1H-detected, gradient-enhanced 15N and 13C NMR experiments for the measurement of small heteronuclear coupling constants and isotopic shifts

G. Otting, B. A. Messerle*, L. P. Soler

*Corresponding author for this work

Research output: Contribution to journalArticle

24 Citations (Scopus)

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/13C 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 significantly larger than the isotope effects mediated via two or more bonds. The one-bond isotope effect thus provides a valuable tool for assigning 15N resonances in cases where coupling constant data yield ambiguous results.

Original languageEnglish
Pages (from-to)5096-5102
Number of pages7
JournalJournal of the American Chemical Society
Volume118
Issue number21
DOIs
Publication statusPublished - 1 Jan 1996
Externally publishedYes

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