Conformational studies by dynamic nuclear magnetic resonance. Part 4. Rotational isomers and torsional barriers of pyridinecarbaldehydes

Abstract

Conformational isomers due to the restricted rotation of the formyl group have been detected in some pyridinecarbaldehydes and non-equivalence has been observed in the symmetric pyridine-4-carbaldehyde, by means of ¹H and ¹³C n.m.r. spectroscopy at low temperature. Assignment of the preferred conformers in pyridine-3-carbaldehyde has been achieved mainly by means of lanthanide shift effects. It has been shown that the carbonyl group shifts the ¹³C resonance of the syn-ortho-carbon upfield. This can be used as a general method for identifying rotational isomers in analogous derivatives. Barriers to internal rotation have been measured for all the derivatives examined by total line shape analysis of ¹H or ¹³C spectral patterns. In one case both measurements were carried out and substantial agreement was obtained.