HMQC
Introduction:
Heteronuclear Multiple Quantum Correlation
is an experiment that identifies protons with their directly bound
carbons. The puise sequence utilizes zero and double quantum coherence
between J-coupled protons and carbons (hence the name) to label each
proton with the frequency of the directly bonded carbon in the F1
dimension of a two dimensional experiment. The conversion of
transverse magnetization is mediated by a delay 1/1JCH
(i.e. the inverse of the one bond coupling constant) and a carbon
90° pulse. The size of 1JCH is dependent on
the carbon hybridisation, being 12O Hz for an sp3 carbon, 16O Hz for
an sp2 carbon and 250 Hz for an sp carbon. As a compromise, we choose
1JCH of 14O Hz for this experiment although, if
there are no aromatic or alkene fragments in the compound,
1JCH = 120 HZ is a better choice. The experiment
is sometimes implemented in its non-decoupled form to measure
1JCH because of the dependency of
1JCH on hybridisation, electronegativity and
ring size.
HMQC is the equivalent of HETCOR but, because it is proton-detected,
HMQC is more sensitive than HETCOR.
The Problem:
Identify the protons that are directly bonded to each carbon in the
complex natural product, Drimentine D. The experiment in combination
with DEPT is particulary useful for identifying diastereotopic
protons.
The Solution:
Each proton-carbon pair has a single crosspeak. 13C nuclei
that have diasteriotopic protons have two correlations (some of these
crosspeak pairs are boxed in the spectrum below).
Australian National University NMR Centre
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