Info

2000 18110 160» 1400 1200 1000 800 600 400 200 0 Hz

Figure 2.1

2000 18110 160» 1400 1200 1000 800 600 400 200 0 Hz

Figure 2.1

scale in parts per million, not hertz, so the positions of peaks will be the same at any field strength.

This is illustrated by the spectra of menthol at 200, 250, 300, 500, and 600 MHz. As we saw in Chapter 1, NMR signals are frequencies measured in hertz, based on the audio frequencies detected in the FID by the Fourier transform. If we plot the spectra on a frequency scale in hertz (Fig. 2.1), we see that the chemical shifts (positions of the proton resonances a-n) in hertz units are proportional to the magnetic field strength, as they should be:

The linewidths and J couplings are independent of field strength, so the appearance of each proton multiplet is the same in each spectrum, regardless of field strength (compare, for example, peak n). If we expand and align all of the peak n multiplets (Fig. 2.2), we can see that they are identical, with a linewidth of about 1.5 Hz and three coupling constants of 10.7, 10.0, and 4.3 Hz (the two broader peaks in the center are unresolved pairs of lines). But this method of displaying the spectrum is impractical because the chemical shift (in hertz) would be different on different NMR instruments, and it would be confusing to make comparisons unless everyone had the same field strength. This is why the ppm scale was developed: to make chemical shifts independent of field strength so that they could be reported on a universal scale. One part per million is one millionth of the fundamental frequency being used for the nucleus being observed. For example, on a Bruker DRX-600, the XH frequency is 600.13 MHz, so 1 ppm is 600 Hz (600 x 106 Hz x 10-6). In Figure 2.1 the frequency corresponding to 3.00 ppm is shown on each spectrum: 3 x 200 = 600 Hz on the 200-MHz instrument, 3 x 250 = 750 Hz on the 250,900 Hz on the 300,1500 Hz on the 500, and 1800 Hz on the 600. It is important to realize that the conversion from Hz to ppm depends on the

5.87 T 250 MHz

Figure 2.2

Figure 2.2

nucleus as well as the field strength: on a "600" the 13C resonant frequency is 150 MHz (roughly one fourth of 600 because yC/yH is about 0.25), so 1 ppm is 150 Hz, not 600 Hz.

Figure 2.3 shows the spectra lined up on a ppm scale rather than a hertz scale (the vertical scale is increased and the tall methyl and OH peaks are clipped off). This is the universally accepted format for presenting NMR data. Although all the peaks (resonances) appear at the same place in the spectrum (same chemical shift in ppm), the multiplet patterns appear to "shrink" horizontally as we go to higher field strength because the J couplings in hertz get smaller and smaller on the ppm scale. For example, on a 200-MHz spectrometer, a typical

7.0-Hz coupling appears as a separation of 7/200 or 0.035 ppm. The same coupling on a 250, 300,500, or 600 MHz spectrometer appears as a separation of 0.028, 0.023,0.014, and 0.012 ppm, respectively. As the 1H coupling patterns "shrink," the footprint of each resonance gets smaller and the chances of overlap get smaller. We say that peaks that are overlapped on the 200-MHz spectrometer are "resolved" (separated by a region of baseline with no intensity) on the 600-MHz spectrometer. This is clearly illustrated by peak g, which at low field is spread out and overlapped with some peaks on its upfield (right-hand) side. This is the more general and more important meaning of "resolution" and explains why people are willing to spend enormous sums of money to achieve even modest gains in magnetic field strength. With smaller footprints we can move to larger, more complex molecules to make use of that "empty space" between the peaks. There are, of course, other ways to avoid overlap—primarily by using 2D and even 3D and 4D experiments, but in each case it is always better to have higher field because the "footprint" size is reduced on the ppm scale. For peaks that are not overlapped at any field strength, there is no change in the appearance of the peak as we move from 200 to 600 MHz, as long as we expand the peak to the same range of chemical shifts in hertz (Fig. 2.2). In this case, the structure of the multiplet depends only on the linewidth (a function of shimming and T2) and /-coupling values, all of which are independent of the field strength, Bo.

A closer look at the methyl region (Fig. 2.4) shows how we can easily mistake two resonances for one. The pattern observed for CH3(b) and CH3(c) at 600 MHz looks like a double doublet. At 500 MHz it looks like a triplet. The reason it changes its form with magnetic field is that it really represents two different resonances with their own chemical shift positions, that is, two doublets. The two vertical lines show the peak positions in ppm, which do not change with field strength. At 600 MHz the two doublets are separated, and

T—i—i—|—i—p-i—i—|—i—i—i—i—|—i—rn—i—|—i—i—i—

T—i—i—|—i—p-i—i—|—i—i—i—i—|—i—rn—i—|—i—i—i—

0.90 0.85 0.80 0.75 ppm we can read the coupling constants from the left-hand side pair (7.0 Hz) and the right-hand side pair (6.6 Hz). At 500 MHz the doublet splittings are "wider" (on the ppm scale) so that the two inner peaks are now overlapped. At 300 MHz and below, the two doublets become intertwined, so that we have to measure the coupling constants from the first and third peaks (7.0 Hz) and the second and fourth peaks (6.6 Hz). If we only saw the spectrum at one field strength, we might be fooled into thinking it was a single resonance split into a triplet or a double doublet.

0 0

Post a comment