The noble gases have a particularly stable electronic configuration, comprising fully filled s and p valence orbitals. This makes these elements relatively non-reactive, and they exist at room temperature as monatomic gases. Pauling predicted in 1933 that the heavier noble gases, whose valence electrons are screened by core electrons and thus less strongly bound, could form stable molecules. This prediction was verified in 1962 by the preparation of xenon hexafluoroplatinate, XePtF6, the first compound to contain a noble-gas atom. Since then, a range of different compounds containing radon, xenon and krypton have been theoretically anticipated and prepared. Although the lighter noble gases neon, helium and argon are also expected to be reactive under suitable conditions, they remain the last three long-lived elements of the periodic table for which no stable compound is known. Here we report that the photolysis of hydrogen fluoride in a solid argon matrix leads to the formation of argon fluorohydride (HArF), which we have identified by probing the shift in the position of vibrational bands on isotopic substitution using infrared spectroscopy. Extensive ab initio calculations indicate that HArF is intrinsically stable, owing to significant ionic and covalent contributions to its bonding, thus confirming computational predictions that argon should form a stable hydride species with properties similar to those of the analogous xenon and krypton compounds reported before.

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Noble-gas chemistry has been undergoing a renaissance in recent years, due in large part to noble-gas hydrides, HNgY, where Ng = noble-gas atom and Y = electronegative fragment. These molecules are exceptional because of their relatively weak bonding and large dipole moments, which lead to strongly enhanced effects of the environment, complexation, and reactions. In this Account, we discuss the matrix-isolation synthesis of noble-gas hydrides, their spectroscopic and structural properties, and their stabilities.This family of species was discovered in 1995 and now has 23 members that are prepared in noble-gas matrices (HXeBr, HKrCl, HXeH, HXeOH, HXeO, etc.). The preparations of the first neutral argon molecule, HArF, and halogen-free organic noble-gas molecules (HXeCCH, HXeCC, HKrCCH, etc.) are important highlights of the field. These molecules are formed by the neutral H + Ng + Y channel. The first addition reaction involving HNgY molecules was HXeCC + Xe + H --> HXeCCXeH, and this led to the first hydride with two noble-gas atoms (recently extended by HXeOXeH). The experimental synthesis of HNgY molecules starts with production of H and Y fragments in solid noble gas via the UV photolysis of suitable precursors. The HNgY molecules mainly form upon thermal mobilization of the fragments.One of the unusual properties of these molecules is the hindered rotation of some HNgY molecules in solid matrices; this has been theoretically modeled. HNgY molecules also have unusual solvation effects, and the H-Xe stretching mode shifts to higher frequencies (up to about 150 cm-1) upon interaction with other species.The noble hydrides have a new bonding motif: HNgY molecules can be represented in the form (H-Ng)+Y-, where (H-Ng)+ is mainly covalent, whereas the interaction between (HNg)+ and Y- is predominantly ionic. The HNgY molecules are highly metastable species representing high-energy materials. The decomposition process HNgY --> Ng + HY is always strongly exoergic; however, the decomposition is prevented by high barriers, for instance, about 2 eV for HXeCCH. The other decomposition channel HNgY --> H + Ng + Y is endothermic for all prepared molecules.Areas that appear promising for further study include the extension of argon chemistry, preparation of new bonds with noble-gas atoms (such as Xe-Si bond), and studies of radon compounds. The calculations suggest the existence of related polymers, aggregates, and even HNgY crystals, and their experimental preparation is a major challenge. Another interesting task, still in its early stages, is the preparation of HNgY molecules in the gas phase.

The relaxation of the higher-energy cis conformer of formic acid to the lower-energy trans form by a tunneling mechanism has been investigated in low-temperature rare gas matrices. In the temperature range 8 -60 K, the tunneling takes place dominantly from the vibrational ground state of the cis form and the temperature dependence of the tunneling rate constant is influenced by the interactions with the environment. The temperature-dependent tunneling rates for HCOOH and DCOOH in solid Ar, Kr, and Xe are measured including data for molecules in different local environments within each host. It was found that the medium and the local environment has a significant influence on the tunneling rate. In reaction kinetics, tunneling of atoms is often negligible compared with over-barrier transitions. At very low temperatures, however, the population of energy states above the barrier becomes exceedingly small and tunneling becomes comparatively more important.1 In a condensed environment, phonons participate in a tunneling reaction and the environment should have some effect on tunneling reactions.2,3 However, in several previous experiments it was found that the tunneling rate constant was unaffected by the change of solvent. 1,4 -6 In this work, we have studied the conversion of cis formic acid ͑HCOOH͒ to trans formic acid in solid rare gases ͑Ar, Kr, Xe͒. This reaction is dominated by tunneling from the vibrational ground state at temperatures below 60 K. The results show that the tunneling rate depends strongly on the environment.The samples were made by mixing vapors of formic acid ͑FA͒ ͑KEBO lab, Ͼ99%͒ or its isotopomers ͑IT Isotope 95%-98% deuteration͒ with rare gases ͑Rg͒ Ar ͑AGA, 99.9999%͒, Kr ͑Air Liquid, 99.95%͒, Xe ͑AGA, 99.997%͒ in the gas phase in a proportion FA/RgϷ1/1000. The gas mixture was deposited on a CsI substrate at 15 K ͑Ar͒, 25 K ͑Kr͒ or 35 K ͑Xe͒ yielding highly monomeric matrices with respect to FA. Thickness of the sample was typically about 100 m. After deposition, the samples were cooled to ϳ8 K which was the lower limit for the cryostat ͑APD DE 202 A͒. The spectra were measured with a FTIR spectrometer ͑Nico-let 60 SX͒ with a resolution of 1 or 0.25 cm Ϫ1 . FA has energy minima in two planar forms differing by orientation of the hydroxyl group as shown in Fig. 1. The interconversion of the conformers involves mainly the torsional motion of the hydroxyl group. In the gas phase, cis-FA is 1365Ϯ30 cm Ϫ1 higher in energy than trans-FA. 7 The barrier from trans to cis has been calculated to be ϳ4200 cm Ϫ1. 8 In this work, cis-FA was prepared by exciting the vibrational transitions of trans-FA in rare-gas matrices with narrowband infrared radiation of an optical parametric oscillator ͑Sunlite, Continuum, FWHM ϳ0.1 cm Ϫ1 ͒. The excitation energy flows into the torsional coordinate inducing the conformer conversion. 9 The IR spectra of cis and trans FA differ significantly from each other making it possible to distinguish them easily in rare-gas matrices.9 FA is trapped in several sites correspo...

Three novel Xe-containing organic compounds, HXeCCH, HXeCC (open-shell species), and HXeCCXeH, are identified using infrared absorption spectroscopy. They are prepared in a low-temperature Xe matrix using UV photolysis of acetylene and subsequent annealing at 40-45 K. The experimental observations are supported by extensive ab initio calculations. This work demonstrates a new way to activate the H-Ctbd1;C- group without use of XeF(2), which can extend the range of organoxenon compounds.

Ultraviolet-irradiation of hydrogen halide containing rare gas matrices yields the formation of linear centrosymmetric cations of type ͑XHX͒ ϩ , ͑XϭAr, Kr, Xe͒. Annealing of the irradiated doped solids produces, along with thermoluminescence, extremely strong absorptions in the 1700-1000 cm Ϫ1 region. Based on isotopic substitution and halogen dependence of these bands, the presence of hydrogen and halogen atom͑s͒ in these species is evident. In the present paper we show the participation of rare gas atom͑s͒ in these new compounds. The evidence is based on studies of the thermally generated species in mixed rare gas matrices. The new species are assigned as neutral charge-transfer molecules HX ϩ Y Ϫ ͑Yϭhalogen͒, and their vibrational spectra are discussed and compared with those calculated with ab initio methods. This is the first time hydrogen and a rare gas atom has been found to make a chemical bond in a neutral stable compound. The highest level ab initio calculations on the existence of compounds of type HXY corroborate the experimental observations. The mechanism responsible for the formation of these species is also discussed.

An organic molecule containing krypton, HKrCCH, is reported. The preparation of HKrCCH includes 193-nm photolysis of H2C2/Kr solid mixtures at 8 K and subsequent thermal mobilization of hydrogen atoms at >/=30 K. The identification is based on infrared absorption spectroscopy and supported by ab initio calculations which show ionic and covalent contributions to the bonding. We believe that a series of similar organokrypton molecules can be prepared as computationally demonstrated for HKrC4H and HKrC3H3. These results feature a generally novel way for activating chemically the H-CC- group, which can find practical applications of the krypton catalysis.

Ultraviolet irradiation of formamide in solid argon forms hydrogen-bonded, carbon- and oxygen-attached complexes NH3−CO and NH3−OC. Computationally, the carbon-attached complex is 1.2 kJ mol-1 more stable than the oxygen-attached complex. However, a thermal equilibrium of the two structures is found experimentally in the matrices. Moreover, the oxygen-attached complex dominates in the 193-nm-induced photolytic complex formation from formamide. In xenon matrices, the photochemistry of formamide increasingly yield the HNCO+H2 binary system. The change in photochemistry of formamide between the argon and xenon enviroments can be attributed to an external heavy-atom effect, where xenon enhances the rate of intersystem crossing from a singlet to a triplet surface.