Continuouswave Lowpower Irradiation Of One Resonance

The following sections examine three techniques that all use low-power irradiation of a proton resonance in a proton-observe experiment. Homonuclear decoupling involves irradiation at a proton peak during the acquisition period in order to eliminate a J coupling to another proton resonance. This is similar to selective heteronuclear decoupling in that it is used to identify J coupling relationships between nuclei. Presaturation and NOE difference involve irradiation of a signal during the relaxation delay period, either to eliminate an unwanted signal (presaturation) or to observe the enhancement of z magnetization (Mz) of protons that are close in space to the irradiated proton (NOE difference). In either case the purpose of the irradiation is "saturation": to equalize the populations of the two energy levels (spin states). All three of these experiments are being replaced by new methods using shaped pulses and pulsed field gradients (Chapter 8): Selective 1D TOCSY for identifying J coupling relationships, "Watergate" for solvent suppression, and selective 1D transient NOE for identifying through-space relationships. Later on we will use the two-dimensional (2D) equivalents of the homonuclear decoupling and NOE difference experiments, which are known as COSY and NOESY, respectively (Chapter 9).

We saw in Chapter 1 how continuous low-power irradiation at a single frequency (the exact frequency of one proton resonance in a spectrum) leads to net absorption of energy by those spins. This happens because there are slightly more spins in the lower energy (a) state to absorb "photons" of RF energy and jump up to the higher energy (ft) state than there are in the ft state to undergo stimulated emission and drop down to the a state. But this absorption of energy rapidly reduces the tiny population difference to zero, at which point the rates of emission and absorption are equal. This state is called saturation, and it differs from the situation after a 90° pulse because no coherence is produced and there is no net magnetization at all. Spins are being promoted all the time during the irradiation period and other spins are dropping down at the same rate, unlike the "starting gun" effect of an RF pulse that gets all the spins moving in phase at the same moment. The relatively slow (~1 s) process of irradiation with low-power (<1 W) RF differs in another way from the rapid and precise rotation that results from a short (tens of ^s), high power (50300 W) pulse of RF energy: the irradiation is fairly selective, affecting only a narrow range of frequencies around the exact frequency of the RF. The lower the RF power level used, the narrower a band of resonances that is saturated. Hence we can use this technique to "wipe out" the population difference (and hence the net magnetization) of just one resonance in the spectrum. Once the saturation condition is established (AP = 0), any particular nucleus in the ensemble is cycling between the a and ft states, and the average time between transitions depends on the amount of RF power used: for very low RF power, transitions are relatively rare, but for higher power the spins are rapidly bouncing back and forth between the a and ft states.

A word about RF power levels would be useful at this point. The amplitude of the RF pulse can be set to a very wide range of values from very weak ("spin tickling") to the high power ("hard") pulses we use to excite all of the spins of a particular type (e.g., XH) in the sample equally. The 90° pulse "width" is the time required for the pulse to rotate the net magnetization by a 90° angle from its equilibrium position (+z axis) down to the x-y plane. The sample magnetization rotates faster during a pulse of higher amplitude (higher power) than during a "weaker" pulse, so the 90° pulse width depends on the pulse amplitude (the "B 1 amplitude"). One way to talk about power levels, then, is to simply specify the 90° pulse width at that power level. You could say, "set the RF power level of the pulse so that the 90° pulse width for 1H is 100 xs" and anyone in the world on any spectrometer could duplicate that power level, without reference to volts or watts or any electronic measurement. We let the spins do the measuring. Having said this, we can give you an idea of the power levels used for CW irradiation of protons. For NOE difference, a typical power level would give a 30 ms 90° pulse, or 3000 times lower RF pulse (B1) amplitude than a "hard" (10 xs) 90° pulse. This is very low power, but we will be applying it for much longer (typically 1.0 or 1.5 s) than the hard pulse (10 xs). For homonuclear decoupling we need to have spins bouncing back and forth rapidly between the a and j states, and this typically requires 10 times higher power, corresponding to a 90° pulse width of 3 ms. This is still 300 times lower RF amplitude than the hard pulse. For presaturation of 90% H2O, a typical power level corresponds to a 90° pulse width of 6 ms, or 600 times lower pulse amplitude than the hard pulse. For waltz-16 heteronuclear decoupling, which must "cover" the entire 1H chemical shift range (0-10 ppm) rather than just a single peak, and must overcome a much large J value (1/CH ~ 150 Hz), a power level is used that corresponds to a 100 xs 90° pulse, only 10 times lower amplitude (100 times lower power) than the hard pulse.

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