THEORETICAL-STUDY OF THE TAUTOMERISM OF 8-AZAADENINE

The prototropic tautomerism of 8-azaadenine (azaade) was studied theor etically by means of ab initio methods, in both the gas phase and aque ous solution. A number of tautomeric forms were not included in the ca lculations after applying a stepwise elimination procedure based on bo th AM1 and HF/6-31G energy values. The tautomers 9H-azaade, 8H-azaade and 7H-azaade survived to this elimination and their optimized geomet ries and energies were calculated at the MP2/6-31//HF/6-31G* level. T o include the solvent effects, two self-consistent reaction field meth od were used: (1) Onsager's SCRF with multipolar expansion up to the h exadecapolar term and (2) the isodensity polarizable continuum method (IPCM). Both methods produce similar results, although the latter repr esents better the situation in aqueous solution. The stability order i n solution, 8H- > 9H- > 7H-azaade, differs slightly from that found in the gas phase, implying that in general the electrostatic effects in solution are important, but the intrinsic stability of these species i n the gas phase overcomes the solvent effect. (C) 1998 John Wiley & So ns, Ltd.