A complete model for the prediction of 1H- and 13C-NMR chemical shifts and torsional angles in phenyl-substituted pyrazoles

Abstract

1H- and 13C-NMR spectra of a number of N-phenyl- and C-phenylpyrazole derivatives have been obtained. The parameter most susceptible to changes in the dihedral angle θ is the difference δ meta-C – δ ortho-C . Values for this parameter have been determined and its usefulness for conformational studies of phenyl-substituted pyrazoles has been demonstrated. A correlation between torsional angles calculated by molecular mechanics and differences in 13C chemical shifts of the ortho and meta carbon atoms of the phenyl groups in 29 N-phenyl-substituted pyrazole derivatives and 11 C-phenyl-substituted pyrazole derivatives has been found. For the N-phenyl-substituted derivatives a correlation between torsional angles and δ meta-H – δ ortho-H values has also been demonstrated. In all cases good correlations between angles and differences in chemical shifts were observed.