title: Proton detection of MAS solid-state NMR spectra of half-integer quadrupolar nuclei.
abstract: Fast magic angle spinning (MAS) and proton detection has found widespread application to enhance the sensitivity of solid-state NMR experiments with spin-1/2 nuclei such as13C,15N and29Si, however, this approach is not yet routinely applied to half-integer quadrupolar nuclei. Here we have investigated the feasibility of using fast MAS and proton detection to enhance the sensitivity of solid-state NMR experiments with half-integer quadrupolar nuclei. The previously described dipolar hetero-nuclear multiple quantum correlation (D-HMQC) and dipolar refocused insensitive nuclei enhanced by polarization transfer (D-RINEPT) pulse sequences were used for proton detection of half-integer quadrupolar nuclei. Quantitative comparisons of signal-to-noise ratios and the sensitivity of proton detected D-HMQC and D-RINEPT and direct detection spin echo and quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) solid-state NMR spectra, demonstrate that one dimensional proton detected experiments can provide sensitivity similar to or exceeding that obtainable with direct detection QCPMG experiments. 2D D-HMQC and D-RINEPT experiments provide less sensitivity than QCPMG experiments but proton detected 2D hetero-nuclear correlation solid-state NMR spectra of half-integer nuclei can still be acquired in about the same time as a 1D spin echo spectrum. Notably, the rarely used D-RINEPT pulse sequence is found to provide similar, or better sensitivity than D-HMQC in some cases. Proton detected D-RINEPT benefits from the short longitudinal relaxation times (T1) normally associated with half-integer quadrupolar nuclei, it can be combined with existing signal enhancement methods for quadrupolar nuclei, and t1-noise in the indirect dimension can easily be removed by pre-saturation of the1H nuclei. The rapid acquisition of proton detected 2D HETCOR solid-state NMR spectra of a range of half-integer quadrupolar nuclei such as17O,27Al,35Cl and71Ga is demonstrated.
authors.affiliation: Iowa State University, Department of Chemistry, Ames, IA 50011, USA; US DOE Ames Laboratory, Ames, IA 50011, USA.
authors.forename: Amrit
authors.initials: A
authors.lastname: Venkatesh
authors.affiliation: Iowa State University, Department of Chemistry, Ames, IA 50011, USA; US DOE Ames Laboratory, Ames, IA 50011, USA.
authors.forename: Michael P
authors.initials: MP
authors.lastname: Hanrahan
authors.affiliation: Iowa State University, Department of Chemistry, Ames, IA 50011, USA; US DOE Ames Laboratory, Ames, IA 50011, USA. Electronic address: arossini@iastate.edu.
authors.forename: Aaron J
authors.initials: AJ
authors.lastname: Rossini
keywords: 2D NMR spectroscopy; Fast MAS; Methods; Quadrupolar nuclei; Structure