Improved Editing of HMBC Spectra

The improved edited HMBC experiment edits into two subspectra according to the number of protons attached to 13C nuclei being odd or even.

Sequence

a) Standard sequence


b) Up-down- sequence


c) Up-down+ sequence

HMBC pulse sequences with a 3rd order low-pass J filter for editing into two subspectra according to the number of directly attached protons being odd or even:

Filled and open bars refer to pi /2 and pi pulses, respectively, while the dashed box represents 13C decoupling. tau = (2×1JCH)-1 and delta is the delay necessary for a gradient. The delay epsilon is equal to the minimum t1 time plus the time required for a proton pi pulse. The delay epsilon' is the same as the delay epsilon but with the addition of the time needed for a carbon pi pulse.

The initial four gradients of the low-pass J filter can be set an order of magnitude weaker than the other gradients for formation of heteronuclear gradient echoes. Sequence a is a standard HMBC pulse sequence where the echo is selected by the filled gradients and the antiecho by the open gradients; b and b are up-down HMBC pulse sequences inverting 13CH and 13CH3 peaks relative to the standard sequence and where the echo is selected by the filled gradients and the antiecho by the open gradients.

The recommended phase cycle is:

  • phi 1 = {x, -x, -x, x},
  • phi 2 = {x, x, 4(-x), x, x},
  • phi 3 = {4(x), 4(y), 4(-x), 4(-y)},
  • with receiver phase {x, -x}.

The delays for the 3rd order low-pass J filter is

  • tau 1 = ½ [1Jmin + 0.07(1Jmax - 1Jmin)]-1,
  • tau = tau 2 = [1Jmax + 1Jmin]-1,
  • tau 3 = ½ [1Jmax - 0.07(1J max - 1Jmin)]-1.

To produce the edited spectra simply combine the subspectra in the following manner:

  • 13C and 13CH2 = 2a - (b + c)
    and
  • 13CH and 13CH3 = 2a + (b + c)
    For optimal results the subspectra should be acquired in such a manner, so that the subspectra resulting from the sequence a is acquired with twice the number of scans as in the others.

    Example Spectra

    In the figure below are overlaid the 13C and 13CH2 spectrum (red) with the 13CH and 13CH3 spectrum (blue) of strychnine in CDCl3.
    Edited HMBC spectra of strychnine

    References


    • Andrew J. Benie and Ole W. Sørensen Improved multiplicity-editing of HMBC NMR spectra Magn. Reson. Chem. in press 2006
    • Nils T. Nyberg and Ole W. Sørensen Multiplicity-edited Broadband HMBC NMR spectra Magn. Reson. in Chem. 44 451-454 2006
    • Thomas Schulte-Herbrüggen, Alex Meissner, Alexandra Papanikos, Morten Meldal, and Ole W. Sørensen,. Optimizing delays in the MBOB, broadband HMBC, and broadband XLOC NMR pulse sequences J.Magn.Reson. 156 282-294 2002

    Code and procedures

    For Varian and Bruker.

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    Document: Improved Editing of HMBC Spectra (index.shtml)
    Last modified: 2006-05-31