A robust method for determining 1H–15N long‐range correlations: 15N optimized CIGAR‐HMBC experiments

An examination of a variety of common nitrogen‐containing systems was undertaken to optimize parameters for observation of 1H–15N long‐range correlations. Because of the diversity of coupling constants encountered with 1H–15N correlations, a modified accordion‐based sequence was used to provide the best results. Optimization of the values for the accordion delay revealed that a range between 3 and 10 Hz provided the best compromise between detection of weak correlations and loss of signal to T2 processes. Multiple bond correlations were readily detected for each class of compound with the exception of anilines. Correlations within heterocyclic systems revealed some general patterns. In general, stronger correlations were observed from protons to ‘pyrrole‐like’ nitrogens than to the ‘pyridine‐type’ nitrogens of imidazoles and pyrazoles. Very long‐range (four‐ and five‐bond) correlations were routinely observed between methyl groups and the nitrogens of aromatic heterocycles.