A MO THEORETICAL-STUDY ON THE REARRANGEMENT OF 1-HYDROXY-1,2,3-TRIAZOLES AND 1-(ACYLOXY)-1,2,3-TRIAZOLES AND THEIR BENZOTRIAZOLE ANALOGS - COMPARISON OF ABINITIO AND SEMIEMPIRICAL CALCULATIONS

Energy differences between 1-hydroxy-1,2,3-triazole (8a) and its 2H- and 3H-tautomers 8b and 8c were calculated by semiempirical (AM1, PM3) and ab initio (from the 3-21G//3-21G up to the MP4SDTQ/MP2//6-31+G* level) methods and compared. With large basis set size and inclusion of electron correlation effects (ECE), these tautomers are found to be quite close in energy. The 2H tautomer 8b is less stable by 2.57 kcal mol-1 and the 3H tautomer 8c by 1.95 kcal mol-1 compared with the N1-hydroxy form 8a. A satisfactory treatment of triazoles containing the -O-N(1)+ moiety by ab initio methods requires inclusion of ECE (at least second-order Moller-Plesset perturbation theory), while for 1H- and 2H-triazole (9a,b) inclusion of MP2 does not change the energy differences significantly. The differential solvent stabilization of 8c compared with that of 8a in the condensed phase was estimated using Tomasi's SCRF theory. The PM3- and AM1-calculated energy differences of isomers 1a/b/c, 2a/b/c, and 3a/b/c (benzotriazole derivatives) and 5a/b/c, 6a/b/c, and 7a/b/c (1,2,3-triazole derivatives) are mutually inconsistent. AM1 predicts the isomers a, but PM3 isomers c to be the most stable. The errors of both methods were estimated by comparison with ab initio calculations for the related model compounds 9a, 9b (1H- and 2H-1,2,3-triazole), 10a, 10b (1H- and 2H-benzotriazole), and (Z)/(E)-diazene (12a, 12b). It is concluded that in almost all cases the PM3 method yields results which are more reliable than AM1. Our experimental finding that the rearrangement of 1a proceeds intermolecularly to 1c, with the isomer 1b remaining undetectable, is understandable on the basis of these PM3-calculations. PM3- and AM1-calculated bond lengths are in acceptable agreement with those resulting from the 3-21G basis set but differ significantly from the respective MP2/6-31+G* values.