Heats of Formation and Gas-Phase Basicity

Although not essentially different from the previous seetion, we have eol-leeted here theoretieal publieations aimed indireetly to the ealeulation of the equilibrium eonstant, either through the heats of eombustion or through aeid-base equilibria. 

Coneerning the enthalpies of formation, whieh are routinely obtained from semiempirieal ealeulations sueh as MNDO or AMI, they are diffieult to ealeulate ab initio and, in general, only relative values are ealeulated for eompounds of the same moleeular eomposition by means of isodesmie reaetions (98JCP692). This preeludes earrying out a ealeulation for the two tautomers and eomparing the ealeulated values with the experimental ones. Some authors have used Hess s law to overeome this diffieulty [97JPC(A)283]. 

The calculation of the proton affinities (PA) for a pair of tautomers and the comparison with experimental data [generally from ICR measurements (Section VII,F)] has been the subject of a series of publications with increasing sophistication (Table IV). Such calculations concerning the annular tautomerism of azoles and benzazoles have been reviewed [87AHC(41)187]. 

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