Óscar Gálvez

Spectroscopic Observation of Matrix-Isolated Carbonic Acid Trapped from the Gas Phase**

) is of fundamental importance, forexample,forregulationofbloodpH,intheacidificationoftheoceans, and in the dissolution of carbonates. This six-atommolecule commonly found in carbonated drinks in submicro-molar concentrations has so far eluded most attempts atisolation and direct detection. Despite the widespread beliefthat it is a highly instable molecule, the pure solid could beprepared previously,

A study of the interaction of CO2 with water ice

Aims. We studied the interaction between CO2 (guest) and H2O (host) molecular ices. Methods. Ices of CO2 and H2O are prepared by four di erent deposition techniques: sequential deposition (amorphous water ice followed by addition of CO2), co-deposition (both gases added simultaneously), inverse sequential deposition (carbon dioxide ice fol- lowed by addition of water) and crystalline sequential deposition (crystalline water ice is prepared first and CO2 is added afterwards). Samples are deposited at 80 K and are studied by temperature programmed desorption and transmission infrared spectroscopy. Results. Two slightly di erent varieties of association of CO2 and H2O are revealed from the di erent spectroscopic properties of the asymmetric stretching band of 12 CO2 and 13 CO2. The two varieties are found to co-exist in some of the samples at 80 K, whereas only the so-called internal CO2 remains after heating at 105 K. At 80 K carbon dioxide is able to adhere to a crystalline water ice surface. Activation energies for the desorption of CO2 from amorphous (Ed = 20:7 2 kJ mol 1 ) and crystalline (Ed = 19:9 2 kJ mol 1 ) water ice are derived from measurements of the sticking of CO2 as a function of ice temperature. Conclusions. These findings may have implications for the study of icy bodies of the Solar System.

Phases of Solid Methanol

The solid phases of methanol were investigated using IR spectroscopy and numerical calculations with the SIESTA method. Improved spectra are reported of amorphous methanol at 90 K, and in particular of the alpha and beta phases at 130 and 165 K, respectively, with assignments of bands not previously measured. The main features of the spectra of each phase are discussed and compared. A study of spectral changes with temperature leads to the conclusion that the metastable phase previously reported might be a mixture of the two known stable phases. Such a mixture could explain all spectral features observed in this investigation. The theoretical calculations provide reasonable agreement with the experimental data for most of the parameters, but predict H-bondings stronger than those observed. Differences between the spectra of the alpha and beta phases are predicted with similar characteristics to the experimental results.

Interaction of CH4 and H2O in ice mixtures

Ice mixtures of methane and water are investigated by means of IR spectroscopy in the 14-60 K range. The spectroscopic research is focused on the symmetry-forbidden nu(1) band of CH(4) and the dangling bond bands of water. The nu(1) band is visible in the spectra of the mixtures, revealing a distorted methane structure which co-exists with the normal crystalline methane. The water dangling bond bands are found to increase their intensity and appear at red-shifted frequency when distorted methane is present. Methane adsorbed on water micropores or trapped inside the amorphous solid water structure is assumed to be responsible for these effects. CH(4) mobility in water ice depends on the deposition method used to prepare the samples and on the temperature. After warming the samples to 60 K, above the methane sublimation point, a fraction of CH(4) is retained in the water ice. An adsorption isotherm analysis is performed yielding the estimation of the desorption energy of CH(4) on H(2)O amorphous surfaces.

AMMONIUM AND FORMATE IONS IN INTERSTELLAR ICE ANALOGS

The so-called hyperquenching technique has been applied to generate water ices containing ammonium and formate ions by sudden freezing of droplets of NH4Cl, NH4COOH, and NaCOOH solutions. Salt deposits were obtained after heating the ices to 210 K to sublimate all water content. All stages are controlled by IR transmission spectroscopy. The NH4 + bands are very much broadened and smeared in the frozen droplets, but stand out strongly when water is eliminated. This fact hints toward the difficulty in ascertaining the presence of this species in astrophysical water-containing ices. Vapor-deposited ices of NH3/HCOOH and H2O/NH3/HCOOH mixtures have also been studied for comparison. HCOO– and NH4 + ions are found to be formed in small proportion even at the lowest temperature, 14 K. By thermal processing, their IR bands become stronger, and at 210 K, after water sublimation, they yield IR spectra similar to those obtained from hyperquenched samples. The observations are interpreted in terms of the varying ion arrangement within the solids along the warming process. A direct comparison to laboratory spectra of irradiated samples, as performed by other groups, is not straightforward.

Ices of CO2/H2O Mixtures. Reflection−Absorption IR Spectroscopy and Theoretical Calculations

Ice mixtures of CO2 and H2O are studied using Fourier transform reflection-absorption infrared (RAIR) spectroscopy. Mixtures are prepared by sequential deposition or co-deposition of the two components from the gas phase onto an Al plate kept at 87 K inside a low-pressure chamber. Two CO2 structures are found in most experiments: a crystalline form similar to pure CO2, which evaporates when warming at 105 K, and a noncrystalline species which remains embedded in amorphous water ice after warming. Significant spectral variations are found depending on the deposition method and the thickness of the solid. Features characteristic of the RAIR technique appear in the spectral regions of the normal modes of the bending and asymmetric stretching CO2 vibrations. Simulations using Fresnel theory and Mie scattering are carried out with acceptable agreement with the experimental spectra of solids of variable thickness, from approximately 1 microm to the limit of nanoparticles. Theoretical calculations of a pure CO2 crystal are performed. The relaxed geometry of the solid and its vibrational spectrum are determined and compared to the experimental results.

SPECTROSCOPIC EFFECTS IN CH4/H2O ICES

Recent observations of CH4 in different astrophysical objects encourage laboratory research on methane/water ice mixtures. An IR spectroscopy laboratory investigation is presented on these systems. Co-deposited samples are formed by vapor deposition of CH4 and H2O on a cold substrate, in a wide range of stoichiometries, from very diluted mixtures to CH4/H2O = 2.5 values. Samples are prepared at 14 K and at 40 K, and their temperature behavior is studied when they are warmed up to 60 K. The spectroscopic analysis is centered on the methane features, and also on the water dangling bonds (DBs) that appear in the spectra of the mixtures. The IR forbidden ν1 band shows up in the spectrum (3.44 μm), indicating some form of distorted methane. The combination bands ν3 + ν4 and ν1 + ν4 are seen at 2.32 and 2.38 μm, and the ν2 + ν3 band weakly at 2.21 μm. Whereas ν3 is not shifted in spectra of mixed samples, the wavenumber peak of ν4 and its combination bands vary in a 6 cm–1 range, providing a possible estimation for the relative methane concentration in the sample. Bands in the spectra of mixtures are always broader than their counterparts in pure CH4 ice. The intensity of ν4 appears to increase in mixed samples with respect to the pure solid. Raising the temperature of the ices up to 60 K liberates part of the methane, but a fraction is retained with a maximum value of ~7% ± 2%. This limit may provide information on the temperature properties of astrophysical objects. The different spectral characteristics of water DBs with increasing methane proportion in mixed samples can also furnish information to estimate the stoichiometry of the mixture.

INFRARED SPECTRA AND THERMODYNAMIC PROPERTIES OF CO2/METHANOL ICES

Ices of mixtures of carbon dioxide and methanol have been studied in a range of temperatures relevant for star-forming regions, comets, polar caps of planets and satellites, and other solar system bodies. We have performed temperature-programmed desorption measurements and recorded IR spectra of various types of samples. The presence of two slightly different structures of CO2 is manifest. A distorted CO2 structure is characterized by bandshifts between 5 cm–1 (ν3) and 10 cm–1 (ν2) with respect to normal CO2. If the samples are heated above 130 K, the distorted CO2 sublimates and only the normal structure remains. The latter can stay trapped until the sublimation of crystalline methanol (150 K). The desorption energy (E d ~ 20 kJ mol–1) of CO2 from methanol ice, and the specific adsorption surface area (6 m2 g–1) of amorphous CH3OH ice, have been determined. CO2 does not penetrate into crystalline ice. Whereas the desorption energy is similar to that of CO2/H2O samples, the specific surface of methanol is much smaller than that of amorphous solid water (ASW). The interaction of CO2 molecules with water and methanol is similar but ices of CH3OH are much less porous than ASW. The inclusion of CO2 into previously formed ices containing these two species would take place preferentially into ASW. However, in processes of simultaneous deposition, methanol ice can admit a larger amount of CO2 than water ice. CO2/CH3OH ices formed by simultaneous deposition admit two orders of magnitude more CO2 than sequentially deposited ices. These findings can have direct relevance to the interpretation of observations from protostellar environments (e.g., RAFGL7009S) and comet nuclei.

On the mechanism of iodine oxide particle formation

The formation of atmospherically relevant iodine oxides IxOy (x = 1,…,3, y = 1,…,7) has been studied experimentally using time-of-flight mass spectrometry combined with a soft ionisation source, complemented with ab initio electronic structure calculations of ionisation potentials and bond energies at a high level of theory presented in detail in the accompanying paper (Galvez et al., 2013). For the first time, direct experimental evidence of the I2Oy (y = 1,…,5) molecules in the gas phase has been obtained. These chemical species are observed alongside their precursors (IO and OIO) in experiments where large amounts of aerosol are also generated. The measured relative concentrations of the IxOy molecules and their dependence on ozone concentration have been investigated by using chemical modelling and rate theory calculations. It is concluded that I2O4 is the most plausible candidate to initiate nucleation, while the contribution of I2O5 in the initial steps is likely to be marginal. The absence of large I3Oy (y = 3,…,6) peaks in the mass spectra and the high stability of the I2O4-I2O4 dimer indicate that dimerisation of I2O4 is the key step in iodine oxide particle nucleation.

Active Site, Catalytic Cycle, and Iodination Reactions of Vanadium Iodoperoxidase: A Computational Study.

A combined computational study using molecular surfaces and Poisson-Boltzmann electrostatic potentials for proteins and quantum calculations on complexes representing the vanadate cofactor throughout the catalytic cycle is employed to study the activity of vanadium iodoperoxidase (VIPO) from alga Laminaria digitata . A model structure of VIPO is compared with available crystal structures of chloroperoxidases (VClPOs) and bromoperoxidases (VBrPOs) focusing on properties of the active site that concern halogen specificity. It is found that VIPO displays distinctive features regarding electrostatic potentials at the site cavity and the local topography of the cavity entrance. Quantum calculations on cofactor stages throughout the catalytic cycle reveal that, while steps involving binding of hydrogen peroxide and halide oxidization agree with available data on VBrPO, final formation and subsequent release of hypohalous acid could follow a different pathway consisting of His476-assisted protonation of bonded hypoiodite and further displacement by a water molecule. Ab initio free energies of reaction computed to explore iodination of organic substrates predict strongly exoergonic reactions with HOI, whereas other possible iodination reagents give thermodynamically disfavored reactions.