Long‐range 19F–15N heteronuclear shift correlation: examination of J‐modulations associated with broad range accordion excitation

Abstract

AbstractThe first demonstrated example of 19F–15N long‐range heteronuclear shift correlation spectroscopy at natural abundance is reported. Because of the very large variation in the size of 2J(N,F) vs 3J(N,F) long‐range heteronuclear couplings, the utilization of one of the new accordion‐optimized long‐range heteronuclear shift correlations experiments is essential if all possible correlations are to be observed in a single experiment. A modified IMPEACH‐MBC pulse sequence was used in conjunction with an optimization range from 4 to 50 Hz to demonstrate the technique using a mixture of 2‐ and 3‐fluoropyridine, which had 2J(N,F) and 3J(N,F) long‐range couplings of −52 and 3.6 Hz, respectively. Because of the size of the 2J(N,F) long‐range coupling constant, a J‐modulation of the long‐range correlation response is observed in the spectrum resulting in a ‘doublet’ in F1 due to amplitude modulation. The size of the ‘doublet’ is shown to be a function of the parameter selection (t1max,Tmax,Tmin and spectral width in F1). This behavior is similar to F1 ‘skew’ associated with long‐range correlation responses in ACCORD‐HMBC spectra which has been analyzed in detail previously. Copyright © 2002 John Wiley & Sons, Ltd.