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

Quantification of anion and cation uptake in ice Ih crystals

While ice has very low solubility for salts compared to water, small amounts of ions are doped into ice crystals. These small ion dopants can alter the fundamental physical and chemical properties of ice, such as its structure and electrical conductivity. Therefore, these results could have a direct impact on the chemical reactivity of ice and ice surfaces. Here, we examine the influence of the uptake of three salts—ammonium chloride (NH4Cl), sodium chloride (NaCl), and ammonium sulfate [(NH4)2SO4]—on ice Ih formation using capillary electrophoresis. Using both cation and anion modes, we observed and quantified the uptake of individual ions into the ice. Our results indicate that anions have a higher propensity for uptake into ice Ih crystals.

Role of an Ice Surface in the Photoreaction of Coumarins

Ice affects many chemical reactions in nature, which greatly influences the atmosphere, climate, and life. However, the exact mechanism of ice in these chemical reactions remains elusive. For example, it is still an open question as to whether ice can act as a catalyst to greatly enhance the reactivity and selectivity, which is essential for the production of some natural compounds in our planet. Here, we discover that ice can lead to high efficiency and stereoselectivity of the [2 + 2] photodimerization of coumarin and its derivatives. The conversion of the [2 + 2] photodimerization of coumarins enhanced by ice is dozens of times higher than that in the unfrozen saturated solution, and the reaction displays a high syn-head-head stereoselectivity (>95%) in comparison with those in the absence of the ice. Note that almost no reaction occurs in the crystal powder and melt of the coumarins, indicating that the role of ice in the photodimerization reaction is not simply due to the usual mechanisms found in the freezing concentration. We further reveal that the reaction rate is found to be proportional to the total area of the ice surface and follows Michaelis-Menten-like kinetics, indicating that the ice surface catalyzes the reaction. Molecular dynamics simulations demonstrate that ice surfaces can induce reactants to form a two-dimensional liquid-crystal-ordered layer with a suitable intermolecular distance and unique side-by-side packing, facilitating stereoselective photodimerization for syn-head-head dimers. These findings give evidence that ice-surface-induced molecular assembly may play an important role in atmospheric heterogeneous photoreaction processes.

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.

Cleavage of carbon–carbon bonds by radical reactions

This review provides insights into the in situ generated radicals triggered carbon–carbon bond cleavage reactions.

Enhanced kinetics of pseudo first-order hydrolysis in liquid phase coexistent with ice

The reaction rate of the hydrolysis of fluorescein diacetate (FDA) is several times larger in the frozen state than that in the unfrozen solution of the same composition at the same temperature. The freeze concentration of reactants in the liquid phase expelled form ice crystals cannot explain the kinetic enhancement of pseudo first order reactions such as the FDA hydrolysis. However, the reaction rate increases as the freeze concentration ratio becomes larger at a constant temperature. Direct pH measurements have revealed that the basicity of the liquid phase is unchanged at any concentration ratio, suggesting that the reactivity enhancement is not caused by increased basicity. The reaction rate enhancement is clearly related to the size of the space in which the liquid phase is confined upon freezing. The ice wall itself or the water structure formed near the wall should thus be responsible for this kinetic enhancement.

Chemistry of small organic molecules on snow grains: the applicability of artificial snow for environmental studies

The utilization of artificial snow for environmentally relevant (photo)chemical studies was systematically investigated. Contaminated snow samples were prepared by various methods: by shock freezing of the aqueous solutions sprayed into liquid nitrogen or inside a large walk-in cold chamber at -35 °C, or by adsorption of gaseous contaminants on the surface of artificially prepared pure or natural urban snow. The specific surface area of artificial snow grains produced in liquid nitrogen was determined using valerophenone photochemistry (400-440 cm(2) g(-1)) to estimate the surface coverage by small hydrophobic organic contaminants. The dynamics of recombination/dissociation (cage effect) of benzyl radical pairs, photochemically produced from 4-methyldibenzyl ketone on the snow surface, was investigated. The initial ketone loading, c = 10(-6)-10(-8) mol kg(-1), only about 1-2 orders of magnitude higher than the contaminant concentrations commonly found in nature, was already well below monolayer coverage. We found that the efficiency of out-of-cage reactions decreased at much higher temperatures than those previously determined for frozen solutions; however, the cage effect was essentially the same no matter what technique of snow production or ketone deposition/uptake was used, including the experiments with collected natural snow. The experimental observation that the contaminant molecules are initially self-associated even at the lowest concentrations was supported by DFT calculations. We conclude that, contrary to frozen aqueous solutions, in which the impurities reside in a 3D cage (micropocket), contaminant molecules located on the artificial snow grain surface at low concentrations can be visualized in terms of a 2D cage. Artificial snow thus represents a readily available study matrix that can be used to emulate the natural chemical processes of trace contaminants occurring in natural snow.

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.

New insights into the mechanism of triplet radical-pair combinations. The persistent radical effect masks the distinction between in-cage and out-of-cage processes

Steady-state and laser-pulsed irradiations of dibenzyl ketone (ACOB0) and derivatives with a p-methyl or a p-hexadecyl chain (ACOB1 and ACOB16, respectively) have been conducted in polyethylene films with 0, 46, and 68% crystallinities. Calculation of the fractions of in-cage combinations of the triplet benzylic radical-pair intermediates based on photoproduct yields, Fc, from ACOB16 are shown to be incorrect as a result of the kinetic consequences of drastically different diffusion coefficients for the benzyl and p-hexadecylbenzyl radicals. Careful analyses of the transient absorption traces, based upon a new model developed here, allow the correct cage effects to be determined even from ACOB0. The model also permits the rate constants for radical-pair combinations and escape from their cage of origin to be calculated using either an iterative fitting procedure or a very simple one which requires only k-CO and the intensities of the transient absorption immediately after the flash and after the in-cage portion of reaction by the benzylic radicals is completed. Values of the rate constant for decarbonylation of the initially formed arylacetyl radicals, k-CO, have been measured from the rise portions of the laser-flash transient absorption traces. They confirm the assertion from results in liquid alkane media that decarbonylation rates are independent of microviscosity. The data separate components of a reaction from an (in-cage) "cage effect" and an (out-of-cage) "persistent radical effect" that are responsible for formation of AB-type (i.e., decarbonylated) products. The effects here are a consequence of vastly different rates of diffusion for coreacting A. and B. benzylic radicals rather than segregation of the radicals in different parts of a hetereogeneous environment (which leads to an excess of AA and BB products). Heretofore, observation of exclusive formation of AB products has been attributed to in-cage combinations of geminate radical pairs. We show that not to be the case here and provide methodologies which may be used for testing the importance of the "persistent radical effect" component of reaction.

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.

Enhanced protonation of cresol red in acidic aqueous solutions caused by freezing

The protonation degree of cresol red (CR) in frozen aqueous solutions at 253 or 77 K, containing various acids (HF, HCl, HNO3, H2SO4, and p-toluenesulfonic acid), sodium hydroxide, NaCl, or NH4Cl, was examined using UV/Vis absorption spectroscopy. CR, a weak organic diacid, has been selected as a model system to study the acid-base interactions at the grain boundaries of ice. The multivariate curve resolution alternating least-squares method was used to determine the number and abundances of chemical species responsible for the overlaying absorption visible spectra measured. The results showed that the extent of CR protonation, enhanced in the solid state by 2-4 orders of magnitude in contrast to the liquid solution, is principally connected to an increase in the local concentration of acids. It was found that this enhancement was not very sensitive to either the freezing rate or the type of acid used and that CR apparently established an acid-base equilibrium prior to solidification. In addition, the presence of inorganic salts, such as NaCl or NH4Cl, is reported to cause a more efficient deprotonation of CR in the former case and an enhanced protonation in the latter case, being well explained by the theory of Bronshteyn and Chernov. CR thus served as an acid-base indicator at the grain boundaries of ice samples. Structural changes in the CR molecule induced by lowering the temperature and a presence of the constraining ice environment were studied by the absorption and 1H NMR spectroscopies. Cryospheric and atmospheric implications concerning the influence of acids and bases on composition and reactivity of ice or snow contaminants were examined.

Cooperative Hydration of Pyruvic Acid in Ice

About 3.5 +/- 0.3 water molecules are still involved in the exothermic hydration of 2-oxopropanoic acid (PA) into its monohydrate (2,2-dihydroxypropanoic acid, PAH) in ice at 230 K. This is borne out by thermodynamic analysis of the fact that QH(T) = [PAH]/[PA] becomes temperature independent below approximately 250 K (in chemically and thermally equilibrated frozen 0.1 < or = [PA]/M < or = 4.6 solutions in D2O), which requires that the enthalpy of PA hydration (DeltaHH approximately -22 kJ mol(-1)) be balanced by a multiple of the enthalpy of ice melting (DeltaHM = 6.3 kJ mol(-1)). Considering that: (1) thermograms of frozen PA solutions display a single endotherm, at the onset of ice melting, (2) the sum of the integral intensities of the 1deltaPAH and 1deltaPA methyl proton NMR resonances is nearly constant while, (3) line widths increase exponentially with decreasing temperature before diverging below approximately 230 K, we infer that PA in ice remains cooperatively hydrated within interstitial microfluids until they vitrify.

Photogeneration of Distant Radical Pairs in Aqueous Pyruvic Acid Glasses

The lambda > 300 nm photolysis of h4- or d4-pyruvic acid aqueous glasses at 77 K yields identical electron magnetic resonance (EMR) spectra arising from distant (r greater or similar 0.5 nm) triplet radical pairs. Spectra comprise: (1) well-resolved quartets, X, at g approximately ge, that closely match the powder spectra of spin pairs interacting across r approximately 1.0 nm with D approximately 3.0 mT, E approximately 0 mT zero field splittings (ZFS), and (2) broad signals, Y, centered at g approximately 2.07 that display marked g-anisotropy and g-strain, exclude D greater or similar 20.0 mT values (i.e., r less or similar 0.5 spin nm separations), and track the temperature dependence of related g approximately 4 features. These results imply that the n-pi excitation of pyruvic acid, PA, induces long-range electron transfer from the promoted carbonyl chromophore into neighboring carbonyl acceptors, rather than homolysis into contact radical pairs or concerted decarboxylation into a carbene. Since PA is associated into hydrogen-bonded dimers prior to vitrification, X signals arise from radical pairs ensuing intradimer electron transfer to a locked acceptor, while Y signals involve carbonyl groups attached to randomly arranged, disjoint monomers. The ultrafast decarboxylation of primary radical ion pairs, 3[PA+* PA-*], accounts for the release of CO2 under cryogenic conditions, the lack of thermal hysteresis displayed by magnetic signals between 10 and 160 K, and averted charge retrotransfer. All EMR signals disappear irreversibly above the onset of ice diffusivity at approximately 190 K.

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.