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Peak splitting

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Anyone care to answer my questions on peak splitting for H NMR? I'm very confused as to what exactly happens..for example, the textbook gives the example of ethanol. The CH3 group is split into 3 peaks because of the neighbouring CH2 group, which has 2 hydrogens. Ok, I'm good with that.

But how come the CH2 group isn't affected by both the OH and the CH3 groups since it's in the middle? And why is the OH group only a singlet and unaffected by the CH2 group?

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[quote name='__inthemaking' post='15108' date='Apr 16 2008, 07:22 PM']Anyone care to answer my questions on peak splitting for H NMR? I'm very confused as to what exactly happens..for example, the textbook gives the example of ethanol. The CH3 group is split into 3 peaks because of the neighbouring CH2 group, which has 2 hydrogens. Ok, I'm good with that.

But how come the CH2 group isn't affected by both the OH and the CH3 groups since it's in the middle? And why is the OH group only a singlet and unaffected by the CH2 group?[/quote]

Well if you look at your data booklet under 'H NMR data, there's R-CH[sub]3[/sub], R-CH[sub]2[/sub]-R, and a R-O-H. So basically the three peaks ur seeing is this.

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Guest The X
Just curious: Would this be for the Further Organic Chemistry option? I can't recall learning this (I'm a Chem HL student, though I haven't had that option). Edited by The X

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[quote name='The X' post='16092' date='May 5 2008, 02:29 PM']Just curious: Would this be for the Further Organic Chemistry option? I can't recall learning this (I'm a Chem HL student, though I haven't had that option).[/quote]

This is for Option G (Modern Analytical Chemistry).

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There are three peaks because the hydrogen are in three different energy environments. This is because the NMR spectrum detects the way the H atom spins, the way in which an H atom spins is changed by the number of H atoms on the adjacent atom. So with CH3-CH3 thewould be only one peak because all the H-atoms are in the same in environment this because on the adjacent C atoms there are three H-atoms, and the situation is the same for the other hydrogens. Whereas for CH3-CH2O-H the CH3 hydrogens have 2 hydrogens adjacent which creates a different spin than the CH2O hydrogens where the adjacent atom has two hydrogens not three which creates a different spin, then the OH hydrogen has the oxygen adjacent and therefore there are no hydrogens adjacent creating a third type of spin which gives a different reading.

So the CH2 is only affected by the CH3 because the O is the adjacent atom.

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[quote name='Bandev' post='16178' date='May 7 2008, 04:54 AM']There are three peaks because the hydrogen are in three different energy environments. This is because the NMR spectrum detects the way the H atom spins, the way in which an H atom spins is changed by the number of H atoms on the adjacent atom. So with CH3-CH3 thewould be only one peak because all the H-atoms are in the same in environment this because on the adjacent C atoms there are three H-atoms, and the situation is the same for the other hydrogens. Whereas for CH3-CH2O-H the CH3 hydrogens have 2 hydrogens adjacent which creates a different spin than the CH2O hydrogens where the adjacent atom has two hydrogens not three which creates a different spin, then the OH hydrogen has the oxygen adjacent and therefore there are no hydrogens adjacent creating a third type of spin which gives a different reading.

So the CH2 is only affected by the CH3 because the O is the adjacent atom.[/quote]

Exactly - nuclear magnetic resonance spectroscopy relies on the chemical microenvironments/chemical moieties of the various funcitonal groups. Here we are doing spin-coupled NMR and hence the microenvironments of certain hydrogen groups within the molecule can be considered to be equivalent. Thus, the peak splitting will be reduced.

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