L13_Molec_spectr_NMR - Molecular Spectroscopy ­part...

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Unformatted text preview: Molecular Spectroscopy ­part III: NMR NMR Spectroscopy •  Nuclear magne,c resonance spectroscopy (NMR) exploits the magne;c proper;es of certain nuclei. NMR is applicable to any nucleus possessing spin. •  When placed in a magne;c field, NMR ac;ve nuclei (such as 1H or 13C) absorb at a frequency characteris;c of the isotope. The resonant frequency, energy of the absorp;on and the intensity of the signal are propor;onal to the strength of the magne;c field. For example, in a 21 tesla magne;c field, protons resonate at 900 MHz. •  Many types of informa;on can be obtained from an NMR spectrum. Much like using IR to iden;fy func;onal groups, analysis of a NMR spectrum provides informa;on on the number and type of chemical en;;es in a molecule. Magne;c resonance imaging •  In an MRI machine a radio frequency transmiQer produces an electromagne;c field. The photons of this field are chosen to have the energy (or resonance frequency) to flip the spin of the aligned protons in the body. As the intensity and dura;on of applica;on of the field increase, more aligned spins are affected. AUer the field is turned off, the protons decay to the original spin state and the difference in energy between the two states is released as a photon. These photons are detected by the scanner. Protons have spin ½ Protons have spin ½ Raising and lowering operators Nuclei in a magne;c field B Electrons shield the nucleus and produce a field B’ opposed to B0 Nuclei in a magne;c field B Chemical shiUs NMR experiments NMR experiments: selec;on rules NMR experiments: selec;on rules NMR experiments: peaks of the spectrum •  Different protons shielded differently •  Area under NMR peaks propor;onal to number of spins involved in the transi;on NMR chemical shits: Carbon NMR chemical shiUs: Carbon NMR Chemical shiUs: proton NMR to determine protein structure •  Developed in 1980 by Kurt Wüthrich •  Won the Nobel prize in 2002 •  Showed NMR is possible for proteins •  Invented “sequen;al assignment” where he determined the distance between any two H atoms in the molecule and paired each peak with a H nucleus in the protein Calcula;ons of NMR chemical shiUs Uncoupled spins The Hamiltonian in separable and energies are: The Hamiltonian in separable and selec;on rules are easy to derive: Allowed transi;ons for uncoupled spins •  Allowed transi;ons: Coupled spins The Hamiltonian in not separable any more and energies contain the coupling term J: Degeneracy present in the uncoupled spin case is liUed Spin dynamics Spin dynamics Larmor precession •  Compute average values using normalized wavefunc;ons Magne;za;on ...
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This note was uploaded on 04/04/2011 for the course CHE 110B taught by Professor Galli during the Winter '11 term at UC Davis.

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