GUIDELINE..2D NMR

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. In some experiment another time interval is added before the detection: the mixing time.

Fig.8 : Scheme for time pulse in a 2D NMR experiment

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 

The 2D NMR - The idea of JEENER

The idea of Jeener consists in the stepwise increasing of the evolution time  .

This will allow to get an NMR signal under the aspect of a sampling of free precession signals of the  type. These FID will differ from each other only by the  period duration written under a matricial form

s(  ,  ). The  delay is the time between the first and the second pulse.

Fig. 9 : Sampling of signals of free precession of the type s(t2)

The first Fourier transform as a function of  gives us an interferogram of the form  (Fig. 10)

Fig. 10 : Interferogram of the form s(t2, w1)

A second Fourier transform, versus the second variable  , gives an NMR spectrum with two frequencies dimensions F1 and F2.

The result of this two fold Fourier transform does not get two spectra  and  but only one spectrum as a function of two independent frequencies, exhibiting a peak with the coordinates  . Thus, an aimantation evolving with the frequency  in the time course  has been converted in another coherence evolving with the frequency  during the period  .

Fig. 11 : NMR spectrum in two dimensions following the second Fourier transform

This double Fourier transform in both dimension yields thus a matrix  (spectrum 3).







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TOCSY and ROESY

These experiments are characterized by a mixing time during which the magnetization of a spin A, submitted to a radio wave frequency field Bl is transferred to a spin linked to A either by a dipolar coupling or by a scalar coupling. During the mixing time the magnetizations are locked according to one direction of the space defined by the direction of the field Bl and the spin frequency shifting versus the main component. The block in the spin system is commonly named spin-lock.

TOCSY ou HOHAHA :

This method allows to demonstrate scalar couplings. The pulse sequence scheme is given Fig. 23. The first 90°x impulsion rotates the magnetization vectors in the xOy plane, in which they freely evolve during the time tl, under the influence of their chemical shift and the scalar coupling constants. We apply then a spin lock on the x axis, that means a low power impulsion or set of impulsions whose duration represents the mixing time tm. During this time, the spin coherence of the system is exchanged. By increasing the mixing time we increase the number of the visible transitions. That is to say that within the same experiment, we get the informations of a COSY, of a LR COSY and of a relayed COSY. The pulse sequence widely used is base upon the MLEV-17 sequence which contains a set of 17 pulses.

Fig. 29 : Saccharine molecule

Fig. 23 : The pulse sequence TOCSY or HOHAHA

The spectrum 10 shows the result of three different mixing time for the saccharose (Fig.29). These are given in milliseconds (20, 60, l00 ms). For each mixing time, we see the appearance of new correlation spots. There is thus an increase of the number of relays with the spin lock time. These ones correspond then to long range correlations.

Spetrum 10: 20ms, 60 ms, 100ms.

Tocsy 20

Tocsy 60

Tocsy 100

ROESY ou CAMELSPIN :

The aim of the dipolar correlation experiments in two dimensions is to take advantage of the vicinity in space of some nuclei. The result of this kind of experiment is a 2D map in which the signals outside of the diagonal arise from the Over Hauser enhancement effect between two space coupled nuclei(Fig.24).

Fig. 24 : The ROESY pulse sequence

In this case, we apply a spin-lock onto the y axis. The correlation peaks arising from the ROE effect are on the opposite sign compared to the diagonal one their intensity is different from 0. However, the correlation peaks coming from a chemical exchange are of the same sign than the crossing one. The ROESY experiment allows by this way the separation of the contributions coming from the exchange and those coming from the dipolar interactions. The ROESY sequence is thus complementary of the NOESY one and it is more often used for the structural determination of the small molecules.