Proton affinity determination

The kinetic method [42,43] is a relative method for thermochemical data determination which is based on measurement of the rates of competitive dissociations of mass-selected cluster ions. This method was introduced by Cooks [44] for proton affinity determination. Later, an extension of this method was proposed by Fenselau [45]. 

Proton affinity determination by the kinetic method, (a) This method is based on competitive dissociation of heterodimer clusters, (b) Potential energy diagram for proton-bound dimer dissociation. 

For proton affinity determination, the kinetic method involves the formation of the proton bound heterodimer between the two bases whose affinities are to be compared. By tandem mass spectrometry, the appropriate cluster ion [BiHB2]+ is selected and its spontaneous or collisional dissociation is observed. As shown in Figure 4.16, the competitive dissociation leading to the two protonated monomers is analysed and the relative abundances of the monomers [BiH]+ and [B2H]+ are measured. From these abundances, the relative proton affinities of the two bases Bi and B2 can be calculated and the proton affinity of one of the two bases can be determined, if the proton affinity of the other is known. 

In an earlier section, measurements were described in which the equilibrium constant, K, for bimolecular reactions involving gas-phase ions and neutral molecules were determined. Another method for determining the proton or other affinity of a molecule is the bracketing method [52]. The principle of this approach is quite straightforward. Let us again take the case of a proton affinity determination as an example. In a reaction... 

It is interesting to compare the possibilities and errors of different hybrid QM/MM schemes. The careful examination and comparison of link atom and LSCF techniques was performed in Ref. [128] using the CHARMM force field [114] and the AMI method [143] as a quantum chemical procedure. In the case of the link atom procedure two options were used QQ - the link atom does not interact with the MM subsystem and HQ - link atom interacts with all MM atoms. The main conclusion of this consideration is that the LSCF and the link atom schemes are of similar quality. The error in the proton affinity determination induced by these schemes is several kcal/mol. It is noteworthy that all the schemes work rather badly in description of conformational properties of n-butane. The large charge on the MM atoms in the proximity of the QM subsystem (especially on the boundary atom) cause significant errors in the proton affinity estimates for all methods (especially, in the case of the LSCF approach where the error can be of tens of kcal/mol). This is not surprising since the stability and transferability of intrabond one- and two-electron density matrix elements Eq. (19) is broken here. It proves that the simple electrostatic model is not well appropriate for these schemes and that a detailed analysis of the... 

Nourse BD, Cooks RG. Proton affinity determination using the kinetic method in an ion trap mass spectrometer. Int J Mass Spectrom Ion Proc. 1991 106 249-72. 

Data on proton affinities (gasphase) ofmany different compounds (see Table 2) demonstrate the high level of accuracy possible in determining energies of related species. In this report by Dewar and Dieter, the enthalpy of formation of is the experimental value (367.2 kcal/mol). The calculated value for is unreliable. 

The proton affinities (gas phase) of thiirane and other three-membered heterocycles have been determined azirane (902.5), thiirane (819.2), phosphirane (815.0), oxirane (793.3 kJ moF ) (80JA5151). Increasing s character in the lone electron pairs decreases proton affinities. Data derived from NMR chemical shifts in chloroform indicate the order of decreasing basicity is azirane > oxirane > thiirane (73CR(B)(276)335). The base strengths of four-, five- and six-membered cyclic sulfides are greater than that of thiirane. 

This technique provides quantitative information about tautomeric equilibria in the gas phase. The results are often complementary to those obtained by mass spectrometry (Section VII,E). In principle, gas-phase proton affinities, as determined by ICR, should provide quantitative data on tautomeric equilibria. The problem is the need to correct the measured values for the model compounds, generally methyl derivatives, by the so-called N-, 0-, or S-methylation effect. Since the difference in stability between tautomers is generally not too large (otherwise determination of the most stable tautomer is trivial) and since the methylation effects are difficult to calculate, the result is that proton affinity measurements allow only semi-quantitative estimates of individual tautomer stabilities. This is a problem similar to but more severe than that encountered in the method using solution basicities (76AHCS1, p. 20). 

MO studies (AMI and AMI-SMI) on the tautomerism and protonation of 2-thiopurine have been reported [95THE(334)223]. Heats of formation and relative energies have been calculated for the nine tautomeric forms in the gas phase. Tire proton affinities were determined for the most stable tautomers 8a-8d. Tire pyrimidine ring in the thiones 8a and 8b has shown a greater proton affinity in comparison with the imidazole ring, or with the other tautomers. In solution, the thione tautomers are claimed to be more stabilized by solvent effects than the thiol forms, and the 3H,1H tautomer 8b is the most stable. So far, no additional experimental data or ab initio calculations have been reported to confirm these conclusions. 

Gas-phase basity and proton affinity values for 3,4,6,7,8,9-hexahydro-2/f-pyrido[l,2-n]pyrimidine were determined and they were compared to other super bases, including its lower and higher piperidine ring homologs (94JP0725, 01JPO25). 

This describes the process during which the monomer is transferred into its cation. This process has proven itself to be the rate determining factor (see part 4.1.3). An extensive collection of proton affinities of relevant olefines is given in Ref.108). 

In principle, the equilibrium approach can be used to measure any of the thermochemical properties listed above. However, in practice, it is most commonly used for the determination of gas-phase acidities, proton affinities, and electron affinities. In addition, equilibrium measurements are used for measuring ion affinities, including halide (F, Cl ) and metal ion (alkali and transition metal) affinities. 

Alternatively, enthalpies of formation of carbenes and biradicals can be measured by using the approach shown in Eq. 5.4b. The key to the measurement is the determination of the proton affinity of the substrate, PA(R), which can be obtained... 

Hehre and co-workers have used this approach for the investigation of biradicals and other reactive neutral molecules. For example, by using the bracketing approach, they were able to determine the proton affinities of o- and p-xylylene (o- and p-quinodimethane (lo and Ip) Figure 5.3), from which they were able to determine the enthalpies of formation of the reactive, Kekule molecules. They found the proton affinity of the meta isomer to be too high to be measured directly by bracketing, but were able to assign a lower limit, and subsequently a lower limit to the enthalpy of formation of the m-xylylene diradicals. 

Thermodynamically it can be stated, if the differences of solvation of the compounds X" and HX between two solvents are neglected, that the difference in the pK values of compound HX in the two solvents is completely determined by the difference in the proton affinities of the two solvents80 hence a comparison of the pjfifj, values of various compounds in the solvents 1,2-DCE, m-cresol, acetic acid, pyridine and water is worth considering (see Table 4.5)80. 

Amad, M. H. Cech, N. B. Jackson, G. S. Enke, C. G. Importance of gas-phase proton affinities in determining the electrospray ionization response for analytes and solvents. I. Mass Spectrom. 2000,35,784-789. 

In these reactivity studies, reactions 22a and b were studied and the rate coefficient and product distribution determined as reported above. This product distribution is at variance with a much earlier study where only an association channel was reported, although with a similar rate coefficient 1(—26) cm6 s-1, equivalent to a binary rate coefficient of 2(—10) cm3 s-1 at 0.5 torr.61 The CHsO+, produced in this way and by reaction 23, was reacted with a series of molecules with proton affinities varying from 166 to 193 kcal mol-1 and encompassing that of CH3OH see Table 3. For the production of CH50+ in the association reaction 22a, sufficient water was... 

The active site is viewed as an acid-base, cation-anion pair, hence, the basicity of the catalyst depends not only on the proton affinity of the oxide ion but also on the carbanion affinity of the cation. Thus, the acidity of the cation may determine the basicity of the catalyst. Specific interactions, i.e., effects of ion structure on the strength of the interaction, are likely to be evident when the carbanions differ radically in structure when this is likely the concept of catalyst basicity should be used with caution. 

The decomposition of NH4HSO4 and the appropriate thermochemical cycle for determining the proton affinity of the HS04-(g) ion can be shown as follows ... 

By means of appropriate thermochemical cycles, it is possible to calculate proton affinities for species for which experimental values are not available. For example, using the procedure illustrated by the two foregoing examples, the proton affinities ofions such as HC03-(g) (1318 k J mol-1) and C032-(g) (2261 kj mol-1) have been evaluated. Studies of this type show that lattice energies are important in determining other chemical data and that the Kapustinskii equation is a very useful tool. 

Considering the abundant evidence for carbene protonation, some quantitative estimate for the base strength of carbenes is clearly desirable. The conventional spectrometric or potentiometric methods of determining the pKa in solution are not applicable, with the exception of some onium ions 1 and their conjugate bases 2 (Section V.B). In favorable cases, equilibria of carbenes with the conjugate carbenium ions have been studied in the gas phase. Proton affinities of various carbenes can be obtained from their enthalpies of formation, and by ab initio computation (Section V.A). Kinetic data have been evaluated to obtain the pKa of carbenes in solution (Section V.B). 

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