Pure shift HMQC: Resolution and sensitivity enhancement by bilinear rotation decoupling in the indirect and direct...

The heteronuclear multiple-quantum coherence in the indirect dimension of the two-dimensional HMQC experiment evolves under the passive ¹H-¹H J-couplings leading to multiplet structures in the F₁ dimension. Besides, ¹H-¹H J-multiplets appear in the direct dimension as well. Thus, multiplets along both dimensions lower the resolution and sensitivity of this technique, when high resolution is required along both dimensions. An efficient broadband homodecoupling scheme along the F₁ dimension of the HMQC experiment has not been realized to date. We have implemented broadband homonuclear decoupling using bilinear rotation decoupling (BIRD) by adding a ¹H SQ evolution period followed by BIRD before the ¹H-¹³C multiple-quantum evolution period in the HMQC. In the direct time domain, BIRD is implemented using a real-time or single-scan scheme, which further improves resolution and sensitivity of this technique. The resulting pure shift HMQC provides singlet peak per chemical site along F₁ as well as F₂ axes and, hence, better resolution and sensitivity than conventional HMQC spectrum for all peaks except diastereotopic methylene protons. Due to the incorporation of the BIRD, the indirect time domain becomes double in length compared to the conventional HMQC. However, slow relaxation of small molecules favors better sensitivity for ps-HMQC relative to conventional HMQC under all conditions. We also found that the sensitivity of ps-HMQC is only slightly less than ps-HSQC for small molecules.