Proton affinities additivity

Additional gas-phase reactivity data, such as gas-phase acidities of alcohols [41], proton affinities of alcohols and ethers [41], and proton affinities of carbonyl compounds [42] could equally well be described by similar equations. 

The proton affinity is defined as the energy released when a proton is added to a system, computed as the energy difference between the system of interest and the same molecule with one additional proton (H ). For example, the proton affinity of PHj is computed as EfPHj) - E(PH/). 

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. 

In many cases, a protonated molecular ion (M - - H)+ is the only ion observed in a thermospray spectrum but if ammonium acetate buffer is used, depending upon the relative proton affinities of the species present, an ammonium adduct (M - - NH4)+ may be the predominant ion. In addition, clusters may be formed with components of the mobile phase. Although the thermospray ionization process involves less energy than conventional Cl, and very little intense fragmentation is usually observed, the presence of ions due to the elimination of small molecules, e.g. water, methanol and ketene, is not unknown. These latter ions are usually of relatively low intensity when compared to the protonated or... 

Humic materials fractionated on the basis of hydrophobicity and proton affinity continue to exhibit two fluorophores as discussed in the section "Exciation-Emission Spectra. Strong evidence to establish the existence of at least two chromophores is seen in the phase-resolved spectra. These spectra are shown in Figures 4 a-f. They consist of the phase-resolved emission spectrum of each of the two fluorophores plotted separately and the normal emission spectrum of the humic fraction. If the nulling out of one fluorophore is exact then the sum of the two separate phase resolved spectra should be additive to equal the normal spectrum. In these figures the normal emission spectrum was measured separately from the two phase resolved emision spectra. The phase resolved spectra were then superimposed onto the scan of the normal emission spectrum. 

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. 

On the basis of the results by Chen and Her [51] it may be expected, however, that the low-proton-affinity PE matrix will be barely detectable, and thus not interfering, in ammonia DCI-MS of a dissolved PE/additive sample. 

The Hartree-Fock calculations used the 6-31G basis set as implemented by the Gaussian-86 computer program.55 Protonated and unprotonated species were calculated and the proton affinities were obtained as the difference between the energy of the protonated species and the energy of the unprotonated ones. In addition, 6-31G calculations of the energy, using the 6-31G obtained geometry were performed. 

Because most species will release energy when a proton is added, the proton addition enthalpy is negative. The proton affinity is the heat associated with the reverse process, the removal of the proton,... 

The reaction of pyridine (py) with bare metal ions (except Fe+) has not been studied widely. The reaction of Fe+ produced by electron ionization of Fe(CO)5 with a mixture of two pyridines (108) was used to compare the proton affinities with the Fe+ affinity. A good correlation was observed. The absolute Fe+ affinity of py was determined to be 49 3 kcal mol-1, which is higher than the value of 44 3 kcal mol 1 for the Fe+-NH3 bond dissociation energy (46). Steric problems with ortho substituted pyridines gave lower than expected affinities. The reaction of py and substituted pyridines showed a maximum addition of four pyridines, similar to the GIB experiments with ammonia (46). 

In addition to the concepts reviewed in the last two sections (appearance energy, ionization energy, and electron affinity), three others are relevant in gas-phase molecular energetics, namely, proton affinity, gas-phase basicity, and gas-phase acidity. 

W1/W2 theory and their variants would appear to represent a valuable addition to the computational chemist s toolbox, both for applications that require high-accuracy energetics for small molecules and as a potential source of parameterization data for more approximate methods. The extra cost of W2 theory (compared to W1 theory) does appear to translate into better results for heats of formation and electron affinities, but does not appear to be justified for ionization potentials and proton affinities, for which the W1 approach yields basically converged results. Explicit calculation of anharmonic zero-point energies (as opposed to scaling of harmonic ones) does lead to a further improvement in the quality of W2 heats of formation at the W1 level, the improvement is not sufficiently noticeable to justify the extra expense and difficulty. 

These bimolecular reactions have provided accurate proton affinities (PAs) for many amines165,166. In addition, cation affinities are accessible, usually by combining the enthalpy of formation (AH[) of cationic species derived from PA measurements with similar data for the bare cation. Thus, the knowledge that the PA of CH3NH2 is 896166 kJmol-1 sets A//f(CH3NH3 + ) = 611 kJmor1. Since A//f(CH3+) = 1092 kJmol-1 and A//f(NH3) = —46 kJmol-1 9, the methyl cation affinity of NH3 may be deduced to be 1092 — 46 — 611 = 435 kJ mol-1. 

Ionization may take place by the interaction with a particle sufficiently high in energy, e.g. an electron or a photon, or by the addition of charged species, e.g. an electron or a proton. The thermochemistry associated with the ionization process provides information on ion structures, since a structure may be assigned based on heat of formation when compared to data of reference ions. Thus, the determination of ionization energy, electron affinity and proton affinity plays a central role in mass spectrometry. 

The use of N.M.R. spectroscopy further requires that the proton affinity of the aromatic substances should not be too low. Benzene and toluene, for instance, have such a low proton affinity that no proton addition complex has so far been demonstrated by means of N.M.R. spectroscopy, even at — 100°C. A combination with other methods is therefore required in order to demonstrate proton addition complexes in these cases. 

The piTj5-values of the methyl derivatives of benz[a]anthracene, summarized in Table 23, are particularly interesting. In the case of the unsubstituted compound, two positions of approximately equal proton affinity have to be taken into account. Thus two isomeric proton addition complexes A and B are present in solution (cf. IID, page 229). 

Methyl cation affinities of benzene and some substituted benzenes have been calculated. These follow a simple additivity rule and the value for benzene shows good agreement with the experimental estimate. Conclusive evidence is presented that these values are linearly related to the corresponding proton affinities. The competition between deuteriation and alkylation in the reaction of radiolytically formed perdeuterio ethyl cations with iV-methylpyrrole and with thiophene has been studied. Deuteriation, the Brpnsted acid pathway, predominates and intramolecular selectivities have been determined for each reaction. ... 

Protons are translocated across the membrane by what is described as a proton pnmp . How does the pump operate The change in redox state experienced by the prosthetic gronps of the enzymes in the chain causes conformational changes in the proteins that alter the affinities of some amino acid side-chain gronps for protons. In addition, there is a change in the direction in which these groups face in the membrane. Consequently, oxidation results in an association with a proton on the matrix side of the membrane whereas reduction results in reversal of the direction that the side-chain groups face and an increase in... 

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