Through-space R3-HETCOR experiments between spin-1/2 and half-integer quadrupolar nuclei in solid-state NMR

Abstract

We present several new methods that allow to obtain through-space 2D HETCOR spectra between spin-1/2 and half-integer quadrupolar nuclei in the solid state. These methods use the rotary-resonance concept to create hetero-nuclear coherences through the dipolar interaction instead of scalar coupling into the HMQC and refocused INEPT experiments for spin n/2 (n>1). In opposite to those based on the cross-polarization transfer to quadrupolar nuclei, the methods are very robust and easy to set-up.

Indirect high-resolution detection for quadrupolar spin-3/2 nuclei in dipolar HMQC solid-state NMR experiments

“…This sequence allows the observation of I-S heteronuclear correlation involving the CT of quadrupolar I nuclei. Therefore, the two pulses sent on the quadrupolar channel of the regular HMQC sequence should be both CTselective p=2-pulses [4]. By CT-selective pulses, one means pulses that act only on AE1/2 spin-states by using rf-fields of the order of the second-order quadrupole interaction.…”

“…They differ by their spectral resolution as well as the number of coherence transfers (one or two). The first proposed HETCOR experiments involving half-integer quadrupolar nuclei were not endowed with isotropic resolution for the quadrupolar nuclei and employed a single coherence transfer [1][2][3][4]. J-HETCOR experiments were performed using J-INEPT sequence [3], whereas D-HETCOR were achieved by using either cross-polarization (CP) transfer [1] or D-INEPT methods like TEDOR [2].…”

“…A better option to enhance the robustness consists in recoupling the dipolar interaction by applying the rf field to the spin-1/2 isotope only. This single channel dipolar recoupling was first used in D-INEPT experiments [2,4].…”

“…An alternative method to design J-and D-HETCOR methods involving half-integer quadrupolar nuclei consists in the use of double quantum coherences as in J-HMQC [7], D-HMQC [4] and D-HSQC [8] experiments. One advantage of this approach is the possibility to do indirect detection through a sensitive isotope with large gyromagnetic ratio c. The indirect detection may enhance the sensitivity of HETCOR experiment depending on the longitudinal relaxation times, T 1 , c and the line widths of correlated nuclei [9].…”

“…We describe both the basic HMQC scheme [7,4] devoid of STMAS filter and the HMQC-ST pulse sequence since these two strategies are compared in the next section.…”