A new gradient-controlled method for improving the spectral width of ultrafast 2D NMR experiments

Ultrafast 2D NMR allows the acquisition of a 2D spectrum in a single scan. Still, a limitation affecting these experiments rests in their inability to cover large spectral ranges while preserving an acceptable resolution, due to limitations in gradient amplitudes and filter bandwidths. Various approaches relying on selective pulses have been recently proposed to overcome this drawback, by “shifting” resonances into arbitrary positions. However, these methods are associated with a number of drawbacks characterizing selective pulses. Here, we propose a new strategy to increase the spectral width accessible in the ultrafast dimension without degrading the resolution. This method leads to “folded-like” spectra along the ultrafast dimension. It does not require any selective pulse, and relies on suitably chosen gradients placed on each side of the mixing period. Our gradient-controlled folding method can be applied to almost any ultrafast 2D experiment, and appears to be particularly promising for heteronuclear 2D NMR. The results obtained on different compounds and pulse sequences (DQF-COSY, TOCSY, HSQC) are presented, highlighting the efficiency and the robustness of our method.