Proton multiplet widths

Many of the practical considerations given above for heteronuclear 7-spectroscopy are equally applicable to the homonuclear case, and the selection of digital resolution in /i follows similar lines of thinking as before. Most proton multiplets will rarely exceed a width of 50 Hz (although those of other nuclides with many homonuclear couplings may do), so at least for proton spectroscopy, an/i spectral width equal to this will suffice. Using 64 and 128... 

Figure 38 shows three fluorine-19 spectra a potassium fluoride in D20 b trifluoroacetic acid and c p-fluorophenol in CDC13 (with expansion). Line-widths are small 1.9 Hz in spectrum a, 1.3 Hz in spectrum b. The computer printout in c shows that what is apparently one single line is in fact a multiplet, and the expansion shows a complex multiplet due to coupling of the fluorine nucleus with the two protons ortho and the two protons meta to it. 

If you ever run a sample which is contaminated with an ammonium salt, in DMSO, you will see 14N-proton coupling, as shown in Spectrum 6.14. Note that the three lines of the multiplet are of equal intensity (the middle line is a little bit taller than the outer ones, but this is because of the width of the peaks at their bases. The central signal is reinforced because it stands on the tails of the outer two). This is because 14N has a spin of 1=1, and the allowed states are therefore -1, 0 and +1. This three line pattern with its 51 Hz splitting is highly characteristic and once seen, should never be forgotten. 

It eould be argued that at say 500 MHz, most multiplets will be less than 0.1 ppm in width. Even if the eentral proton were not extraeted accurately, the error in chemical shift would be less than the error in the predicted shifts. The errors are additive however, and this will compromise the certainty ascribed to the yes/no decision. The inability to separate convoluted multiplets would seriously prejudice this approach. 

The selectivity of INEPT can be achieved in the most straightforward way, as suggested by Bax and coworkers301 302, by replacing all the hard non-selective proton pulses of INEPT with selective ( soft ) pulses (this pulse sequence is sometimes denoted as SPINEPT). To retain full sensitivity, the selective pulses must cover the selected multiplet and its 29Si satellites, i.e. the excitation band should be approximately equal to the width of the multiplet plus the value of /(29Si—XH) to be used for polarization transfer. For the choice of suitable selective pulses, see Reference 135. 

The coupling constants of even a seemingjy complex multiplet can be discerned in a relatively simple manner. First, the total width (outside peak to outside peak) of a first-order multiplet is equal to the sum of aU the coupling constants, keeping in mind that, for example, a triplet of /= 7 Hz is really a doublet of doublets with both / values equal to 7 Hz. The expansion of the proton spectrum of ethyl vinyl ether is presented as an example in Figure 8.23. Integration data are displayed between the spectrum and the horizontal axis in Figure 8.23a. 

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