Broadband Cross-Polarization to Half-Integer Quadrupolar Nuclei: Wideline Static NMR Spectroscopy
James J. Kimball, Adam R. Altenhof, Michael J. Jaroszewicz, Robert W. Schurko
Abstract
Cross-polarization (CP) is a technique commonly used for the signal enhancement of NMR spectra; however, applications to quadrupolar nuclei have heretofore been limited due to a number of problems, including poor spin-locking efficiency, inconvenient relaxation times, and reduced CP efficiencies over broad spectral bandwidths─this is unfortunate, since they constitute 73% of NMR-active nuclei in the periodic table. The Broadband Adiabatic Inversion CP (BRAIN-CP) pulse sequence has proven useful for the signal enhancement of wideline and ultra-wideline (i.e., 250 kHz to several MHz in breadth) powder patterns arising from stationary samples; however, a comprehensive investigation of its application to half-integer quadrupolar nuclei (HIQN) is currently lacking. Herein, we present theoretical and experimental considerations for applying BRAIN-CP to acquire central-transition (CT, +1/2 ↔ −1/2) powder patterns of HIQN. Consideration is given to parameters crucial to the success of the experiment, such as the Hartmann–Hahn (HH) matching conditions and the phase modulation of the contact pulse. Modifications to the BRAIN-CP sequence such as flip-back (FB) pulses and ramped contact pulses applied to the 1 H spins are used for the reduction of experimental times and increased CP bandwidth capabilities, respectively. Spectra for a series of quadrupolar nuclei with broad CT powder patterns, including 35 Cl ( S = 3/2), 55 Mn ( S = 5/2), 59 Co ( S = 7/2), and 93 Nb ( S = 9/2), are acquired via direct excitation (CPMG and WCPMG) and indirect excitation (CP/CPMG and BRAIN-CP) methods. We demonstrate that proper implementation of the sequence can enable 1 H- S broadband CP over a bandwidth of 1 MHz, which to the best of our knowledge is the largest CP bandwidth reported to date. Finally, we establish the basic principles necessary for simplified optimization and execution of the BRAIN-CP pulse sequence for a wide range of HIQNs.