cruvalcaba
Registered: 10/27/09
Posts: 6
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Reply with quote | #1 |
How do we distinguish all the funtional groups in different zones?..some peaks are misleading. |
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echiu
Registered: 09/29/09
Posts: 19
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Reply with quote | #2 |
I have trouble determining what some peaks represent as well. I usually begin by using the molecular formula to eliminate as many functional groups as possible. This usually leaves one or two possibilities for each zone. Then I calculate the DBE and eliminate all the functional groups that contain too many pi bonds or those that contain a ring. Usually if the DBE is 4 or more, there is a beneze ring, but not always, which is why you have to look in zone 5 to see if there are two long, sharp peaks at around 1500 and 1600. If peaks in other zones do not fall neatly into any of the functional groups, check to see if any highly conjugated structures such as benzene rings are present. If there is a lot of conjugation, then the peak should be located at a slightly lower stretching frequency than the table says it should be. Hope you find this helpful. |
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cruvalcaba
Registered: 10/27/09
Posts: 6
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cruvalcaba
Registered: 10/27/09
Posts: 6
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Reply with quote | #4 |
In C-NMR can the carbons, in some cases, also couple with hydrogens? |
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jaymeecastillo
Registered: 11/02/09
Posts: 6
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Reply with quote | #5 |
Hmm... I believe it depends on what kind of C you are talking about. It was brought up in class today (Pg. 171 of Lecture Supplement), and Dr. H clarified that 1H does not couple with 12C in NMR because only nuclei with a spin quantum numbers, I, that aren't equal to zero (have an odd number of either protons or neutrons) can be observed by NMR. Since 12C has 6 protons and 6 neutrons, it's "NMR inactive," and therefore cannot couple with 1H.
BUT, 1H can couple with 13C (6 protons and 7 neutrons), because 13C has an I not equal to zero.
I don't know if I'm just being redundant and repeating what was said in class, but I hope this helps!
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DrH
Moderator
Registered: 09/22/08
Posts: 437
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