GIAO AB-INITIO CALCULATIONS OF NUCLEAR SHIELDINGS OF MONOSUBSTITUTED BENZENES AND N-SUBSTITUTED PYRAZOLES

Ab initio coupled GIAO calculations were carried out on 13 mono-substi tuted benzenes a and 21 1-substituted pyrazoles b, using four differen t ab initio methods which led to the final selection of the hybrid B3L YP/6-311+G(2d,p)//B3LYP/6-31+G(d) basis set. Comparison with experimen tal chemical shifts in solution (taking into account the calculated sh ieldings of the corresponding references) shows the excellent agreemen t between both sets if one allows for a factor of proportionality of a bout 0.96. Some structural problems such as conformation, tautomerism and structure of salts are discussed using a combination of calculated and experimental values. (C) 1998 Elsevier Science B.V. All rights re served.