Abstract
The nature and extent of hydrogen bonding in water has been scrutinized for decades, including how it manifests in optical properties. Here we report vacuum ultraviolet absorption spectra for the lowest-lying electronic state of subcritical and supercritical water. For subcritical water, the spectrum redshifts considerably with increasing temperature, demonstrating the gradual breakdown of the hydrogen-bond network. Tuning the density at 381 °C gives insight into the extent of hydrogen bonding in supercritical water. The known gas-phase spectrum, including its vibronic structure, is duplicated in the low-density limit. With increasing density, the spectrum blueshifts and the vibronic structure is quenched as the water monomer becomes electronically perturbed. Fits to the supercritical water spectra demonstrate consistency with dimer/trimer fractions calculated from the water virial equation of state and equilibrium constants. Using the known water dimer interaction potential, we estimate the critical distance between molecules (ca. 4.5 Å) needed to explain the vibronic structure quenching.
Highlights
The nature and extent of hydrogen bonding in water has been scrutinized for decades, including how it manifests in optical properties
This is true of high-temperature and supercritical water (SCW), which have increasingly been touted as alternative reaction media for green chemistry[1,2], and whose participation in fundamental geochemistry[3,4,5] and abiogenesis[6,7] are considered crucial
We have reported VUV absorption data on subcritical water and SCW to investigate the lowest-lying A~1B1 X~1A1 first continuum electronic transition
Summary
The nature and extent of hydrogen bonding in water has been scrutinized for decades, including how it manifests in optical properties. The density dependence of the VUV spectrum near and above the critical temperature was explored, and a slight blueshift of the absorption band was noted with decreasing density, likely due to a narrowing of the spectrum caused by changes in the local solvent environment These conclusions were drawn based on examination of the absorption edge, and the position of the absorption maximum was merely extrapolated based on fitting. In complementary X-ray absorption studies examining the same water electronic states, the pre-edge absorption (4a1’1s) of supercooled liquid water has been observed to redshift with increasing temperature[28] This has been interpreted as clear weakening of the H-bond network at higher temperatures[33]
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