01272010 - CH 310N MWF 8:00 Lecture 4 W 1/27/2010 Reading...

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CH 310N – MWF 8:00 – Lecture 4 – W 1/27/2010 Graded Homework : HW#01 deadline = 4:00 p.m. on Th 1/28 HW#02 deadline = 2:00 p.m. on F 1/29 HW#03 deadline = 4:00 p.m. on Tu 2/2 Reading Assignment : Sections 13.8, 13.9, 13.10 Textbook Problems : 13.2-13.5 Last Time : Infrared spectroscopy continued (end Chapter 12) Introduction to NMR spectroscopy (begin Chapter 13) If you have an excusable conflict for ANY of the three midterm exams, the deadline to sign up to take a conflict exam is 5:00 p.m. on Friday, February 5 . Today : NMR theory (continued…) Equivalence of protons NMR integration Relationship of chemical shift to structure
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If a sample is irradiated with electromagnetic radiation of energy E 0 , exactly equal to D E, energy can be absorbed by some of the nuclei in the spin = +½ state The result is “spin flip” – nuclei that absorb energy end up in the higher energy spin = –½ state. apply B 0 D E spin = –½ irradiate with E 0 = D E D E spin = random orientation of spins, no B 0 , equal energy When E 0 = D E and “spin flip” occurs, magnetic nuclei are said to be in resonance with the applied radiation “Nuclear magnetic resonance” [This type of “resonance” has nothing to do with the “resonance structures” you have learned about previously…]
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The exact electromagnetic frequency required for resonance depends on B 0 and on the identity of the nuclei being probed. In practice, superconducting magnets having B 0 ranging from 1.41–14.1 tesla (T) are used. For example, if B 0 = 7.05 T, 1 H nuclei resonate at n ≈ 300 MHz 13 C nuclei resonate at n ≈ 75 MHz radio waves (“rf energy”) Do all 1 H nuclei absorb rf energy at exactly the same frequency? NO!!! (if they did, NMR would not be useful for structure determination…) Why not??? All nuclei in molecules are surrounded by electrons. Application of an external All nuclei in molecules are surrounded by electrons . Application of an external magnetic field causes these electrons to circulate, which sets up a local magnetic field ( B local ) of their own. B local opposes the applied field ( B 0 ). The effective magnetic field ( B eff ) experienced by a particular nucleus is a bit smaller than the applied field ( B 0 ): B eff = B 0 B local Nuclei in a molecule are shielded from the full effect of B 0 by the presence of nearby circulating electrons. Since each nucleus exists in a slightly different
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01272010 - CH 310N MWF 8:00 Lecture 4 W 1/27/2010 Reading...

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