The INADEQUATE experiment is a special case of double-quantum spectroscopy that allows unambiguous determination of 13C-13C spin-spin connectivities through the DQ transitions of the spin system.

Easy implementation on any AVANCE spectrometer. However, high sample concentrations are required.

The basic 2D INADEQUATE pulse sequence consists of a multiple-quantum preparation period  (90º-delay-45º or 90º-delay-135º cluster) followed by a variable evolution period in which 13C-13C DQC evolution takes place. A final mixing pulse transfers polarization creates 13C SQC that can be detected under broadband proton decoupling ( 80JMR478-43 , 81JACS2102 ). The 1H version of the INADEQUATE experiment has been largely used (see 2D DQ experiment). However, a number of  variants of the INADEQUATE experiment have been proposed. Autocorrelated experiments reduce the size of the F1 dimension by half ( 81MOP1051 , 82JMR175-49 ), the use of a 135º read pulse provide quadrature detection in the F1 dimension without need of 45º phase shifts ( 82JMR158-48 , 83JMR259-53 ), composite-pulse version to minimize resonance offset effects ( 83MOP1109 )

Several approaches to improve the inherent low sensitivity of INADEQUATE  experiments:

The initial heteronuclear polarization transfer takes advantages of the higher proton polarization and the usually shorter proton longitudinal relaxation times, which allows shorter intervals between subsequent scans ( 82JMR180-46 , 85JMR1-62 ).

The final polarization transfer takes advantage of the higher sensitivity of proton detection as incorporated in inverse techniques ( 86JMR389-68 ). See, for instance, the ge-2D ADEQUATE experiment.

A version using heteronuclear cross-polarization has been proposed ( 94JMRA113-109 ).

Improved versions have been proposed using pulsed field-gradients (see ge-2D INADEQUATE experiment).

A complete revision on 2D INADEQUATE experiments has been published ( 02MRC3 )

The 2D INADEQUATE experiment can be recorded in routine/automation modes. The most important parameters to consider are the delay of the MQ period which is optimized to 1/2JHH in order to obtain anti-phase coherence and the number of scans to achieve good signal-to-noise ratio. The spectral width in F1 must be twice the largest possible DQ frequency. Large data matrices will be required to encompass complete spectra and therefore low digital resolution in both dimensions will be the rule. A shortening of T1 via relaxation agents can be quite useful, especially for nonprotonated carbons.
 
More details on practical implementation of the INADEQUATE experiment on AVANCE spectrometers can be found in

Tutorials: 2D X-detected experiments

The 2D matrix contains a series of correlation multiplets centered at the SQ 13C chemical shift in F2 and at the DQ frequencies (sum of chemical shifts between coupled 13C spins) in F1. Artifacts include axial peaks, F1 noise and SQC peaks along SQ diagonals.

Extensive description of the 2D INADEQUATE experiment can be found in 87ANG625 and 88BOOK1 (pages 348-379)

Related experiments:

2D X-detected experiments