Spin-dynamics and efficiency of single ¹⁴N-¹H cross-polarization at fast magic angle spinning in solids

The naturally abundant ¹⁴N isotope (>99 %) is sparingly employed for characterization in solid-state nuclear magnetic resonance (NMR) despite the importance of nitrogen atoms in shaping molecular structures and properties. This inhibition can be attributed to large quadrupolar couplings (∼several MHz), resulting in more involved spin methodologies for ¹⁴N nuclei. Experimentally, spin-½ nuclei are utilized for excitation and detection through two-way (¹H→¹⁴N→¹H) polarization transfer between spin-½ nuclei and ¹⁴N. Herein, we show direct ¹⁴N spin excitation followed by ¹⁴N→¹H cross-polarization (CP) is an efficient method for polarization transfer even for ¹⁴N spins with a large quadrupolar coupling constant (3-4 MHz). This contrasts previous studies, which indicate that ¹H-¹⁴N spectra can only be observed with a pair of at least a rotor period-long symmetric ¹⁴N pulses (J. Chem. Phys. 151 (2019) 154202). The ¹⁴N→¹H CP spin dynamics have been experimentally established and can be explained in analogy to spin-½ Hartmann-Hahn CP if visualized in the quadrupolar jolting frame. The ¹⁴N→¹H CP is ∼1.9-2.7 times more efficient in polarization transfer than other ¹⁴N edited experiments. Considering shorter ¹⁴N T₁ relaxation times compared to protons, ¹⁴N edited spectra were recorded using ¹⁴N→¹H CP, resulting in enhanced sensitivity per unit of time.