Disentangling Coupling Effects in the Infrared Spectra of Liquid Water

Abstract

Effects in the Infrared Spectra of Liquid Water. ChemRxiv. Preprint.A quantitative characterization of intermolecular and intramolecular couplings that modulate the OH-stretch vibrational band in liquid water has so far remained elusive. Here, we take up this challenge by combining the centroid molecular dynamics (CMD) formalism, which accounts for nuclear quantum effects, with the MB-pol potential energy function, which accurately reproduces the properties of water across all phases, to model the infrared (IR) spectra of various isotopic water solutions with different levels of vibrational couplings, including those that cannot be probed experimentally. Analysis of the different IR OH-stretch lineshapes provides direct evidence for the partially quantum-mechanical nature of hydrogen bonds in liquid water, which is emphasized by synergistic effects associated with intermolecular coupling and many-body electrostatic interactions. Furthermore, we quantitatively demonstrate that intramolecular coupling, which results in Fermi resonances due to the mixing between HOH-bend overtones and OH-stretch fundamentals, are responsible for the shoulder located at ∼3250 cm -1 of the IR OH-stretch band of liquid water. File list (2) download file view on ChemRxiv manuscript.pdf (1.43 MiB) download file view on ChemRxiv supporting_info.pdf (0.93 MiB)

First-principles study of the infrared spectrum in liquid water from a systematically improved description of H-bond network

“…Moreover, a shoulder around ∼2300 cm −1 on the lefthand side of the peak is identified in the stretching band in experimental spectra of water. According to recent theoretical analyses 41,42 , this shoulder feature is attributed to the so-called Fermi resonance which describes the spectral enhancement due to the intramolecular vibrational coupling between the OD stretching mode and the overtone of bending mode. At both PBE and SCAN levels of studies, the shoulder appears roughly at the same position around 2300 cm −1 .…”

“…On the other hand, one may expect that the hybrid functional SCAN0 mitigates the self-interaction error in DFT, and brings the calculated IR spectra closer to the experiments. Furthermore, recent studies 42,46,47 suggested that nuclear quantum ef-fects due to light protons play a crucial role in obtaining accurate liquid water properties. The role played by quantum nuclei awaits the future investigation which is likely to broaden the spectral features and slightly reduce the frequencies of stretching band due to the delocalized protons.…”

Which quantum statistics–classical dynamics method is best for water?

“…imaginary-time Feynman paths) [33][34][35] to describe the quantum Boltzmann statistics, and obtains classical trajectories from a Liouvillian that conserves the ring-polymer phase-space density. These 'ring-polymer methods' include centroid molecular dynamics (CMD) [20][21][22][23][24][25] and (thermostatted) ring-polymer molecular dynamics ((T)RPMD). 18,[26][27][28][29][30][31][32] Unlike LSC-IVR, the ring-polymer methods conserve the quantum Boltzmann distribution by construction.…”

Active Learning of Many-Body Configuration Space: Application to the Cs+–water MB-nrg Potential Energy Function as a Case Study

“…Within the MB-nrg framework, the water-water interactions are described by the MB-pol PEF, [9][10][11] which has been shown to correctly reproduce the properties of water 34,35 from small clusters in the gas phase, [36][37][38][39][40][41][42][43][44][45][46][47][48] to bulk water, [49][50][51][52] the air/water interface, [53][54][55][56] and ice. [57][58][59] The interactions between Cs + ions and water molecules are described through the MBE of Eq.…”