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Double resonance

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Presentation on double resonance in NMR

Publicada em: Saúde e medicina
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Double resonance

  1. 1. DOUBLE RESONANCE IN NMR PRESENTED BY GUIDED BY PRIYANKA GOSWAMI Dr. RAJA CHAKROBARTY M.PHARM ASSOCIATE PROFESSOR DEPARTMENT OF PHARMACY DEPARTMENT OF PHARMACY ASSAM DOWN TOWN UNIVERSITY ASSAM DOWN TOWN UNIVERSITY
  2. 2. SPIN DECOULING • Decoupling is a special technique used in NMR spectroscopy to avoid the splitting of the signals by eliminating partially or fully the observed coupling. • Which involve the irradiation of a proton with sufficiently intense radiofrequency energy, so that it prevents the coupling with the neighbouring proton and gives spectral line as a singlet. • Decoupling can help to determine structures of chemical compounds. 2
  3. 3. EFFECT OF DECOUPLING IN NMR SPECTRA • Decoupling is used to simplify a complex NMR spectrum. • It is possible to irradiate each coupled protons in the molecule to produce the spectrum with less complexity • Decoupling causes the multiplet to collapse to a doublet or singlet and give spectra which are easy to interrupt 3
  4. 4. • If we examine the proton NMR spectrum of pure ethanol , it will look like the NMR spectrum drawn above, however most ethanol is found mixed in either acid or base, this acid will readily react with alcohol, there by changing the concentration of ethanol in the sample. this in turn affect the NMR spectrum 4
  5. 5. • The base react with the alcohol, forming the alkoxide molecule, if we add a bit of water in which the H is replace by deuterium so that we can labile the water it will react with alkoxide to form the final product, placing it into the proton NMR spectrum, we see that all the 1H spectral signals disappear because it is exchanged for deuterium. 5
  6. 6. • When the ethanol molecule and the base/acid exchange the hydrogen atom, there will be an equal probability of the new hydrogen having either a +1/2 or -1/2 spin, as this exchange takes place the average spin of hydrogen will be zero. 6
  7. 7. • This means that since the spin of hydroxide hydrogen is zero, it will not undergo spin-spin interaction with the adjacent methylene hydrogen. in such case, the methylene hydrogen is said to be spin decoupled. 7
  8. 8. DECOUPLING BY IRRADIATION • Spin decoupling is useful phenomenon. It can be used by an NMR spectrum to enhance spectral signals to get a clear distinction between certain signals 8
  9. 9. • The range for coupling constant for cis 6-12 that for trans molecule is 12-18 Hz. However the given molecule contain a methyl group where hydrogen atom undergo coupling with h1 and h2 atom. this in turn leads to complex signal that involve much spectral signal. • What we can do simply the signal is irradiate the methyl hydrogen with right frequency. this will cause the methyl hydrogen to flip and change spins. the rapid interconversion of spin up to spin down will lead to an average spin of zero, thereby removing coupling between methyl hydrogen and H1 and H2. 9

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