The ADEQUATE (Adequate Sensitivity Double-Quantum Spectroscopy) experiment correlates two directly bonded carbons with a proton directly bonded to one of the carbons. With this experiment, one obtains connectivity information betwen carbons and protons that are two bonds away.
One problem with the HMBC experiment is that correlations are seen between protons and carbons that are two, three, and sometimes four or more bonds away. This failure to establish the number of bonds between correlated nuclei often limits the ability to distinguish between different molecular structures. In the ADEQUATE experiment, carbons and protons that are ONLY two bonds apart are correlated. This is becuase one-bond carbon-carbon and one-bond carbon-proton couplings are used.
Shown below is an ADEQUATE spectrum of sucrose at 500 MHz. In this version, cross peaks are interpreted like the familiar HMBC cross peaks. For example, the anomeric proton, 1, has a cross peak (lableled 2-1) at the carbon frequency of 2. Likewise, the proton 2 has a cross peak (labeled 1-2) at the carbon frequency of 1.
Since one-bond carbon-carbon couplings are used, signal only arrises
from molecules containing two, adjacent 13 C nuclei. For natural abundance
work, this affects the sensitivity drastically since only 0.01% of molecules
are seen. The original INADEQUATE experiment that used carbon detection
was so insensitive that almost pure samples were necessary. The ADEQUATE
is proton-detected and thus, has much higher, although still poor, sensitivity.
Typically, 35 -50 mg of a small molecule are necessary for an overnight
accumulation at 500 MHz with a standard probe.