A theoretical study of the photochemistry of 1,3-cyclopentadiene and its cyano derivatives bound to a water dimer: Assessing reactivity of ionized clusters and possible photoproducts

We theoretically investigate the photoionization scenarios of molecular complexes involving cyclopentadiene and cyanocyclopentadiene bound to water dimers. Using electronic structure calculations within density-functional theory (DFT) and time dependent DFT (TD-DFT), we explore the potential photochemical pathways following ionization, and determine the charge transfer excitations related to the possible subsequent reactions. Our findings suggest that the investigated photochemical pathways of the hydrated complexes take place in two well-defined ultraviolet regions: (i) 8.2-9.5 eV for the cyclic compounds and (ii) 11.2-11.4 eV for the bound water dimer. We quantify how H-bonding effects can influence the photoionization channels. Before forming possible photoproducts, we also examine the regiospecificity of OH addition to 1,3-cyclopentadiene and its cyano derivatives We analyze our results in light of photoionization studies of jet-cooled molecular complexes and possible implications in astrochemical environments.

Competition between Water-Water Hydrogen Bonds and Water-π Bonds in Pyrene-Water Cluster Anions

We present infrared spectra and density functional theory calculations of hydrated pyrene anion clusters with up to four water molecules. The experimental spectra were acquired by using infrared Ar messenger photodissociation spectroscopy. Water molecules form clusters on the surface of the pyrene, forming hydrogen bonds with the π-system. The structures of the water clusters and their interaction with the π-system are encoded in OH stretching vibrational modes. We find that the interactions between water molecules are stronger than the interactions between water molecules and the π-system. While all clusters show multiple conformers, three- and four-membered rings are the lowest energy structures in the larger hydrates.

Water-Hydrocarbon Interactions in Anionic Pyrene Monohydrate

Interactions between water and polycyclic aromatic hydrocarbons are essential in many aspects of chemistry, from interstellar and atmospheric processes to interfacial hydrophobicity and wetting phenomena. Despite their growing importance, the intermolecular potentials of the water-hydrocarbon interactions are underdeveloped compared to the water-water potentials, and there are similarly few experimental probes that are sensitive to the details of the water-hydrocarbon potential. We present a combined experimental and computational study of anionic pyrene monohydrate, one of the simplest water/hydrocarbon clusters. The action spectrum in the OH region of the mass-selected cluster ion provides a rigorous benchmark for intermolecular potentials and computational methodologies. We identify missing intermolecular interactions and shortcomings in conventional dynamics calculations by comparing experimental data to density functional theory and classical molecular dynamics calculations. Kinetic trapping is prevalent, even for one water molecule and one pyrene molecule, leading to slow equilibration in conventional molecular dynamics calculations, even on nanosecond time scales and at low temperatures (50 K). At constant energy, temperature fluctuations for the pair of molecules are substantial. Immersing the system in a bath of soft spheres and employing parallel tempering alleviates kinetic trapping and dampens temperature fluctuations, bringing the system closer to the thermodynamic limit. With such augmented sampling, a simple, flexible water model reproduces the line width and the asymmetric broadening of the symmetric OH stretching mode, which we assign to spectral diffusion. In the OH stretching region, dynamics calculations predict a more intense antisymmetric peak than experiments observe but do not predict the bimodal split symmetric peak that the experiments show. Our work suggests that electronic polarization, missing in the empirical force field, is responsible for the first discrepancy and that quantum nuclear effects, captured neither in density functional theory nor in classical dynamics, may be responsible for the second.

Wetting of a Hydrophobic Surface: Far-IR Action Spectroscopy and Dynamics of Microhydrated Naphthalene

The interaction of water and polycyclic aromatic hydrocarbons is of fundamental importance in areas as diverse as materials science and atmospheric and interstellar chemistry. The interplay between hydrogen bonding and dipole-π interactions results in subtle dynamics that are challenging to describe from first principles. Here, we employ far-IR action vibrational spectroscopy with the infrared free-electron laser FELIX to investigate naphthalene with one to three water molecules. We observe diffuse bands associated with intermolecular vibrational modes that serve as direct probes of the loose binding of water to the naphthalene surface. These signatures are poorly reproduced by static DFT or Møller-Plesset computations. Instead, a rationalization is achieved through Born-Oppenheimer Molecular Dynamics simulations, revealing the active mobility of water over the surface, even at low temperatures. Therefore, our work provides direct insights into the wetting interactions associated with shallow potential energy surfaces while simultaneously demonstrating a solid experimental-computational framework for their investigation.

Competition between In-Plane vs Above-Plane Configurations of Water with Aromatic Molecules: Non-Covalent Interactions in 1,4-Naphthoquinone-(H2O)1-3 Complexes

Non-covalent interactions between aromatic molecules and water are fundamental in many chemical and biological processes, and their accurate description is essential to understand molecular relative configurations. Here we present the rotational spectroscopy study of the water complexes of the polycyclic aromatic hydrocarbon 1,4-naphthoquinone (1,4-NQ). In 1,4-NQ-(H₂O)_1,2, water molecules bind through O-H···O and C-H···O hydrogen bonds and are located on the plane of 1,4-NQ. For 1,4-NQ-(H₂O)₃, in-plane and above-plane water configurations are observed exhibiting O-H···O, C-H···O, and lone pair···π-hole interactions. The observation of different water arrangements for 1,4-NQ-(H₂O)₃ allows benchmarking theoretical methods and shows that they have great difficulty in predicting energy orderings due to the strong competition of C-H···O binding with π and π-hole interactions. This study provides important insight into water interactions with aromatic systems and the challenges in their modeling.

A combined theoretical and experimental study of small anthracene-water clusters

Water-cluster interactions with polycyclic aromatic hydrocarbons (PAHs) are of paramount interest in many chemical and biological processes. We report a study of anthracene monomers and dimers with water (up to four)-cluster systems utilizing molecular beam vacuum-UV photoionization mass spectrometry and density functional calculations. Structural loss in photoionization efficiency curves when adding water indicates that various isomers are generated, while theory indicates only a slight shift in energy in photoionization states of different isomers. Calculations reveal that the energetic tendency of water is to remain clustered and not to disperse around the PAH. Theoretically, we observe water confinement exclusively in the case of four water clusters and only when the anthracenes are in a cross configuration due to optimal OH⋯π interactions, indicating dependence on the size and structure of the PAH. Furthermore theory sheds light on the structural changes that occur in water upon ionization of anthracene, due to the optimal interactions of the resulting hole and water hydrogen atoms.

An infrared spectroscopic study of trifluoromethoxybenzene⋯methanol complexes formed in superfluid helium nanodroplets

We have studied the intermolecular complex formation between trifluoromethoxybenzene and methanol (CD₃OD) in superfluid helium droplets by infrared spectroscopy in the spectral range of 2630-2730 cm^-1, covering the O-D stretches of methanol-d₄ (CD₃OD). The cluster size associated with the observed bands is deduced from the variation of infrared intensity of a particular band with the partial pressures of trifluoromethoxybenzene and methanol. Quantum chemical calculations are performed at the MP2/6-311++G(d,p) level of theory to complement the experimental results. As a result, we have identified six different conformers of the trifluoromethoxybenzene⋯methanol intermolecular complex: three bound via O-H⋯O hydrogen bonds and the other three via O-H⋯π hydrogen bonds. Furthermore, to access the effect of fluorination on the methyl unit of anisole molecules, we compare the IR spectrum of trifluoromethoxybenzene (C₆H₅OCF₃)⋯methanol with our earlier reported spectrum of anisole (C₆H₅OCH₃)⋯methanol.

Vibrational Spectroscopy of Benzonitrile-(Water)1-2 Clusters in Helium Droplets

Polycyclic aromatic hydrocarbons are considered as primary carriers of the unidentified interstellar bands. The recent discovery of the first interstellar aromatic molecule, benzonitrile (C₆H₅CN), suggests a repository of aromatic hydrocarbons in the outer earth environment. Herein, we report an infrared (IR) study of benzonitrile-(D₂O)_n clusters using mass-selective detection in helium nanodroplets. In this work, we use isotopically substituted water, D₂O, instead of H₂O because of our restricted IR frequency range (2565-3100 cm^-1). A comparison of the experimental and predicted spectra computed at the MP2/6-311++G(d,p) level of benzonitrile-(water)_1-2 clusters reveals the formation of a unique local minimum structure, which was not detected in previous gas-phase molecular beam experiments. Here, the solvent water forms a nearly linear hydrogen bond (H-bond) with the nitrile nitrogen of benzonitrile, while the previously reported most stable cyclic H-bonded isomer is not observed. This can be rationalized by the stepwise aggregation process of precooled monomers. The addition of a second water molecule results in the formation of two different isomers. In one of the observed isomers, a H-bonded water chain binds linearly to the nitrile nitrogen similar to the monohydrated benzonitrile-water complex. In the other observed isomer, the water dimer forms a ring-type structure, where a H-bonded water dimer simultaneously interacts with the nitrile nitrogen and the adjacent ortho CH group. Finally, we compare the water-binding motif in the neutral benzonitrile-water complex with the corresponding positively and negatively charged benzonitrile-water monohydrates to comprehend the charge-induced alteration of the solvent binding motif.