it is slow enough and if no side reactions occur. The rate of spontaneous ionization of trityl derivatives depends on the leaving group, the substituents on the trityl moiety, the solvent, and the temperature [42]. The rate constants of ionization of trityl chloride in 10 1 SO2/CD2CI2 are kt = 270 sec 1 at - 53° C and kt = 35 sec 1 at - 80° C the rates of recombination of counterions within the ion pair are kr = 320 sec 1 at — 53° C and kr = 8 sec-1 at -80° C. This also indicates that ionization is exothermic in this system. 

Figure 9 shows evolution of molecular weights with conversion for a hypothetical system, in which ions and ion pairs have the same reactivities (kp+ = kp = 10s mol L sec-1), covalent species are inactive, the ionization equilibrium constant is Ki = 10 5 mol-, L, and the dissociation constant is Kd = 10 7 mol/L. Kf is defined by the ratio of the rate constant of ionization to that of recombination of counterions within the ion pair (Kj = kj/kr). Kd is defined by the ratio of the rate constants of the dissociation of ion pair and that of the association of free ions (Kd... 

When one compares the rate constants of ionization of ketones and sulphones in water which have virtually the same reaction energy (vide Table 3), one concludes that the reactions for sulphones are, intrinsically, two orders of magnitude faster than for ketones. Both kinds of reactions can be studied within the ISM-formalism in the same manner as hydride transfers. The calculated bond extensions, taken from ref.36, are presented in Table 3. The plots of T] versus AG presented in Figure 7 reveal the existence of two families of reactions, each characterized by n and X ketones n = 0.56 and A,=3(X) kJ mol and sulphones n = 0.70 and =127 kJ mol L Amines, not presented in Table 3 and in Figure 7, have nt= 0.85 and X=87 kJ mol l [36a]. 

The full kinetic scheme (Scheme 30) including rate constants of ionization and deactivation (temporary termination), rate constants of propagation and all involved equilibrium constants have been resolved, as discussed in detail in several reviews. " ... 

Previous theoretical kinetic treatments of the formation of secondary, tertiary and higher order ions in the ionization chamber of a conventional mass spectrometer operating at high pressure, have used either a steady state treatment (2, 24) or an ion-beam approach (43). These theories are essentially phenomenological, and they make no clear assumptions about the nature of the reactive collision. The model outlined below is a microscopic one, making definite assumptions about the kinematics of the reactive collision. If the rate constants of the reactions are fixed, the nature of these assumptions definitely affects the amount of reaction occurring. 

Figure 5. Comparison between the experimental variations of R, the ratio CH3OD2 V CHjOHD +, with ionization chamber concentration of CHsOD and theoretical predictions of the kinematic theory for assumed velocity-independent rate constants of the reaction CtUOH2 + + CH5OH - CH3OH + CH3OH2+ for both the complex-formation and proton-stripping mechanisms...

Several 1 -phosphates of deoxyfluoro sugars were prepared, and their acid-catalyzed hydrolysis was studied. 2-Deoxy-2-fluoro- (580), 3-deoxy-3-fluoro- (582), 4-deoxy-4-fluoro- (583), and 6-deoxy-6-fluoro-a-D-gluco-pyranosyl phosphates (584) were prepared by treatment of the corresponding per-( -acetylated )9-D-glucopyranoses with phosphoric acid [the p anomer (581) of 580 was prepared by a different method]. The first and second ionization constants (pA a, and pA a2) of these compounds were determined potentiometrically, as well as by the F-n.m.r. chemical shifts at a series of pH values, and then the rate constants of hydrolysis for neutral (B) and monoanion (C) were decided. The first-order rate-constants (k) for 580-584 and a-D-glucopyranosyl phosphate (in Af HCIO4,25 °) were 0.068, 0.175, 0.480, 0.270, 1.12, and 4.10 (all as x lOVs), respectively. The rate... 

Opeida [46] compared the values of the rate constants of peroxyl radical reactions with hydrocarbons with the BDE of the oxidized hydrocarbon, electron affinity of peroxyl radical, EA(R02 ) ionization potential of hydrocarbon (/Rn), and steric hindrance of a-substituent R(Fr). They had drawn out the following empirical equation ... 

Equilibrium constant on the molar concentration scale Michaelis constant Ionization constant of water Equilibrium constant on the mole fraction scale General rate constant Boltzmann s constant Rate constant of step i... 

The reactant must undergo ionization to form a carbocation intermediate in water with a rate constant of between 10 and 10 s in order to be able to conveniently monitor this reaction. 

The rate constant for ionization of aqua species can be easily estimated from a knowledge of the pK of the coordinated water. It can then be verified whether H exchange between the complex species and water is controlled by this ionization or by dissociation of water from the complex. Calculate which path is responsible for the H exchange of... 

Rate constants for ionization of carbon acids (chloroform-r and acetophenone-r) in alkaline aqueous sulfolane have been determined and their dependence on solvent composition and temperature has been interpreted for this highly basic medium. °... 

Chemical/Physical. Atkinson et al. (2000) studied the kinetic and products of the gas-phase reaction of 2-heptanone with OH radicals in purified air at 25 °C and 740 mmHg. A relative rate constant of 1.17 x 10 " cmVmolecule Sec was calculated for this reaction. Reaction products identified by GO, FTIR, and atmospheric pressure ionization tandem mass spectroscopy were (with respective molar yields) formaldehyde, 0.38 acetaldehyde, L0.05 propanal, X0.05 butanal, 0.07 pentanal, 0.09 and molecular weight 175 organic nitrates. 

The TRAPI was developed by Matsuoka and co-workers " and has been used to determine the rate constants of about a dozen IM reactions at atmospheric pressure. As a first approximation, the TRAPI experiment might be described as an atmospheric pressure version of the PHPMS with initial ionization caused by a pulsed X-ray source. The X-rays cause relatively even ionization throughout the 6.4-cm ion source volume by penetrating through thin sections of the ion source walls formed by 25- im thick molybdenum foil. A 16-pm ion-sampling aperture is located at the center of one of these thin walls. The ions that pass through this aperture are measured by an associated mass spectrometer as a function of time after the X-ray pulse. 

In aqueous solutions, irradiation of 15 results in the formation of a transient absorption due to 16, with A av at 390 nm. " This absorption band is shifted to 405 nm, indicating ionization of 16 to form 17. The rate constant for forming 17 was measured to be 2 x 10 s whereas 17 decayed with a rate constant of 1.67 s In addition, photolysis of 15 in D2O leads to deuterium incorporation at the benzylic position, thus verifying the mechanism of solvent-assisted reformation of 15 from 16. °... 

Fluorene and 9,9-dideuteriofluorene oxidized at the same rate in DMSO and in tert-butyl alcohol solution. This observation is consistent with a rapid, reversible ionization step. In tert-butyl alcohol the exchange of alpha deuterium atoms of dideuteriofluorene was measured (see experimental section) and a second-order rate constant for ionization calculated to be 0.12 = = 0.01 mole"1 per second. Under the conditions of this experiment the rate of oxygen absorption of undeuterated fluorene was approximately 1/50 the rate of deuterium exchange from the 9,9-dideuteriofluorene. 

The catalyst in the absence of fluorene absorbed oxygen, and in Table V a correction for this process has been attempted. The corrected data indicate an ionization rate constant of fluorene of 1.2 0.2 mole"1 per second at 29.5°C. The isotope effect in ionization of fluorene and 9,9-dideuteriofluorene is thus about 10. 

Final confirmation of this interpretation was attempted by studying the electron transfer between fluorene and m-trifluoromethylnitrobenzene in basic solution monitored by ESR spectroscopy in the absence of oxygen. Table VI summarizes data yielding an ionization rate constant of 0.9... 

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