1H-Detected, Gradient-Enhanced 15N and 13C NMR Experiments for the Measurement of Small Heteronuclear Coupling Constants and Isotopic Shifts

Abstract

A suite of 1H-detected, gradient-enhanced experiments is presented for the measurement of small 1H−15N and 13C−15N coupling constants in 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 were applied to obtain a complete set of coupling constants in tris(1-pyrazolyl)methane (1), where 6% of the aromatic ring systems were doubly enriched with 15N. The absolute sign of the 1H−1H, 1H−15N, 13C−15N, and 15N−15N coupling constants was determined by relating them to the sign of a one-bond 1H−13C coupling constant. The experiments also yield multiple bond 13C/12C isotope effects on the 15N chemical shifts. The 1H−15N and 13C−15N 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 13C/12C isotope effects on the 15N chemical shifts were observed to be...