lecture16x-hetcor

lecture16x-hetcor - CHEM BCMB 4190/6190/8189 Introductory...

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1 CHEM / BCMB 4190/6190/8189 Introductory NMR Lecture 16 supplement
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2 Lets consider the two-spin AX system ( 13 CHCl3) with A= 1 H = sensitive nuclei and X= 13 C = insensitive nuclei A) At equilibrium: N4 = N N3 = N + ± C N2 = N + ± H N1 = N + ± C + ± H N2 - N4 ± N1 - N3 = ± H N3 - N4 ± N1 - N2 = ± C ± H = 4 * ± C For 13 C spectrum: X1 transition: N3 – N4 = ± C X2 transition: N1 – N2 = ± C X 2 X 1 N2 N1 N3 N4 X1 ²³ ³² ²² ³³ X2 A1 A2
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3 A) After a selective 180 ° pulse exciting the A2 transition: The populations of N1 and N3 are inverted: before 180 A2 pulse after 180 A2 pulse N4 = N N N3 = N + ± C N + ± C + ± H N2 = N + ± H N + ± H N1 = N + ± C + ± H N + ± C After 180 ° A2 pulse: X1 transition: N3 – N4 = ± C + ± H = 5 ± C X2 transition: N1 – N2 = ± C - ± H = -3 ± C After selective inversion of the A1 or A2 transition, the signal amplification factors for the spectra of X are given by: 1 + ² A / ² X and 1 - ² A / ² X 180 ° x N2 N1 N3 N4 X1 ³´ ´³ ³³ ´´ X2 A1 A2 180 ° x on A2 y µ x µ M H C ´ M H C ³ z y µ x µ M H C ´ M H C ³ X 2 X 1 180 ° x on A2 z y µ x µ M C H ´ M C H ³ z y µ x µ M C H ´ M C H ³ 90 ° x on 13 C
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4 B) How about if we “selectively” apply a 180 ° pulse to both the A1 and A2 transitions: this is essentially, therefore, just a 180 pulse on 1 H The populations of N1 and N3, and N2 and N4 are inverted: before 180 ° pulse after 180 ° pulse N4 = N N + ± H N3 = N + ± C N + ± C + ± H N2 = N + ± H N N1 = N + ± C + ± H N + ± C X1 transition: N3 – N4 = ± C X2 transition: N1 – N2 = ± C As you would expect, a non-selective 180 ° 1 H pulse does not change the sensitivity of the 13 C spectrum: What if t ² 0? 180 ° x 180 ° x N2 N1 N3 N4 X1 ³´ ´³ ³³ ´´ X2 A1 A2 X 2 X 1 X 2 X 1 13 C 1 H 90x 90x 90x t 13 C 1 H 90x X 2 X 1 FT FT, t=0
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5 HETCOR or C, H-COSY: correlates 1 H and 13 C chemical shifts -in this case t 1 varies (increasing t 1 allows for 1 H chemical shift evolution) -we will not ignore chemical shift evolution: M H C ± ² M H C ³ ² ´ H -the vectors M H C ± and M H C ³ precess and diverge according to ´ H , J CH and t 1
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6 Let’s look at a simple case: assume that during t1, M H C ± rotates 360 ° (so that
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lecture16x-hetcor - CHEM BCMB 4190/6190/8189 Introductory...

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