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Heteronuclear 2D correlation spectroscopy with inverse detection

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Title: Heteronuclear 2D correlation spectroscopy with inverse detection


1
Heteronuclear 2D correlation spectroscopy with
inverse detection
NMR 2003 NMR applications to Structural Chemistry
2
Heteronuclear 2D correlation spectroscopy
  • Basis
  • Heteronuclear correlation 2D experiments
    establish connectivities between any pair of
    nuclei, in the majority of cases, between a
    proton and an heteronucleus (13C, 15N), which
    are connected by a scalar coupling.
  • Applications
  • Assignment of the observed resonances in the NMR
    spectra to the specific atoms in the molecule.
  • Identification of the number of protons directly
    bonded to a given carbon atom.
  • Transfer of established protons assignments onto
    the directly bonded heteronucleus or, on
    occasions, vice versa.

3
  • Heteronuclear 2D correlation spectroscopy with
    inverse detection
  • Direct detection and inverse detection
  • Sensitivity advantages of inverse detection
  • 2D HSQC
  • 2D HMQC
  • Suppression of 1H bound to nuclides with I?1/2
    (1H-12C,1H-14N) in HMQC and HSQC
  • Phase-cycling
  • Pulsed field gradients gradient-enhanced HMQC
    and gradient-enhanced HSQC
  • 2D HMBC

4
Heteronuclear 2D correlation spectroscopy
5
Heteronuclear 2D correlation spectroscopy
direct detection
  • i) Proton excitation (I)
  • ii) Frequency labeling of proton during the
    evolution time t1
  • ii) Polarisation transfer of proton (I) to the
    heteronucleus (S)
  • v) Heteronucleus S detection (during acquisition
    time t2)

6
Heteronuclear 2D correlation spectroscopyinverse
detection
  • i) Proton excitation (I)
  • ii) Polarisation transfer of proton (I) to the
    heteronucleus (S)
  • iii) Frequency labeling of the heteronucleus,
    during evolution time t1
  • iv) Polarisation transfer of the heteronucleus to
    the proton
  • v) Proton detection (during acquisition time t2)

7
Heteronuclear 2D correlation spectroscopy
relative sensitivity
8
Heteronuclear 2D correlation spectroscopy
Comparison HETCOR (direct detection)/HSQC
(inverse detection)
HSQC spectrum Santonine (20mg /0.7 ml CDCl3) Exp
time 1h 8 min, ns4
HETCOR spectrum Santonine (65mg /0.7 ml
CDCl3) Exp time 1h 15 min, ns32
9
Heteronuclear 2D correlation spectroscopy with
inverse detection technical aspects
  • Technical problems associated with the design of
    heteronuclear correlation experiments
  • a) Suitable hardware
  • The spectrometer, probe and control software must
    have two or more independent rf channels, so that
    one can pulse 1H and 13C (or 15N) and detect 1H
    magnetisation while performing 13C decoupling.
  • The use of inverse probes allow a large
    improvement in sensitivity. The probe is designed
    with the 1H coil wound closest to the sample, for
    maximum detected 1H signal.

10
Heteronuclear 2D correlation spectroscopy with
inverse detection technical aspects
11
Heteronuclear 2D correlation spectroscopy with
inverse detection technical aspects
12
Heteronuclear 2D correlation spectroscopy with
inverse detection technical aspects
  • b) 13C decoupling
  • 13C decoupling while sampling 1H magnetisation is
    a very demanding requirement, since the 13C
    spectral bandwith is much larger than the 1H
    bandwith.
  • There are decoupling methods (GARP) that afford
    very effective 13C decoupling over wide
    bandwidths.
  • c) Selective detection of 1H signals associated
    with protons in 13CHn fragments, while
    suppressing the 1H signals associated with
    protons in 12CHn fragments.
  • This can be very difficult since the natural
    abundance of 13C is only 1.1, so one must
    suppress signals that may be 100 times more
    intense than the desired signals.
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