Matrix-Isolation in Cryogenic Water-Ices: Facile Generation, Storage, and Optical Spectroscopy of Aromatic Radical Cations

Effect of Microhydration on the Temporary Anion States of Pyrene

The influence of incremental hydration (≤4) on the electronic resonances of the pyrene anion is studied using two-dimensional photoelectron spectroscopy. The photoexcitation energies of the resonances do not change; therefore, from the anion's perspective, the resonances remain the same, but from the neutral's perspective of the electron-molecule reaction, the resonances decrease in energy by the binding energy of the water molecules. The autodetachment of the resonances shows that hydration has very little effect, showing that even the dynamics of most of the resonances are not impacted by hydration. Two specific resonances do show changes that are explained by the closing of specific autodetachment channels. The lowest-energy resonance leads to efficient electron capture as observed through thermionic emission and evaporation of water molecules (dissociative electron attachment). The implications of low-energy electron capture in dense molecular interstellar clouds are discussed.

How to Predict Excited State Geometry by Using Empirical Parameters Obtained from Franck-Condon Analysis of Optical Spectrum

Excited state geometries of molecules can be calculated with highly reliable wavefunction schemes. Most of such schemes, however, are applicable to small molecules and can hardly be viewed as error-free for excited state geometries. In this study, a theoretical approach is presented in which the excited state geometries of molecules can be predicted by using vibrationally resolved experimental absorption spectrum in combination with the theoretical modelling of vibrational pattern based on Franck-Condon approximation. Huang-Rhys factors have been empirically determined and used as input for revealing the structural changes occurring between the ground and the excited state geometries upon photoexcitation. Naphthalene molecule has been chosen as a test case to show the robustness of the proposed theoretical approach. Predicted 1B_2u excited state geometry of the naphthalene has similar but slightly different bond length alternation pattern when compared with the geometries calculated with CIS, B3LYP, and CC2 methods. Excited state geometries of perylene and pyrene molecules are also determined with the presented theoretical approach. This powerful method can be applied to other molecules and specifically to relatively large molecules rather easily as long as vibrationally resolved experimental spectra are available to use.

Micro- vs Macrosolvation in Reichardt's Dyes

Solvation is a complex phenomenon involving electrostatic and van der Waals forces as well as chemically more specific effects such as hydrogen bonding. To disentangle global solvent effects (macrosolvation) from local solvent effects (microsolvation), we studied the UV-vis and IR spectra of a solvatochromic pyridinium-N-phenolate dye (a derivative of Reichardt's dye) in rare gas matrices, in mixtures of argon and water, and in water ice. The π-π* transition of the betaine dye in the visible region and its C-O stretching vibration in the IR region are highly sensitive to solvent effects. By annealing argon matrices of the betaine dye doped with low concentrations of water, we were able to synthesize 1:1 water-dye complexes. Formation of hydrogen-bonded complexes leads to small shifts of the π-π* transition only, as long as the global polarity of the matrix environment does not change. In contrast, changes of the global polarity result in large spectral band shifts. Hydrogen-bonded complexes of the betaine dye are more sensitive to global polarity changes than the dye itself, explaining why E_T values determined with Reichardt's dyes are very different for protic and nonprotic solvents, even if the relative permittivities of these solvents are similar.

Advances in Cryochemistry: Mechanisms, Reactions and Applications

It is counterintuitive that chemical reactions can be accelerated by freezing, but this amazing phenomenon was discovered as early as the 1960s. In frozen systems, the increase in reaction rate is caused by various mechanisms and the freeze concentration effect is the main reason for the observed acceleration. Some accelerated reactions have great application value in the chemistry synthesis and environmental fields; at the same time, certain reactions accelerated at low temperature during the storage of food, medicine, and biological products should cause concern. The study of reactions accelerated by freezing will overturn common sense and provide a new strategy for researchers in the chemistry field. In this review, we mainly introduce various mechanisms for accelerating reactions induced by freezing and summarize a variety of accelerated cryochemical reactions and their applications.

Probing solvation and reactivity in ionized polycyclic aromatic hydrocarbon–water clusters with photoionization mass spectrometry and electronic structure calculations

Synchrotron based mass spectrometry coupled with theoretical calculations provides insight into polycyclic aromatic hydrocarbon water interactions.

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.

Photochemistry of coronene in cosmic water ice analogs at different concentrations

This work presents the photochemistry of ultraviolet (UV) irradiated coronene in water ices at 15 K, studied using mid-infrared Fourier transform (FTIR) spectroscopy for C24H12:H2O at concentrations of (1:50), (1:150), (1:200), (1:300) and (1:400). Previous UV irradiation studies of anthracene:H2O, pyrene:H2O and benzo[ghi]perylene:H2O ices at 15 K have shown that aromatic alcohols and ketones, as well as CO2 and H2CO are formed at very low temperatures. Like-wise, here, in addition to the coronene cation, hydroxy-, keto-, and protonated coronene (coronene-H+) are formed. The rate constants for the decay of neutral coronene and for the formation of photoproducts have been derived. It is shown that PAHs and their UV-induced PAH:H2O photoproducts have mid-infrared spectroscopic signatures in the 5-8 μm region that can contribute to the interstellar ice components described by Boogert et al. (2008) as C1-C5. Our results suggest that oxygenated and hydrogenated PAHs could be in UV-irradiated regions of the ISM where water-rich ices are important.

The highly reactive benzhydryl cation isolated and stabilized in water ice

Diphenylcarbene (DPC) shows a triplet ground-state lying approximately 3 kcal/mol below the lowest singlet state. Under the conditions of matrix isolation at 25 K, DPC reacts with single water molecules embedded in solid argon and switches its ground state from triplet to singlet by forming a strong hydrogen bond. The complex between DPC and water is only metastable, and even at 3 K the carbene center slowly inserts into the OH bond of water to form benzhydryl alcohol via quantum chemical tunneling. Surprisingly, if DPC is generated in amorphous water ice at 3 K, it is protonated instantaneously to give the benzhydryl cation. Under these conditions, the benzhydryl cation is stable, and warming to temperatures above 50 K is required to produce benzhydryl alcohol. Thus, for the first time, a highly electrophilic and extremely reactive secondary carbenium ion can be isolated in a neutral, nucleophilic environment avoiding superacidic conditions.

The impact of recent advances in laboratory astrophysics on our understanding of the cosmos

An emerging theme in modern astrophysics is the connection between astronomical observations and the underlying physical phenomena that drive our cosmos. Both the mechanisms responsible for the observed astrophysical phenomena and the tools used to probe such phenomena-the radiation and particle spectra we observe-have their roots in atomic, molecular, condensed matter, plasma, nuclear and particle physics. Chemistry is implicitly included in both molecular and condensed matter physics. This connection is the theme of the present report, which provides a broad, though non-exhaustive, overview of progress in our understanding of the cosmos resulting from recent theoretical and experimental advances in what is commonly called laboratory astrophysics. This work, carried out by a diverse community of laboratory astrophysicists, is increasingly important as astrophysics transitions into an era of precise measurement and high fidelity modeling.

Chemical Kinetics of Reactions in the Unfrozen Solution of Ice

Some reactions are accelerated in ice compared to aqueous solution at higher temperatures. Accelerated reactions in ice take place mainly due to the freeze-concentration effect of solutes in an unfrozen solution at temperatures higher than the eutectic point of the solution. Pincock was the first to report an acceleration model for reactions in ice,1 which successfully simulated experimental results. We propose here a modified version of the model for reactions in ice. The new model includes the total molar change involved in reactions in ice. Furthermore, we explain why many reactions are not accelerated in ice. The acceleration of reactions can be observed in the cases of (i) second- or higher-order reactions, (ii) low concentrations, and (iii) reactions with a small activation energy. Reactions with a buffer solution or additives in order to adjust ion strength, zero- or first-order reactions, or reactions containing high reactant concentrations are not accelerated by freezing. We conclude that the acceleration of reactions in the unfrozen solution of ice is not an abnormal phenomenon.

Rise in the pH of an unfrozen solution in ice due to the presence of NaCl and promotion of decomposition of gallic acids owing to a change in the pH

Oxidative decomposition of gallic acid occurs in alkaline solutions but hardly arises in acidic solutions. We have found that the addition of sodium chloride promotes the decomposition of gallic acid caused by freezing even under neutral and acidic conditions. Even at pH 4.5, gallic acid was decomposed by freezing in the presence of NaCl; however, in the absence of NaCl, it was hardly decomposed by freezing at pH lower than 7. Chloride ions are more easily incorporated in ice than sodium ions when the NaCl solution is frozen. The unfrozen solution in ice becomes positively charged, and as a result, protons transfer from the unfrozen solution to the ice. We measured the pH in the unfrozen solution which coexists with single-crystal ice formed from a 5 mmol dm(-3) NaCl solution and determined the pH to be 8.6 at equilibrium with CO(2) of 380 ppm or 11.3 in the absence of CO(2) compared to pH 5.6 in the original solution. From the model calculation performed for gallic acid solution in the presence of 5 mmol dm(-3) NaCl, it can be estimated that the amount of OH(-) transferred from the ice to the solution corresponds to 1.26 x 10(-5) mol dm(-3). The amount of OH(-) transferred is concentrated into the unfrozen solution and affects the pH of the unfrozen solution. Therefore, the pH in an unfrozen gallic acid solution in ice becomes alkaline, and the decomposition of gallic acid proceeds. It is expected that other base-catalyzed reactions in weakly acidic solutions also proceed by freezing in the presence of NaCl without the need for any alkaline reagents.

Photodecarbonylation of dibenzyl ketones and trapping of radical intermediates by copper(II) chloride in frozen aqueous solutions

This paper presents a quantitative and qualitative study of the Norrish type I reaction of dibenzyl ketone (DBK) and 4-methyldibenzyl ketone (MeDBK), producing the benzyl radicals and consequently recombination products, in frozen aqueous solutions over a broad temperature range (-80 to 20 degrees C). This work extends previous research on the cage effects in various constrained media to provide information about the dynamics and reactivity of the photochemically generated intermediates at the grain boundaries of ice matrix. As the temperature of aqueous solutions decreases, the solute concentrations become high at layers covering ice crystals, causing efficient molecular segregation. The cage effect experiments have shown that diffusion of the benzyl radicals within such reaction aggregates is still remarkably efficient at temperatures below -50 degrees C, independently of the initial ketone concentration in the range of 10(-6)-10(-4) mol L(-1). In addition, the study of trapping the benzyl radicals formed in situ by CuCl2 was used as a qualitative probe of heterogeneous bimolecular reactions in the frozen aqueous matrix and on its surface. Molecules of both solutes were found to be segregated from the ice phase to the same location and underwent chemical reactions within diffusion and intermediates lifetimes limits. Understanding the fundamental physicochemical processes in ice is unquestionably important in related environmental or cosmochemical investigations.

Oxidation of aromatic and aliphatic hydrocarbons by OH radicals photochemically generated from H2O2 in ice

Oxidation of aromatic and saturated aliphatic hydrocarbons (c = 10(-3)-10(-5) mol L(-1)) by the hydroxyl radicals, photochemically produced from hydrogen peroxide (c = 10(-1)-10(-5) mol L(-1)), in frozen aqueous solutions was investigated in the temperature range of -20 to -196 degrees C. While aromatic molecules (benzene, phenol, naphthalene, naphthalen-2-ol, or anthracene) underwent primarily addition-elimination reactions to form the corresponding hydroxy compounds, saturated hydrocarbons (cyclohexane, butane, methane) were oxidized to alcohols or carbonyl compounds via hydrogen abstraction and termination reactions. The results suggest that these photoreactions, taking place in a highly concentrated liquid or solidified layers covering the ice crystals, are qualitatively similar to those known to occur in liquid aqueous solutions; however, that probability of any bimolecular reaction in the environment ultimately depends on organic contaminant concentrations and oxidants availability at specific locations of the ice matrix, temperature, wavelength, and photon flux. They, moreover, support hypotheses that oxidation of organic impurities in the snowpack can produce volatile hydroxy and carbonyl compounds, which may consequently be released to the atmosphere.

Double Ionization of Quaterrylene (C40H20) in Water-Ice at 20 K with Lyα (121.6 nm) Radiation

Polycyclic aromatic hydrocarbon (PAH) molecules undergo facile ionization in cryogenic water-ices resulting in near quantitative conversions of neutral molecules to the corresponding singly charged radical cations. Here we report, for the first time, the production and stabilization of a doubly ionized, closed shell PAH in water-ice. The large PAH quaterrylene (QTR, C40H20) is readily photoionized and stabilized as QTR 2+ in a water-ice matrix at 20 K. The kinetic analysis of photolysis shows that the QTR 2+ is formed at the expense of QTR +, not directly from QTR. The long-axis polarized S1-S0 (1(1)B(3u) <-- 1(1)Ag) transition for QTR 2+ falls at 1.59 eV (782 nm). TD-DFT calculations at the B3LYP level predict that this transition falls at 1.85 eV (670 nm) for free gas-phase QTR 2+, within the 0.3 eV uncertainty associated with these calculations. This red shift of 0.26 eV is quite similar to the 0.24 eV red shift between the TD-DFT computational prediction for the lowest energy transition for QTR + (1.68 eV) and its value in a water matrix (1.44 eV). These results suggest that multiple photoionization of such large PAHs in water-ice can be an efficient process in general.

Aggregation of Methylene Blue in Frozen Aqueous Solutions Studied by Absorption Spectroscopy

The paper presents a qualitative as well as quantitative spectroscopic study of methylene blue (MB) aggregation that occurs upon freezing the aqueous solutions over a wide concentration range. The Gaussian curve analysis and the multivariate curve resolution-alternating least squares method were used to determine the number and concentration of chemical species responsible for the overlaying absorption visible spectra measured. The results show the extent of aggregation for the concentrations above 10(-7) mol L(-1), being dependent on the freezing rate and the initial concentration. While the local concentration of MB at the grain boundaries of polycrystalline ice increased by approximately 3 orders of magnitude upon fast freezing at 77 K compared to the liquid phase, the concentration raised at least by 6 orders of magnitude upon slow freezing at 243 K. Since enhancement of the local concentration of solutes plays an important role in (photo)chemical transformations in solid aqueous media, this work helps to understand how the initial conditions control the course of the process. The results are relevant in other interdisciplinary fields, such as environmental chemistry, cosmochemistry, or geochemistry.