Gas-phase acidities and basicities

Acidities and basicities of organic compounds are expected to be different in the gas phase and in solution. Whereas in the gas phase acidity and basicity are intrinsic properties of the individual molecules, in the liquid state these properties belong to the phase 

Gas-phase acidities and basicities for many organic compounds are now available, primarily due to the development within the past decades of three new experimental techniques pulsed high-pressure i.e. 0.1... 1300 Pa) mass spectrometry (HPMS) [22, 23, 118], the flowing afterglow (FA) technique with a fast-flowing gas like helium in the pressure range of ca. 10 . .. 10 Pa [119], and pulsed electron beam, trapped ion cell, ion cyclotron resonance (ICR) spectrometry, carried out at ca. 10 ... 10 Pa [24-26, 115]. 

The development of these ion-molecule equilibrium measurements has completely changed the status of acid/base reactions (and of other reactions cf. Sechon 5.2) in the gas phase. It is now possible to compare the complex and poorly understood situahon in solution with the simple state in the gas phase. It is also possible to determine the acidity of all acids in the gas phase, from the weakest such as methane to the strongest. In solution, however, due to the levelling effect of the solvent or solubility problems, only a certain range of acids can be measured in a given solvent. 

Using these methods, relative intrinsic acidities and basicihes of molecules in the gas phase have been determined by measuring equilibrium constants, K = [A ] [BH]/[HA] [B ], for proton-transfer reactions such as 

Similar measurements for proton-transfer reactions such as (S = solvent molecules) 

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Relationships between Hydration Energies and Gas-Phase Acidities and Basicities... 

Proton Transfer and Electron Transfer Equilibria. The experimental determination used for the data discussed in the above subsections of Section IV.B were obtained from ion-molecule association (clustering) equilibria, for example equation 9. A vast amount of thermochemical data such as gas-phase acidities and basicities have been obtained by conventional gas-phase techniques from proton transfer equilibria,3,7-12-87d 87g while electron affinities88 and ionization energies89 have been obtained from electron transfer equilibria. 

Cooks and co-workers developed an alternative method for the experimental determination of relative gas phase acidities and basicities. 

The early work on gas-phase acidities and basicities (Brauman and Blair, 1970 Brauman et al., 1971), obtained by probing the preferred direction of proton transfer reactions in the gas phase, established a benchmark in physical organic chemistry. The results pointed out, for example, that polarizability arguments could satisfactorily account for the smooth trends observed within a homologous series, unlike the -values in condensed media where solvation effects may reverse the relative orders of acidities and basicities. 

Secondary isotope effects on gas-phase acidity and basicity 146... 

Other means of manipulating ions trapped in the FTMS cell include photodissociation (70-74), surface induced dissociation (75) and electron impact excitation ("EIEIO")(76) reactions. These processes can also be used to obtain structural information, such as isomeric differentiation. In some cases, the information obtained from these processes gives insight into structure beyond that obtained from collision induced dissociation reactions (74). These and other processes can be used in conjunction with FTMS to study gas phase properties of ions, such as gas phase acidities and basicities, electron affinities, bond energies, reactivities, and spectroscopic parameters. Recent reviews (4, 77) have covered many examples of the application of FTMS and ICR, in general, to these types of processes. These processes can also be used to obtain structural information, such as isomeric differentiation. 

Burk, P. Koppel, I. A. Koppel, I. Leito, I. Travnikova, O. Critical test of pafor-mance of B3LYP functional for prediction of gas-phase acidities and basicities, Chem. Phys. Lett. 2000,323,482-489. 

Proton transfer reactions play very important role in chemistry and biochemistry [1-3]. Considerable attention has been focused on the gas phase reactions in the last decades, since they are free of the solvent pollution thus being related to the intrinsic reactivity [4 6]. In particular, investigations of gas-phase acidities and basicities were some of the major undertakings in the field [7,8]. The proton affinity (PA), on the other hand is an interesting thermodynamic property by itself. It gives useful information on the electronic structure of base in question and serves as an indicator of the electrophilic substitution susceptibility of aromatic compounds [9]. It is the aim of this article to describe some recent advances in theoretical calculations of the proton affinities of substituted aromatics. We shall particularly dwell in more detail on the additivity rules, which enable simple and quick estimates of PAs in heavily substituted benzenes and naphthalenes. Some prospects for future developements will be briefly discussed too. 

We will not discuss the general literature on the theory and techniques of gas-phase acidity and basicity here (see, e.g.. Bowers, 1979 Gal and Maria, 1990). They are defined as the Gibbs free energies (AG°add and AG°base) of the ionization for the gas-phase acidity of an acid into the corresponding base and a proton, and analogously for the basicity (see Gal and Maria, 1990, p. 169 ff). The ther-... 

Table 2. Gas-phase acidities and basicities of inorganic molecules... 

The purest measures of acid-base strength are gas-phase acidity and basicity parameters, where solvent effects are not applicable ... 

Gas-Phase Acidity and Basicity of Common Deuterating Agents... 

Measurement of gas phase acidity and basicity values allows a reexamination of the relationship of structure to acidity and basicity. The order of acidity of alcohols in the gas phase (Table 7.4) is f-butyl > isopropyl > ethyl > methyl, with methyl alcohol being more acidic them water. This is opposite the order of acidity of these compounds in aqueous solution (Table 7.1). The decrease of acidity in solution with increasing alkyl substitution was traditionally... 

Much of the available thermodynamic data, snch as bond dissociation energies, gas-phase acidities and basicities, and heats of formation for ions and neutral target species, has been determined nsing TCID [37, 38]. These data inclnde a variety of metals (alkalis, magnesitrm, altrmintrm, and first and second row transition metals), and many types of target molectrles. For instance, Armentront [39] stndied an abso-Inte cation affinity scale, thermochemistry of alkali-metal cation interactions with histidine,and host-gnest interactions of crown ethers with alkali ions nsing TCID. 

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