Water proton affinity

The formation of either the radical cation M,+ or the protonated [M + H]+ molecule, or both together, will depend on the relative ionization energies or proton affinities of the sample molecules and the solvent components. Concerning the solvent, the charge exchange is favoured for solvents with low proton affinity (water, chloroform, cyclohexane, etc.), while solvents with higher proton affinities (methanol, acetonitrile, etc.) will favour proton transfer. 

Table 1. Examples of Volatile Substances Typically Present in Air Mixtures to be Analyzed by PTR-MS, their Molecular Formulas, Protonated Masses, and Proton Affinities. Water and Ammonia (bold) can Serve as Primary Ions.

Water For most mass spectroscopists, water is a problematic substance. However, as a reagent gas, water has extraordinary properties. Because of the high conversion rate into HjO ions, and due to its low proton affinity, water achieves a high response for many compounds when used as the reagent gas. The spectra obtained usually have few fragments and concentrate the ion beam on a dominant ion. 

Many of the inorganic oxoacids are strong (i.e. have negative PX3 values) in aqueous solution. But, as we have seen, use of a solvent with a lower proton affinity than water (for example pure ethanoic (acetic) acid makes it possible to differentiate between the strengths of these acids and measure pX values. The order of strength of some typical oxoacids is then found to be (for H X -> H , X- + H") ... 

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... 

Most reported triazine LC applications are reversed-phase utilizing C-8 and C-18 analytical columns, but there are also a few normal-phase (NH2,CN) and ion-exchange (SCX) applications. The columns used range from 5 to 25-cm length and from 2 to 4.6-mm i.d., depending on the specific application. In general, the mobile phases employed for reversed-phase applications consist of various methanol and/or acetonitrile combinations in water. The ionization efficiency of methanol and acetonitrile for atmospheric pressure chemical ionization (APcI) applications were compared, and based on methanol s lower proton affinity, the authors speculated that more compounds could be ionized in the positive ion mode when using methanol than acetonitrile in the mobile phase. 

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... 

To verify the mechanism presented, the quantum-chemical calculations of proton affinity, Aa, were carried out for modifiers, since the corresponding experimental data are quite rare. The calculations were performed for isolated molecules, since the properties of species in the interlayer space are probably closer to the gas phase rather than the liquid. The values of Ah were calculated as a difference in the total energy between the initial and protonated forms of the modifier. Energies were calculated using the TZV(2df, 2p) basis and MP2 electron correlation correction. Preliminarily, geometries were fully optimized in the framework of the MP2/6-31G(d, p) calculation. The GAMESS suite of ah initio programs was employed [10]. Comparison between the calculated at 0 K proton affinities for water (7.46 eV) and dioxane (8.50 eV) and the experimental data 7.50 eV and 8.42 eV at 298 K, respectively (see [11]), demonstrates a sufficient accuracy of the calculation. 

However, in more complicated amines, this straight correlation is violated. The bicyclic tertiary amine l-azabicyclo[4.4.4]tetradecane (22) and the acyclic tertiary amine n-Bu3N have nearly the same first IP (7.84 and 7.90 eV, respectively), but the proton affinity of the bicyclic amine is 20 kcal mol 1 lower than that of the acyclic52. On the other hand, for other bridge-head tertiary amines like l-azabicyclo[2.2.2]octane (quinuclidine, 20) and l-azabicyclo[3.3.3]undecane (manxine, 21) the expected relation between proton affinities and IP values is observed. The extraordinary properties of l-azabicyclo[4.4.4]tetradecane (22) are caused by its unusual conformation the nitrogen lone-pair is directed inward into the bicycle where protonation is not possible. In the protonated form, the strained out-conformation is adopted. This makes it the least basic known tertiary amine with purely saturated alkyl substituents. Its pKa, measured in ethanol/water, is only +0.693. Strain effects on amine basicities have been reviewed by Alder88. 

In most papers the experimental proton affinity difference between water and ammonia is taken as being equal to 37.5 kcalmol-1. Various calculated values of this gap have appeared. The PA values of ammonia and water were 205.6 and 168 or 168.6 kcalmol-1 respectively (APA = 37 or 37.6 kcalmol-1) according to Dixon and Lias26 the work of Defrees and McLean27 led to similar calculations of P/HN1I3) = 204.0 and PA (H30) = 165.1, with A PA = 38.9 kcalmol-1, although Pople and coworkers28 predicted a A PA = 39.4 kcalmol-1. 

A term applied to a solvent exhibiting affinity for protons and therefore acting as a proton acceptor. Water and ethanol are both protophihe moreover, both are proto-genic (i.e., they can release protons in ionization reactions). See Protogenic Amphiprotic... 

As a strong polar proton donor, water is strongly absorbed in most ionic liquids. In the extreme case, it is fully miscible with ionic liquids, such as [BMIM]Bp4 and [BMIMJCFsCOj. Even the hydrophobic ionic liquids can pick up a low concentration of water, resulting in a reduced absorption affinity for some organic compounds (27). The presence of water may, therefore, influence the performance of an ionic liquid in catalytic applications. 

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