H-1-DETECTED, GRADIENT-ENHANCED N-15 AND C-13 NMR EXPERIMENTS FOR THEMEASUREMENT OF SMALL HETERONUCLEAR COUPLING-CONSTANTS AND ISOTOPIC SHIFTS

A suite of H-1-detected, gradient-enhanced experiments is presented fo r the measurement of small H-1-N-15 and C-13-N-15 coupling constants i n small organic compounds, where no protons are directly bound to the nitrogens, The experiments are based on modified HMQC pulse sequences and a novel gradient supported half-filter element. The experiments we re applied to obtain a complete set of coupling constants in tris(1-py razolyl)methane (1), where 6% of the aromatic ring systems were doubly enriched with N-15. The absolute sign of the H-1-H-1, H-1-N-15, C-13- N-15, and N-15-N-15 coupling constants was determined by relating them to the sign of a one-bond H-1-C-13 coupling constant, The experiments also yield multiple bond C-13/C-12 isotope effects on the N-15 chemic al shifts, The H-1-N-15 and C-13-N-15 coupling constants in 1 are non- uniform, with some of the short-range coupling constants smaller than the longer range coupling constants. In contrast, all one-bond C-13/C- 12 isotope effects on the N-15 chemical shifts were observed to be sig nificantly larger than the isotope effects mediated via two or more bo nds. The one-bond isotope effect thus provides a valuable tool for ass igning N-15 resonances in cases where coupling constant data yield amb iguous results.