Spin Decoupling Methods Double Resonance

Spin decoupling, sometimes called double resonance, is another technique used to simplify NMR spectra. It is very useful for determining which spin-spin multiplets are acmally linked with one another and for locating absorptions that may be hidden, or buried, under other group absorptions. 

In spin decoupling, a second radiofrequency signal is applied while the spectrum is scanned in the usual fashion. This radiofrequency can be set to irradiate a selected group of protons in the molecule being measured. Irradiation causes the selected group of protons to undergo rapid transitions 

FIGURE 6.18 The 300-MHz H spectrum of a 50-50 mixture of resolved and unresolved (racemic) or-phenylethylamine in CDCI3. The chiral resolving agent (5)-(+)-0-acetyhnandelic acid was added. 

FIGURE 6.19 NMR spectra of propyl bromide (A) the normal spectrum at 60 MHz (B) the effect of decoupling (irradiating) the a CH2 protons (C) the effect of decoupling the p CH2 protons (D) the effect of decoupling the 7CH3 protons. 

FIGURE 6.20 A spectrum of the vinyl protons of allyl bromide (CH2=CH—CH2—Br) (a) the normal spectrum (b) the spectrum with the —CH2—Br protons decoupled. (From McFarlane, W., Chem. Britain, 

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Although these chemical methods are useful, the most versatile and unambiguous aid is undoubtedly double resonance (or spin decoupling). This method allows the unequivocal identification of protons which are mutually coupled. As an extension, this method can be used for measuring chemical shifts when the signal of a proton is partially or completely obscured by... 

Boum describe their use of the line shape method in the temperature range —24° to — 82°C (deuterium decoupling was used so that only the unperturbed single proton resonance was observed) and of a double-resonance method in the temperature range —97° to — 116°C. This involved the observation of recovery of magnetization of one of the two lines in the spectrum after a saturating r.f. field applied to the other line was removed. Consistent rates of inversion were found from both methods as evidenced by linearity of the Arrhenius plot. The results do not agree with the spin-echo results of Allerhand et al In this type of work, while fairly consistent results of rate constants may be obtained, there is dispute as to how the thermodynamic parameters should be derived, even in the relatively simple case of cyclohexane. ... 

Double resonance experiments include a group of techniques in which a sample is simultaneously irradiated with two or more signals of different radio frequencies. Among these methods are spin decoupling, the nuclear Overhauser effect, spin tickling, and inter-nuclear double resonance. These procedures aid in the interpretation of complex NMR spectra and enhance the information that can be obtained fromjhem. Only the first of these techniques, spin decoupling, will be described here. The nuclear Overhauser effect is discussed briefly in Section 19E-1. 

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