2D IR spectroscopy of high-pressure phases of ice

Abstract

We present experimental and simulated 2D IR spectra of some high-pressure forms of isotope-pure D 2 O ice and compare the results to those of ice Ih published previously [F. Perakis and P. Hamm, Phys. Chem. Chem. Phys. 14, 6250 (2012); L. Shi et al., ibid. 18, 3772 (2016)]. Ice II, ice V, and ice XIII have been chosen for this study, since this selection covers many aspects of the polymorphism of ice. That is, ice II is a hydrogen-ordered phase of ice, in contrast to ice Ih, while ice V and ice XIII are a hydrogen-disordered/ordered couple that shares essentially the same oxygen structure and hydrogenbonded network. For the transmission 2D IR spectroscopy, a novel method had to be developed for the preparation of ultrathin films (1-2 µm) of high-pressure ices with good optical quality. We also simulated 2D IR spectra based on molecular dynamics simulations connected to a vibrational exciton picture. These simulations agree with the experimental results in a semi-quantitative manner for ice II, while the same approach failed for ice V and ice XIII. From the perspective of 2D IR spectroscopy, ice II appears to be more inhomogeneously broadened than ice Ih, despite its hydrogen-order, which we attribute to the fact that ice II is structurally more complex with four distinguishable hydrogen bonds that mix due to exciton coupling. Ice V and ice XIII, on the other hand, behave as expected with the hydrogen-disordered case (ice V) being more inhomogenously broadened. Furthermore, in all hydrogen-ordered forms (ice II and ice XIII), cross peaks could be identified in the anisotropic 2D IR spectrum, whose signs reveal the relative direction of the corresponding excitonic states. Published by AIP Publishing. https://doi

Molecular-Level Interpretation of Vibrational Spectra of Ordered Ice Phases

“…Rapid cooling of ice III yields ice IX, as expected, though slower cooling results in ice II 22,85. However, ice II transforms only to the disordered form of ice XIII, ice V upon heating 55. Furthermore, both ice II and ice XIII display unique distortions to their local tetrahedral hydrogen bonded networks with the root mean squared deviation from the ideal tetrahedral bond angle of ice II and ice XIII being 16.8• 36 and 18.5 • ,35,86 respectively.…”

“…This delocalization and strong intermolecular coupling also suggests that a simple assignment of vibrational modes to individual spectral peaks in water and ice is not appropriate. 48,50,52,54,55 Recently, experimental and computational two dimensional IR (2DIR) spectroscopy studies have been used to disentangle some of the vibrational cross-couplings. [55][56][57][58] The extra frequency dimension of 2DIR spectra provides information on the couplings and dynamics of different modes.…”

“…aqueous systems. As Raman spectroscopy is generally experimentally better suited to probing the ordered ice phases due to requiring lower powered lasers and the higher degree of scattering created,55 Raman spectroscopy is a more common experimental technique thaninfrared spectroscopy. As in Ref.…”

“…8,55 It is the only ice with no known disordered counterpart. Ice IX is a metastable phase and the ordered form of ice III, with an oxygen lattice structure containing five-membered rings joined together by four-membered rings 85.…”

Vibrational echo spectroscopy of aqueous sodium bromide solutions from first principles simulations

“…All the calculations of spectral dynamics are done at the levels of both cumulant and Condon approximations. We note that while some of the existing studies have considered spectral diffusion and vibrational dynamics of local OH/OD modes, there have also been studies which incorporated the effects of resonant coupling amongst the vibrating OH/OD modes in pure H 2 O or D 2 O . Since each OD mode is treated as a local mode and also the effects of fast population relaxation have not been included in our calculations, the present study refers to vibrational spectral diffusion calculations of local OD stretch modes in aqueous NaBr solutions.…”