Solid para hydrogen

Control of Vibrational Wave Functions in Solid Para-Hydrogen... 

We observed ESR spectra of new reactive species produced by y-rays irradiation of solid para-hydrogen at 4K [1], The species was assigned as H2 anions by experimental results [2-4]. Quantum tunneling diffusion of the H2 anions was reported [5]. The stabilization of H2 anions was discussed theoretically by the vacancy-assisted model in the hydrogen crystal [6,7]. The recent paper of Suter et al. [8] does not explain the important experimental results that H2 anions change into electron bubbles. 

Fig. 10. Pressure dependence of the Raman-active phonon and roton frequencies in solid para-hydrogen, as calculated [57] and measured [100]...

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Turkevich, J., Selwood, P. W. Solid Free Radical as Catalyst for Ortho-Para-Hydrogen Conversion. J. Am. Chem. Soc. 63, 1077 (1941). 

Isotopic disorders (e.g. in solid chlorine atoms 35C1 and 37C1 isotopes can arise) and nuclear spin degeneracy (ortho and para hydrogen) are two rather less important factors which can also give rise to residual disorder in solids at 0 K. 

At one time Taylor considered7 that if adsorption on a solid surface would catalyse the interconversion of ortho and para hydrogen, it was an indication that chemisorption with dissociation into atoms occurred the temperature at which such interconversion took place would (if this were true) have been some indication of the activation energy for chemisorption. No doubt chemisorption does promote this spin isomerization but it is now known that adsorption of the van der Waals type can result in speedy attainment of the equilibrium mixture of ortho and para hydrogen, provided that the surfaces are of some paramagnetic substance the close proximity to a paramagnetic surface can catalyse the interconversion,8 without dissociation of the adsorbed molecules. 

Saccharin (84) was discovered in 1878 and can be manufactured from toluene (Scheme 18). In the synthesis, the initial chlorosulfonation yields a mixture of the o-and p-toluenesulfonyl chlorides the required ortho-isomer is obtained by freezing out the solid para-isomer to leave the liquid o-sulfonyl chloride behind. The imino hydrogen atom in saccharin is acidic owing to the electron-attracting (-1) properties of the adjacent carbonyl and sulfonyl moieties (Scheme 18). Consequently, saccharin readily forms the sodium salt, sodium saccharin (84), on treatment with sodium hydroxide. 

This model has been used for the description of Arrhenius curves of H-transfers as described in more detail in Chapter 6. The next two chapters show applications of these symmetry effects. First the para-hydrogen induced polarization (PHIP) experiments are discussed. There the symmetry induced nuclear spin polarization creates very unconventional NMR lineshape patterns, which are of high diagnostic value for catalytic studies. Then in Section 21.4 symmetry effects on NMR line-shapes and relaxation data of intramolecular hydrogen exchange reactions are discussed and examples from iH-liquid state and H-solid state NMR are presented and compared to INS spectra. The last section gives an outlook on possible future developments in the field. 

Solid para-Hi Matrix and Hydrogen Molecular Ions... 

An alternative possibility to increase the sensitivity of solid-state NMR for the study of catalytic processes is the application of spin-polarized para-hydrogen. The most prominent example of this technique is the so-called Para Hydrogen Induced Polarization or short PHIP experiment, originally invented for liquid-state NMR studies of homogeneous catalysis. PHIP is not restricted to homogeneous hydration reactions and liquid state NMR, but is also applicable to studies of heterogeneous hydration... 

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