Improved NMR transfer of magnetization from protons to half-integer spin quadrupolar nuclei at moderate and high magic-angle spinning frequencies

Abstract

Half-integer spin quadrupolar nuclei are the only magnetic isotopes for the majority of the chemical elements. Therefore, the transfer of polarization from protons to these isotopes under magic-angle spinning (MAS) can provide precious insights into the interatomic proximities in hydrogen-containing solids, including organic, hybrid, nanostructured and biological solids. This transfer has recently been combined with dynamic nuclear polarization (DNP) in order to enhance the NMR signal of half-integer quadrupolar isotopes. However, the cross-polarization transfer lacks robustness in the case of quadrupolar nuclei, and we have recently introduced as an alternative technique a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>D</mml:mi></mml:math>-RINEPT (through-space refocused insensitive nuclei enhancement by polarization transfer) scheme combining a heteronuclear dipolar recoupling built from adiabatic pulses and a continuous-wave decoupling. This technique has been demonstrated at 9.4 T with moderate MAS frequencies, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>ν</mml:mi><mml:mi>R</mml:mi></mml:msub><mml:mo>≈</mml:mo><mml:mn>10</mml:mn></mml:mrow></mml:math>-15 kHz, in order to transfer the DNP-enhanced <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi></mml:mi><mml:mn>1</mml:mn></mml:msup></mml:math>H polarization to quadrupolar nuclei. Nevertheless, polarization transfers from protons to quadrupolar nuclei are also required at higher MAS frequencies in order to improve the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi></mml:mi><mml:mn>1</mml:mn></mml:msup></mml:math>H resolution. We investigate here how this transfer can be achieved at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>ν</mml:mi><mml:mi>R</mml:mi></mml:msub><mml:mo>≈</mml:mo><mml:mn>20</mml:mn></mml:mrow></mml:math> and 60 kHz. We demonstrate that the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>D</mml:mi></mml:math>-RINEPT sequence using adiabatic pulses still produces efficient and robust transfers but requires large radio-frequency (rf) fields, which may not be compatible with the specifications of most MAS probes. As an alternative, we introduce robust and efficient variants of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>D</mml:mi></mml:math>-RINEPT and PRESTO (phase-shifted recoupling effects a smooth transfer of order) sequences using symmetry-based recoupling schemes built from single and composite <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>π</mml:mi></mml:math> pulses. Their performances are compared using the average Hamiltonian theory and experiments at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>B</mml:mi><mml:mn>0</mml:mn></mml:msub><mml:mo>=</mml:mo><mml:mn>18.8</mml:mn></mml:mrow></mml:math> T on <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>γ</mml:mi></mml:math>-alumina and isopropylamine-templated microporous aluminophosphate (AlPO<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi></mml:mi><mml:mn>4</mml:mn></mml:msub></mml:math>-14), featuring low and significant <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi></mml:mi><mml:mn>1</mml:mn></mml:msup></mml:math>H-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi></mml:mi><mml:mn>1</mml:mn></mml:msup></mml:math>H dipolar interactions, respectively. These experiments demonstrate that the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi></mml:mi><mml:mn>1</mml:mn></mml:msup></mml:math>H magnetization can be efficiently transferred to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi></mml:mi><mml:mn>27</mml:mn></mml:msup></mml:math>Al nuclei using <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>D</mml:mi></mml:math>-RINEPT with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>SR</mml:mi><mml:msubsup><mml:mn>4</mml:mn><mml:mn>1</mml:mn><mml:mn>2</mml:mn></mml:msubsup></mml:mrow></mml:math>(270<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi></mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math>90<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi></mml:mi><mml:mn>180</mml:mn></mml:msub></mml:math>) recoupling and using PRESTO with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>R</mml:mi><mml:msubsup><mml:mn>22</mml:mn><mml:mn>2</mml:mn><mml:mn>7</mml:mn></mml:msubsup></mml:mrow></mml:math>(180<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi></mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math>) or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>R</mml:mi><mml:msubsup><mml:mn>16</mml:mn><mml:mn>7</mml:mn><mml:mn>6</mml:mn></mml:msubsup></mml:mrow></mml:math>(270<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi></mml:mi><mml:mn>0</mml:mn></mml:msub></mml:math>90<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi></mml:mi><mml:mn>180</mml:mn></mml:msub></mml:math>) schemes at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>ν</mml:mi><mml:mi>R</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>20</mml:mn></mml:mrow></mml:math> or 62.5 kHz, respectively. The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>D</mml:mi></mml:math>-RINEPT and PRESTO recoupling schemes complement each other since the latter is affected by dipolar truncation, whereas the former is not. We also analyze the losses during these recoupling schemes, and we show how these magnetization transfers can be used at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>ν</mml:mi><mml:mi>R</mml:mi></mml:msub><mml:mo>=</mml:mo><mml:mn>62.5</mml:mn></mml:mrow></mml:math> kHz to acquire in 72 min 2D HETCOR (heteronuclear correlation) spectra between <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi></mml:mi><mml:mn>1</mml:mn></mml:msup></mml:math>H and quadrupolar nuclei, with a non-uniform sampling (NUS).