Proton affinities gas-phase

Szule]ko J E and McMahon T B 1993 Progress toward an absolute gas-phase proton affinity scale J. Am. Chem. Soc. 115 7839-48... 

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

In considering the following isoelcctronic sequence (8 valence electrons) and the corresponding gas-phase proton affinities (An / kJ mor )3 >... 

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). 

This approach may be possible for small molecules because the gas-phase proton affinity can be obtained quantum mechanically with an accuracy of 1-2 kcal/mol [19]. However, the solvation free energy of H+ cannot be calculated and the experimental value is only known approximately, from 259.5 to 262.5 kcal/mol [60]. Also, because the proton affinity and solvation free energies in Eq. (10-7) are on the order of hundreds of kcal/mol, small percentage errors in the calculation can give rise to large error in AGaqP and pKa. Thus, this method for calculation of absolute pifa s remains impractical at the present time [6],... 

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. 

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). 

The basicity of dimethoxycarbene (197) in solution is similar to that of pyrrolidine (pATa = 12.3). The gas phase proton affinity of pyrrolidine, 227... 

Figure 5. The complexation of phenylhalocarbenes with tetrabutylammonium halides in CH3CN. The numbers in parentheses are gas phase proton affinities.

It is well known that alkyl substitution changes the basicity of amines. However, solvation effects lead to an anomalous order of basicities in solution (NH3 tertiary amine < primary amine < secondary amine). From gas-phase proton affinity data the intrinsic effects of alkyl substituents can be evaluated and a quite regular order (NH3 < primary amine < secondary amine < tertiary amine) is obtained91. 

The advent of techniques that enable the study of fast reactions in the gas phase, such as ion cyclotron resonance (ICR) spectrometry, Fourier-transform ion cyclotron resonance spectrometry (FT-ICR) and high pressure mass spectrometry (HPMS), allowed the measurement of the gas-phase proton affinities for strong bases84-86 as well as for... 

Gas-phase proton affinity measurements are not included unless further clustering steps are also reported. 

Amad, M.H. Cech, N.B. Jackson, G.S. Enke, C.G. Importance of Gas-Phase Proton Affinities in Determining the ESI Response for Analytes and Solvents. J. Mass Spectrom. 2000, 35, 784-789. 

The intrinsic basicities of cyclopentenone and cyclohexenone (59), and their lactone analogues (60), have been accessed via measurement of their gas-phase proton affinities, and compared with the saturated carbonyl compounds in both cases. The results... 

HEATS OF FORMATION OF GASEOUS IONS GAS-PHASE PROTON AFFINITIES... 

J. Proton Affinity Retrieval Prograno. With the current high level of interest in chemical ionization mass spectrometry, there is a need for a reliable file of gas phase proton affinities. No data base of this sort has previously been assembled and for these reasons, the task of gathering and evaluating all published gas phase proton affinities has been undertaken by Rosenstock and co-workers at NBS. This file [28], which has about 400 critically evaluated gas phase proton affinities drawn from the open literature, and can be searched on the basis of compound type or the proton affinity value. It will be appended to the MSSS and the bibliographic component will be merged with the Mass Spectrometry Bulletin Search System. 

HPMS has been used by Meot-Ner (79JA2396) and Kebarle (73JA3504) for the determination of the gas-phase proton affinities of the compounds listed in Tables V and VI (Section IV,A). 

In both cases [that is, AH(B ) = constant and AH(B ) oc IP(B )] one can rationalize the linear relationships found between solution pK and IP values (74HCA546 74JA3314 79JOC2093 84JHC269). They appear as a consequence of the corresponding linear relationships between solution pK values and gas phase proton affinities. 

The first condition can be easily verified with the aid of the extensive data available on gas-phase proton affinities (Wolf el al., 1977). Likewise, the second condition can be verified with present knowledge of heats of formation of ions obtained by several mass spectrometric techniques. The chemistry of alcohols is one case where reactions (55 R = CH3, C2H5, C3H7 and... 

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). 

The gas phase proton affinities of azetidine and (3-lactams were measured by FTICR MS. Azetidine presents a gas phase basicity practically equal to that of /V-methylethanamine (92JA4728). 

Part of the difficulties encountered in comparing these two approaches results from the different ways in which they are used. The E-C approach treats the interaction of only two species ala time to the extent that the nonpolar solvents used in these studies minimize solvation effects, the results are comparable to gas-phase proton affinities. In contrast, the HSAB principle is usually applied to exchange or competition reactions of the sort ... 

Correlate the behavior of various solutes in superacids" with their gas-phase proton affinities. What factors besides proton affinities affect their solution chemistry Predict what species will be present when XHj (Group VA, 15). H2X (Group VIA, 16) and HX (Group VIIA. 17) are dissolved in "superacids. ... 

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