AROMATIC NITROGEN HETEROCYCLIC HEATS OF FORMATION - A COMPARISON OF SEMIEMPIRICAL AND AB-INITIO TREATMENTS

Ab initio calculated azole, azine and benzoazole heats of formation ar e within 0-4 kcal mol(-1) of experimental results for isolobal reactio n schemes. The ab initio heats of formation are compared with calculat ed semiempirical (MNDO, AM1 PM3) heats of formation. The MNDO method s ystematically underestimates all nitrogen heterocyclic heats of format ion. The AM1 method overestimates azole and underestimates azine heats of formation. The PM3 method performs well on azoles and benzoazoles, but underestimates azine heats of formation. Overall, the PM3 method is the most accurate semiempirical method for calculating nitrogen het erocyclic heats of formation. Correction terms for the semiempirical a zine heats of formation are suggested, which bring semiempirical value s into agreement with experimental results. Semiempirical and ab initi o methods are used to predict pentazole, pentazine, hexazine, pyrazepi ne, indazine, purine and azolotriazine heats of formation. (C) 1997 El sevier Science B.V.