Water Clusters in an Argon Matrix: Infrared Spectra from Molecular Dynamics Simulations with a Self-Consistent Charge Density Functional-Based Tight Binding/Force-Field Potential

Abstract

The present theoretical study aims at investigating the effects of an argon matrix on the structures, energetics, dynamics, and infrared (IR) spectra of small water clusters (H2O)n (n = 1-6). The potential energy surface is obtained from a hybrid self-consistent charge density functional-based tight binding/force-field approach (SCC-DFTB/FF) in which the water clusters are treated at the SCC-DFTB level and the matrix is modeled at the FF level by a cluster consisting of ∼340 Ar atoms with a face centered cubic (fcc) structure, namely (H2O)n/Ar. With respect to a pure FF scheme, this allows a quantum description of the molecular system embedded in the matrix, along with all-atom geometry optimization and molecular dynamics (MD) simulations of the (H2O)n/Ar system. Finite-temperature IR spectra are derived from the MD simulations. The SCC-DFTB/FF scheme is first benchmarked on (H2O)Arn clusters against correlated wave function results and DFT calculations performed in the present work, and against FF data available in the literature. Regarding (H2O)n/Ar systems, the geometries of the water clusters are found to adapt to the fcc environment, possibly leading to intermolecular distortion and matrix perturbation. Several energetical quantities are estimated to characterize the water clusters in the matrix. In the particular case of the water hexamer, substitution and insertion energies for the prism, bag, and cage are found to be lower than that for the 6-member ring isomer. Finite-temperature MD simulations show that the water monomer has a quasifree rotation motion at 13 K, in agreement with experimental data. In the case of the water dimer, the only large-amplitude motion is a distortion-rotation intermolecular motion, whereas only vibration motions around the nuclei equilibrium positions are observed for clusters with larger sizes. Regarding the IR spectra, we find that the matrix environment leads to redshifts of the stretching modes and almost no shift of the bending modes. This is in agreement with experimental data. Furthermore, in the case of the water monomer and dimer, the magnitudes of the computed shifts are in fair agreement with the experimental values. The complex case of the water hexamer, which presents several low-energy isomers, is discussed.

Density-functional tight-binding: basic concepts and applications to molecules and clusters

“…Using the same approach, it was found that the influence of the coordination of water clusters on PAHs led to a modification of the anharmonicity of the γ CH mode, and that this could be a fingerprint of the edge-coordination of the water cluster on the PAH [283]. In the case of a water molecule (described at the DFTB level) surrounded by a rare gas matrix (described with a force field FF), it was shown from MD// DFTB/FF simulations that at low temperature (,10 K), the water molecule rotates inside the matrix (in agreement with experimental results at low concentration of water), and that leads to red shifts and broadening of the water stretching modes [335] (for a review, see ref [326]).…”

“…Structural and energetic changes were reported when potassium clusters up to 20 atoms adsorb on a potassium surface K (110) or K (100) [333], the interaction energy being found to dominate the structural reorganization one. MgO supported Au islands were also studied [334,335]. In these islands, the inner atoms were found to remain neutral while the perimeter ones were found to be negatively charged.…”

“…Finally, structural properties and IR spectroscopy of carbonaceous molecules, water molecules and complexes embedded in cryogenic argon matrix were investigated via the DFTB-MM model described in section 3.5 [70,131,335]. The structuration of a water dimer/coronene complex within the argon matrix is illustrated in Figure 9.…”

“…Fine effects such as the modification of the energetic order of the (H 2 O) 6 isomers with respect to the gas phase were shown. Besides, MD simulations using the DFTB-MM model allowed to show the influence of (even low) temperature (10 K) on the IR spectrum of a single water molecule embedded in the Ar matrix: red shifts and broadening experimentally observed with respect to the gas phase could be interpreted [335].…”

“…Vibrational spectra at the DFTB2 level were benchmarked on small molecules Figure 9. Structure of a water dimer interacting with coronene within an argon rare gas matrix subpiece treated via a DFTB-MM scheme [335].…”