Study of Chemical Exchange by 2D NMR

ZZ exchange experiment

Because of the partial double bond character of the amide C-N bond in N,N dimethylformamide (DMF), free rotation around C-N is hindered and a slow exchange can be established between the two distinct methyl proton signals (states A and B). Cross peaks (exchange peaks) with the same phase with the diagonal peaks can be observed in 2D NOESY and 2D ROESY experiments (Fig.1). 

Fig 1： 2D NOESY (left) and 2D ROESY (right) of DMF in DMSO-d₆ at 70°C.

This slow exchange system was further examined with the newly prepared ZZ exchange experiment. Briefly, in the ¹H¹³C ZZ exchange experiment, following ¹³C frequency-labeling during t₁ (indirect dimension), magnetization is changed into in phase ¹³C Z magnetization and is allowed to exchange. Then magnetization is transferred to proton and labeled with its proton frequency during FID (direct dimension) (Fig.2).

Fig 2： ¹H¹³C ZZ exchange experiment.

The results with DMF are shown in Fig.3 for different values of mixing time. Exchange peaks were observed between the two ¹H-¹³C cross peaks (auto peaks) representing the two exchanging states. Auto peaks represent magnetization that didn’t exchange during mixing time. Exchange peak denoted as B->A in Fig.3 represents B magnetization at the start of ZZ exchange (labeled with the B carbon frequency during t₁) that was exchanged during mixing time and therefore it was transferred to A state (labeled with A proton frequency during t₂). Similarly, the A->B exchange peak represents A state carbon magnetization that was transferred to B state and labeled with the B proton frequency.

Fig 3： ¹H¹³C ZZ exchange experiment for DMF in DMSO-d₆. The 2D experiment was executed as array measurement for the acquisition parameter "mixing time". The result is a single pseudo-3D data file with the recorded values of mixing time on the Z-axis

Phase cycling combined with proton decoupling and in phase carbon magnetization during mixing time ensure that there is no NOE contribution to exchange peaks. Therefore, the above experiment can confirm the presence of slow exchange or even provide under favorable conditions, quantitative description of the exchange event.

It is necessary in the above experiment that the exchange states A and B show different chemical shifts for both proton and carbon. If this is not the case then the exchange peaks will overlap with the autopeaks. In the special case where exchange states show same proton but different carbon chemical shifts, exchange might be demonstrated with a ¹³C-¹³C NOESY type of experiment (Fig.4).

Fig 4： ¹³C-¹³C NOESY for DMF in DMSO-d₆ at 70°C.

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