Gas-phase basicity scales

The FT-ICR gas-phase basicity scale for the weak bases87,93 can be compared with the results obtained by McMahon and coworkers91,92 using ICR and HPMS spectromet-ric techniques. Satisfactory agreement was found with the existing ICR data, but some variances were observed with the HMPS results. 

These two equilibria provide the relative strength of an acid and base, respectively, relative to a reference compound (BrefH or Bref) of known acidity or basicity. The reference with respect to which the gas-phase basicity scale was constructed is ammonia, which features AG + = 195.6 kcal mol -1 and PA = 204 kcal mol -1 4 All the data given in this review are referred to these values also, the standard states of the different species involved in equilibria 3-5 above—perfect gases—shall be assumed to have unit fugacity. 

Kaljurand, I., Koppel, I.A., Kiitt, A. et al. (2007) Experimental gas phase basicity scale of superbasic phosphazenes. Journal of Physical Chemistry A, 111, 1245-1250. 

The most comprehensive study of lithium cation basicities for organic bases was conducted by Taft, Gal and coworkers who investigated the effect of molecular structure on the gas-phase cation and proton basicities. Taft s LCA scale was revised and extended, and the lithium cation basicity scale now includes over 200 compounds. In the same work the correlations between gas-phase basicities toward lithium cation (LCB) and proton (GB) were examined. Good correlations are obtained provided that separate lines are drawn for homogeneous families and the differences in slopes are traced back to the different sensitivities to structural effects. Large deviations are explained by either a different attachment center for Li+ and H+ or a chelation effect toward Li+. Figure 5 describes three types of interactions that involve chelation of a lithium cation. 

Schwesinger, R. and Schlemper, H. (1987) Peralkylated polyaminophosphazenes - extremely strong, neutral nitrogen bases. Angewandte Chemie - International Edition, 26, 1167-1170. Kaljurand, I., Rodima, T, Pihl, A. et al. (2003) Acid-base equilibria in nonpolar media. 4. Extension of the self-consistent basicity scale in THF medium. Gas phase basicities of phosphazenes. The Journal of Organic Chemistry, 68, 9988-9993. 

The gas-phase basicity of triphenylamine (876.4 kJ/mol) is less than that of triphenylphosphine (940.4 kJ/mol). Do you expect electronic or steric factors to contribute more to this difference Explain. (Data from C. Laurence and J.-F. Gal, Lewis Basicity and Affinity Scales Data and Measurement, John Wiley and Sons, United Kingdom, 2010, p. 5.)... 

In certain cases, it is not possible to establish a proton transfer equilibrium, for example, if the subject compound, M, is unstable, or if MH+ undergoes a fast reaction with M, or a reaction other than proton transfer with B. In these instances, other strategies have been adopted to determine relative gas-phase basicities or proton affinities. The simplest such approach is called the bracketing technique the ion MH+ is generated and the occurrence or nonoccurrence of proton transfer with a series of molecules, Bj, B2, etc., is observed. Reference compounds are chosen whose position in the relative scale of gas basicities is known. 

The results of these recent experimental proton transfer equilibrium studies indicated that portions of the 1984 gas-phase basicity/proton affinity scale were constricted. For example, in the 1984 scale the difference between the proton affinities of isobutene and ammonia was given as 33.5 kj moH, while according to the newer results, this interval is actually about 50 kJ moH. Also, these experimental results indicated that the portion of the 1984 scale representing gas-phase basicities/proton affinities higher than that of ammonia (a portion of the scale that was not anchored by data for a primary standard) was seriously constricted. 

Table 1 Backbone scale of proton affinities and gas-phase basicities ...

Table 1 shows the results of the 1998 evaluation of the gas-phase basicity and proton affinity scales as exemplified by the backbone scale. The results shown include the evaluated proton affinity, gas-phase basicity and entropy of protonation values, the ab initio value reported by Smith and Radom and, for comparison, the proton affinity value cited in the... 

Compounds are transformed into each other by chemical reactions that can be run under a variety of conditions from gas-phase reactions in refineries that produce basic chemicals on a large scale, through parallel transformations of sets of compounds on well-plates in combinatorial chemistry, all the way to the transformation of a substrate by an enzyme in a biochemical pathway. This wide range of reaction conditions underlines the complicated task of imderstanding and predicting chemical reaction events. 

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