Photochemistry of coronene in cosmic water ice analogs at different concentrations

Microhydration of small protonated polyaromatic hydrocarbons: a first principles study

Using first principles methodology, we investigate the microsolvation of protonated benzene (BzH+), protonated coronene (CorH+) and protonated dodecabenzocoronene (DbcH+). Gas phase complexes of these small protonated polyaromatic hydrocarbons (H+PAHs) with mono-, di-, and tri-hydrated water molecules are considered. Their most stable forms are presented, where we discuss their structural, energetic aromaticity and IR and UV spectral features. In particular, we focus on the analysis of the bonding and various non-bonded interactions between these protonated aromatics and water clusters. The strength of non-bonded interactions is quantified and correlated with their electron density profiles. Furthermore, insights into the interfacial interactions and stability of these complexes were obtained through non-covalent index and symmetry-adapted perturbation theory (SAPT0) analyses. We also discuss the effects of the extension of the π aromatic cloud on the water solvation of these protonated aromatics. In particular, we extended our predictions for the S0 → S1 and S0 → T1 wavelength transitions of micro hydrated H+PAHs to deduce those of these species solvated in aqueous solution. The present findings should be useful for understanding, at the microscopic level, the effects of water interacting with H+PAHs, which are relevant for organic chemistry, astrochemistry, atmospheric chemistry, combustion and materials science.

Hydrophobic hydration of the hydrocarbon adamantane in amorphous ice

Hydrophobic molecules are by definition difficult to hydrate. Previous studies in the area of hydrophobic hydration have therefore often relied on using amphiphilic molecules where the hydrophilic part of a molecule enabled the solubility in liquid water. Here, we show that the hydrophobic adamantane (C10H16) molecule can be fully hydrated through vapour codeposition with water onto a cryogenic substrate at 80 K resulting in the matrix isolation of adamantane in amorphous ice. Using neutron diffraction in combination with the isotopic substitution method and the empirical potential structure refinement technique, we find that the first hydration shell of adamantane is well structured consisting of a hydrogen-bonded cage of 28 water molecules that is also found in cubic structure II clathrate hydrates. The four hexagonal faces of the 51264 cage are situated above the four methine (CH) groups of adamantane whereas the methylene (CH2) groups are positioned below the edges of two adjoining pentagonal faces. The oxygen atoms of the 28 water molecules can be categorised on the basis of symmetry equivalences as twelve A, twelve B and four C oxygens. The water molecules of the first hydration shell display orientations consistent with those expected for a clathrate-hydrate-type cage, but also unfavourable ones with respect to the hydrogen bonding between the water molecules. Annealing the samples at 140 K, which is just below the crystallisation temperature of the matrix, removes the unfavourable orientations and leads to a slight increase in the structural order of the first hydration shell. The very closest water molecules display a tendency for their dipole moments to point towards the adamantane which is attributed to steric effects. Other than this, no significant polarisation effects are observed which is consistent with weak interactions between adamantane and the amorphous ice matrix. FT-IR spectroscopy shows that the incorporation of adamantane into amorphous ice leads to a weakening of the hydrogen bonds. In summary, the matrix-isolation of the highly symmetric adamantane in amorphous ice provides an interesting test case for hydrophobic hydration. Studying the structure and spectroscopic properties of water at the interface with hydrophobic hydrocarbons is also relevant for astrophysical environments, such as comets or the interstellar medium, where amorphous ice and hydrocarbons have been shown to coexist in large quantities.

Infrared Spectroscopy and Photochemistry of Anthracoronene in Cosmic Water Ice

We present a laboratory study of the polycyclic aromatic hydrocarbon (PAH) anthracoronene (AntCor, C₃₆H₁₈) in simulated interstellar ices in order to determine its possible contribution to the broad infrared absorption bands in the 5-8 μm wavelength interval. The Fourier transform infrared (FTIR) spectrum of AntCor, codeposited with water ice, was collected. The FTIR spectrum of the sample irradiated with ultraviolet photons was also collected. Unirradiated and UV-irradiated AntCor embedded in water ice have not been studied before; therefore, the molecule's band positions and intensities were compared to published data on AntCor in an argon matrix and theoretical calculations (DFT), as well as the published results of its parent molecules, coronene and anthracene, in water ice. The experimental band strengths for unirradiated AntCor exhibit variability as a function of PAH:H₂O concentration, with two distinct groupings of band intensities. AntCor clustering occurs for all concentrations and has a significant effect on PAH degradation rates and photoproduct variability. Near-IR spectra of irradiated AntCor samples show that AntCor⁺ production increases as the concentration of AntCor in water ice decreases. Photoproduct bands are assigned to AntCor⁺, cationic alcohols, protonated AntCor, and ketones. We report the rate constants of the photoproduct production for the 1:1280 AntCor:H₂O concentration. CO₂ production from AntCor is much less than what was previously reported for Ant and Cor and exhibits two distinct regimes as a function of AntCor:H₂O concentration. The contribution of AntCor photoproducts to astronomical spectra can be estimated by comparison with the observed intensities in the 7.4-8.0 μm range.

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.

Far-IR and UV spectral signatures of controlled complexation and microhydration of the polycyclic aromatic hydrocarbon acenaphthene

UV and IR spectroscopic study of the controlled complexation and microhydration of a polycyclic aromatic hydrocarbon under isolated conditions using free electron laser FELIX.

Theoretical determination of adsorption and ionisation energies of polycyclic aromatic hydrocarbons on water ice

In dense interstellar environments, Polycyclic Aromatic Hydrocarbons (PAHs) are likely to condense onto or integrate into water ice mantles covering dust grains. Understanding the role of ice in the photo-induced processes involving adsorbed PAHs is therefore a key issue in astrochemistry. This requires (i) the knowledge of PAH-ice interactions, i.e. PAH-ice adsorption energies and local structures at the PAH-ice interface, as well as (ii) the understanding of the fate of electrons in the PAH-ice system upon excitation. Regarding (i), in this work, we determined the lowest energy structures of PAH-ice systems for a variety of PAHs ranging from naphthalene to ovalene on three types of ice - crystalline (Ih and Ic) and amorphous (low density) - using an explicit description of the electrons and a finite-sized system. The electronic structure was determined using the Self Consistent Charge Density Functional based Tight Binding (SCC-DFTB) scheme with modified Mulliken charges in order to ensure a good description of the studied systems. Regarding (ii), the influence of the interaction with ice on the Vertical Ionisation Potentials (VIPs) of the series of PAHs was determined using the constrained SCC-DFTB scheme benchmarked against correlated wavefunction results for PAH-(H₂O)_n (n = 1-6, 13) clusters. The results show a deviation equal, at most, to ∼1.4 eV of the VIPs of PAHs adsorbed on ice with respect to the gas phase values. Our results are discussed in the light of experimental data and previous theoretical studies.

Infrared Spectroscopy of Matrix-Isolated Neutral and Ionized Anthracoronene in Argon

The matrix-isolated mid-IR (MIR) spectrum of neutral and ionized anthracoronene (C₃₆H₁₈, AnthCor) in argon has been measured experimentally, compared to the spectrum of its parent molecules, coronene and anthracene, and analyzed by comparison to a theoretical spectrum computed using density functional theory (DFT). The experimental and theoretical band positions generally agree within 0-10 cm^-1. Anthracoronene exhibits extremely intense cation and anion bands around 1330 and 1318 cm^-1. The intensity of these two bands approaches what is traditionally observed over the entire 1000-1600 cm^-1 range for a typical PAH cation or anion. The matrix-isolated near-IR (NIR) through overlap region (OVR) spectrum of ionized AnthCor in argon has been reported for the first time and compared to the spectrum of its parent molecules, coronene and anthracene. The spectrum of AnthCor contains a very strong electronic transition around 6175 cm^-1, placing it outside the range of the electronic transitions typically observed for PAHs. Anthracoronene is one of the few PAHs studied to date which has exhibited the formation of anions upon UV photolysis.

Microhydration of PAH+ cations: evolution of hydration network in naphthalene+-(H2O) n clusters (n ≤ 5)

The interaction of polycyclic aromatic hydrocarbon molecules with water (H2O = W) is of fundamental importance in chemistry and biology. Herein, size-selected microhydrated naphthalene cation nanoclusters, Np+-W n (n ≤ 5), are characterized by infrared photodissociation (IRPD) spectroscopy in the C-H and O-H stretch range to follow the stepwise evolution of the hydration network around this prototypical PAH+ cation. The IRPD spectra are highly sensitive to the hydration structure and are analyzed by dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ) to determine the predominant structural isomers. For n = 1, W forms a bifurcated CH···O ionic hydrogen bond (H-bond) to two acidic CH protons of the bicyclic ring. For n ≥ 2, the formation of H-bonded solvent networks dominates over interior ion solvation, because of strong cooperativity in the former case. For n ≥ 3, cyclic W n solvent structures are attached to the CH protons of Np+. However, while for n = 3 the W3 ring binds in the CH···O plane to Np+, for n ≥ 4 the cyclic W n clusters are additionally stabilized by stacking interactions, leading to sandwich-type configurations. No intracluster proton transfer from Np+ to the W n solvent is observed in the studied size range (n ≤ 5), because of the high proton affinity of the naphthyl radical compared to W n . This is different from microhydrated benzene+ clusters, (Bz-W n )+, for which proton transfer is energetically favorable for n ≥ 4 due to the much lower proton affinity of the phenyl radical. Hence, because of the presence of polycyclic rings, the interaction of PAH+ cations with W is qualitatively different from that of monocyclic Bz+ with respect to interaction strength, structure of the hydration shell, and chemical reactivity. These differences are rationalized and quantified by quantum chemical analysis using the natural bond orbital (NBO) and noncovalent interaction (NCI) approaches.

Probing the interaction between solid benzene and water using vacuum ultraviolet and infrared spectroscopy

Benzene exhibits strong concentration dependent energy shifts of electronic states in solid phase mixtures with water ice.