Insight into the Binding of Argon to Cyclic Water Clusters from Symmetry-Adapted Perturbation Theory

This work systematically examines the interactions between a single argon atom and the edges and faces of cyclic H2O clusters containing three-five water molecules (Ar(H2O)n=3-5). Full geometry optimizations and subsequent harmonic vibrational frequency computations were performed using MP2 with a triple-ζ correlation consistent basis set augmented with diffuse functions on the heavy atoms (cc-pVTZ for H and aug-cc-pVTZ for O and Ar; denoted as haTZ). Optimized structures and harmonic vibrational frequencies were also obtained with the two-body-many-body (2b:Mb) and three-body-many-body (3b:Mb) techniques; here, high-level CCSD(T) computations capture up through the two-body or three-body contributions from the many-body expansion, respectively, while less demanding MP2 computations recover all higher-order contributions. Five unique stationary points have been identified in which Ar binds to the cyclic water trimer, along with four for (H2O)4 and three for (H2O)5. To the best of our knowledge, eleven of these twelve structures have been characterized here for the first time. Ar consistently binds more strongly to the faces than the edges of the cyclic (H2O)n clusters, by as much as a factor of two. The 3b:Mb electronic energies computed with the haTZ basis set indicate that Ar binds to the faces of the water clusters by at least 3 kJ mol-1 and by nearly 6 kJ mol-1 for one Ar(H2O)5 complex. An analysis of the interaction energies for the different binding motifs based on symmetry-adapted perturbation theory (SAPT) indicates that dispersion interactions are primarily responsible for the observed trends. The binding of a single Ar atom to a face of these cyclic water clusters can induce perturbations to the harmonic vibrational frequencies on the order of 5 cm-1 for some hydrogen-bonded OH stretching frequencies.

Vibrational Spectroscopy of the Water Dimer at Jet-Cooled and Atmospheric Temperatures

The vibrational spectroscopy of the water dimer provides an understanding of basic hydrogen bonding in water clusters, and with about one water dimer for every 1,000 water molecules, it plays a critical role in atmospheric science. Here, we review how the experimental and theoretical progress of the past decades has improved our understanding of water dimer vibrational spectroscopy under both cold and warm conditions. We focus on the intramolecular OH-stretching transitions of the donor unit, because these are the ones mostly affected by dimer formation and because their assignment has proven a challenge. We review cold experimental results from early matrix isolation to recent mass-selected jet expansion techniques and, in parallel, the improvements in the theoretical anharmonic models. We discuss and illustrate changes in the vibrational spectra of complexes upon increasing temperature, and the difficulties in recording and calculating these spectra. In the atmosphere, water dimer spectra at ambient temperature are crucial. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A Rather Universal Vibrational Resonance in 1:1 Hydrates of Carbonyl Compounds

When the lower frequency OH stretching fundamental of a water molecule is shifted to the 3500 cm^-1 spectral range by the solvation of a carbonyl compound, in this case a ketone, its infrared intensity is shared with a dark state. It is shown by chemical and isotope substitution for more than a dozen systems that the location of this resonance is remarkably substitution-independent. Harmonic and anharmonic model calculations support its assignment to a combination of the water bending overtone and in-plane water libration. This previously unrecognized intramolecular-intermolecular coupling in single solvent water has a strength of 7-10 cm^-1. It may have been sporadically observed before in a few other carbonyl compounds such as amides, without any previous exploration of its potential universality. The resulting generic picosecond energy redistribution channel for aqueous solutions may represent a slow counterpart and doorway model of what happens on a subpicosecond time scale when the hydrogen bonds become stronger, such as in carboxylic acid dimers or protonated water clusters.

How many water molecules are needed to solvate one?

Many efforts undertaken to study the solvation process have led to general theories that may describe mean properties, but are unable to provide a detailed understanding at the molecular level. Remarkably, the basic question of how many solvent molecules are necessary to solvate one solute molecule is still open. By exploring several water aggregates of increasing complexity, in this contribution we employ semiclassical spectroscopy to determine on quantum dynamical grounds the minimal network of surrounding water molecules to make the central one display the same vibrational features of liquid water. We find out that double-acceptor double-donor tetrahedral coordination constituting the standard picture is necessary but not sufficient, and that particular care must be reserved for the quantum description of the combination band due to the coupling of the central monomer bending mode with network librations. It is actually our ability to investigate the combination band with a quantum-derived approach that allows us to answer the titular question. The minimal structure eventually responsible for proper solvation is made of a total of 21 water molecules and includes two complete solvation shells, of which the whole first one is tetrahedrally coordinated to the central molecule.

How quantum spectroscopic simulations can explain water solvation by comparison with experimental spectra.

Observation of the Low-Frequency Spectrum of the Water Trimer as a Sensitive Test of the Water-Trimer Potential and the Dipole-Moment Surface

Intermolecular interactions in bulk water are dominated by pairwise and non‐pairwise cooperative interactions. While accurate descriptions of the pairwise interactions are available and can be tested by precise low‐frequency spectra of the water dimer up to 550 cm⁻¹, the same does not hold for the three‐body interactions. Here, we report the first comprehensive spectrum of the water trimer in the frequency region from 70 to 620 cm⁻¹ using helium‐nanodroplet isolation and free‐electron lasers. By comparison to accompanying high‐level quantum calculations, the experimentally observed intermolecular bands can be assigned. The transition frequencies of the degenerate translation, the degenerate in‐plane and the non‐degenerate out‐of‐plane libration, as well as additional bands of the out‐of‐plane librational mode are reported for the first time. These provide a benchmark for state‐of‐the‐art water potentials and dipole‐moment surfaces, especially with respect to three‐body interactions.

Rotational energy relaxation quantum dynamics of a diatomic molecule in a superfluid helium nanodroplet and study of the hydrogen isotopes case

The rotational energy relaxation (RER) of a molecule X2(j,mj) in a 4He superfluid nanodroplet [HeND or (4He)N; T = 0.37 K] has been investigated using a hybrid quantum dynamics approach recently proposed by us. As far as we know, this is the first theoretical study about rotational relaxation inside HeNDs, and here several (real and hypothetical) isotopes of H2 have been examined, in order to analyze the influence of the rotational constant Be of these fast rotors on the dynamics. The structure of the nanodroplet practically does not change during the RER process, which approximately takes place according to a cascade mechanism j → j - 2; j - 2 → j - 4; …; 2 → 0, and mj is conserved. The results are consistent with the very scarce estimated experimental data available. The lifetime of an excited rotational state (≈1.0-7.6 ns) increases when: (a) Be increases; (b) j increases; and (c) N decreases (above N = 100 there is a small influence of N on the lifetime). This also applies to the global relaxation time and transition time. The analysis of the influence of the coupling between the j and j - 2 rotational states (due to the X2-helium interaction) and the X2 angular velocity on the lifetime and related properties has been helpful to better understand the dynamics. In contrast to the RER results, for the vibrational energy relaxation (VER) in HeNDs, when the quantum number v increases a decrease is observed in the lifetime of the excited vibrational state. This difference can be interpreted taking into account that RER and VER are associated with very different types of motion. Besides, in VER the intermediate excited states show metastability, differing from the RER case. We hope that the present study will encourage more studies to be developed on the RER dynamics in HeNDs, a basic, interesting and difficult to study physical phenomenon about which we still know very little.

Observation of the Low-Frequency Spectrum of the Water Dimer as a Sensitive Test of the Water Dimer Potential and Dipole Moment Surfaces

Using the helium nanodroplet isolation setup at the ultrabright free-electron laser source FELIX in Nijmegen (BoHeNDI@FELIX), the intermolecular modes of water dimer in the frequency region from 70 to 550 cm^-1 were recorded. Observed bands were assigned to donor torsion, acceptor wag, acceptor twist, intermolecular stretch, donor torsion overtone, and in-plane and out-of-plane librational modes. This experimental data set provides a sensitive test for state-of-the-art water potentials and dipole moment surfaces. Theoretical calculations of the IR spectrum are presented using high-level quantum and approximate quasiclassical molecular dynamics approaches. These calculations use the full-dimensional ab initio WHHB potential and dipole moment surfaces. Based on the experimental data, a considerable increase of the acceptor switch and a bifurcation tunneling splitting in the librational mode is deduced, which is a consequence of the effective decrease in the tunneling barrier.

Acid solvation versus dissociation at "stardust conditions": Reaction sequence matters

Chemical reactions at ultralow temperatures are of fundamental importance to primordial molecular evolution as it occurs on icy mantles of dust nanoparticles or on ultracold water clusters in dense interstellar clouds. As we show, studying reactions in a stepwise manner in ultracold helium nanodroplets by mass-selective infrared (IR) spectroscopy provides an avenue to mimic these "stardust conditions" in the laboratory. In our joint experimental/theoretical study, in which we successively add H₂O molecules to HCl, we disclose a unique IR fingerprint at 1337 cm^-1 that heralds hydronium (H₃O⁺) formation and, thus, acid dissociation generating solvated protons. In stark contrast, no reaction is observed when reversing the sequence by allowing HCl to interact with preformed small embryonic ice-like clusters. Our ab initio simulations demonstrate that not only reaction stoichiometry but also the reaction sequence needs to be explicitly considered to rationalize ultracold chemistry.

A combined experimental and theoretical study on the terahertz vibrations of water vapors

Water vapor is of significance in global greenhouse effect. Its strong absorption at infrared region characterizes the fingerprint of water structure. We hereby performed THz time-domain spectroscopy (0.2-2 THz) study on water vapors at relative humidity from 10% to 50%, in which the intensities of vibrations were found not proportionally increased with relative humidity. Besides rotation of water monomer, the absorption at THz frequency was probably attributed to small water clusters (N < 9) based on quantum mechanics calculations, where the distinct absorption peaks were matched with the vibrations of clusters. Moreover, planar structures were found to prevail in clusters less than eight waters owing to increasingly strong hydrogen bonds while cube structure dominated in eight-water cluster. This study thus promotes our understanding of not only absorptions of water vapor at far-infrared region but also their structures.

Quantum nature of the hydrogen bond from ambient conditions down to ultra-low temperatures

Quantum simulations reveal strong temperature effects for weak hydrogen bonds and differences in quantum delocalization between various hydrogen-bonded systems.

A helium nanodroplet setup for mid and far-infrared spectroscopy using pulsed-free-electron lasers: vibrational spectra of propargyl alcohol

Helium nanodroplets spectroscopy in combination with free electron laser. The combination is suitable for studying molecular clusters in 66–3600 cm⁻¹ spectral range, covering the important low-frequency large amplitude intra and inter-molecular motions.