Separation of ortho‐ and para‐Hydrogen in Van der Waals Complex Formation

Abstract

Natural hydrogen gas is a mixture of molecules in two distinct forms-ortho-and para-hydrogen-corresponding to the different total spin of the nuclei. [1,2] Physical and chemical properties of ortho-and para-hydrogen are different and the development of efficient techniques for separation or interconversion of the hydrogen spin-isomers is important for hydrogen gas production and storage technologies with particular relevance on the eve of the hydrogen fuel era; [2, 3] it is also essential for understanding the chemistry of the Universe [4] and critical for fundamental studies of low-temperature physics.[1] The lowerenergy para-form can be obtained by cryogenic distillation through ortho-para conversion, [5] whereas a separation technique is used to enrich the gas to obtain the more energetic ortho-form. Conventionally, the separation of the hydrogen spin-isomers (as well as the isotopes) is achieved by differential adsorption on surfaces [6,7] or by sieving in nanoporous materials. [8,9] The fractionation of the spin-isomers is known to occur due to the difference in their adsorption heats arising mostly from hindrance of the rotational motion of adsorbed or confined molecules [6,7, 10] as a result of the anisotropic interactions between the molecule and the adsorbent surface. These interactions are very complicated and are rarely known wellenough to allow for quantitative simulations of adsorption equilibria. Herein we suggest that similar separation of the spin-isomers may occur in the gas phase through the formation of van der Waals (vdW) complexes so the analysis of vdW complexes involving H 2 may elucidate the mechanisms of ortho--para separations on surfaces.The total nuclear spin of para-hydrogen, pH 2 , is zero, while the total nuclear spin of ortho-hydrogen, oH 2 , is one. Ortho-hydrogen may therefore exist in three nuclear spin states with different spin projections, whereas only one state is allowed for pH 2 . The corresponding statistical weights of the nuclear spin levels are g o = 3 and g p = 1 for oH 2 and pH 2 , respectively. The rotational angular momentum j of H 2 can be either even or odd depending on the parity of the total nuclear spin. The zero-point energy of oH 2 (j = 1) is therefore larger than that of pH 2 (j = 0) by 118.7 cm À1 (171 K). The natural ortho-para spin conversion is a very slow process and, despite the energetic difference, the relative abundance of the ortho-and para-isomers in natural gas remains close to the ratio of the statistical weights g o : g p = 3:1.[1]The gas of H 2 near an adsorbent surface is described by the equilibrium relations shown in Equations (1) and (2):and the enrichment of the gas phase by the ortho-isomer is described in Equation (3) in which the separation coefficient [7] is equal to the ratio of the equilibrium constants from Equations (1) and (2)where O and P denote the absolute partition functions Q(T) of ortho-and para-isomers in the gas phase and Õ and P are the same quantities for the adsorbed species. In a mixture of H 2 with a gas of inert ...

Predissociation spectra of the 35Cl−(H2) complex and its isotopologue 35Cl−(D2)

“…as it was first proposed by Wild et al 1 and subsequently investigated in more detail by Buchachenko et al 3 and by Grinev et al 17 Using the dissociation energies calculated in this work, the reactions ( 14) and ( 15) are found to be exoergic by 109.8 cm À1 Fig. 4 Experimental (top) and theoretical (bottom) predissociation spectra of Cl À (H 2 ) at 8 K. Since all transition occur from the vibrational ground state, the bands have been labeled using the final state vibrational quantum numbers (v 1 ,v 2 ,l 2 ).…”

Rotationally resolved infrared spectrum of the Na+-D2 complex: An experimental and theoretical study

“…Additionally, the slightly greater stability of complexes containing j = odd H 2 /D 2 means that they become enriched in the ion source through ligand switching reactions that are slightly more exothermic for H 2 (46 cm −1 ) than for D 2 (26 cm −1 ). 36,37…”

Ab Initio Characterization of the Electrostatic Complexes Formed by H2 Molecule and Cr+, Mn+, Cu+, and Zn+ Cations

“…This leads to further enrichment of the complexes containing oH 2 and pD 2 in ligand-exchange reactions. 46 Rotationally resolved IR photodissociation spectra allow one to distinguish the complexes containing the two spin-isomers. However, cation, anion, and neutral complexes containing pH 2 are seldom observed due to their low abundance.…”