8 Proton Nuclear Magnetic Resonance HNMR

Organic Chemistry: Structure and Function

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1 Proton Nuclear Magnetic Resonance ( 1 H-NMR) Spectroscopy Part 1 Lecture Supplement: Take one handout from the stage
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2 Midterm 1 High score = 97 Average = 73.5 Grade scale posted on course web site Exam return: Last name Returned by Farnosh Robyn Steve JC Richard Dr H A - Cl Co-He Ho-Le Li-Ng Ni-Si Sk-Z
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3 1 H-NMR Spectroscopy Background and Theory Fundamental principle The energy required to cause nuclear spin flip is a function of the magnetic environment of the nucleus. Protons, electrons, neutrons have “spin” ( I ) Motion of charged particle creates magnetic field In absence of external influence, magnetic poles (spin axis) randomly oriented No external magnetic field Spin alignment random With external magnetic field Spins aligned B o •Add external magnetic field ( B o ): spins align add magnetic field
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4 Background and Theory Nuclear Spin Flip I = +1/2 parallel to B o (lower energy); I = -1/2 antiparallel to B o (higher energy) Addition of energy results in nuclear spin flip Excited state Nuclear spin antiparallel to B o Higher energy I = +1/2 I = -1/2 Ground state Nuclear spin parallel to B o Lower energy Increasing energy Absorb energy Release energy (relaxation) E ~ 0.02 cal mol -1 = radio wave photons
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5 Background and Theory Magnetic Field Controls E E influenced by magnetic field strength at nucleus I = -1/2 I = +1/2 Spin state energy Magnetic field strength at nucleus Large magnetic field large E E Small magnetic field small E E Energy required for spin flip ( E)
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6 Background and Theory The NMR Spectrum Spectrum = plot of photon energy versus photon quantity Intensity of signal (photon quantity) Spin flip energy (photon energy) NMR signal Shielded (upfield) High magnetic field strength Deshielded (downfield) Low magnetic field strength
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7 Background and Theory The NMR Spectrum Nuclear: manipulation of nuclear spin Magnetic: magnetic field strength influences E C O C O X Resonance: tendency of a system to oscillate at maximum amplitude at a certain frequency NMR 1 H nucleus = a proton 1 H-NMR = proton NMR
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8 Background and Theory Spectrum Structure How do we deduce structure from NMR spectrum? Information from NMR spectrum Number of signals number of nonequivalent proton groups in molecule Position of signals (chemical shift) magnetic environment of protons Relative intensity of signals (integration) ratio of equivalent proton types Splitting of signals (spin-spin coupling) proton neighbors
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9 Number of Signals Proton Equivalency NMR signal due to photon absorption Photon energy controlled by magnetic environment of nucleus Nuclei in same magnetic environment = equivalent Multiple magnetic environments multiple signals Number of signals = number of equivalent proton sets Protons equivalent One NMR signal O H H Protons not equivalent Two NMR signals Cl CN H H
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10 Number of Signals Proton Equivalency Equivalent = proton magnetic environments identical in every way
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