Real-time pure shift measurements for uniformly isotope-labeled molecules using X-selective BIRD homonuclear decoupling

We introduce a novel selective inversion element for chunked homonuclear decoupling that combines isotope selection via BIRD-filtering with band-selective inversion on the X-heteronucleus and allows efficient real-time decoupling of homonuclear and heteronuclear couplings. It is especially suitable for uniformly isotope-labeled compounds. We discuss in detail the inversion element based on band-selective refocusing on the X-nuclei (BASEREX), highlighting in particular the role of appropriate band-selective shaped refocusing pulses and the application of broadband X-pulses for an effective BIRD^d element during homodecoupling. The approach is experimentally verified and studied in detail using uniformly ¹³C-labeled glucose and a uniformly ¹⁵N,¹³C-labeled amino acid mixture.