Nuclear Magnetic Resonance Spectra of the ABX3 Type: Field Dependence and Relative Signs of Spin-Spin Coupling Constants

The ABX3 spin system is studied as a function of applied magnetic field. A method for the determination of the relative signs of JAX and JBX, together with the relative chemical shifts and coupling constants is described. The results are applied to the cis crotonic acid, the trans crotonic acid, its methyl ester, and N-methylformamide. In the latter case it is shown that the relative chemical shift of the A and B protons can be estimated from the field behavior of the X3 group of lines.