Phase-sensitive multidimensional NMR with CPMG detection

Abstract

Nuclear magnetic resonance (NMR) spectra are often complicated by overlapping signals from heterogeneous interactions. Multidimensional NMR is used to separate signals from different interactions and improve resolution. If the heterogeneous interactions are refocusable, windowed Carr–Purcell–Meiboom–Gill (CPMG) detection can provide signal enhancement; however, it has previously not been straightforward to combine multidimensional NMR with windowed CPMG detection, particularly when split-t1 or delayed acquisition is used. Here, we use generalized hypercomplex acquisition schemes to restore valid phase encoding. It is not necessary to control (or even consider) the details of coherence scrambling during CPMG pulse trains. Instead, it is sufficient to treat 2D NMR experiments with CPMG detection as if they were phase-sensitive 3D experiments. Our schemes are not unique; other phase-cycling schemes can yield equivalent results. Nevertheless, our algorithms are general and may be applied without rewriting the base phase cycle of the pulse sequence. We highlight two examples: (1) removal of heterogeneous broadening from water-exchange experiments in a nonuniform field and (2) I = 3/2 87Rb 3QMAS CPMG of RbNO3.