Proton affinities absolute

Alternatively, the translational energy threshold for endothermic proton transfer from MH+ to R can be measured using a flowing afterglow triple quadrupole instrument.127 These data define the proton affinity of M, relative to that of R. Thus, the PA of cyclopropenylidene was found to exceed that of ammonia by 23.3 1.8 kcal/mol (Table 6).128 In order to obtain absolute proton affinities, the enthalpies of formation of both the base and the conjugate acid must be known from other measurements (Eq. 9). Numerous reference compounds with known absolute PA are available.124... 

Evaluation of the only appropriate Fukui function is required for investigating an intramolecular reaction, as local softness is merely scaling of Fukui function (as shown in Equation 12.7), and does not alter the intramolecular reactivity trend. For this type, one needs to evaluate the proper Fukui functions (/+ or / ) for the different potential sites of the substrate. For example, the Fukui function values for the C and O atoms of H2CO, shown above, predicts that O atom should be the preferred site for an electrophilic attack, whereas C atom will be open to a nucleophilic attack. Atomic Fukui function for electrophilic attack (fc ) for the ring carbon atoms has been used to study the directing ability of substituents in electrophilic substitution reaction of monosubstituted benzene [23]. In some cases, it was shown that relative electrophilicity (f+/f ) or nucleophilicity (/ /f+) indices provide better intramolecular reactivity trend [23]. For example, basicity of substituted anilines could be explained successfully using relative nucleophilicity index ( / /f 1) [23]. Note however that these parameters are not able to differentiate the preferred site of protonation in benzene derivatives, determined from the absolute proton affinities [24],... 

According to Del Bene (85MI4), the reproduction by theoretical methods of the experimentally determined absolute proton affinities of molecules such as CHjOH, H2CO, CO, CH3NH2, and HCN requires (1) the optimization of the molecular geometries of the neutral and protonated forms at the Hartree-Fock 6-31G(d) (82JPC1529) level (2) the evaluation of the zero-point energy... 

TABLE 10. Absolute proton affinities of some silyl compounds... 

Compound Absolute proton affinity" (kcal mol ) Experimental ... 

Absolute Proton Affinities 2.1. Experimental Basicity Scales... 

Theoretical absolute proton affinities obtained for monosubstituted benzenes C H X are presented in Table 1. The examined substituents encompass X = C//3, OH, OGH3,... 

Maksic, Z.B. and Kovacevic, B. (2000) Absolute proton affinity of some polyguanides. The Journal of Organic Chemistry, 65, 3303-3309. 

The gas phase basicities and pKa values of tris(phosphazeno) substituted azacalix[3](2,6) pyridine in acetonitrile and some related compounds were examined by the density functional theory (DFT) computational method. It was shown that the hexakis(phospha-zeno) derivative of azacalx[3](2,6)pyridine is a hyperstrong neutral base, as evidenced by the absolute proton affinity of 314.6kcal/mol and pKa (MeCN) of 37.3 units. It is a consequence of the very strong bifurcated hydrogen bond (32kcal/mol) and substantial cationic resonance effect [14]. 

The heats of adduct formation with many Lewis acids have been measured experimentally in the gas phase using the technique of ion cyclotron resonance [10]. The absolute proton affinity (PA) of a gaseous base molecule has a precise thermodynamic definition in relation to the negative of the enthalpy variation for a hypothetieal reaction of attachment of an isolated proton to a molecule M in the gas phase. 

The absolute proton affinity (PA) of a base molecule in the gas phase has a precise thermodynamic definition in relation to the enthalpy variation of the hypothetical reaction... 

Adams NG, Smith D, Tichy M, Javahery J, Twiddy ND, Ferguson EE (1989) An absolute proton affinity scale in the 130-140 kcal mol range. J Chem Phys 91 4037-4042... 

Czak6 G, Matyus E, Simmonnett AG, Csaszar G, Schaefer HF in, Allen WD (2008) Anchoring the absolute proton affinity scale. J Chem Theory Comput 4 1220-1229... 

The evaluation of such a body of interrelated thermodynamic data involves first an evaluation of the thermochemical scales for internal consistency in the three parameters, AG° (at different temperatures), AH° and AS°. Final values assigned for the proton affinities and entropy changes must be consistent with what is known about the thermochemistry of M and MFI+. The lengths of segments of the scale linking different primary standards (compounds for which Eqn [2] can be used to derive an absolute proton affinity value) must of course match the known interval between the known proton affinity values. 

Szulejko J and McMahon TB (1993) Progress towards an absolute, proton affinity scale. Journal of the American Chemical Society 115 7839-7848. 

Traeger JC (1996) The absolute proton affinity for isobutene. Rapid Communications in Mass Spectrometry 10 119-121. 

Eckert-Maksic, M., Hodoscek, M., Kovacek, D., et al. (1997) Theoretical model calculations of the absolute proton affinities of benzonitrile, nitroso- and nitrobenzene. Journal of Molecular Structure (Theochem), 417,131-143. 

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