Basicity and acidity in gas phase

Understanding the behavior of organic bases in solution requires some knowledge of their gas phase (intrinsic) basicities (proton affinities (PA)). These can be determined by ICR methods or by variable-temperature pulsed high-pressure mass spectrometry. Both methods afford basicities (termed thermodynamic vs. kinetic basicity), which have been compared in (91JOC179). 

Reactivity of Five-membered Rings with Two or More Heteroatoms 

Although imidazole is a stronger acid than pyrazole in the gas phase (by 2.6 pKa units) and in DMSO (by 1.3 pA j, units), pyrazole is a slightly stronger acid (by 0.2 pK units) in aqueous solution. This can be explained in terms of the equilibrium shown in Equation (1). In the gas phase electrostatic repulsion between adjacent lone pairs of electrons shifts the equilibrium to the left (86JA3237, 88CHE469). 

Many examples are known of complexes between metal cations and both neutral azoles and azole anions. Overlap between the d-orbitals of the metal atom and the azole rr-orbitals is believed to increase the stability of many of these complexes. 

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25)147). Many transition metal cations form complexes with imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thiazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(2l)323). 

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