Inept-Inadequate
Introduction:
The only unambiguous experiment that correlates carbons based on the
number of bonds between the two coupled nuclei is the INADEQUATE
experiment. Unfortunately, the 1D version is rarely useful and the 2D
version is very insensitive since INADEQUATE detects only one in
10,000 of the molecules in the NMR tube. The sensitivity and the
selectivity of the experiment can be improved by combining the INEPT
experiment with the INADEQUATE experiment to create the
INEPT-INADEQUATE experiment. In the application described below, the
tertiary carbons are selectively excited by the INEPT sequence and
this polarization is transferred to the directly bonded carbons.
The Problem:
The connectivity at the carbonyl groups of the compound below could
not be unambiguously assigned by long-range HETCOR or by HMBC because
of the uncertainty of the relative magnitude of the two-bond and
three-bond 1H-13C coupling constants. INADEQUATE
is the obvious experiment but when the assignment of only one or two
resonances remains then a 2D expeniment is usually not the method of
choice. The INEPT-INADEQUATE experiment can be optimized to excite
only tertiary carbons and detect unprotonated carbons. only one of the
carbonyls in this compound has a one-bond correlation to a tertiary
carbon.
The Solution:
The upper spectrum is the normal 13C spectrum of the
carbonyl regions the lower spectrum is the INEPT-INADEQUATE
spectrum. The carbonyl resonance that shows a
13C-13C doublet (the outer two peaks of the
carbonyl at 174.8 ppm) is the carbonyl adjacent to the tertiary
carbon.
A word of caution: Although the experiment is
more sensitive than INADEQUATE, a minimum of 2OO mg of material is required.
REF:
Kessler etal. J. Magn. Reson. 62, 573 (1985).
Australian National University NMR Centre
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