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).

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