Solid‐State NMR Double‐Quantum Dipolar Recoupling Enhanced by Additional Phase Modulation

Abstract

Additional phase modulation (APM) is proposed to generally enhance the theoretical efficiency of homonuclear double‐quantum (DQ) recoupling in solid‐state NMR. APM applies an additional phase list to DQ recoupling in steps of an entire block. The sine‐based phase list can enhance the theoretical efficiency by 15–30 %, from 0.52 to 0.68 (non‐γ‐encoded recoupling) or from 0.73 to 0.84 (γ‐encoded recoupling), with doubled recoupling time. The genetic‐algorithm (GA) optimized APM can adiabatically enhance the efficiency to ∼1.0 at longer times. The concept of APM has been tested on SPR‐51, BaBa, and SPR‐31, which represent γ‐encoded recoupling, non‐γ‐encoded recoupling, and another kind beyond the former two, respectively. Simulations reveal that enhancements from APM are due to the activation of more crystallites in the powder. Experiments on 2,3‐13C labeled alanine are used to validate the APM recoupling. This new concept shall shed light on developing more efficient homonuclear recoupling methods.