Separating Para and Ortho Water

Abstract

Water exists as two nuclear-spin isomers, para and ortho, determined by the overall spin of its two hydrogen nuclei. For isolated water molecules the conversion between these isomers is forbidden and they act as different molecular species. Yet, these species are not readily separated and no pure para sample has been produced. Accordingly, little is known about their specific physical and chemical properties, conversion mechanisms, or interactions. Here, we demonstrate the production of isolated samples of both spin isomers in pure beams of para and ortho water in their respective absolute ground state. These single-quantum-state samples are ideal targets for unraveling spin-conversion mechanisms, for precision spectroscopy and fundamentalsymmetry-breaking studies, and for spin-enhanced applications, e. g., laboratory astrophysics and -chemistry or hypersensitized NMR experiments.Significant efforts have been undertaken to separate and study the nuclear-spin isomers of water, motivated by their importance in a wide variety of scientific disciplines. This ranges from the astronomical importance of the ortho-para ratio [1][2][3][4][5] , to studies of nuclear-spin conversion [6,7] , selection rules and reactive collisions [8][9][10] or symmetry breaking [11] . Spin-enriched samples furthermore would allow for hypersensitized NMR experiments via polarization transfer reactions [12][13][14] . However, unlike other small polyatomic molecules exhibiting spin isomerism, such as fluoromethane or ethylene [15,16] , which were spin-isomerically enriched using the light induced drift technique [7] , this has not been achieved for water. Separation through selective adsorption on surfaces was reported [17] , but these results remain controversial and could not be reproduced [18][19][20][21] . Thus, studies of spin-conversion dynamics in water have been limited to water embedded in rare gas matrices, with relative spin populations modified by the sample temperature [22] . A recent study investigated nuclear-spin conversion in the gas-phase and found no spin conversion for water monomers [23] .Recently, the production of a single spin isomer of water in a magnetic-hexapole-focuser setup was demonstrated [24] . One of the magnetically active spin projections (m = +1) of ground-state ortho water was magnetically focused into the interaction volume, while all other spin-projection states were defocused or diverged unaffected by the field. The purity of the produced ortho beam was later evaluated as 93 %, with simulations suggesting an upper limit for the achievable purity of 97 % [24,25] .Here, we experimentally demonstrate the production of pure samples of both, para and ortho water, the latter further separated into its M = 0 and M = 1 angular momentum projections, in the gas-phase. The produced single-quantum-states are ideally suited for further experiments on nuclear-spin conversion under collision-free conditions, nuclear-spin-dependent reactivity [8] , trapping of single spin-isomer samples in electromagnetic traps [26] ...

Quantum‐rotor‐induced polarization

“…The electrical polarizability of a water molecule is obtained as the second derivative of its energy with respect to the applied field and depends on the rotational state. It is therefore possible to seperate ortho and para-water using electric field gradients, [52] and it is also possible to monitor the spin conversion by measuring the dielectric constant of the material. [53] This is a convenient method in practice as capacitive detection is simple and rather sensitive.…”

Collisional cross-section of water molecules in vapour studied by means of 1H relaxation in NMR

“…In supercritical water171819, spin-rotation significantly contributes to NMR relaxation despite the high density. In the context of our attempts to prepare para -water20212223242526 and related spin states in other molecules272829303132, we have measured longitudinal relaxation rates R 1  = 1/ T 1 of gaseous H 2 O and HDO at different temperatures and pressures. To the best of our knowledge, this is the first time that such observations are reported.…”

Spatial separation of 2-propanol monomer and its ionization-fragmentation pathways

“…Such spatial separation of molecular conformers was previously achieved using inhomogeneous electric fields [29,30]. Different species of complex molecules can be spatially separated within a cold molecular beam by the electrostatic deflector [29], which was demonstrated in a number of pioneering experiments on the separation of individual quantum states [31][32][33], structural isomers [34,35], or specific cluster sizes [36][37][38][39][40]. Here, the 2-propanol monomer is spatially separated from the original molecular beam using the deflector and the purified samples are exploited in femtosecond-laser ionization studies.…”