The use of three-bond spin-spin coupling constants in the determination of conformations of molecules in solution

Abstract

Over the last 15 years, the use of three-bond 1 H - 1 H spin coupling constants (3 -/HH) for obtaining stereochemical and conformational information has become a well-established procedure. The method relies on the relation (originally pointed out by Karplus) between 3J HH and cos 2 < Ø) where Ø is the dihedral angle in the H -X -Y -H fragment. This method is particularly successful when applied to a series of related molecules for which the form of 3J HHØ relation has been determined experimentally. The method can be used (1) for defining dihedral angles in molecules of fixed conformation; (2) for determining the fractional populations of conformers in a mixture of rapidly interconverting conformers and thus to check potential energy calculations; (3) in conjunction with studies of lanthanide induced shifts (l.i.s.) since any acceptable conformational solution must agree with both the coupling constant and l.i.s. data; (4) to monitor any conformational changes which might take place on lanthanide ion binding. Three-bond coupling constants involving other nuclei such as 13 C, 14 N and 31 P can also be used for conformational analysis. The various applications of three-bond coupling constants to conformational studies of peptides and choline analogues will be considered.