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Introduction to NMR in Chemistry: Understanding the Basics |

NMR, nuclei, spin, magnetic, resonance, spectroscopy, chemical shift

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Introduction to NMR in Chemistry: Understanding the Basics |

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  1. Intro to NMRfor CHEM 645we also visited the website:The Basics of NMR by Joseph P. Hornak, Ph.D.http://www.cis.rit.edu/htbooks/nmr/ Brian Bahnson Department of Chemistry & Biochemistry University of Delaware

  2. Nuclei Unpaired Protons Unpaired Neutrons Net Spin (MHz/T) 1H 1 0 1/2 42.6 2H 1 1 1 6.5 31P 1 0 1/2 + 17.3 23Na 1 2 3/2 11.3 15N 0 1 1/2 4.3 13C 0 1 1/2 10.7 Tesla Natural abundance 19F 1 0 1/2 40.1 99.98 0.36 1.11 A Spinning Nucleus Gives a Magnetic Moment gyromagnetic ratio  =  unpaired electron spin Spinning nucleus photo frequency applied magnetic field

  3. The energy gap between states is small and corresponds to radio frequency waves E = h  Planck’s constant For I = ½ nuclei have two spin states M = -1/2 M = +1/2

  4. The energy gap gives rise to a slight excess of the +1/2 state -1/2 E +1/2 Boltzmann Distribution Nupper-/Nlower+ = e-E/kT DE = h  N direction of applied static magnetic field - + S

  5. The Net Magnetization Vector N S A 90º pulse places the net vector along the y-axis

  6. The spin-lattice relaxation time – T1 For a clearer description click:The Basics of NMR and follow the links – spin physics – T1 processes The return to equilibrium along the z-axis is the T1

  7. The de-phasing of this precession is mediated by the spin-spin relaxation T2 The spin-spin relaxation time – T2 For a clearer description click:The Basics of NMR and follow the links – spin physics – T2 processes Following a 90º pulse

  8. What is actually measured in NMR experiment? But due to the T1 relaxation: FT

  9. Definition of Chemical Shift d = (n - nREF) x106 / nREF units of ppm

  10. CA CB CA CB Spin-Spin Coupling is Through Bonds a.k.a. scalar-coupling or J-coupling HA HB B0 CA CB +1/2 -1/2 +1/2 -1/2 Looking at only HA JA-B shielded d (ppm) deshielded If HB not there

  11. NOE Effects are Through Space C C C C C C NOEs are highly distant dependent, seen from 2-5Å NOE  1/r6.f (tc) Correlation time is a combination of rotational motions, molecule size + shape dependent, and intra-molecular motions

  12. A Protein’s 1H-NMR Spectra is Complex To assign peaks of proteins, this needs to be spread into a 2nd or 3rd dimension.

  13. 2-D and 3-D Separates Out Spectra

  14. Two Papers to Read for next Thursday’s Class Wuthrich, (1989) “Protein Structure Determination in Solution by Nuclear Magnetic Resonance Spectroscopy” Science 243, 45-50. Ikura et al., (1989) “A Novel Approach for Sequential Assignment of 1H, 13C, and 15N Spectra of Larger Proteins: Heteronuclear Triple-Resonance Three-Dimensional NMR Spectroscopy. Application to Calmodulin” Biochemistry 29, 4659-4667.

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