Ch1b2010_week3

Ch1b2010_week3 - Introduction to Nuclear Magnetic Resonance...

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1 Introduction to Nuclear Magnetic Resonance (NMR) • NMR is a powerful tool for the study of molecular structure and dynamics • with mass spectrometry, provides high resolution methods for chemical analysis • Magnetic Resonance Imaging (MRI) methods provide non-invasive probes of living organisms 04-1
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2 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 T5 x 1 0 +1 k B x 1 0 +6 k B T 04-2
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3 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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5 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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6 •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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7 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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8 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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9 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, γ : μ= γ m I (h/2 π ) Transitions between different levels are coupled to absorption or emission of radiation E = h ν = - γ m I (h/2 π ) B 0 or ν = - γ m I B 0 / 2 π with the selection rule m I = + 1 (absorption (-1) or emission (+1)) ν = γ B 0 / 2 π for hydrogen ( 1 H), γ = 26.7522 x 10 7 rad sec -1 T -1 so with B 0 = 22.27 T, ν = 9.5 x 10 8 sec -1 = 950 MHz (T = “Tesla”; earth’s magnetic field ~ 50 μT ~ 0.5 Gauss) 04-8
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10 Each isotope has a specific γ : nuclei spin γ (10 7 rad sec -1 T -1 ) 1 H 1/2 26.7522 2 H 1 4.1066 13 C 1/2 6.7283 hence the NMR frequencies are isotope-specific.
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Ch1b2010_week3 - Introduction to Nuclear Magnetic Resonance...

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