Molecules proton affinity

Mixed cluster ions of the type H CEhCN (HiOIb react also with other large proton affinity molecules such as CHiOH via... 

Figure 2. Plot of ionization potentials of neutral molecules against their proton affinities. Molecules are C, N, O, and F donors. Data are from references 20 and 21.

It is possible to detemiine the equilibrium constant, K, for the bimolecular reaction involving gas-phase ions and neutral molecules in the ion source of a mass spectrometer [18]. These measurements have generally focused on tln-ee properties, proton affinity (or gas-phase basicity) [19, 20], gas-phase acidity [H] and solvation enthalpies (and free energies) [22, 23] ... 

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

However, many substances, notably alcohols, have a greater proton affinity than the hydrogen fluoride molecule, and so behave as bases, for example ethanol ... 

Morokuma K 1977. Why Do Molecules Interact The Origin of Electron Donor-Acceptor Complexes, Hydrogen Bonding, and Proton Affinity. Accounts of Chemical Research 10 294-300. 

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/). 

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... 

This is either an (M + 1)+ or an (M - -18)+ ion, depending upon the relative proton affinities of the analyte and ammonia. The molecular weight is therefore either 221 or 204 Da. Since the molecule contains an even number of nitrogen atoms, it must have an even molecular weight. The molecular species must therefore be (M - -18)+ and so the molecular weight is 204 Da. 

In the present work, we shall investigate the problem of the amount of correlation accounted for in the DF formalism by comparing the molecular electrostatic potentials (MEPs) and dipole moments of CO and N2O calculated by DF and ab initio methods. It is indeed well known that the calculated dipole moment rf these compounds is critically dependent on the level of theory implemented and, in particular, that introduction of correlation is essential for an accurate prediction [13,14]. As the MEP property reflects reliably the partial charges distribution on the atoms of the molecule, it is expected that the MEP will exhibit a similar dependence and that its gross features correlate with the changes in the value of dipole moment when switching from one level of theory to the other. Such a behavior has indeed been reported recently by Luque et al. [15], but their study is limited to the ab initio method and we found it worthwhile to extend it to the DF formalism. Finally, the proton affinity and the site of protonation of N2O, as calculated by both DF and ab initio methods, will be reported. 

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

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. 

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],... 

Hunter, E. P. L. Lias, S. Evaluated gas phase basicities and proton affinities of molecules An update. I. Phys. Chem. Ref. Data 1998,27,413-457. 

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... 

Two other molecules considered in the same context, but with proton affinities larger than ammonia, were pyridine (C5H5N) and trimethylamine (TMA) (CH3)3N whose proton affinities are 220.8 cal mol-1 and 225.1 cal mol-1, respectively.190 Metastable decomposition studies of NH3(C5H5N)mH+ (m = 1-5) yield the following results ... 

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