Jain, Sheetal K., Guinevere Mathies, and Robert G. Griffin. “Off-Resonance NOVEL.” The Journal of Chemical Physics 147, no. 16 (October 28, 2017): 164201.
Dynamic nuclear polarization (DNP) is theoretically able to enhance the signal in nuclear magnetic resonance (NMR) experiments by a factor gamma_e/gamma_n, where gamma’s are the gyromagnetic ratios of an electron and a nuclear spin. However, DNP enhancements currently achieved in high-field, high-resolution biomolecular magic-angle spinningNMRare well below this limit because the continuous-wave DNP mechanisms employed in these experiments scale as w0^(-n) where n ~ 1–2. In pulsed DNP methods, such as nuclear orientation via electron spin-locking (NOVEL), the DNP efficiency is independent of the strength of the main magnetic field. Hence, these methods represent a viable alternative approach for enhancing nuclear signals. At 0.35 T, the NOVEL scheme was demonstrated to be efficient in samples doped with stable radicals, generating 1H NMR enhancements of 430. However, an impediment in the implementation of NOVEL at high fields is the requirement of sufficient microwave power to fulfill the on-resonance matching condition, omega_0I = omega_1S, where omega_0I and omega_1S are the nuclear Larmor and electron Rabi frequencies, respectively. Here, we exploit a generalized matching condition, which states that the effective Rabi frequency, omega_1Seff, matches omega_0I . By using this generalized off-resonance matching condition, we generate 1H NMR signal enhancement factors of 266 (70% of the onresonanceNOVEL
enhancement) with omega_1S/2pi = 5 MHz.We investigate experimentally the conditions for optimal transfer of polarization from electrons to 1H both for the NOVEL mechanism and the solid-effect mechanism and provide a unified theoretical description for these two historically distinct forms of DNP.