Broadband homodecoupled heteronuclear multiple bond correlation spectroscopy

Abstract

A general concept for removing proton-proton scalar J couplings in 2D NMR spectroscopy is proposed. The idea is based on introducing an additional J resolved dimension into the pulse sequence of a conventional 2D experiment to design a pseudo 3D NMR experiment. The practical demonstration is exemplified on the widely used gradient coherence selected heteronuclear long-range correlation spectroscopy (HMBC). We refer to this type of pulse sequence as tilt HMBC experiment. For every (13)C chemical shift evolution increment, a homonuclear J resolved experiment is recorded. The long-range defocusing delay of the HMBC pulse sequence is exploited to implement this building block. The J resolved evolution period is incremented in a way very similar to ACCORDION spectroscopy to accommodate the buildup of heteronuclear long-range antiphase magnetisation as well. After Fourier transformation in all dimensions the spectra are tilted in the J resolved dimension. Finally, a projection along the J resolved dimension is calculated leading to almost disappearance of proton-proton spin multiplicities in the 2D tilt HMBC spectrum. The tilt HMBC experiment combines sensitivity with simple experimental setup and can be recorded with short recycle delays, when combined with Ernst angle excitation. The recorded spectra display singlet proton signals for long-range correlation peaks making an unambiguous signal assignment much easier. In addition to the new experiment a simple processing technique is applied to efficiently suppress the noise originating from forward linear prediction in the indirect evolution dimensions. In case of issues with fast repetition times, probe heating and RF power handling most of the RF pulses can be replaced by broadband, frequency swept pulses operating at much lower power.