X nuclei

Heteronuclear chemical shift-correlated spectroscopy, commonly called H-X COSY or HETCOR has, as the name implies, different and F frequencies. The experiment uses polarization transfer from the nuclei to the C or X nuclei which increases the SNR. Additionally, the repetition rate can be set to 1—3 of the rather than the longer C. Using the standard C COSY, the ampHtude of the C signals are modulated by the... 

Nonselective polarization transfer, as implied by the term, represents a process that allows simultaneous polarization transfer from all protons to a//X nuclei. In sefectmepolarization transfer, however, the population of just one nucleus is inverted at any one time. The selective polarization transfer sequence therefore cannot be used to generate a proton-decoupled C-spectrum containing all sensitivity-enhanced C resonances. 

X/Y coherence transfer steps as in the original HNCA experiment.41 As in the previous case, X nuclei are selectively irradiated by low power rectangular or shaped pulses, and coherence selection is accomplished by the matched pulsed field gradients Gb G2 and further assisted by a spoil gradient Gs. Owing to the need to avoid -pulses in the X-channel, the spectra are processed in magnitude mode in the fft dimension. 

Suppose the X nuclei have spin The possible values of each quantum... 

A development of the method in order to study A A, XX M systems, where it is not convenient to observe A, has been reported. (475) In this situation the M resonance is a triplet, because of coupling to the two X nuclei, and the outer lines have energy levels in common with one or other of the groups of weak outer lines at a distance J(XX ) from the centre of the X spectrum. Hence when M-(X experiments are used to locate the outer lines the signs of J(MX) and J(XX ) are compared. Compounds such as (Ph2P)2CMc2 were studied in this way. (475) These methods do not work when J(XX ) is very small. The problem (476) of tetramethylbiphosphine disulphide has been finally solved by means of P) experiments. (75) Tickling of X transitions... 

It is discriminatory, in the sense that only the X nuclei located near hydrogen atoms are excited. 

Figure 3 Selective HMQC pulse sequence employing a time reversed q-SNEEZE pulse for de-excitation of X nuclei qs is a q-SNEEZE pulse and re is a RE-BURP pulse...

In the case of relatively slow relaxation of quadrupolar X nuclei (as defined by the quadrupolar relaxation time and/or large coupling constants... 

A selection of coupling constants V( Sn,X) for X nuclei throughout the periodic table is listed in Table 1, together with the reduced coupling constants... 

The AA XX notation may be interpreted as follows. The chemical shifts of the A and X nuclei are very far from each other (at opposite ends of the alphabet). The A and A nuclei are chemically equivalent, but magnetically nonequivalent, as are the X and X nuclei. Figure 4-2 illustrates the proton AA part of the spectrum of 1,1-difluoroethene, in which 10 peaks are visible. This appearance is quite different from the simple 1 2 1 triplet expected in the first-order case. The multiplicity of peaks in Figure 4-2 in fact permits the measurement of 7aa Ihe coupling between the equivalent protons. 

---