Abstract
The CO2 molecule is weakly bound in water. Here we analyze the influence of a dissolved CO2 molecule on the structure and OH vibrational spectra of the surrounding water. From the analysis of ab initio molecular dynamics simulations (BLYP-D3) we present static (structure, coordination, H-bonding, tetrahedrality) and dynamical (OH vibrational spectra) properties of the water molecules as a function of distance from the solute. We find a weakly oscillatory variation (“ABBA”) in the ‘solution minus bulk water’ spectrum. The origin of these features can largely be traced back to solvent–solute hard-core interactions which lead to variations in density and tetrahedrality when moving from the solute’s vicinity out to the bulk region. The high-frequency peak in the solute-affected spectra is specifically analyzed and found to originate from both water OH groups that fulfill the geometric H-bond criteria, and from those that do not (dangling ones). Effectively, neither is hydrogen-bonded.
Highlights
CO2−water systems are among the most frequently encountered fluid mixtures in and around the Earth [Duan and Zhang, 2006],1 where they govern geological, environmental, and biological processes that affect many aspects of our daily lives
This is in strong contrast to the contribution from those H atoms within 2.5 Å from the solute that are hydrogen-bonded to other water molecules; their harmonic frequencies are severely downshifted with respect to the gas-phase value as is the case in bulk water
We have examined the perturbation that the almost-hydrophobic CO2 solute causes on the surrounding hydration shell, using ab initio MD (AIMD) simulations and a variety of analysis tools including power spectra for open systems
Summary
CO2−water systems are among the most frequently encountered fluid mixtures in and around the Earth [Duan and Zhang, 2006],1 where they govern geological, environmental, and biological processes that affect many aspects of our daily lives. The present study deals with a very dilute CO2−water system, with emphasis on the influence exerted by the solute on the surrounding water With this particular focus, several papers have already been published. We will show that by tiling these spectral slices together and subtracting the standard OH power spectrum of bulk water we obtain a difference spectrum that displays a shell-like variation of the OH frequency as a function of distance from the CO2 solute. The method has been developed and successfully applied in experimental vibrational spectroscopy It was first introduced as the “doubledifference method” by Lindgren et al.[21,22] to extract the OH spectrum originating from the first hydration shell around cations and anions in aqueous solution. In a collaborative experimental− computational effort,[18] this approach was turned into a tool to provide evidence for a phase transition in the water layer surrounding a CO2 molecule at ambient temperatures, that
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