DFT and experimental prediction of negative chemical shifts of methyl protons in some piperidines.

The high resolution (1)H NMR spectra of four 3-ethyl-4-hydroxy-4-phenylpiperidines 1-4 have been recorded in CDCl(3), C(6)D(6), and (CD(3))(2)CO and analysed. These compounds exist in chair conformation with axial orientation of hydroxyl group at C(4) and equatorial orientation of the remaining substituents. The chemical shifts of methyl protons of ethyl group are quiet novel and surprising and are closer to TMS in CDCl(3) and somewhat higher in C(6)D(6). Changing the solvent from CDCl(3) to (CD(3))(2)CO alters the chemical shifts of these protons and they are very closer to TMS (0.01 ppm) in 3-4 and negative in 2 i.e., less than zero. The results are interpreted in terms of the magnetic anisotropic effect of the phenyl rings at C(2) and C(4) which in turn depends on the conformations of the ethyl group at C(3) and hydroxyl group at C(4). DFT studies were also performed to predict the favoured conformations of ethyl group at C(3) and hydroxyl group at C(4). Chemical shifts were also computed theoretically in the favoured conformation and they also support negative chemical shifts of 2 in acetone-d(6).