Mechanistic study of proton transfer and HD exchange in ice films at low temperatures (100-140 K)

We have examined the elementary molecular processes responsible for proton transfer and HD exchange in thin ice films for the temperature range of 100-140 K. The ice films are made to have a structure of a bottom D(2)O layer and an upper H(2)O layer, with excess protons generated from HCl ionization trapped at the D(2)OH(2)O interface. The transport behavior of excess protons from the interfacial layer to the ice film surface and the progress of the HD exchange reaction in water molecules are examined with the techniques of low energy sputtering and Cs(+) reactive ion scattering. Three major processes are identified: the proton hopping relay, the hop-and-turn process, and molecular diffusion. The proton hopping relay can occur even at low temperatures (<120 K), and it transports a specific portion of embedded protons to the surface. The hop-and-turn mechanism, which involves the coupling of proton hopping and molecule reorientation, increases the proton transfer rate and causes the HD exchange of water molecules. The hop-and-turn mechanism is activated at temperatures above 125 K in the surface region. Diffusional mixing of H(2)O and D(2)O molecules additionally contributes to the HD exchange reaction at temperatures above 130 K. The hop-and-turn and molecular diffusion processes are activated at higher temperatures in the deeper region of ice films. The relative speeds of these processes are in the following order: hopping relay>hop and turn>molecule diffusion.     

Adsorption of hydrogen on zeolite NaA at low temperatures

A volumetric method was used to measure the hydrogen adsorption isotherms at 23–100°K and from 10^–2 to 65 kPa. A thermodynamic analysis was carried out for the experimental results, which were compared with literature data.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2140–2142, September, 1990.     

Adsorption of molecular hydrogen on aluminophosphate zeolites at 77 K

The H₂ sorption properties of the aluminophosphate zeolites AlPO-5, AlPO-31, AlPO-11, AlPO-36, and AlPO-8 at 77 K have been investigated. A series of H₂ adsorption isotherms has been obtained for cylindrical micropore channels in the aluminophosphate zeolites. The absolute values of the amount adsorbed α(P) for the mesoporous aluminophosphate materials and the effective density of adsorbed H₂ in the micropore space β*(P, d) have been determined. It has been demonstrated experimentally that the sorbate density depends on the size of the micropore channel of the zeolite d. Hydrogen sorption isotherms have been calculated from experimental isotherms. A procedure allowing β*(P, d) to be estimated for intermediate d values is presented.     

Adsorption of hydrogen on multiwalled carbon natotubes at 77 K

Adsorption of H₂ on multiwalled carbon nanotubes was measured at 77 K by a volumetric method. Adsorption and desorption isotherms are largely reversible. The adsorption capacity increased remarkably after receiving heat treatment at 400 °C and being pressed into pellets. The isotherms show typical feature of supercritical adsorption and were satisfactorily modeled by the model that applied for usual supercritical adsorption. The maximal adsorption capacity of hydrogen under the condition tested is less than 0.5 wt%.     

Adsorption and hydrolysis of alcohols and carbonyls on ice at temperatures of the upper troposphere

The uptake of gaseous ethanol, 1,1,1-trifluoroethanol, acetone, chloral (CCl(3)CHO), and fluoral (CF(3)CHO) on ice films has been investigated using a coated-wall flow tube at temperatures 208-228 K corresponding to the upper troposphere (UT), with a mass spectrometric measurement of gas concentration. The uptake was largely reversible and followed Langmuir-type kinetic behavior, i.e., surface coverage increased with the trace gas concentration approaching a maximum surface coverage at a gas phase concentration of N(max) ～ (2-4) × 10(14) molecules cm(-3), corresponding to a surface coverage of ～30% of a monolayer (ML). The equilibrium partition coefficients, K(LinC), were obtained from the experimental data by analysis using the simple Langmuir model for specific conditions of temperature and concentration. The analysis showed that the K(LinC) depend only weakly on surface coverages. The following expressions described the temperature dependence of the partition coefficients (K(LinC)) in centimeters, at low coverage for ethanol, trifluoroethanol, acetone, chloral, and fluoral: K(LinC) = 1.36 × 10(-11)?exp(5573.5/T), K(LinC) = 3.74 × 10(-12)?exp(6427/T), K(LinC) = 3.04 × 10(-9)?exp(4625/T), K(LinC) = 7.52 × 10(-4)?exp(2069/T), and K(LinC) = 1.06 × 10(-2)?exp(904/T). For acetone and ethanol the enthalpies and entropies of adsorption derived from all available data showed systematic temperature dependence, which is attributed to temperature dependent surface modifications, e.g., QLL formation. For chloral and fluoral, there was an irreversible component of uptake, which was attributed to hydrate formation on the surface. Rate constants for these surface reactions derived using a Langmuir-Hinshelwood mechanism are reported.     

Adsorption study of acetone on acid-doped ice surfaces between 203 and 233 K

Adsorption studies of acetone on pure ice surfaces obtained by water freezing or deposition or on frozen ice surfaces doped either with HNO3 or H2SO4 have been performed using a coated wall flow tube coupled to a mass spectrometric detection. The experiments were conducted over the temperature range 203-233 K and freezing solutions containing either H2SO4 (0.2 N) or HNO3 (0.2-3 N). Adsorption of acetone on these ice surfaces was always found to be totally reversible whatever were the experimental conditions. The number of acetone molecules adsorbed per ice surface unit N was conventionally plotted as a function of acetone concentration in the gas phase. For the same conditions, the amount of acetone molecules adsorbed on pure ice obtained by deposition are about 3-4 times higher than those measured on frozen ice films, H2SO4-doped ice surfaces lead to results comparable to those obtained on pure ice. On the contrary, N increases largely with increasing concentrations of nitric acid in ice surfaces, up to about 300 times under our experimental conditions and for temperatures ranging between 213 and 233 K. Finally, the results are discussed and used to reestimate the partitioning of acetone between the ice and gas phases in clouds of the upper troposphere.     

Vacuum ultraviolet photodissociation and surface morphology change of water ice films dosed with hydrogen chloride

Time-of-flight (TOF) spectra of photofragment H atoms from the photodissociation of water ice films at 193 nm were measured for amorphous and polycrystalline water ice films with and without dosing of hydrogen chloride at 100-145 K. The TOF spectrum is sensitive to the surface morphology of the water ice film because the origin of the H atom is the photodissociation of dimerlike water molecules attached to the ice film surfaces. Adsorption of HCl on a polycrystalline ice film was found to induce formation of disorder regions on the ice film surface at 100-140 K, while the microstructure of the ice surface stayed of polycrystalline at 145 K with adsorption of HCl. The TOF spectra of photofragment Cl atoms from the 157 nm photodissociation of neutral HCl adsorbed on water ice films at 100-140 K were measured. These results suggest partial dissolution of HCl on the ice film surface at 100-140 K.     

Release of oxygen atoms and nitric oxide molecules from the ultraviolet photodissociation of nitrate adsorbed on water ice films at 100 K

Production of O((3)P(J), J = 2, 1, 0) atoms from the 295-320 nm photodissociation of NO(3)- adsorbed on water polycrystalline ice films at 100 K was directly confirmed using the resonance-enhanced multiphoton ionization technique. Detection of the O atom signals required an induction period after deposition of HNO3 onto the ice film held at 130 K due to the slow ionization rate of HNO(3) to H+ and NO(3)- with a rate constant of k = (5.3 +/- 0.2) x 10(-3)s(-1). Translational energy distributions of the O atoms were represented by a combination of two Maxwell-Boltzmann energy distributions with translational temperatures of 2000 and 100 K. Direct detection of NO from the secondary photodissociation process was also successful. On the atmospheric implications, the influence of the direct release of the oxygen atoms into the air from NO(3)- adsorbed on the natural snowpack was included in an atmospheric model calculation on the mixing ratios of ozone and nitric oxide at the South Pole, and the results compared favorably with the field data.     

Complex formation of vinylidene chloride with hydrogen chloride in the solid phase at low temperatures

Moscow State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 2, pp. 170–174, March–April, 1992.     

The infrared spectra of amorphous solid water and ice Ic between 10 and 140 K

Absorption spectra from 4000 to 1200 cm^?1 of amorphous solid water and polycrystalline ice I_c have been measured between 10 K and 140 K. Warm up and recooling of an H₂O sample prepared at 10 K gives rise to both irreversible and reversible changes in the peak frequency, band width, and peak height as well as the integrated intensity of the OH stretching band. These spectral effects are related to structural differences. The structure of amorphous solid water also depends on deposition conditions. The optical constants of amorphous so water are determined at 10 K and 80 K from a Kramers-Kronig analysis of the transmission spectra taking into account reflection and interference losses. The astrophysical implication of the temperature dependence of peak frequency and band width of the 3250 cm^?1 band in amorphous solid water is discussed briefly.     

Radiative association of He+ with H2 at temperatures below 100 K

The paper presents a theoretical study of the low-energy dynamics of radiative association processes in the He+ + H2 collision system. Formation of the triatomic HeH2(+) ion in its bound rotation-vibration states on the potential-energy surfaces of the ground and of the first excited electronic states is investigated. Close-coupling calculations are performed to determine detailed state-to-state characteristics (bound <-- free transition rates, radiative and dissociative widths of resonances) as well as temperature-average characteristics (rate constants, photon emission spectra) of the two-state (X <-- A) reaction He+(2S) + H2(X1sigma(g)+) --> HeH2(+)(X2A') + h nu and of the single-state (A <-- A) reaction He+(2S) + H2(X1sigma(g)+) --> HeH2(+)(A2A') + h nu. The potential-energy surfaces of the X- and A-electronic states of HeH2(+) and the dipole moment surfaces determined ab initio in an earlier work [Kraemer, Spirko, and Bludsky, Chem. Phys. 276, 225 (2002)] are used in the calculations. The rate constants k(T) as functions of temperature are calculated for the temperature interval 1 < or = T < or = 100 K. The maximum k(T) values are predicted as 3.3 x 10(-15) s(-1) cm3 for the X <-- A reaction and 2.3 x 10(-20) s(-1) cm3 for the A <-- A reaction at temperatures around 2 K. Rotationally predissociating states of the He+-H2 complex, correlating with the upsilon = 0, j = 2 state of free H2, are found to play a crucial role in the dynamics of the association reactions at low temperatures; their contribution to the k(T) function of the X <-- A reaction at T < 30 K is estimated as larger than 80%. The calculated partial rate constants and emission spectra show that in the X <-- A reaction the HeH2(+)(X) ion is formed in its highly excited vibrational states. This is in contrast with the vibrational state population of the ion when formed via the (X <-- X) reaction He(1S) + H2(+)(X2sigma(g)+) --> HeH2(+)(X2A') + h nu.     

Direct observation of OH radicals ejected from water ice surface in the photoirradiation of nitrate adsorbed on ice at 100 K

Production of gaseous OH radicals in the 248-350 nm photoirradiation of NO3(-) doped on amorphous ice at 100 K was monitored directly by using resonance-enhanced multiphoton ionization. The translational energy distribution of the OH product was represented by a Maxwell-Boltzmann energy distribution with the translational temperature of 3250 +/- 250 K. The rotational temperature was estimated to be 175 +/- 25 K. We have confirmed that the OH production should be attributed to the secondary photolysis of H2O2 produced on ice surface on the basis of the results of controlled photolysis experiments for H2O2 doped on ice surface.     

Temperature, composition, and hydrogen isotope effect in the hydrogenation of CO on amorphous ice surface at 10-20 K

An experiment on the addition reaction of a D atom (deuteration) to CO on a cold ice surface is performed by deuterium atom exposure of three types of samples (pure solid CO, CO-capped H2O ice, and CO-H2O mixed ice) at 10-20 K. The variation of IR absorption spectra for the samples was measured by a Fourier transform infrared spectrometer during exposure to deuterium atoms. Reactions on pure solid CO were observed only at 10 K, while reactions on CO-capped H2O ice and CO-H2O mixed ice were observed to proceed even at 20 K. This indicates that the coexistence of H2O at the surface raises the reactive temperature. In addition, the experiment on H atom exposure was also carried out at 15 K to compare the reaction rate constant between the H and D atoms. The ratio of reaction rate constant kD/kH obtained is about 0.08 at 15 K. The authors provide information on the potential energy for the H+CO reaction at the surface by using the ratio kD/kH and by a model calculation of the potential tunneling with the asymmetric Eckart potential.     

Spectroscopic and computational evidence for SO2 ionization on 128 K ice surface

The experimental part of this study focuses on FTIR spectroscopy of SO(2) adsorbate on the surface of ice nanoparticles at 128 K, in the 0.5-1 monolayer coverage range. In addition to the infrared spectroscopic features due to molecular SO(2), a structured band is observed at approximately 1030 cm(-1). A similar band was observed in past spectroscopic studies of SO(2) aqueous solutions, and assigned to anionic products of SO(2) ionization. Ab initio normal mode analysis of HSO(3)(-) yielded intense SO stretch bands in the vicinity of the observed "ionic" feature. The relative intensities of the molecular and the anionic bands indicate that 0.3 approximately 0.5 of the adsorbate is ionized. These results are consistent with the previously published data on isotopic exchange in SO(2)-covered ice nanoparticles (Devlin and Buch, J. Chem. Phys., 2007, 127, 091101) which pointed towards substantial SO(2) ionization at low temperatures. Density functional theory modeling of molecular and ionized adsorbate on a crystal ice slab suggests that anion solvation by molecular SO(2) adsorbate facilitates the SO(2) ionization process at the ice surface.     

Adsorption of neon on zeolite NaA at low temperatures

A volumetric method was used to measure the adsorption isotherms of neon at from 28 to 100 K and from 10^–2 Pa to 65 kPa. A thermodynamic analysis was carried out for the experimental data and these results were compared with literature data.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2137–2139, September, 1990.     

Hydrogen peroxide formation following the vacuum ultraviolet photodissociation of water ice films at 90 K

The production of gaseous OH radicals from the 300-350 nm photodissociation of H(2)O(2) that was photolytically produced on a water ice surface following the 157 nm photolysis of water ice at 90 K was directly monitored using resonance-enhanced multiphoton ionization. The translational energy distribution estimated by the time-of-flight spectrum of the OH products is represented by a Maxwell-Boltzmann energy distribution with a translational temperature of 3750+/-250 K. The rotational temperature was estimated by a spectral simulation to be 225+/-25 K. Surface defects produced by HCl deposition on the water ice contributed to the higher production rate of H(2)O(2) in the 157 nm photoirradiation of water ice while surface coverage caused by CD(3)OH deposition decreased the H(2)O(2) production rate.     

Reactions of methyl radicals with propene at temperatures between 750 and 1000 K

The pyrolysis of propene, initiated by methyl radicals, has been studied in the temperature range 750-1000 K and at a pressure of 0.13 bar in a quasi-wall-free reactor using laser heating by fast vibrational-translational (V-T) energy transfer. This is a convenient method to study homogeneous high-temperature kinetics since the reactor walls remain cold. The radial temperature distribution in the reactor has been investigated by four different methods: a stationary heat balance, optical absorption, pressure rise, and the temperature dependence of the rate of an isomerization reaction. Methyl radicals were produced via the fast thermal dissociation of di-tert-butyl-peroxide and the products were analysed using GC-MS. The main products of the overall reaction of the model system propene and methyl (C3H6 + CH3) were isopentane (iso-C5H12) and but-1-ene (1-C4H8), whereas allene (C3H4), trans-but-2-ene (trans-2-C4H8) and cis-but-2-ene (cis-2-C4H8) were minor components, all showing a strong dependence on temperature. The product distribution and the temperature dependence were analysed by a kinetic model of 61 species and 166 reactions developed for the high-temperature oxidation of butane and the low-temperature oxidation of n-pentane and isopentane. It was necessary to include a few missing reactions and to adjust some rate constants to make the modeling agree with the experimental investigations. This extended mechanism has to be evaluated further in forthcoming experiments.