Accordion BIRD‐HMBC experiments: improved one‐bond correlation suppression in accordion heteronuclear multiple‐bond correlation‐type experiments

Modifications of the BIRD-HMBC experiment, in which the accordion principle and a constant time period have been introduced, are presented. With the simple accordion BIRD-HMBC, the use of a BIRD(r, X) element in the middle of the variable long-range couplings evolution period efficiently eliminates unwanted (1)J(CH) signals and samples a broad range of long-range heteronuclear couplings. While the accordion BIRD-HMBC experiment shows reduced F(1) modulation, superior suppression of unwanted (1)J(CH) signals and sensitivity comparable to the ACCORD-HMBC, the addition of another constant time period suppresses F(1) modulation due to (1)H-(1)H coupling, while preserving the excellent suppression of unwanted (1)J(CH) signals. The constant-time (CT) accordion BIRD-HMBC experiment combines sensitivity similar to the CIGAR-HMBC and superior suppression of unwanted (1)J(CH) signals. Results of identically optimized ACCORD-HMBC, accordion BIRD-HMBC, CT accordion BIRD-HMBC and CIGAR-HMBC experiments performed on a sample of strychnine are compared and discussed.