Infrared spectra and UV-induced photochemistry of methyl aziridine-2-carboxylate isolated in argon and xenon matrices

Methyl aziridine-2-carboxylate (MA2C) has been isolated in low temperature argon and xenon matrices and its structure and photochemistry were studied by FTIR spectroscopy. The reactant as well as the main photoproducts were characterized by comparison of their experimental IR spectra with spectra calculated at the DFT(B3LYP)/6-311++G(d,p) level. The theoretical calculations predicted the existence of two low energy MA2C conformers, differing by the orientation of the OCCN dihedral angle. Both conformers were identified in the studied matrices. Both narrowband tunable and broadband UV irradiations of matrix-isolated MA2C yielded isomerization photoproducts resulting from cleavage of the CC and weakest CN bonds of the aziridine ring. Irradiation with UV laser-light at λ = 235 nm resulted in the formation of the E isomer of methyl 2-(methylimino)-acetate (MMIA) and the Z isomer of methyl 3-iminopropanoate (M3IP). Subsequent irradiation at 290 nm led to observation of new bands resulting from E → Z isomerization of MMIA, while bands due to M3IP remained unchanged. The photoproduced Z isomer of MMIA could be subsequently consumed upon higher-wavelength irradiation (λ = 330 nm). The initially produced MMIA conformer was found to obey the nonequilibrium of excited rotamers (NEER) principle. No photoproducts resulting from the cleavage of the strongest CN bond of the MA2C aziridine ring were observed, nor that of methyl 3-aminoacrylate (M3AA), which could in principle be obtained also by cleavage of the weakest CN bond of the MA2C aziridine ring, but would imply a different H-atom migration simultaneous with the ring opening process. These results indicate that both the differential electronic characteristics of the CN bonds of substituted aziridine rings and the type of required H-atom migration are major factors in determining the specific photochemistries of substituted aziridines. Photofragmentation reactions of MA2C were also observed, through identification of various related products, e.g., acetonitrile, methanol, methane, CO and CO₂.     

The far from equilibrium structure of argon clusters doped with krypton or xenon

Heterogeneous clusters created by doping Ar host clusters with Kr or Xe are shown to have radically different structures from the mixed clusters of the same type created by co-expansion of Ar-Kr or Ar-Xe gas mixtures. In contrast to the co-expansion case, the doped mixed clusters can be produced with Kr or Xe on the surface and Ar in the bulk. With the doping technique it is thus possible to control the surface composition of a specific cluster. A study of the cluster properties as a function of the doping pressure is also reported for the case of Ar clusters doped with Xe. The clusters have been studied by means of synchrotron radiation based X-ray photoelectron spectroscopy.     

Hexagonal close packed krypton and xenon

Hexagonal close packed krypton and xenon were observed in vapour phase grown thin films. At 26 K, a₀ = 4.00 Å and c = 6.53 Å for Kr; a₀ = 4.34 Å and c = 7.09 Å for Xe.     

The vaporization properties of krypton and xenon

The vaporization properties of krypton and xenon are reassessed from the experimental results published in the literature, more sparsely since the 1970s. The measured vapour pressure and the saturated liquid density of both substances are adjusted to empirical, reliable equations. Results are presented for the calculated vapour pressures and the orthobaric densities of the two substances. The coordinates of the triple point, the normal boiling temperature, and the critical point are discussed as well as the corresponding values of the enthalpies of vaporization. Argon, for which recent experimental, accurate work has been published, was kept out of the present review.     

Infrared intensity in small ammonia and water clusters

Helium droplet technique has been used in order to measure the strength of the infrared absorption in small ammonia and water clusters as a function of size. Hydrogen bonding in ammonia and water dimers causes an enhancement of the intensity of the hydrogen stretching bands by a factor of four and three, respectively. Two types of the hydrogen bonded clusters show different size dependence of the infrared intensity per hydrogen bond. In ammonia (NH3)2 and (NH3)3 it is close to the crystal value. In water clusters, it increases monotonically with cluster size being in tetramers, a factor of two smaller than in the ice. The measured infrared intensity in water clusters is found to be a factor of two to three smaller as compared to the results of numerical calculations.     

Infrared absorption spectra of carbon monoxide in rare gas matrices

Medium and high resolution infrared spectra of CO trapped in solid Ne, Ar, Kr and Xe are presented. Spectra of low optical density samples are identical to those previously observed but a different assignment is proposed. Monomeric absorption is clearly identified by double doping experiments and by sample deposition through a nitrogen cold trap. In addition other broad weak bands are observed on the spectra of high optical density samples. These bands are assigned to combinations between internal vibration, libration and lattice vibrations. The barrier hindering rotation due to site distortion is estimated to 30–50 cm^?1. Librational motion is strongly perturbed by coupling with lattice motion. Carbon monoxide is also an efficient probe to study the dynamical properties of rare gas lattices.     

High-resolution spectra of the 1(1,1) ← 0(0,0) rotational transition of water in argon and krypton matrices

The high-resolution spectra of H₂¹⁶O, H₂¹⁸O, HD¹⁶O, HD¹⁸O, D₂¹⁶O, and D₂¹⁸O isolated in argon and krypton matrices are reported in the 10–60 cm^?1 region. The high resolution is obtained by observing very dilute mixtures of water in the rare gases. No discrete absorbances attributable to pure argon or krypton are found in this region. In all cases the relative gas—matrix shifts of the isotopimers are consistent with the rotation—translation coupling model. The linewidths are found to correlate with rotation—translation coupling. The integrated intensity of the 1(1,1) — 0(0,0) transition of HDO exhibits a temperature dependence which does not deviate significantly from that expected from a Boltzmann distribution in rigid rotor energy levels.     

Photochemical reaction of sulfur hexafluoride with water in low-temperature xenon matrices

Sulfur hexafluoride SF(6) is a very stable molecule with which very few reactions with other molecules have been reported. Here, we report a photochemical reaction of SF(6) with water molecules using a matrix-isolation technique, where SF(6) and H(2)O were co-condensed in Xe matrices, and the products were observed using infrared spectroscopy. Irradiation at 193 nm from an ArF excimer laser caused the simultaneous decomposition of SF(6) and H(2)O, which resulted in the production of novel species. Infrared spectra and molecular orbital calculations of the species showed that the product was a SF(4)?HF?HOF complex, which consists of hydrogen bonds and charge transfer interaction between S and F atoms. The assignment of the species was confirmed by isotope shifts using D and (18)O isotope substitutions.     

Infrared spectra of arene matrices containing some atomic metals

Codeposition of atomic iron, cobalt, or nickel at liquid nitrogen temperature with benzene or toluene produced unstable arene—metal π-complexes whose i.r. spectra were analogous to those of the known and thermally stable bis-(arene)chromium. The presence of both a 1:1 precursor and a more stable 2:1 arene—metal complex is suggested by the temperature dependence of the spectra of some of these systems.     

Some reactions with krypton difluoride and xenon fluorides

The VF₅XeF_x (x being 2,4 and 6) system was sistematically investigated. Besides XeF_2^·.VF₅ (1) and 2XeF_6^·.VF₅ (2), new adducts XeF_6^·.VF₅ and XeF_6^·.2VF₅ were also isolated. The obtained adducts were characterized by following mass balance throughout the experiment, by Raman and IR spectroscopy, by recording the melting point - composition diagram, etc.The results of reactions in a system with some binary fluorides (e.g. TiF₄, MnF₂, CrF₃) and KrF₂ are also discussed.     

The sublimation of argon, krypton, and xenon

In this paper, the sublimation pressure of Ar, Kr, and Xe are obtained as functions of temperature from an exactly integrated form of the Clapeyron equation. No fitting to experimental data of the equilibrium pressure has been necessary. The deviation plots of the sublimation pressure show that the results are satisfactory. The derived enthalpy of sublimation of the three rare gases from T = 0 K up to their respective triple point temperatures are asymmetric, distorted parabolas showing maxima for . The , which is a measure of the cohesion energy of the solid crystals, is easily calculated. A general equation has been obtained for as a function of temperature which also gives the enthalpies of sublimation of neon and radon. The (s + ? + g) triple point coordinates of Rn are reassessed.     

Infrared spectra of 1,3-butadiene matrices containing some atomic metals

Low-temperature matrices of 1,3-butadiene containing Ni, Pd, Fe or Mg atoms were prepared by codeposition at − 190°C, and their i.r. spectra were examined as the matrices were warmed to various temperatures. With Ni atoms, a charge-transfer complex, NiC₄H₆, whose i.r. spectrum was indistinguishable from that of solid C₄H₆, was formed initially. It rearranged at about −130°C into a yellow π-complex Ni(C₄H₆)₂ which in turn transformed rapidly into a more stable red polymeric π-complex (NiC₄H₆)_n. The spectra of these π-complexes showed that the butadiene ligands were in the cis configuration. The polymer decomposed at about −80°C into butadiene, metallic nickel and traces of organonickel residues. It also reacted with water to give butane and butenes. Matrices with Pd or Fe atoms behaved similarly but only a charge-transfer complex was observed with Mg atoms.     

Infrared spectra of monomeric thioformaldehyde in Ar,N2 and Xe matrices

Pyrolysis products of 1,3-dithietane were trapped in low-temperature matrices and their infrared spectra were recorded. The vibrational bands of monomeric H₂CS, which was not perturbed by other decomposition products, were identified in Ar, N₂ and Xe matrices. These results will be useful to evaluate the frequency shifts of H₂CS induced by the interaction with other molecules.     

The fusion curves of xenon,krypton, and argon

The experimental results on the fusion of the heavier rare gases at very high pressures, obtained in the last 20 years, are examined and analysed in conjunction with the measurements made at lower pressures from 1940 onwards. The parameters in the Simon equation for the melting curves of Xe, Kr, and Ar are determined, and the coordinates of a possible high-pressure {s(fcc) + s(hcp) + ?} triple-point are identified for each one of these three elements. The enthalpies of transition of the transformations involved are estimated as well as their respective values of the entropies of transition.     

Photoluminescence spectra of clusters of group IV elements embedded in SiO2 matrices

Using a rf co-sputtering technique, SiO₂ films containing C, Si and Ge atoms were prepared and their Raman and photoluminescence spectra were measured. Raman spectra obtained are very broad indicating that the group IV elements are embedded in the form of clusters. The samples containing C clusters showed a very strong photoluminescence peak at around 2.1～2.2eV (visible with naked eye). The films containing Ge and Si clusters showed luminescence peaks at around 1.6～1.8eV. Strong luminescence was not observed for the samples containing well grown microcrystals a few nanometers in diameter.     

Electron impact fragmentation of size-selected krypton clusters

Clusters of krypton are generated in a supersonic expansion and size selected by deflection from a helium target beam. By measuring angular distributions for different fragment masses and time-of-flight distributions for fixed deflection angles and fragment masses, the complete fragmentation patterns for electron impact ionization at 70 eV are obtained from the dimer to the heptamer. For each of the neutral Kr(n) clusters studied, the main fragment is the monomer Kr(+) ion with a probability f(n)(1) > 90%. The probability of observing dimer Kr(2)(+) ions is much smaller than expected for each initial cluster size. The trimer ion Kr(3)(+) appears first from the neutral Kr(5), and its fraction increases with increasing neutral cluster size n, but is always much smaller than that of the monomer or dimer. For neutral Kr(7), all possible ion fragments are observed, but the monomer still represents 90% of the overall probability and fragments with n > 3 contribute less than 1% of the total. Aspects of the Kr(n) cluster ionization process and the experimental measurements are discussed to provide possible reasons for the surprisingly high probability of observing fragmentation to the Kr(+) monomer ion.     

Adsorption of polar molecules on krypton clusters

The formation process of binary clusters has been studied using synchrotron based core level photoelectron spectroscopy. Free neutral krypton clusters have been produced by adiabatic expansion and doped with chloromethane molecules using the pickup technique. The comparison between the integrated intensities, linewidths, and level shifts of the cluster features of pure krypton and of chloromethane-krypton clusters has been used to obtain information about the cluster geometry. We have shown that most of the chloromethane molecules remain on the surface of the clusters.     

The density dependence of hot-electron thermalization in liquid argon,krypton and xenon

The time required for thermalization of hot electrons in liquid argon, krypton and xenon increases with decreasing density to a minimum value of ≈ 7 ns at a density of ≈ 1.2 × 10²² cm^?3. Previous data for the solids are found to lie an extrapolations of the liquid-phase density dependence.     

The density dependence of the quasi-free electron state in fluid xenon and krypton

Photoinjection measurements show that in liquid and gaseous krypton and xenon V₀ is negative. V₀ has a minimum (?0.77 eV for Xe and ?0.53 eV for Kr) near the density of the mobility maximum, becoming zero at densities of the order of 10²⁰ atoms/cm³.