Confinement Effects on the Nuclear Spin Isomer Conversion of H2O

Fast ortho-to-para conversion of molecular hydrogen in chemisorption and matrix-isolation systems

Molecular hydrogen has two nuclear-spin modifications called ortho and para. Because of the symmetry restriction with respect to permutation of the two protons, the ortho and para isomers take only odd and even values of the rotational quantum number, respectively. The ortho-to-para conversion is promoted in condensed systems, to which the excess rotational energy and spin angular momentum are transferred. We review recent studies on fast ortho-to-para conversion of hydrogen in molecular chemisorption and matrix isolation systems, discussing the conversion mechanism as well as rotational-relaxation pathways.

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.

Matrix Isolation Spectroscopy and Nuclear Spin Conversion of Propyne Suspended in Solid Parahydrogen

Parahydrogen (pH₂) quantum solids are excellent matrix isolation hosts for studying the rovibrational dynamics and nuclear spin conversion (NSC) kinetics of molecules containing indistinguishable nuclei with nonzero spin. The relatively slow NSC kinetics of propyne (CH₃CCH) isolated in solid pH₂ is employed as a tool to assign the rovibrational spectrum of propyne in the 600-7000 cm^-1 region. Detailed analyses of a variety of parallel (ΔK = 0) and perpendicular (ΔK=±1) bands of propyne indicate that the end-over-end rotation of propyne is quenched, but K rotation of the methyl group around the C₃ symmetry axis still persists. However, this single-axis K rotation is significantly hindered for propyne trapped in solid pH₂ such that the energies of the K rotational states do not obey simple energy-level expressions. The NSC kinetics of propyne follows first-order reversible kinetics with a 287(7) min effective time constant at 1.7 K. Intensity-intensity correlation plots are used to determine the relative line strengths of individual ortho- and para-propyne rovibrational transitions, enabling an independent estimation of the ground vibrational state effective A″ constant of propyne.

Terahertz and mid-infrared spectroscopy of matrix-isolated clusters and matrix-sublimation ice of D2O

We have established an apparatus for terahertz and mid-infrared spectroscopy in an ultrahigh vacuum and have measured absorption spectra of D₂O clusters trapped in solid Ar. To assign terahertz absorption peaks due to the D₂O dimer, trimer, and tetramer, the dependence of the spectrum on the annealing temperature and D₂O dilution was analyzed. The assignment was also examined by ab initio calculations with the use of the "our own N-layered integrated molecular orbital and molecular mechanics" method, where the flexibility of surrounding Ar atoms was systematically incorporated. We identified all the intermolecular fundamentals of the dimer and those with significant intensities of the trimer and tetramer, whose structural symmetries were revealed to be broken down. After isolating the D₂O clusters in solid Ar, we sublimated only Ar atoms to leave behind matrix-sublimation ice, which was found to be amorphous- or crystal-like depending on the formation conditions: the dilution and sublimation temperature. The crystallinity of matrix-sublimation ice was determined by decomposing its terahertz spectrum into the spectra of amorphous and crystalline ices. Since the crystallinity got higher by raising the dilution and sublimation temperature, the diffusion of the D₂O monomer on the surface of sublimating solid Ar was found to be crucial to the crystallization of the sublimation ice.

Nuclear Spin Symmetry Conservation in 1H216O Investigated by Direct Absorption FTIR Spectroscopy of Water Vapor Cooled Down in Supersonic Expansion

We report the results of an experimental study related to the relaxation of the nuclear spin isomers of the water molecule in a supersonic expansion. Rovibrational lines of both ortho and para spin isomers were recorded in the spectral range of H₂O stretching vibrations at around 3700 cm^-1 using FTIR direct absorption. Water vapor seeded in argon, helium, or oxygen or in a mixture of oxygen and argon was expanded into vacuum through a slit nozzle. The water vapor partial pressure in the mixture varied over a wide range from 1.5 to 102.7 hPa, corresponding to a water molar fraction varying between 0.2 and 6.5%. Depending on expansion conditions, the effect of water vapor clustering was clearly seen in some of our measured spectra. The Boltzmann plot of the line intensities allowed the H₂O rotational temperatures in the isentropic core and in the lateral shear layer probed zones of the planar expansion to be determined. The study of the OPR, i.e., the ratio of the ortho to para absorption line intensities as a function of T_rot, did not reveal any signs of the OPR being relaxed to the sample temperature. In contrast, the OPR was always conserved according to the stagnation reservoir equilibrium temperature. The conservation of the OPR was found irrespective of whether water molecule clustering was pronounced or not. Also, no effect of the paramagnetic oxygen admixture enhancing OPR relaxation was observed.