Photooxidation of Aniline Derivatives Can Be Activated by Freezing Their Aqueous Solutions

A combined experimental and computational approach was used to investigate the spectroscopic properties of three different aniline derivatives (aniline, <i>N</i>,<i>N</i>-dimethylaniline, and <i>N</i>,<i>N</i>-diethylaniline) in aqueous solutions and at the air–ice interface in the temperature range of 243–298 K. The absorption and diffuse reflectance spectra of ice samples prepared by different techniques, such as slow or shock freezing of the aqueous solutions or vapor deposition on ice grains, exhibited unequivocal bathochromic shifts of 10–15 nm of the absorption maxima of anilines in frozen samples compared to those in liquid aqueous solutions. DFT and SCS-ADC(2) calculations showed that contaminant–contaminant and contaminant–ice interactions are responsible for these shifts. Finally, we demonstrate that irradiation of anilines in the presence of a hydrogen peroxide/O₂ system by wavelengths that overlap only with the red-shifted absorption tails of anilines in frozen samples (while having a marginal overlap with their spectra in liquid solutions) can almost exclusively trigger a photochemical oxidation process. Mechanistic and environmental considerations are discussed

Clustering of Uracil Molecules on Ice Nanoparticles

We generate a molecular beam of ice nanoparticles (H₂O)_<i>N</i>, <i>N̅</i> ≈ 130–220, which picks up several individual gas phase uracil (U) or 5-bromouracil (BrU) molecules. The mass spectra of the doped nanoparticles prove that the uracil and bromouracil molecules coagulate to clusters on the ice nanoparticles. Calculations of U and BrU monomers and dimers on the ice nanoparticles provide theoretical support for the cluster formation. The (U)_<i>m</i>H⁺ and (BrU)_<i>m</i>H⁺ intensity dependencies on <i>m</i> extracted from the mass spectra suggest a smaller tendency of BrU to coagulate compared to U, which is substantiated by a lower mobility of bromouracil on the ice surface. The hydrated U_<i>m</i>·(H₂O)_<i>n</i>H⁺ series are also reported and discussed. On the basis of comparison with the previous experiments, we suggest that the observed propensity for aggregation on ice nanoparticles is a more general trend for biomolecules forming strong hydrogen bonds. This, together with their mobility, leads to their coagulation on ice nanoparticles which is an important aspect for astrochemistry

Biomolecule Analogues 2‑Hydroxypyridine and 2‑Pyridone Base Pairing on Ice Nanoparticles

Ice nanoparticles (H₂O)_<i>N</i>, <i>N</i> ≈ 450 generated in a molecular beam experiment pick up individual gas phase molecules of 2-hydroxypyridine and 2-pyridone (HP) evaporated in a pickup cell at temperatures between 298 and 343 K. The mass spectra of the doped nanoparticles show evidence for generation of clusters of adsorbed molecules (HP)_<i>n</i> up to <i>n</i> = 8. The clusters are ionized either by 70 eV electrons or by two photons at 315 nm (3.94 eV). The two ionization methods yield different spectra, and their comparison provides an insight into the neutral cluster composition, ionization and intracluster ion–molecule reactions, and cluster fragmentation. Quite a few molecules were reported <i>not to coagulate</i> on ice nanoparticles previously. The (HP)_<i>n</i> cluster generation on ice nanoparticles represents the first evidence for coagulating of molecules and cluster formation on free ice nanoparticles. For comparison, we investigate the coagulation of HP molecules picked up on large clusters Ar_<i>N</i>, <i>N</i> ≈ 205, and also (HP)_<i>n</i> clusters generated in supersonic expansions with Ar buffer gas. This comparison points to a propensity for the (HP)₂ dimer generation on ice nanoparticles. This shows the feasibility of base pairing for model of biological molecules on free ice nanoparticles. This result is important for hypotheses of the biomolecule synthesis on ice grains in the space. We support our findings by theoretical calculations that show, among others, the HP dimer structures on water clusters

Spectroscopic Properties of Naphthalene on the Surface of Ice Grains Revisited: A Combined Experimental–Computational Approach

An experimental-computational method is used to investigate the spectroscopic behavior of naphthalene on the surface of ice grains. UV–vis diffuse reflectance and fluorescence spectroscopies of naphthalene combined with DFT and ADC(2) calculations provide evidence for the occurrence of excited-state associates. The measured and calculated bathochromic shifts of the S₀ → S₁ electronic transitions related to naphthalene dimers or naphthalene–ice interactions do not exceed 3 nm. The bands observed in the emission spectrum of frozen naphthalene solutions are assigned to excited dimers of different mutual orientations, naphthalene phosphorescence, and fluorescence of anthracene present as a trace impurity and populated by the energy transfer from excited naphthalene. Photochemical reactivity in/on ice and snow is dependent on the absorption properties and speciation of the compounds present in these media. Hence, within this study, we exploit frozen solutions of naphthalene to demonstrate both the absence of considerable bathochromic shift and a strong tendency to aggregate

Spectroscopic Properties of Naphthalene on the Surface of Ice Grains Revisited: A Combined Experimental–Computational Approach

An experimental-computational method is used to investigate the spectroscopic behavior of naphthalene on the surface of ice grains. UV–vis diffuse reflectance and fluorescence spectroscopies of naphthalene combined with DFT and ADC(2) calculations provide evidence for the occurrence of excited-state associates. The measured and calculated bathochromic shifts of the S₀ → S₁ electronic transitions related to naphthalene dimers or naphthalene–ice interactions do not exceed 3 nm. The bands observed in the emission spectrum of frozen naphthalene solutions are assigned to excited dimers of different mutual orientations, naphthalene phosphorescence, and fluorescence of anthracene present as a trace impurity and populated by the energy transfer from excited naphthalene. Photochemical reactivity in/on ice and snow is dependent on the absorption properties and speciation of the compounds present in these media. Hence, within this study, we exploit frozen solutions of naphthalene to demonstrate both the absence of considerable bathochromic shift and a strong tendency to aggregate

Photochemistry of 4‑Chlorophenol in Liquid and Frozen Aqueous Media Studied by Chemical, Compound-Specific Isotope, and DFT Analyses

The photochemistry of 4-chlorophenol in liquid and frozen aqueous solutions and on the surface of ice grains yields substantially different photoproducts. Several complementary experimental and theoretical methods, such as trace analyses of the photoproducts, trapping experiments, compound-specific isotope analyses, and quantum chemical calculations, were used to study the reaction mechanism differences. A similar carbon kinetic isotope effect determined for the photolysis of 4-chlorophenol samples in the temperature range of 20 to −40 °C and the results of trapping experiments suggest that heterogeneous cleavage of the C–Cl bond in the excited state is probably a common key step that leads to the formation of carbene and hydroxyphenyl cation intermediates. We conclude that the subsequent specific reactions of these species under various conditions are responsible for the formation of different final photoproducts

Spectroscopic Properties of Benzene at the Air–Ice Interface: A Combined Experimental–Computational Approach

A combined experimental and computational approach was used to study the spectroscopic properties of benzene at the ice–air interface at 253 and 77 K in comparison with its spectroscopic behavior in aqueous solutions. Benzene-contaminated ice samples were prepared either by shock-freezing of benzene aqueous solutions or by benzene vapor-deposition on pure ice grains and examined using UV diffuse reflectance and emission spectroscopies. Neither the absorption nor excitation nor emission spectra provided unambiguous evidence of benzene associates on the ice surface even at a higher surface coverage. Only a small increase in the fluorescence intensity in the region above 290 nm found experimentally might be associated with formation of benzene excimers perturbed by the interaction with the ice surface as shown by ADC(2) excited-state calculations. The benzene associates were found by MD simulations and ground-state DFT calculations, although not in the arrangement that corresponds to the excimer structures. Our experimental results clearly demonstrated that the energy of the S₀ → S₁ electronic transition of benzene is not markedly affected by the phase change or the microenvironment at the ice–air interface and its absorption is limited to the wavelengths below 268 nm. Neither benzene interactions with the water molecules of ice nor the formation of dimers and microcrystals at the air–ice interface thus causes any substantial bathochromic shift in its absorption spectrum. Such a critical evaluation of the photophysical properties of organic contaminants of snow and ice is essential for predictions and modeling of chemical processes occurring in polar regions

Unexpected Photoreactivity in a NO₂‑Functionalized Aluminum-MOF

The metal–organic framework CAU-10-NO₂ [Al­(OH)­BDC-NO₂] (CAU stands for Christian-Albrechts-University; H₂BDC-NO₂ is 5-nitroisophthalic acid) was observed to exhibit unexpected photochemical reactivity. Upon irradiation of the MOF with UV light with a wavelength of 365 nm (or with sunlight), guest molecules inside the pore system of the MOF can be oxidized and stable radicals are formed from the organic linker molecules. The reactivity toward different alcohols was studied by UV/vis spectroscopy and EPR spectroscopy. The amount of generated radicals depends on the size of the solvent molecules; however, as an exception, methanol shows a much lower reactivity than ethanol. DFT calculations were carried out to gain insights into these photochemical reactions. The results indicate that the nitro group is reduced to form a nitroso moiety. This was confirmed by means of NMR spectroscopy. The exact nature of the radical could not be revealed, but the results indicate that it could be a further reduced anionic nitroso radical. Methanol and ethanol can be distinguished using this photochemical reaction simply by the coloring of the irradiated MOF. Such a property is characteristic for a sensor; therefore, a synthesis procedure was developed to implement the MOF into a device by which the compound was directly grown onto gold substrates