The detection of muonium in water

We report the first direct observation of the muonium atom (μ⁺e^?) in a liquid sample. Using the transverse field μSR technique muonium spin precession signals were detected in water at six different fields between 4 and 80 G.     

Observation of muonium substituted free radicals in a durene single crystal

Two chemically inequivalent 2,3,5,6-tetramethyl cyclohexadienyl radicals are observed in a durene single crystal by means of the muon spin rotation technique. Orientations and anisotropies of the muon hyperfine interactions are determined.     

Long-lived muonium in water revealed by pulsed muons

The relaxation rate of muonium in pure water has been found to be smaller than 0.05 × 10⁶ s^?1. With the aid of pulsed muong. μSR and MuSR measurements have been carried out over a longer time range than ever achieved. The results indicate a stability of thermal muonium in water from 3 to 29°C.     

Direct Raman evidence for a weak continuous phase transition in liquid water

This paper presents the Raman depolarization ratio of degassed ultrapure water as a function of temperature, in the range 303.4-314.4 K (30.2-41.2 degrees C). The pressure of the sample was the vapor pressure of water at the measurement temperature. The data provide a direct indication of the existence of a phase transition in the liquid at 307.7 K, 5.8 kPa (34.6 degrees C, 0.057 atm). The minimum in the heat capacity, C(p)(), of water occurs at 34.5 degrees C, 1.0 atm (J. Res. Natl. Bur. Stand. 1939, 23, 197(1)). The minimum in C(p)() is shallow, and the transition is a weak-continuous phase transition. The pressure coefficient of the viscosity of water changes sign as pressure increases for temperatures below 35 degrees C (Nature 1965, 207, 620(2)). The viscosity minimum tracks the liquid phase transition in the P, T plane where it connects with the minimum in the freezing point of pure water in the same plane (Proc. Am. Acad. Arts Sci. 1911-12, 47, 441(3)). Previously we argued (J. Chem. Phys. 1998, 109, 7379(4)) that the minimum in the pressure coefficient of viscosity signaled the elimination of three-dimensional connectivity in liquid water. These observations coupled with recent measurements of the coordination shell of water near 300 K (Science 2004, 304, 995(5)) suggest that the structural component that changes during the phase transition is tetrahedrally coordinated water. At temperatures above the transition, there is no tetrahedrally coordinated water in the liquid and locally planar water structures dominate the liquid structure. Water is a structured liquid with distinct local structures that vary with temperature. Furthermore, liquid water has a liquid-liquid phase transition near the middle of the normal liquid range.     

Identification of free radicals induced by UV irradiation in collagen water solutions

Electron paramagnetic resonance (EPR) method has shown that hydrogen atoms and acetic acid free radicals appear in surrounding acetic acid-water solution of collagen under ultraviolet (UV) irradiation. These free radicals interact with the collagen molecule; consequently, seven superfine components of EPR spectrum with the split of aH = 11.3G and g-factor 2.001 appear. It is assumed that this spectrum is related to the free radical occurred on the proline residue in collagen molecule. In order to discover .OH hydroxyl radicals even in minor concentration, spin trap 5.5-dimethyl-1-pyrroline N-oxide (DMPO) has been applied. During the irradiation of collagen water solution in the presence of spin trap, EPR spectrum of the DMPO/.OH adduct has not been identified, while the above mentioned spectrum has been observed once the hydrogen peroxide H2O2 and FeSO4 were added to the sample. That means that water photolysis does not take place in collagen water-solution due to UV irradiation. It was suggested that occurrence of hydrogen radical is connected with the electron transmission to the hydrogen ion. The possible source of free electrons can be aromatic residues, photo ionization of which takes place in collagen molecule due to UV irradiation.     

Experimental evidence for an inverse hydrogen migration in arginine radicals

Radicals formed by electron transfer to protonated arginine have been predicted by theory to undergo an inverse migration of the hydrogen atom from the C(alpha) position to the guanidine carbon atom. Experiments are reported here that confirm that a fraction of arginine and arginine amide radicals undergo such an inverse hydrogen migration. The rearranged arginine and arginine amide C(alpha) radicals are detected as stable anions after charge inversion by collisions with Cs atoms of precursor cations at 3 and 50 keV kinetic energies. RRKM calculations on the B3-PMP2/aug-cc-pVTZ potential energy surface indicate that arginine radicals undergo rapid rotations of the side chain to reach conformations suitable for C(alpha)-H transfer, which is calculated to be fast (k > 10(9) s(-1)) in radicals formed by electron transfer. By contrast, H-atom transfer from the guanidine group onto the carboxyl or amide C=O groups is >50 times slower than the C(alpha)-H atom migration. The guanidine group in arginine radicals is predicted to be a poor hydrogen-atom donor but a good H-atom acceptor and thus can be viewed as a radical trap. This property can explain the frequent observation of nondissociating cation radicals in electron capture and electron transfer mass spectra of arginine-containing peptides.     

Direct evidence for hydrated protons as the active species in artificial photocatalytic water reduction into hydrogen

Photocatalytic water reduction to hydrogen over oxide semiconductors is one of the most extensively investigated artificial photocatalytic reactions, but the nature of the active species has not yet been elucidated. Here, we successfully prepared Pt/rutile TiO_2(110) surfaces with hydrated proton species via co-adsorption of hydrogen and water and observed the photocatalytic reduction of hydrated protons to H_2 upon UV light illumination. These results provide experimental evidence to prove hydrated protons as the active species for photocatalytic water reduction to hydrogen and demonstrate the occurrence of photocatalytic reduction of hydrated protons to H_2 within the H-bonding network on the catalyst surface instead of directly on the catalyst surface. The Pt-TiO_2 interface is capable of dissociating water to form hydroxyl groups that facilitate the formation of H-bonding network on the catalyst surface to enhance the photocatalytic H_2 production. Our results greatly advance fundamental understanding of artificial photocatalytic water reduction.     

Direct evidence for symmetry control in cyclodextrin-water interactions

Abstract

The cyclodextrins, cyclic oligosaccharides possessing generally 6, 7, or 8 α, 1 –4 linked glucopyranose units, are widely used for the solubilisation and transport of organic molecules in aqueous media.³ Their solubilities 145 gL^?1, α-Cd; 20 gL^?1, β-CD; 220 gL^?1, γ-CD, are considerably lower than those of simple saccharides and in particular that of β-CD, the most widely available compound, is anomalously low. We have previously shown that the solubilities arise from symmetry determined interactions of the CDs with the dynamic hexagonal structure of water. In the case of the unmodified cyclodextrins all three compounds exist in solution as large aggregates, thus the solubility of these systems is controlled by the interactions between these hydrogen bonded aggregates and the hydrogen bonding networks present in water. In the case of α- or γ-CD there are favorable overlaps with the hexagonal water structure, however for the seven-fold symmetry of β-CD no such favorable interactions can occur between the odd symmetry element and the even symmetry elements of water. Hence the observed solubilities of α- and γ-CD are higher than that of β-CD.     

Rate constants for one-electron oxidation by methylperoxyl radicals in aqueous solutions

Rate constants for one-electron oxidation by the methylperoxyl radicals (CH₃O₂, HOCH₂O₂, ^?O₂CCH₂O₂, and CCI₃O₂) in aqueous solutions have been measured by pulse radiolysis and found to be in the range of 3 × 10⁵ to 6 × 10⁸ M^?1 s^?1 for compounds with redox potentials between 0.6 and 0.1 V. Substitution on the methylperoxyl radical with OH or CO₂^? has only a minor effect on the rate of oxidation but substitution with three chlorines increases the rate constants by two orders of magnitude. The redox potential of the CH₃O₂ radical is estimated to be 0.6–0.7 V.     

High electric field induced radicals in organic solutions

Experiments for production of radicals by high electric field emission from nitrobenzene in polar and non-polar solutions are reported. A sequence of radicals has been identified by their ESR spectra and their relation to possible charge carriers in organic liquids is discussed.     

Quantum diffusion of hydrogen and muonium atoms in liquid water and hexagonal ice

We have used the ring polymer molecular dynamics method to study the diffusion of muonium, hydrogen, and deuterium atoms in liquid water and hexagonal ice over a wide temperature range (8-361 K). Quantum effects are found to dramatically reduce the diffusion of muonium in water relative to that predicted by classical simulation. This leads to a simple explanation for the lack of any significant isotope effect in the observed diffusion coefficients of these species in the room temperature liquid. Our results indicate that the mechanism of the diffusion in liquid water is similar to the intercavity hopping mechanism observed in ice, supplemented by the diffusion of the cavities in the liquid. Within the same model, we have also been able to simulate the observed crossover in the c-axis diffusion coefficients of hydrogen and deuterium in hexagonal ice. Finally, we have been able to obtain good agreement with experimental data on the diffusion of muonium in hexagonal ice at 8 K, where the process is entirely quantum mechanical.     

Direct evidence for distinct colour origins in ROY polymorphs

ROY is one of the most well-studied families of crystal structures owing to it being the most polymorphic organic material on record. The various red, orange, and yellow colours of its crystal structures are widely-believed to originate from molecular conformation, though the orange needle (ON) polymorph is thought to be an exception. We report high-pressure, single-crystal X-ray measurements which provide direct experimental evidence that the colour origin in ON is intermolecular, revealing that the molecule undergoes minimal deformation but still exhibits a pronounced, reversible, pale orange → dark red colour change between ambient pressure and 4.18 GPa. Our experimental data are rationalised with band structures, calculated using an accurate hybrid DFT approach, where we are able to account for the variation in colour for five polymorphs of ROY. We highlight the outlier behaviour of ON which shows marked π⋯π stacking interactions that are directly modified through application of pressure. Band structure calculations confirm these intermolecular interactions as the origin of the colour change.

Alternative colour origins in ROY polymorphs are conclusively determined for the first time, using high-pressure diffraction and hybrid DFT.