7- NMR

7- NMR - Stuff covered today is on upcoming quiz 1...

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1 Stuff covered today is on upcoming quiz
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2 Introduction to Nuclear Magnetic Resonance (NMR) ± NMR is a powerful tool for the study of molecular structure and dynamics ±w i t h m a s s s p e c t r o m e t r y , p r o v i d e s h i g h resolution methods for chemical analysis ± Magnetic Resonance Imaging (MRI) methods provide non-invasive probes of living organisms 04-1
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Introduction to Nuclear Magnetic Resonance (NMR) k B T= 4.14 x 10 -21 J = .026 eV = 200 cm -1 at 300 K E 5 x 10 -5 k B T 5 x 10 +1 k B T 5 x 10 +6 k B T 04-2
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4 Because the energies for NMR spectroscopy are so small, NMR is very sensitive to small changes in the chemical environment of molecules. This is what makes it to powerful. E 5 x 10 -5 k B T
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NMR take home lessons ±nuclei of certain atoms have non-zero magnetic moments and spins ±NMR results from the absorption of energy by a nucleus changing its spin orientation in a magnetic field ±the energy of this transition is sensitive to the nuclear environment ±NMR spectra can be used to characterize structure and dynamics of molecular systems 04-3
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6 web resources Introduction to NMR: www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr1.htm www.chembio.uoguelph.ca/driguana/NMR/TOC.HTM More thorough, including 2D NMR: http://www.cis.rit.edu/htbooks/nmr/bnmr.htm NMR references OGC section 20.4 Roberts and Caserio, section 9:10 04-4
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7 ±many nuclei have non-zero spin ±a spinning charge has a magnetic dipole moment, µ ±magnetic moments can interact with an external magnetic field B 0 and with nearby nuclei with non-zero spin. They behave in some ways like small bar magnets. Nuclear Magnetic Resonance (NMR) Spectroscopy 04-5
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8 Nuclear Spin a non-zero nuclear spin is required for NMR I = 1/2: 1 H, 13 C, 19 F, 29 Si, 31 P, . .. I > 1/2: 2 H (I=1), 14 N (I=1), 23 Na (I=3/2), 17 O (I=5/2),± I = 0: 12 C, 16 O, 32 S, . .. No NMR! direction of the externally applied magnetic field B 0 I = 1/2 I = 1 m I = -1 m I = 0 m I = +1 m I = +1/2 m I = -1/2 A nucleus with the spin quantum # I has 2 I +1 possible spin states: m l = - I , - I +1 . .. I -1, I , that give possible orientations of the nuclear spin angular momentum 04-6 the spins precess around B 0
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9 The energy of a spinning nucleus depends on the orientation in a magnetic field ±the energy of the magnetic dipole depends on the orientation in a magnetic field, B 0 : E = -µ± B 0 ±the lowest energy state is when µ and B 0 are parallel m I = +1/2 m I = -1/2 change in orientation when ¨ E is absorbed B 0 Energy B 0 m I = -1/2 m I = +1/2 ¨ E = 2µ B 0 04-7
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10 Energy Levels of Nuclei in a Magnetic Field The magnetic moment of a nucleus depends on both the spin state m I and the magnetogyric ratio, J : µ= J m I (h/2 S ) Transitions between different levels are coupled to absorption or emission of radiation ¨ E = h Q = - J ¨ m I (h/2 S ) B 0 or Q = - J ¨ m I B 0 / 2 S with the selection rule ¨ m I = + 1 (absorption (-1) or emission (+1)) ? Q = J B 0 / 2 S for hydrogen ( 1 H), J = 26.7522 x 10 7 rad sec -1 T
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7- NMR - Stuff covered today is on upcoming quiz 1...

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