The NMR spectra and conformations of cyclic compoundsV: Proton couplings and chemical shifts in bridged cyclobutanes

Abstract

Abstract The PMR spectra of twelve pinene derivatives are reported, analysed and assigned. The proton couplings in the bridged cyclobutane group are compared with those of other strained cyclobutanes, and the relationship between 2 J HH and the C.CH 2 .C angle is shown to be anomalous in these systems, suggesting unusually small H. C. H. angles in cyclobutanes. The very large values of 4 J HH (eq‐eq) in buckled cyclobutanes are interpreted in terms of current M. O. theory and also given a simple geometric rationalisation based on the direct mechanism. The various couplings in the pinene skeleton are discussed in terms of present theories and minor conformational effects in these molecules. Substituent chemical shift (SCS) values for Me and OH groups around the pinene skeleton are obtained, and shown not to agree with calculations based on present theories of chemical shifts.