A quantum-chemical study of the possibility to synthesize nuclear spin isomers of water by homogeneous catalytic hydrogenation of compounds containing semipolar X → O bonds (X = N, P, S) with hydrogen

Quantum chemical simulation in the framework of the density functional theory (PBE functional, TZV2p basis set for valence electrons and SBK JC pseudopotentials for core electrons) of the mechanism of catalytic hydrogenation of N 2 O on palladium cis dihydride (Me 2 PCH 2 CH 2 PMe 2 )PdH 2 by para hydrogen predicts that the process proceeds with reten tion of proton spin correlation inherent in para hydrogen and is characterized by a relatively low activation energy (17.7 kcal mol -1 ) in the key step. This offers the principal opportunity for the chemical synthesis of para water, the nuclear spin isomer of water with antiparallel proton spins.Key words: spin isomers, para hydrogen, ortho and para water, homogeneous catalysis, catalytic hydrogenation, palladium complexes, density functional theory, quantum chemical calculations, quantum chemical simulation of reaction mechanism. Similar to two nuclear spin isomers of hydrogen (ortho Н 2 and para Н 2 ), water exists as two isomers that differ in total nuclear spin moment of protons I: ortho Н 2 О (I = 1) and para Н 2 О (I = 0). Owing to spin rotation interaction these isomers have different rovib rational spectra and total energies. They were detected in vapor at low concentrations and in cryogenic matrices. 1-3 For the spin isomers of water, the energy difference, ΔЕ o p , between two lowest rotational states with different symmetry is much smaller than for the spin isomers of hydrogen (23.8 vs. 118.7 cm -1 , respectively). 4At T > 50 K, the equilibrium concentration ratio of the spin isomers of water is 3 : 1, but at T < 50 К, it becomes strongly temperature dependent. Based on this temperature dependence, water samples with a non equilibrium spin isomer ratio were prepared by trapping water in cryogenic neon, argon, and krypton matrices at 30 К and lower temperatures. 5,6 By measuring the spin isomer ratio using the Fourier IR spectra, it was shown 5,6 that at 6 К (in a neon matrix; water : neon molar ratio = 1 : 1000) a typical conversion time is 70 min, but it is abruptly reduced to 35 min upon double doping the sample with paramagnetic O 2 and as the temperature increases to 30 К.The role of water is central to most natural processes and therefore the separation of its nuclear spin isomers or even preparation of mixtures highly enriched with one of them at ambient temperatures could offer attractive pros pects for solving basic and applied problems in various fields (medicine, biology and chemistry, geosciences, meteorology, etc.). However, spin nonselective proton exchange between water molecules in liquids results in a fast interconversion of the nuclear spin isomers of water (the lifetime is shorter than 1 ms at room temperature 7 ), which makes the separation and investigation of the prop erties of ortho and para water a very complicated task.Publications 8,9 concerning the preparation of bulk amounts of liquid water with a nonequilibrium spin iso mer ratio at ambient temperatures using GC separation on alumina or activated charcoal have attracted great ...

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“…19,20 This gave an impetus to a computer based search for more active catalytic systems for this reaction.…”

mentioning

confidence: 99%

“…19, 20 In the present work, we report the results of calculations of the mechanism and kinetic parameters of N 2 O hydrogenation on a model palladium cis dihydride (Me 2 PCH 2 CH 2 PMe 2 )PdH 2 , which makes it possible to carry out the reaction at a lower activation energy.…”

mentioning

confidence: 99%