exam3-2007

exam3-2007 - Name _________________________ Exam 3, 2007:...

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Unformatted text preview: Name _________________________ Exam 3, 2007: CHEM/BCMB 4190/6190/8189 1 Exam 3: CHEM/BCMB 4190/6190/8189 (103 points) Thursday, 1 November, 2007 1 ). Consider the effect of the normal spin-echo pulse sequence (right) and the effect of this pulse sequence on the 1 H spin in a simple heteronuclear coupled system (i.e. 1 H- 13 C in 13 CHCl 3 ), ignoring the effects of magnetic field inhomogeneity . We will assume that the delay ¡ is equal to 1/(4 ¢ 1 J CH ). a. In the vector diagrams shown below, the effects of the spin echo sequence to point ‘c’ are shown. During the delay ¡ , the vector M H C £ moved 180 ° to the position shown in ‘c’, whereas the vector M H C ¤ move 90 ° . In terms of the reference frequency, ¥ rf , and the coupling constant, 1 J CH , what is the Larmor frequency, ¥ H , for this 1 H spin (in radians/second)? ( 6 points ) As shown in ‘c’, in 1/(4 ¡ 1 J CH ) seconds ( ¢ ), the M H C £ vector moved 180 ° , or ¤ radians, away from (faster than) the reference frequency ( ¥ rf ). The M H C ¦ vector moved 90 ° , or ¤ /2 radians. The average of these represents the position of a vector, M H , corresponding to the Larmor frequency ( ¥ C ), which is 3 ¤ /4 radians. Thus, relative to ¥ rf , the vector M H has moved 3 ¤ /4 radians in 1/(4 ¡ 1 J CH ) seconds, and, thus, its frequency relative to ¥ rf is 3 ¤ 1 J CH (radians/second). Thus, ¥ H = ¥ rf + 3 ¤ 1 J CH . b. Complete the vector diagrams for points ‘d’ and ‘e’ in the spin-echo sequence. Make sure to indicate the directions of rotation of the vectors in ‘d’. Then, sketch the signal that you would observe for data acquisition beginning at point ‘e’. ( 6 points ) c. On the same vector diagrams shown in ‘a’, draw the vectors for the case where ¥ H = ¥ rf . ( 4 points ) These are shown as the open arrows (labeled M H2 C ¦ and M H2 C £ ) on the axes above. ¡ ¡ a b c d e 180 ° x 90 ° x 1 H C ¤ 1 H C £ ¥ H transverse plane M y z x 90 ° x ¡ M H C ¤ a y z x b M H C £ y x c 180 ° x transverse plane M H C ¤ M H C £ x d y x e ¡ M H M H2 C ¤ M H2 C £ M H2 C ¤ M H2 C £ Name _________________________ Exam 3, 2007: CHEM/BCMB 4190/6190/8189 2 2 ). The spin-echo pulse sequence for the J- modulated spin-echo experiment (Attached Proton Test, APT), is shown (right). When the delay ¡ is set properly, this pulse sequence can be used to discriminate between 13 C nuclei with even (0, 2) versus odd (1, 3) numbers of bound 1 H nuclei. a. Explain how the APT works (in all cases, assume that the Larmor frequency ( ¢ C ) is equal to the reference frequency ( ¢ rf )). To do this, you must: 1 – state what the proper value of ¡ is 2 – draw vector diagrams for points ‘b’, ‘c’, ‘d’ (before decoupling is begun), ‘e’ (after decoupling is begun), and ‘f’ in the pulse sequence, AND, sketch the observed signal....
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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 UGA.

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exam3-2007 - Name _________________________ Exam 3, 2007:...

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