AMX spin system

Similarly, cross peak positions in RELAY spectra give valuable information about remote connectivities (spins coupled to common partner but not to each other) and are very useful in resolving chemical shift degeneracies. An AMX spin system with no coupling between A and X shows a cross peak at ... 

Figure 5.28 A schematic drawing of an AMX spin system representing coupling interactions recorded at low digital resolution so that no fine splittings are visible. Note the symmetrical appearance of the cross-peaks on either side of the diagonal.

Their 31P NMR spectra show characteristic AMX spin systems with typical downfield shifts of the twofold coordinated phosphorus atom (120-160 ppm). The 1Jpj gj of 4a (203 Hz) is remarkably high and even the Jp2 si (141 Hz) shows a value that used to characterize a P=Si bond [1,2],... 

Each ratio is greater than 3 so a first order analysis is justified and the 100 MHz spectrum of the aliphatic protons of styrene oxide is indeed a first order spectmm and could be labelled as an AMX spin system. 

Let us now look at a more complex spin system, in which we have two different coupling constants to the same proton. In the trisubstituted aromatic 3-amino-4-methoxybenzoic acid shown in Figure 4.18, we have an AMX spin system and this leads to a 1,3- (or meta) coupled doublet for at 6 7.26, a 1,2- (or ortho) coupled doublet for at 5 6.85 and a... 

It might be possible to measure the side-chain dihedral angle xi (chi-1), defined by the N-Ca-Cp-Y angle, if we can measure the H -H 7-coupling constants. In the AMX spin system, there are three protons and each appears as a double doublet Ha is split by and is split by H / and Ha and H / is split by and Ha. Assuming that and Hp< have... 

What about the COSY-35 experiment (Fig. 9.34) We can now show with product operators why it simplifies the crosspeak fine structure. Consider again the AMX spin system of a peptide residue in D2O ND-CHa-CH H -Y. For the crosspeaks shown in Figure 9.33 (left) let s focus on the lower one F = Hp/F2 = Hq, (H -> Hq, coherence transfer). We start the t period with -ly and write down the terms that result from J coupling, keeping in mind that there are two / couplings affecting H Jap and Jppr. 

FIGURE 6.9 (a) Schematic representation of an AMX spectrum, showing the spin states associated with each spectral line. The situation depicted is that for all three fs positive. The signs of m are indicated. (b) Spectrum of 2,3-dichloropyridine at 60 MHz, an example of an AMX spin system. (c-e) Results of selective decoupling in 2,3-dichloropyridine, with decoupling frequencies centered between the following lines (c) Mi and M2, (d) M3 and M4, (e) and A2. 

Fig. 8. Experimental spectra of the AMX spin system of 1,2-dibromo-propanoic acid with coupling constants / m = 10 Ha Jax = 4.6 Hz, nd = H Hz and offset differences l X mI 232 Hz, x xl 497 Hz, and — i/ l = 265 Hz at a spectrometer frequency of 500 MHz. The spectra were obtained by selectively exciting spin A followed by a mixing period of increasing duration The mixing schemes were broadband DIPSI-2 with...

Energy level schemes are very instructive representations for describing the line splitting of simple spin systems such as AX or AMX spin systems and for visualizing the quantum mechanical allowed and forbidden transitions. The relative population of each state can be calculated and the signal enhancement by polarization transfer using a double-quantum, forbidden transition calculated. 

The Selective Population Transfer (SPT) experiment is usually used in spin system analysis with a FT spectrometer. Normally the experimental SPT spectra are compared with calculated SPT spectra simulated using different combinations of coupling constant signs. In common with many textbooks the AMX spin system 2,3-dibromopropionic acid will be used to introduce the concepts behind the SPT experiment. The IH spin system parameters for 2,3-dibromopropionic acid are shown below. The only difference between Spin System A and Spin System B is the sign of the coupling constant J(H(2), H(3)), the results of SPT experiments will be used to distinguish between the two possible spin systems. 

Spin system energy level scheme of the alternative AMX spin systems A and B which vary in the sign of the J(H(2), H(3)) coupling constant. 

From Fig. 5.9 it is apparent that the 12 transitions of an AMX spin system correspond to the edges of a cube. Four transitions slope from right to left, 1, 2, 3 and 4 four transitions are vertical, 5, 6, 7 and 8 four transitions slope from left to right, 9, 10, 11 and 12. The effect of the relative sign J(M, X) is obvious in the comparing spin systems A and B transitions 9, 10, 11 and 12 are unaffected but the other transitions are exchanged in pairs, 1 with 3 and 2 with 4, 5 with 6 and 7 with 8. 

Figure 4. Intensity patterns in an AX2 spin system (left, resonances of A right, of X2) resulting from different longitudinal spin order. Spin 1 is that of the A nucleus. Circles at line positions mean that the respective transition is unobservable owing to cancellation (e.g., 2I2zI3z would be observable in an AMX spin system). For further explanation, see the text.

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