HCNMBC--a pulse sequence for H-(C)-N Multiple Bond Correlations at natural isotopic abundance.

We propose a pulse sequence, HCNMBC for multiple-bond H-(C)-N correlation experiments via one-bond (1)J(C,H) and one- or multiple bond (n)J(N,C) coupling constants (typically n=1-3) at the natural isotopic abundance. A new adiabatic refocussing sequence is introduced to provide accurate and robust refocussing of both chemical shift and J-evolution over wide ranges of C-13 and N-15 frequencies. It is demonstrated that the proposed pulse sequence provides high quality spectra even for sub-milligram samples. We show that when a 1.7 mm cryoprobe is available as little as 10 μg of glycine in D2O is sufficient to obtain the HCNMBC spectrum in ca. 12 h. The preliminary results indicate that the pulse sequence has a great potential in the structure determination of nitrogen heterocycles especially in cases where synthesis produces regioisomers.