The 2D NMR experiment belongs as well to the Fourier transform spectroscopy than to the impulsion one and relies on a sequence of three time intervals: preparation, evolution and detection. In some experiment another time interval is added before the detection: the mixing time.
The preparation time
Upon the preparation time the spin system under study is firstly prepared, for example it is submitted either to a decoupling experiment or just to a transverse magnetization by the means of a 90° impulsion. It allows the excited nuclei to get back their equilibrium state between two successively executed pulse sequence.
The evolution time t1
During the evolution time , the spin system is evolving under the effect of different factors, each coherence evolves at its own characteristic frequency as a function of the chemical shift and of the scalar coupling of the corresponding nucleus.
The mixing time
It is made of a pulse sequence which achieves coherence transfers in such a way that different frequencies can be correlated.
The detection time
The acquisition of the modulated signal takes place during the detection period.
The sequence we just described does not constitute by itself a 2D NMR experiment.