Proton affinity table

The basicities of saturated heterocycles are similar to those of analogous open chain systems, with the exception of three-membered heterocycles, in which the basicity is markedly reduced. Table 1 gives pvalues for the equilibria between free and monoprotonated heterocycles. As the ring size increases, the protonated species become more stable and the pKa values approach those of the open chain analogues. Increasing basicity (thiirane < oxirane < aziridine) prevails in gas phase proton affinities (Table 2) (80JA5151). 

In RPLC-APCl-MS, where the mobile phase consists of a mixture of water and methanol or acetonitrile, and eventually a buffer, the formation of protonated water clusters can be considered as a starting point in a series of even-electron ion-molecule reactions. The protonated water clusters transfer their proton to any species in the gas mixture with a higher proton affinity (Table 6.1). The mass spectrum of acetonitrile (MeCN)-water mixture shows protonated MeCN-water clusters, [(MeCN), (HjO) + H]", with /w-values of 1-3, and -values of 0-1. The addition of aimnonium acetate to MeCN-water results in the observation of mixed solvent clusters, e.g., [(MeCN), + and [(MeCN) , (HjO) +... 

The proton affinities (Table 22.1) of methanol (754 kJ moTi) and water (691 kJ moTi) are significantly lower than that of ammonia (854 kJ mol i) and whether or not these molecules are protonated in H-zeolites has created lively debates in the literature. 

Like protons, transition-metal ions are strongly acidic and they can, in principle, add to both the C—H and C—C bonds of alkanes. As already noted in the section on proton affinities (Table 1) strained cycloalkanes are intrinsically more basic than open-chain alkanes, and the reaction of cyclopropane with Pt((II) to form a platinacyclo-butane (equation 14) was the first reaction of a formally saturated hydrocarbon with a transition-metal ion . The driving force in this reaction is relief of the strain associated with the small ring. The resulting metallacyclobutane is essentially free of ring strain. Many low-valent transition-metal complexes have been found to react with cycloalkanes. Metal ions convet the strained hydrocarbons quadricyclane , cubane , bicyclo-[2.1.0]pentanes , bicyclo[3.1.0]hexanes , bicyclo[4.1.0]heptanes and bicyclo-butanes into less strained isomers (usually cyclohexanes). 

In mass spectrometry, the proton affinity parameter is used more than basicity. The proton affinity of a molecule M is the standard enthalpy of reaction [designated delta H (AH)] associated with the reaction MH+ M + H+ in the gas phase. The higher the AH value, the more energy that must be supplied to remove a proton from M. The basicity of M corresponds to AG (standard free enthalpy of reaction) of the same reaction. Thus, AG = AH - TAS, where T is the temperature of the system and AS is the entropy variation. AS is generally difficult to determine. For that reason, mass spectrometrists generally utilize proton affinities. Table 3.1 in Chapter 3 lists the proton affinity values of the main families of organic compounds. 

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. 

Table 8—2. Proton affinities of selected compounds, from AMI calculations...

Table 3 Gas Phase Proton Affinities of Small Heteroeyeles (sojASiSi)...

The calculation of the proton affinities (PA) for a pair of tautomers and the comparison with experimental data [generally from ICR measurements (Section VII,F)] has been the subject of a series of publications with increasing sophistication (Table IV). Such calculations concerning the annular tautomerism of azoles and benzazoles have been reviewed [87AHC(41)187]. 

Table 1.1. Proton affinities for selected reagent gases...

Table 3 Calculated proton affinities for the N2O molecule [kcal/molj...

Table 3 presents the proton affinities calculated (without zero-point and thermodynamic contributions) for both N- and 0-ends of N2O. It is seen that indeed the SCF result leads... 

The main features of DCI-MS and DCI-MS/MS are given in Table 6.14. DCI has gained rapid popularity because it is relatively simple to adapt to almost any mass spectrometer and gives results similar to FD, but in a more simple manner. It is not a substitute for FD, but it is less expensive and generally produces more fragmentation information than FD. For many compounds, molecular ions will be obtained where conventional solids probes would not do the job. DCI is known for the specificity provided by choosing reagent gas with different proton affinities. The major... 

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. 

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... 

Figure 3. Comparison between the calculated proton affinities and stability indices. Numerals correspond to the line numbers in the Table.

As seen in Table V, there is a clear dependence of the equilibrium position on the basicity of amines, excluding triethylamine. However, it is necessary to take into account not only the proton affinity of the amine, but also the ability of the amine to form a dative bond with a boron atom. The equilibrium position also depends on the structure of the phosphorus-... 

Although it is not surprising that anions have a rather high affinity for protons, it is also found that neutral molecules bind protons with the release of energy. Table 9.3 shows the proton affinities for some neutral molecules having simple structures. 

The data shown in Tables 9.2 and 9.3 reveal some interesting facts. We know that HI is a strong acid in aqueous solution. However, the proton affinity of 1(g)- is 1314kJ/mol, whereas that of H20 is 686kJ/ mol. Therefore, for the reaction... 

Table 6. Experimental Gas Phase Basicities (GB) and Proton Affinities (PA) of Carbenes and Reference Compounds (298 K)...

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