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Parahydrogen induced polarization

Parahydrogen induced polarization. Thomas Theis University of California Berkeley Physics 250 04/17/2008 . The para -hydrogen phenomenon. Used to create large polarization from large population differences  10.000 fold NMR signal enhancement

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Parahydrogen induced polarization

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  1. Parahydrogen induced polarization Thomas Theis University of California Berkeley Physics 250 04/17/2008

  2. The para-hydrogen phenomenon • Used to create large polarization from large population differences •  10.000 fold NMR signal enhancement • High polarization can be exploited in numerous applications • Detection of reaction intermediate (especially in hydrogenations) • Characterization of gas flows (e.g. micro engines, catalyst beds) • Characterization of fuel cells

  3. Outline • Production of para-H2 • Density Matrix description • Pasadena vs. Altadena • Focus on basics rather than applications • overcoming hydrogenations • Reference: Clifford R. Bowers, Sensitivity Enhancement Utilizing Parahydrogen, Encyclopedia of Nuclear Magnetic Resonance 2002,9, 750-770.

  4. Dihydrogenwavefunctions Nuclear wavefunctions: symmetric antisymmetric total =  e r n has to be symmetric (aacording to Dima, antisymmetric according to the literature)  ortho states live in the odd rotational states  para state lives in the even rotational states (including J=0)

  5. Production of non-equilibrium ortho/para hydrogen mixtures • Transitions between ortho and para hydrogen are symetrically forbidden (para singletortho triplet)  non-equilibrium mixtures are long lived • To induce the transition catalysts at low temperatures break the symmetry • Once the hydrogen desorbs from the catalyst the ortho/para ratio is conserved and given by

  6. Para-hydrogen enrichment 51% para @ 77K 99.9% para @ 4K Percentage composition of ortho and para hydrogen as function of temperature

  7. Pairwise Hydrogenation using a catalyst e.g. Wilkinsons catalyst

  8. Pasadena Parahydrogen and Synthesis Allow Dramatically enhanceed Nuclear allignment. First published in 1986 from Bowers and Weitekamp at Caltech in Pasadena

  9. Direct product space reminder

  10. General Hamiltonian for two spin systems and it’s Eigenstates Rotating field Hamiltonian: ωz1, ωz2: rotating frame chemical shifts D: dipolar coupling constant J: scalar coupling constant Eigenstates: Weak coupling limit:

  11. Transition probabilities

  12. Density Matrix for pure para-H2 adduct For ®0 (weak coupling)

  13. Adduct Density Matrix from para-H2 with mole fraction χp For f =0 i.e. thermalized ortho/para mixture where p = ¼ and ®0

  14. Pasadena correlation diagram

  15. Time evolution of the adduct density Matrix observed NMR signal

  16. Pasadena signal from weakly coupled spin systems Evolution under J coupling and detection: Maximized for 45° pulse

  17. Obtained spectra

  18. Comparison to thermal signal After 90° pulse Evolution under J coupling and detection For f=1 the ratio evaluates to 2ε = 31240 !

  19. Altadena (just next to Pasadena) Adiabatic Longitudinal Transport After Dissociation Engenders Net Allignment

  20. What have we learned? Density Matrix formalism is a very poweful tool to make accurate predictions of the NMR signals

  21. A step further from Hydrogenations

  22. Thank‘s for your attention Thank‘s to • Scott Burt and Louis Bouchard • Hattie, Pete, Ngok Do • Dima

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