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l15b - NMR spectroscopy: Chemical-shift equivalence ©...

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Unformatted text preview: NMR spectroscopy: Chemical-shift equivalence © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I NMR spectroscopy: Chemical-shift equivalence It frequently happens that a molecule gives rise to an NMR spectrum in which the number of peaks does not match the number of protons in that molecule. A case in point is benzene, which has six protons yet only one peak in its spectrum: Benzene’s six protons all produce an NMR signal at the same chemical shift because they are all chemical-shift equivalent to each other. There are two ways in which protons are rendered chemical-shift equivalent: (1) by symmetry and (2) by a rapid mechanism. We will first consider chemical-shift equiva- lence by symmetry and then proceed to the more intricate chemical-shift equivalence by a rapid mechanism. Chemical-shift equivalence by symmetry When we discussed molecular vibrations, we encountered the concept of symme- try-equivalence. Recall that two atoms in a molecule are symmetry-equivalent if they are related by one of the symmetry elements (the center of symmetry i , the plane of symmetry ! , or the axis of symmetry C n ) belonging to that molecule. Symmetry-equivalent protons are always in the same chemical envi- ronment and thus absorb at the same value of B lab , that is, symmetry- equivalent protons are chemical-shift equivalent and produce the same NMR peak. Problem There are three dibromobenzenes. One produces one NMR signal, one produces two NMR signals, and one produces three NMR signals. Which is which? NMR spectroscopy: Chemical-shift equivalence © Bruno I. Rubio 2 Answer Br Br H a H a H a H a 1 signal H a H a Br Br H b H b 2 signals H a H c Br H b Br H b 3 signals Problem How many NMR signals does each of these compounds exhibit? (a) Cl Br (b) Cl Br (c) Br Cl (d) Cl Cl Cl (e) Cl Cl Cl (f) Cl Cl Cl (g) (h) Cl Cl (i) Cl Cl (j) Cl Cl (k) Cl Cl (l) N N (m) N N (n) N N (o) N N NMR spectroscopy: Chemical-shift equivalence © Bruno I. Rubio 2 Answer Br Br H a H a H a H a 1 signal H a H a Br Br H b H b 2 signals H a H c Br H b Br H b 3 signals Problem How many NMR signals does each of these compounds exhibit? (a) Cl Br (b) Cl Br (c) Br Cl (d) Cl Cl Cl (e) Cl Cl Cl (f) Cl Cl Cl (g) (h) Cl Cl (i) Cl Cl (j) Cl Cl (k) Cl Cl (l) N N (m) N N (n) N N (o) N N NMR spectroscopy: Chemical-shift equivalence © Bruno I. Rubio 3 (p) Cl Cl (q) Cl Cl (r) Cl Cl (s) (t) O (u) O (v) O O (w) N O H (x) O OH Answer (a) four (b) four (c) two (d) one (e) two (f) three (g) two (h) three (i) three (j) three (k) six (l) two (m) four (n) four (o) eight (p) three (q) two (r) one (s) two (t) three (u) six (v) four (w) three (x) four Chemical-shift equivalence by a rapid mechanism The six protons of benzene are always chemical-shift equivalent because they are always symmetry-equivalent: the rigid ring in which they reside keeps them permanently so. However, there are innumerable cases in which internal rotation about...
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This note was uploaded on 02/27/2012 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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l15b - NMR spectroscopy: Chemical-shift equivalence ©...

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