Double quantum and cross correlation peaks in COSY NMR spectra

Recently, Warren and co-workers have shown that double quantum, gradient selective, 2D NMR experiments can give rise to apparently unreconcilable double quantum signals and intermolecular cross peaks. These experiments have been used to challenge the basic assumptions of room temperature NMR. We re-examine the gradient-induced multiple spin echo explanation, first discussed by Warren. It is shown that simple analytical explanations can be provided for (i) double quantum peaks, (ii) cross correlation peaks, (iii) the amplitudes of the observed cross peaks, and (iv) the disappearance of the cross peaks when the solvent and solute are placed in different tubes.