318-4 - Organic Lecture Series CH 318 N LECTURE 4 Textbook...

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Organic Lecture Series 1 CH 318 N LECTURE 4 Textbook Assignment: Chapter 13-Begin Homework (for credit): POW 1 posted-due on Feb 1 Today’s Topics: NMR-theory and instrumentation Notice & Announcements: Office Hrs- Kyle Williams: Wed 3-4 WEL 4.308 Organic Lecture Series 2 N N uclear uclear M M agnetic agnetic R R esonance esonance
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Organic Lecture Series 3 Molecular Spectroscopy • Nuclear magnetic resonance (NMR) Nuclear magnetic resonance (NMR) spectroscopy spectroscopy : a spectroscopic technique that gives us information about the number and types of atoms in a molecule, for example, about the number and types of – hydrogen atoms using 1 H-NMR spectroscopy – carbon atoms using 13 C-NMR spectroscopy – phosphorus atoms using 31 P-NMR spectroscopy Organic Lecture Series 4 • An electron has a spin quantum number of 1/2 with allowed values of +1/2 and -1/2 – this spinning charge creates an associated magnetic field – in effect, an electron behaves as if it is a tiny bar magnet and has what is called a magnetic moment • The same effect holds for certain atomic nuclei – any atomic nucleus that has an odd mass number, an odd atomic number, or both also has a spin and a resulting nuclear magnetic moment – the allowed nuclear spin states are determined by the spin quantum number, I , of the nucleus Nuclear Spin States Nuclear Spin States
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Organic Lecture Series 5 Nuclear Spin States Nuclear Spin States – a nucleus with spin quantum number I has 2 I + 1 spin states; if I = 1/2, there are two allowed spin states – Table 13.1 gives the spin quantum numbers and allowed nuclear spin states for selected isotopes of elements common to organic compounds 1 H 2 H 12 C 13 C 14 N 16 O 31 P 32 S Element Nuclear spin quantum number ( I ) Number of spin states 1/2 1 0 0 0 1/2 1 23 1 2 1/2 21 Organic Lecture Series 6 Nuclear Spins in Nuclear Spins in B 0 – within a collection of 1 H and 13 C atoms, nuclear spins are completely random in orientation – when placed in a strong external magnetic field of strength: B 0 , however, interaction between nuclear spins and the applied magnetic field is quantized, with the result that only certain orientations of nuclear magnetic moments are allowed
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Organic Lecture Series 7 –for 1 H and 13 C, only two orientations are allowed E = h ν ν is called the resonance frequency or the Larmor frequency. Organic Lecture Series 8 • In an applied field strength of 7.05T, which is readily available with present-day superconducting electromagnets, the difference in energy between nuclear spin states for 1 H is approximately 0.120 J (0.0286 cal)/mol, which corresponds to electromagnetic radiation of 300 MHz (300,000,000 Hz) 13 C is approximately 0.030 J (0.00715 cal)/mol, which corresponds to electromagnetic radiation of 75MHz (75,000,000 Hz) Nuclear Spins in Nuclear Spins in B 0
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Organic Lecture Series 9 – the energy difference between allowed spin states increases linearly with applied field strength – values shown here are for 1 H nuclei α β 300 MHz 60 MHz Organic Lecture Series 10 – when nuclei with a spin quantum number of
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318-4 - Organic Lecture Series CH 318 N LECTURE 4 Textbook...

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