ACCORD-HMBC: advantages and disadvantages of static versus accordion excitation

Long-range correlations observed with the recently reported ACCORD-HMBC pul se sequence using both static and accordion optimization of the long-range coupling delay are discussed. The latter affords a substantial improvement in both the number and intensity of the long-range correlations observed. A ccordion optimization, however, introduces a modulation in F-1 that is depe ndent on the range of potential long-range couplings sampled during the exp eriment. Modulation in F-1 is moderated by a scaling factor, N, the value o f which is determined by the number of increments of the evolution time and the dwell time in F-1. Although F-1 modulation can be used as a determinan t of the authenticity of correlations in noisy spectra, it can also impede the interpretation and assignment of the spectra when wide ranges of potent ial long-range couplings are sampled. Nominally usable ranges for the accor dion optimization of the long-range delay are suggested when it is necessar y to resort to limited digitization in the second frequency domain. Long-ra nge correlations observed for strychnine, which was used as a model compoun d for this study, are surveyed; numerous four-bond [(4)J(C,H)] correlations are observed by acquiring ACCORD-HMBC data with broad (e.g. 2-25 Hz) optim ization ranges. Finally, the observation of homonuclear 2D J responses due to strong homonuclear coupling during the variable delay are also observed and accounted for. The offset of these responses, relative to the chemical shift of the carbon by which they are sorted, is also governed by the scali ng factor N. Copyright (C) 1999 John Wiley & Sons, Ltd.