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

2 The para-hydrogen phenomenon Used to create large polarization from large population differences   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-H 2 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,

4 Dihydrogen wavefunctions 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 Percentage composition of ortho and para hydrogen as function of temperature 51% 77K 99.9% 4K

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: Eigenstates: ωz1, ωz2: rotating frame chemical shifts D: dipolar coupling constant J: scalar coupling constant Weak coupling limit:

11 Transition probabilities

12 Density Matrix for pure para-H 2 adduct For  ®0 (weak coupling)

13 Adduct Density Matrix from para- H 2 with mole fraction χ p 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 2 ε = !

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