Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion

Buntkowsky, Gerd; Walaszek, Bernadeta; Adamczyk, Anna; Xu, Yiling; Limbach, Hans−Heinrich; Chaudret, Bruno

doi:10.1039/b601594h

Cited by 87 publications

(88 citation statements)

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“…A complex chemical network with numerous formation and destruction pathways makes it difficult to disentangle to what other molecules' OPR that of a given species is related (e.g., Kahane et al 1984). The physics of spin exchange processes for binary collisions, but even more so for surface interactions (e.g., Buntkowsky et al 2006), is very complex and not fully understood. Despite these challenges, the OPR has been observed using molecules in a variety of conditions and the observed ratios used to model current and past conditions.…”

Section: Introductionmentioning

confidence: 99%

WATER ABSORPTION IN GALACTIC TRANSLUCENT CLOUDS: CONDITIONS AND HISTORY OF THE GAS DERIVED FROMHERSCHEL/HIFI PRISMAS OBSERVATIONS

Flagey¹,

Goldsmith²,

Lis³

et al. 2012

ApJ

100

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ABSTRACT2 O for the lower energy level of each transition observed. The total column density of water in translucent clouds is usually about a few 10 13 cm −2 . We find that the abundance of water relative to hydrogen nuclei is 1 × 10 −8 in agreement with models for oxygen chemistry in which high cosmic ray ionization rates are assumed. Relative to molecular hydrogen, the abundance of water is remarkably constant through the Galactic plane with X(H 2 O) = 5 × 10 −8 , which makes water a good traced of H 2 in translucent clouds. Observations of the excited transitions of H 2 O enable us to constrain the abundance of water in excited levels to be at most 15%, implying that the excitation temperature, T ex , in the ground state transitions is below 10 K. Further analysis of the column densities derived from the two ortho ground state transitions indicates that T ex 5 K and that the density n(H 2 ) in the translucent clouds is below 10 4 cm −3 . We derive the water ortho-to-para ratio for each absorption feature along the line of sight and find that most of the clouds show ratios consistent with the value of 3 expected in thermodynamic equilibrium in the high-temperature limit. However, two clouds with large column densities exhibit a ratio that is significantly below 3. This may argue that the history of water molecules includes a cold phase, either when the molecules were formed on cold grains in the well-shielded, low-temperature regions of the clouds, or when they later become at least partially thermalized with the cold gas (∼25 K) in those regions; evidently, they have not yet fully thermalized with the warmer (∼50 K) translucent portions of the clouds.

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Section: Introductionmentioning

confidence: 99%

WATER ABSORPTION IN GALACTIC TRANSLUCENT CLOUDS: CONDITIONS AND HISTORY OF THE GAS DERIVED FROMHERSCHEL/HIFI PRISMAS OBSERVATIONS

Flagey¹,

Goldsmith²,

Lis³

et al. 2012

ApJ

100

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“…Indeed, the transfer of both hydrogen atoms of an H 2 molecule to the same product molecule is a necessary condition for the observation of PASADENA or ALTADENA, but is not a sufficient one. In the transient dihydride complex formed upon interaction of H 2 with the catalyst, the equivalence of the two H atoms is lost, and the initial coherence of nuclear spins starts to decay due to spin relaxation processes [42].…”

Section: Discussionmentioning

confidence: 99%

“…Typically, a paramagnetic powder is used to break the symmetry of the nuclear spin Hamiltonian when the H 2 molecule interacts with the surface; evolution under this Hamiltonian makes the transition allowed and results in a much faster equilibration of the ortho/para ratio [42]. I use a paramagnetic powder, FeO(OH) (Sigma Aldrich #371254), for this process.…”

Section: Methodsmentioning

confidence: 99%

MRI of Heterogeneous Hydrogenation Reactions Using Parahydrogen Polarization

Burt

2008

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The power of magnetic resonance imaging (MRI) is its ability to image the internal structure of optically opaque samples and provide detailed maps of a variety of important parameters, such as density, diffusion, velocity and temperature. However, one of the fundamental limitations of this technique is its inherent low sensitivity. For example, the low signal to noise ratio (SNR) is particularly problematic for imaging gases in porous materials due to the low density of the gas and the large volume occluded by the porous material. This is unfortunate, as many industrially relevant chemical reactions take place at gas-surface interfaces in porous media, such as packed catalyst beds. Because of this severe SNR problem, many techniques have been developed to directly increase the signal strength. These techniques work by manipulating the nuclear spin populations to produce polarized (i.e. non-equilibrium) states with resulting signal strengths that are orders of magnitude larger than those available at thermal equilibrium. 2This dissertation is concerned with an extension of a polarization technique based on the properties of parahydrogen. Specifically, I report on the novel use of heterogeneous catalysis to produce parahydrogen induced polarization and applications of this new technique to gas phase MRI and the characterization of micro-reactors. First, I provide an overview of nuclear magnetic resonance (NMR) and how parahydrogen is used to improve the SNR of the NMR signal. I then present experimental results demonstrating that it is possible to use heterogeneous catalysis to produce parahydrogen-induced polarization.These results are extended to imaging void spaces using a parahydrogen polarized gas. In the second half of this dissertation, I demonstrate the use of parahydrogen-polarized gasphase MRI for characterizing catalytic microreactors. Specifically, I show how the improved SNR allows one to map parameters important for characterizing the heat and mass transport in a heterogeneous catalyst bed. This is followed by appendices containing detailed

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“…3,5,37 However, the glass wall of an NMR tube (e.g. as used for a SABRE hyperpolarisation experiment) offers a surface for the heterogeneous conversion between spin isomers, which is more rapid.…”

Section: In-situ Monitoring Of the Conversion Of Ph 2 In Nmr Tubesmentioning

confidence: 99%

“…However, as the conversion between oH 2 and pH 2 is forbidden, conversion between isomers is very slow unless a catalyst is used. [1][2][3][4][5] The pH 2 isomer is useful for a wide range of applications, including: liquid fuels, [6][7][8] matrix isolation spectroscopy, 9,10 certain hyperpolarisation methods for nuclear magnetic resonance (NMR) spectroscopy, 3,4,11,12 and as a moderator for spallation neutron sources. 13,14 For many of these applications the proportion of the pH 2 isomer is of vital importance.…”

Section: Introductionmentioning

confidence: 99%

Quantitative In Situ Monitoring of Parahydrogen Fraction Using Raman Spectroscopy

Parrott

Dallin²,

Andrews³

et al. 2018

Appl Spectrosc

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Raman spectroscopy has been used to provide a rapid, non-invasive and non-destructive quantification method for determining the parahydrogen fraction of hydrogen gas. The basis of the method is the measurement of the ratio of the first two rotational bands of hydrogen at 355 cm −1 and 586 cm −1 corresponding to parahydrogen and orthohydrogen, respectively. The method has been used to determine the parahydrogen content during a production process and a reaction. In the first example, the performance of an in-house liquid nitrogen cooled parahydrogen generator was monitored both at-line and on-line. The Raman measurements showed that it took several hours for the generator to reach steady state and hence, for maximum parahydrogen production (50 %) to be reached. The results obtained using Raman spectroscopy were compared to those obtained by at-line low-field NMR spectroscopy. While the results were in good agreement, Raman analysis has several advantages over NMR for this application. The Raman method does not require a reference sample, as both spin isomers (ortho and para) of hydrogen can be directly detected, which simplifies the procedure and eliminates some sources of error. In the second example, the method was used to monitor the fast conversion of parahydrogen to orthohydrogen in-situ. Here the ability to acquire Raman spectra every 30 s enabled a conversion process with a rate constant of 27.4 × 10 −4 s −1 to be monitored. The Raman method described here represents an improvement on previously reported work, in that it can be easily applied on-line and is approximately 500 times faster. This offers the potential of an industrially compatible method for determining parahydrogen content in applications that require the storage and usage of hydrogen.

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Mechanism of nuclear spin initiated para-H2 to ortho-H2 conversion

Cited by 87 publications

References 50 publications

WATER ABSORPTION IN GALACTIC TRANSLUCENT CLOUDS: CONDITIONS AND HISTORY OF THE GAS DERIVED FROMHERSCHEL/HIFI PRISMAS OBSERVATIONS

WATER ABSORPTION IN GALACTIC TRANSLUCENT CLOUDS: CONDITIONS AND HISTORY OF THE GAS DERIVED FROMHERSCHEL/HIFI PRISMAS OBSERVATIONS

MRI of Heterogeneous Hydrogenation Reactions Using Parahydrogen Polarization

Quantitative In Situ Monitoring of Parahydrogen Fraction Using Raman Spectroscopy

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