Probing Confinement Effects on the Infrared Spectra of Water with Deep Potential Molecular Dynamics Simulations

MC Andrade and TA Pham, JOURNAL OF PHYSICAL CHEMISTRY LETTERS, 14, 5560-5566 (2023).

DOI: 10.1021/acs.jpclett.3c01054

The hydrogen-bond network of confined water is expectedto deviatefrom that of the bulk liquid, yet probing these deviations remainsa significant challenge. In this work, we combine large-scale moleculardynamics simulations with machine learning potential derived fromfirst-principles calculations to examine the hydrogen bonding of waterconfined in carbon nanotubes (CNTs). We computed and compared theinfrared spectrum (IR) of confined water to existing experiments toelucidate confinement effects. For CNTs with diameters >1.2 nm,wefind that confinement imposes a monotonic effect on the hydrogen-bondnetwork and on the IR spectrum of water. In contrast, confinementbelow 1.2 nm CNT diameter affects the water structure in a complexfashion, leading to a strong directional dependence of hydrogen bondingthat varies nonlinearly with the CNT diameter. When integrated withexisting IR measurements, our simulations provide a new interpretationfor the IR spectrum of water confined in CNTs, pointing to previouslyunreported aspects of hydrogen bonding in this system. This work alsooffers a general platform for simulating water in CNTs with quantumaccuracy on time and length scales beyond the reach of conventionalfirst-principles approaches.

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