Parahydrogen induced polarization Thomas Theis University of California

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

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 2

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, 750 -770. 3

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) 4

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 5

Para-hydrogen enrichment 51% para @ 77 K 99. 9% para @ 4 K Percentage composition of ortho and para hydrogen as function of temperature 6

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

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

Direct product space reminder 9

General Hamiltonian for two spin systems and it’s Eigenstates Rotating field Hamiltonian: ωz 1, ωz 2: rotating frame chemical shifts D: dipolar coupling constant J: scalar coupling constant Eigenstates: Weak coupling limit: 10

Transition probabilities 11

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

Adduct Density Matrix from para -H 2 with mole fraction χp For f =0 i. e. thermalized ortho/para mixture where p = ¼ and ® 0 13

Pasadena correlation diagram 14

Time evolution of the adduct density Matrix observed NMR signal 15

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

Obtained spectra 17

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

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

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

A step further from Hydrogenations 21

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