310NSp0801182008 - CH 310N MWF 8am Lecture 3 F Reading...

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CH 310N – MWF 8am – Lecture 3 – F 1/18/2008 Reading Assignment : Sections 13.5–13.7 Textbook Problems : No new problems…we’ll need to discuss most of the material in Chapter 13 before you’ll be able to tackle the problems!!! Graded Homework : HW#01 deadline = 3:00 p.m. on Mon 1/28 Last Time : Infrared spectroscopy (Chapter 12) Today : Introduction to nuclear magnetic resonance (NMR) spectroscopy (begin Chapter 13) TA office hours and discussion sessions begin on Tuesday, 1/22. Click on the “Help” tab on the course website for the complete schedule… Enjoy the three day weekend!!! No lecture or Bocknack office hours on Monday…
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Nuclear Magnetic Resonance (NMR) Spectroscopy (start Chapter 13 material) First introduced in the 1950’s – rapidly became the most useful spectroscopic technique for the elucidation of organic structures. Recall: IR – functional groups mass spectronomy – size of molecule & molecular formula (Chapter 14) NMR – “map” of carbon–hydrogen framework Many atomic nuclei behave as though they are spinning about an axis (like the Earth) Since the nucleus of an atom has a positive charge, and since a charge in motion generates a magnetic field, a spinning nucleus behaves like a tiny bar magnet. A spinning nucleus will therefore interact with an applied magnetic field (denoted B 0 ). Requirements for nuclear spin: Odd number of protons in nucleus Odd number of neutrons in nucleus or Nuclei that have an even number of protons and an even number of neutrons do not have a spin associated with them Magnetic Nuclei (spin) 1 H 2 H 13 C 14 N 19 F 31 P Nonmagnetic Nuclei (no spin) 12 C 16 O 32 S Focus on 1 H NMR…
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A 1 H nucleus has two allowed spin states: spin = spin = –½ or (similar to allowed spin states for an electron) In the absence of an applied magnetic field : spins of magnetic nuclei oriented randomly all nuclei have same magnetic spin energy apply B 0 In the presence of an applied magnetic field ( B 0 ) : B 0 spin = +½ aligns parallel to B 0 spin = –½ aligns antiparallel to B 0 parallel spin is lower in energy than antiparallel spin more nuclei have spin = +½ than spin = –½
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In the presence of an applied magnetic field, for every 1,000,000 protons, an excess of 10–20 will have spin = +½ vs. spin = –½ Not a big difference, but enough of a difference to be detected!!!
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