Mass Spectrometry and Ion Cyclotron Resonance Studies

Thanks to the work of Taft in California and Abboud in Madrid, the gas-phase basicity of many pyrazoles has been determined by ICR and in some cases, using Cooks method, by mass spectrometry 840MS(19)627 . Both methods afford basicities (termed thermodynamic vs. kinetic basicity) which have been compared in 91JOC179 . 

The experimental proton affinity of 3(5)-methylpyrazole (11), 216.0 kcal mol is in agreement with those calculated (see Section 3.01.2.1) for tautomers 3-methyl (11a) 217.2 kcal mol , and 5-methyl (11b) 217.6 kcal mol 89MI 301-01 . 

Other structural studies by ICR include basicity of methylpyrazoles (13 compounds) 90JA1303 acid-base properties of trifluoromethylpyrazoles in the gas phase 9lJOC3942 the determination of the gas-phase proton affinities of 32 A-H and A -methyl pyrazoles and the analysis of these data by comparison with ab initio 6-3IG calculations 92JOC3938 . The gas-phase basicities of eight pyrazoles substituted only at position 4 (R = H, NO2, F, Cl, C02Et, CH3, NHj, 1-adamantyl) were measured and the experimental values discussed in two ways (i) by comparing these values with AMI calculated proton affinities and (ii) within the framework of the Taft-Topsom analysis of substituent effects 95JCS(P2)1875 . 

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The cation-TT interaction involves binding of a cation to Ji-electrons (e.g., benzene, acetylene). Early mass spectrometry and ion cyclotron resonance studies have established that alkali-metal cations (e.g., Na+) bind strongly to simple aromatics. Later, Deakyne, and Meolner showed that organic ions (e.g., alkylammoniums) display affinities for aromatics. Moreover, extensive work by Dougherty and CO workers has established models of the cation- n- interaction that have been successfully demonstrated in a variety of synthetic receptors. ... 

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