Nuclear spin conversion of water confined in solid parahydrogen

Ortho-para interconversion of nuclear states of H2O through replica transition state: prospect of quantum entanglement at homodromic Bjerrum defect site

CONTEXT

From a nuclear spin prospective, water exists as para and ortho nuclear spin isomers (isotopomers). Spin interconversions in isolated molecules of water are forbidden, but many recent reports have shown them to happen in bulk, through dynamic proton exchanges happening between interconnected networks of a large array of water molecules. In this contribution, a possible explanation for an unexpected slow or delayed interconversion of ortho-para water in ice observed in an earlier reported experiment is provided. Using the results of quantum mechanical investigations, we have discussed the roles played by Bjerrum defects in the dynamic proton exchanges and ortho-para spin state interconversions. We guess that at the sites of the Bjerrum defects, there are possibilities of quantum entanglements of states, through pairwise interactions. Based on the perfectly correlated exchange happening via a replica transition state, we speculate that it can have significant influences on ortho-para interconversions of water. We also conjecture that the overall ortho-para interconversion is not a continuous process, rather can be imagined to be happening serendipitously, but within the boundary of the rules of quantum mechanics.

METHODS

All computations were performed with Gaussian 09 program. B3LYP/6-31++G(d,p) methodology was used to compute all the stationary points. Further energy corrections were computed using CCSD(T)/aug-cc-pVTZ methodology. Intrinsic reaction coordinate (IRC) path computations were carried out for the transition states.

Infrared Spectroscopic Studies of Oxygen Atom Quantum Diffusion in Solid Parahydrogen

The thermally induced diffusion of atomic species in noble gas matrices was studied extensively in the 1990s to investigate low-temperature solid-state reactions and to synthesize reactive intermediates. In contrast, much less is known about the diffusion of atomic species in quantum solids such as solid parahydrogen (p-H₂). While hydrogen atoms were shown to diffuse in normal-hydrogen solids at 4.2 K as early as 1989, the diffusion of other atomic species in solid p-H₂ has not been reported in the literature. The in situ photogeneration of atomic oxygen, by ArF laser irradiation of an O₂-doped p-H₂ solid at 193 nm, is studied here to investigate the diffusion of O(³P) atoms in a quantum solid. The O(³P) atom mobility is detected by measuring the kinetics of the O(³P) + O₂ → O₃ reaction after photolysis via infrared spectroscopy of the O₃ reaction product. This reaction is barrierless and is thus assumed to be diffusion-controlled under these conditions such that the reaction rate constant can be used to estimate the oxygen atom diffusion coefficient. The O₃ growth curves are well fit by single exponential expressions allowing the pseudo-first-order rate constant for the O(³P) + O₂ → O₃ reaction to be extracted. The reaction rates are affected strongly by the p-H₂ crystal morphology and display a non-Arrhenius-type temperature dependence consistent with quantum diffusion of the O(³P) atom. The experimental results are compared to H(²S) atom reaction studies in p-H₂, analogous studies in noble gas matrices, and laboratory studies of atomic diffusion in astronomical ices and surfaces.

Formation of Para-H2O by Vacuum-UV Photolysis of O2 in Solid Hydrogen: Implication for Astrochemistry

The observation that the ortho to para ratio (OPR) of interstellar H₂O is smaller than 3 is an important yet unresolved subject in astronomy. We irradiated O₂ embedded in solid H₂ at 3 K with vacuum-ultraviolet (VUV) light and observed IR lines associated with para-H₂O (denoted as pH₂O) and nonrotating H₂O-(oH₂)_n (where oH₂ denotes ortho-H₂) but no lines associated with ortho-H₂O (denoted as oH₂O). After maintaining the matrix in darkness for ∼30 h, the amount of pH₂O decreased, accompanied by an increase in H₂O-(oH₂)_n via diffusion of oH₂. After that, the continuous nuclear-spin conversion from oH₂ to para-H₂ (denoted as pH₂) in solid H₂ over time resulted in the conversion of nonrotating H₂O-(oH₂)_n to rotating pH₂O in solid pH₂. The observation of the formation and conversion of pH₂O in our experiment suggests a plausible route in which VUV irradiation of O₂ and H₂ adsorbed on grain surfaces might be responsible for the smaller OPR of interstellar H₂O.

VUV photochemistry and nuclear spin conversion of water and water-orthohydrogen complexes in parahydrogen crystals at 4 K

Samples of H2O, HDO, and D2O were isolated in solid parahydrogen (pH2) matrices and irradiated by vacuum ultraviolet (VUV) radiation at 147 nm. Fourier-Transform Infrared (FTIR) spectra showed a clear depletion of D2O and an enrichment of both HDO and H2O by 147 nm irradiation. These irradiation-dependent changes are attributed to the production of OH and/or OD radicals through photodissociations of H2O, HDO, and D2O. The radicals subsequently react with the hydrogen matrix, leading to the observed enrichment of H2O. No trace of isolated OH or OD was detected in the FTIR spectra, indicating that the OH/OD radicals react with the surrounding matrix hydrogen molecules via quantum tunneling within our experimental timescale. The observed temporal changes in concentrations, especially the increase of HDO concentration during VUV irradiation, can be interpreted by a model with a rapid conversion from orthohydrogen (oH2) to pH2 in water-oH2 complexes upon VUV photodissociation, indicating either the acceleration of the nuclear spin conversion (NSC) of H2 due to the magnetic moment of the intermediate OH/OD radical, or the preferential reaction of the OH/OD radical with a nearby oH2 molecule over other pH2 molecules. We have also identified and quantified an anomalously slow NSC of H2O and D2O complexed with oH2 in solid pH2.

Hydrogen atom quantum diffusion in solid parahydrogen: The H + N2O → cis-HNNO → trans-HNNO reaction

The diffusion and reactivity of hydrogen atoms in solid parahydrogen at temperatures between 1.5 K and 4.3 K are investigated by high-resolution infrared spectroscopy. Hydrogen atoms are produced within solid parahydrogen as the by-products of the 193 nm in situ photolysis of N₂O, which induces a two-step tunneling reaction, H + N₂O → cis-HNNO → trans-HNNO. The second-order rate constant for the first step to form cis-HNNO is found to be inversely proportional to the N₂O concentration after photolysis, indicating that the hydrogen atoms move through solid parahydrogen via quantum diffusion. This reaction only readily occurs at temperatures below 2.8 K, not due to an increased rate constant for the first reaction step at low temperatures but rather due to an increased selectivity to the reaction. The rate constant for the second step of the reaction mechanism involving unimolecular isomerization is shown to be independent of the N₂O concentration as expected. The inverse concentration dependence of the rate constant for the reaction step that involves the hydrogen atom demonstrates clearly that quantum diffusion influences the reactivity of the hydrogen atoms in solid parahydrogen, which does not have an analogy in classical reaction kinetics.