Double-resonance - It is often advantageous to reverse or remove the splitting caused by spin-spin coupling This is called spin decoupling Spin

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It is often advantageous to reverse or remove the splitting caused by spin-spin coupling This is called spin decoupling Spin decoupling (or just “decoupling) can be used for several reasons - to simplify spectra - to assist in identi f cation of coupling between nuclei - to improve signal-to-noise How do we decouple spins from one another? Remember, coupling occurs because the transitions of one spin ( α to β or β to α ) ensue when the other (coupled) spin is in either the α or β state - thus, to observe the coupling of one spin to another, the lifetime of the coupled spin in a given state must be long enough for the transitions of the other to occur - this lifetime ( τ 1 ) must be greater than 1/ J ( J is the scalar coupling constant) 1 > 1/ J - if this lifetime is signi f cantly shortened, the coupling (splitting of the signal) will not be observed Two main types of decoupling: selective and broad-band
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selective decoupling - selective excitation is accomplished by a very long pulse centered at the Larmor frequency of the signal of interest (remember, the bandwidth is proportional to 1/ τ p , so a long pulse is highly selective ) - the long pulse causes rapid α β and β α transitions such that the lifetime of any particular spin in any given state is short (i.e. τ 1 < 1/ J ) - selective excitation of one signal (spin) in an NMR spectrum leads to multiplet collapse and simpli f cation of the spin-spin splitting patterns of the signals from the nuclei to which the spin is coupled 90 d 1 1 H 1 H low-power long pulse (frequency selective)
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often it is useful to remove all coupling of a given type for instance, it is often useful in 13 C NMR/spectra to remove all coupling from 1 H 90 d 1 1 H 13 C broad-band or “cpd” sequence broad-band decoupling (or composite pulse decoupling , cpd) - excitation of a large frequency range using a train of short pulses (short pulses excite a large bandwidth) applied in rapid succession - the rapid succession of pulses causes rapid α β and β α transitions such that the lifetime of any particular spin in any given state is short (i.e. τ 1 < 1/ J ) - broadband excitation of all frequencies corresponding to a particular nucleus type (i.e. all hydrogens) leads to multiplet collapse and simpli f cation of the spin-spin splitting patterns of the signals from coupled nuclei
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O || CH 3 -C-O-CH 2 -CH 3 ethyl acetate trivial example - selective decoupling of methyl hydrogens from methylene hydrogens in the ethyl moiety of ethyl acetate
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selective decoupling can identify signals from coupled nuclei, thus assisting to establish connectivity/structure intensities of signals from coupled nuclei are improved (due to multiplet collapse) - integrals are unchanged for signals from coupled nuclei selective decoupling no decoupling
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( right ) selective decoupling of the methyl hydrogens (1.3 ppm) of propylene oxide con f rms identity of signal from methine H (2.8 ppm)
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This note was uploaded on 11/13/2011 for the course CHEM 4190 taught by Professor Staff during the Fall '08 term at University of Georgia Athens.

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Double-resonance - It is often advantageous to reverse or remove the splitting caused by spin-spin coupling This is called spin decoupling Spin

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