Delay Incrementation - Constant Time and Accord-Principle

2 Delay Incrementation - Constant Time Principle and Accord-Principle 

Delay incrementation is a prerequisite of n-dimensional NMR spectroscopy. During a selected free precession period, called the i period in a 2D experiment, either coupling or chemical shift evolution of a particular NMR-active species i.e. IR, l c occurs. In a J-resolved (JRES) experiment the coupling evolves whilst in a chemical shift resolved (COSY) experiments the chemical shift evolves. The selected free precession period is then incremented in subsequent experiments to generate a modulation of the directly detected response during the t2 period. 

A perennial problem with coherence transfer experiments is the choice of coupling constant to use for calculating the free precession period prior to the coherence transfer step. The same problem arises when calculating the refocusing delay after a coherence transfer step prior to data acquisition under decoupling. 

HMBC experiment. However as already discussed decreasing the corresponding free precession period prior to the coherence transfer reintroduces splitting due to IH, IH coupling in the fl dimension. So alternative experiments such as IMPEACH-HMBC and CIGAR-HMBC experiment have been designed to suppress this splitting [5.173, 5.174, 5.175]. 

In Check it 5.5.2.2 the ACCORD-HMQC experiment is simulated [5.176]. This experiment is characterized by a low sensitivity of resulting signals to varying X, coupling constants, but an unwanted modulation in the f 1 dimension that is a function of the incrementation and the underlying heteronuclear coupling constant. The ACCORD-HSQC experiment [5.177] has been suggested to overcome this modulation in the fl dimension. 

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