Observation of nuclear spin species conversion inside the 1593 cm−1 structure of H2O trapped in argon matrices: Nitrogen impurities and the H2O:N2 complex

We have investigated, at high resolution (0.03 cm−1), the 1593 cm−1 structure observed in the IR absorption spectrum of water trapped in solid argon doped with nitrogen. It exhibits a doublet at 1592.59 ± 0.05 and 1593.08 ± 0.05 cm−1 and a line centered at 1592.93 ± 0.05 cm−1. The central component, which increases irreversibly upon annealing and when the concentration is increased, is due to the proton acceptor submolecule of the H2O dimer, as mentioned in the literature. The doublet is assigned to the H2O:N2 complex. After a fast cooling of the sample from 20 to 4 K, the low frequency line of the doublet decreases with time and the high frequency one increases, the total integrated absorption increasing slightly. The ratio of the integrated intensities between the low frequency component and the high frequency one reaches a constant limit of 0.5 ± 0.1 at infinite time. This time behavior, perfectly exponential with a time constant τ of about 680 min, is reproducible. As the nitrogen molecule cannot rotate in an argon substitutional site, and as the H2O submolecule seems to preserve somewhat its identity, this is interpreted as nuclear spin species conversion between ortho and para states of the H2O submolecule within the complex. The order of magnitude of the energy difference between the ortho and para lowest levels, about 5 cm−1, is too weak to imply any, even very hindered, rotational motion of H2O, but it could be the energy range of a tunneling effect. When the temperature is increased, the two components coalesce at 25 K into a single symmetrical line pointing at 1593.3 cm−1 and the conversion time shortens dramatically. An Arrhenius plot leads to a weak activation energy of the conversion process (about 30 cm−1). A possible geometry of the complex in solid argon, different from the gas phase one, is proposed.