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-substituted benzenes a and 21 1-substituted pyrazoles b, using four different ab initio methods which led to the final selection of the hybrid B3LYP/6-311+G(2d,p)//B3LYP/6-31+G(d) basis set. Comparison with experimental chemical shifts in solution (taking into account the calculated shieldings of the corresponding references) shows the excellent agreement between both sets if one allows for a factor of proportionality of about 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 reserved.