Proton transfer bracketing

The gas-phase acidity of some of the simplest organogermanes and of trimethylstannane have been determined by a combination of gas-phase equilibrium measurements and proton-transfer bracketing experiments using FTMS185 190. The experimental values of A//°cjd are shown in Table 6 and refer to the enthalpy change associated with reaction 29. 

Proton-transfer equilibrium measurements and proton-transfer bracketing methods are sources for proton affinity values of organometallic complexes. The determination of the site of proto-nation, i.e., metal atom vs. a ligand, is a fundamental dilemma of any study on the protonation of metal complexes. It was demonstrated, for example, that Fe(CO)5 was protonated exclusively at the metal atom, whereas the results for the proton transfer to ferrocene can be explained by the formation of a metal-protonated form and a ring-protonated form, involving the agostic interaction of the proton with the metal atom. 

The methods summarized by Eqs. 8 and 9 have both been applied to halocarbenes. The proton affinities obtained by ICR bracketing125 (Table 6) are consistently lower than those derived from enthalpies of formation (Table 7). The case of dichlorocarbene, with a difference in AH of 15 kcal/mol), is particularly disturbing and has been analysed in some detail.134 Notably, the PA of CCl2 from an earlier bracketing experiment126 was closer to the enthalpy-derived PA. The discrepant results from similar experiments125126 indicate that HCClJ is not a good substrate for proton transfer studies. 

The equilibrium constant for Eq. 9-102, calculated from the pKa of 7.0 for imidazole, is 10 7 M. Since Keq is also the ratio of the overall rate constants for the forward and reverse reactions, we see that for the forward reaction kj = 10 7 x 1.5 x 1010 = 1.5 x 103 s . This slow rate results from the fact that in the intermediate complex (in brackets in Eq. 9-102) the proton is on the imidazole group most of the time. For a small fraction of the time it is on the coordinated molecule HzO but reverts to being on the imidazole many times before the imidazole and OH3+ separate (see also Eqs. 9-97 and 9-98). Because of this unfavorable equilibrium within the complex, the diffusion-controlled rate of proton transfer from a protonated imidazole to water is far less than for proton transfer in the reverse direction. 

Because of the high instability of enamines (particularly primary and secondary ones, which rearrange to their corresponding imines), they are handled with ICR, in its drift mode, and a bracketing technique in order to detect charged forms on altering the basicity of the reference base used, on the assumption that exothermic proton transfer processes will be observed whereas endothermic processes will not be observed. 

The gas phase acid/base properties of molecules have been subject to equilibrium or bracketing measurements employing mass spectrometric techniques like ion cyclotron resonance (ICR) [4], Fourier transform ion cyclotron resonance (FT-ICR) [5,6], Flowing afterglow (FA) and Selected ion flow tube (SIFT) [7], and high pressure mass spectrometry (HPMS) [8]. Proton transfer between neutral molecules are then investigated by measurements of reactions... 

Bronsted and Pedersen [20] indicated that the rate constant for proton transfer from acid to a base cannot continue to increase in accord with a linear Bronsted law but must be limited by an encounter rate. This prediction was confirmed by Eigen s school [21] who showed that changed from 1 to zero as the p/f of the donor acid fell below that of the acceptor base (Fig. 5). Eigen [21] considered the following scheme (sometimes called the Eigen mechanism) for proton transfer from HX to Y where reactions in brackets occur in the encounter complex (Eqn. 28). The overall rate constants are given in Eqns. 29 and 30. 

Bracketing methods involving exothermic proton transfer have yielded both cationic and neutral metal-ligand bond energies. For example, M0H (M - Fe and Co) were reacted with a series of reference bases, reaction 5 (42). For CoOH , proton transfer was observed with... 

Equilibrium in the ion source of a mass spectrometer cannot be achieved in certain ion-molecule systems because of rapid competitive reactions or because one of the species in the equilibrium of interest is an unstable species such as a free radical. In such cases, it is sometimes possible to obtain an experimental estimate of the enthalpy change of a particular reaction (charge transfer, proton transfer, etc.) by use of a technique known as bracketing, in which the ion of interest is reacted with a series of molecules selected to provide a range of values for the relevant thermochemical parameter of interest, e.g., ionization energy, electron affinity, gas-phase basicity, or acidity. Reaction is presumed to occur for exothermic processes and not to occur for endothermic processes. 

The brackets around the H indicate that the acid functions as a catalyst. In each of the reactions above, the mechanism involves a proton transfer as the final step of the mechanism. 

First let s focus our attention on all of the proton transfers in the entire meehanism. Four of the steps above are proton transfer steps. Two of them involve protonation and two involve deprotonation. So, in the end, the acid is not consumed by the reaction. It is a catalyst here. From now on, we will place brackets around the acid to indicate that its function is catalytic ... 

The methods for the determination of GBs and PAs (Chap. 2.12) make use of their relation to (Eq. 9.23) and the shift of upon change of [AH] or B, respectively [187,188]. Basically, the value of GB or PA is bracketed by measuring eq with a series of several reference bases ranging from lower to higher GB than the unknown. There are two methods we should address in brief, a detailed treatment of the topic being beyond the scope of the present book, however. The kinetic method makes use of the dissociation of proton-bound heterodimers, and the ther-mokinetic method determines the equilibrium constant of the acid-base reaction of gaseous ions. In general, proton transfer plays a crucial role in the formation of protonated molecules, e.g., in positive-ion chemical ionization mass spectromehy (Chap. 7). 

In certain cases, it is not possible to establish a proton transfer equilibrium, for example, if the subject compound, M, is unstable, or if MH+ undergoes a fast reaction with M, or a reaction other than proton transfer with B. In these instances, other strategies have been adopted to determine relative gas-phase basicities or proton affinities. The simplest such approach is called the bracketing technique the ion MH+ is generated and the occurrence or nonoccurrence of proton transfer with a series of molecules, Bj, B2, etc., is observed. Reference compounds are chosen whose position in the relative scale of gas basicities is known. 

Equation (3) incorporates relativistic effects, effects of target density, and corrections to account for binding of inner-shell electrons, as well as the mean excitation energy C/Z is determined from the shell corrections, S/2 is the density correction, Ifj accounts for the maximum energy that can be transferred in a single collision with a free electron, m/M is the ratio of the electron mass to the projectile mass, and mc is the electron rest energy. If the value in the bracket in Eq. (4) is set to unity, the maximum energy transfer for protons... 

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