TL;DR: It is proposed that this phenomenon could be explained by the neutralization of the solvated protons by electrons, and the proposed electron-tunneling mechanism should be considered as a possible alternative to the literature models based on tunneling-assisted or radiation-induced diffusion of protons in noble-gas solids.
Abstract: The (NgHNg)+ cations (Ng=Ar and Kr) produced via the photolysis of HF∕Ar, HF∕Kr, and HBr∕Kr solid mixtures are studied, with emphasis on their decay mechanisms. The present experiments provide a large variety of parameters connected to this decay phenomenon, which allows us to reconsider various models for the decay of the (NgHNg)+ cations in noble-gas matrices. As a result, we propose that this phenomenon could be explained by the neutralization of the solvated protons by electrons. The mechanism of this neutralization reaction probably involves tunneling of an electron from an electronegative fragment or another trap to the (NgHNg)+ cation. The proposed electron-tunneling mechanism should be considered as a possible alternative to the literature models based on tunneling-assisted or radiation-induced diffusion of protons in noble-gas solids. As a novel experimental observation of this work, the efficient formation of HArF molecules occurs at 8K in a photolyzed HF∕Ar matrix. It is probable that the low-t...
TL;DR: This critical review describes fascinating experimental and theoretical advances in 'noble gas' chemistry during the last twenty years, and has taken a somewhat unexpected course since 2000.
Abstract: In this critical review I describe fascinating experimental and theoretical advances in ‘noble gas’ chemistry during the last twenty years, and have taken a somewhat unexpected course since 2000. I also highlight perspectives for further development in this field, including the prospective synthesis of compounds containing as yet unknown Xe–element and element–Xe–element bridging bonds, peroxide species containing Xe, adducts of XeF2 with various metal fluorides, Xe–element alloys, and novel pressure-stabilized covalently bound and host–guest compounds of Xe. A substantial part of the essay is devoted to the—as yet experimentally unexplored—behaviour of the compounds of Xe under high pressure. The blend of science, history, and theoretical predictions, will be valued by inorganic and organic chemists, materials scientists, and the community of theoretical and experimental high-pressure physicists and chemists (151 references).
TL;DR: The matrix-isolation synthesis of noble-gas hydrides, their spectroscopic and structural properties, and their stabilities are discussed, including the existence of related polymers, aggregates, and even HNgY crystals, and areas that appear promising for further study include the extension of argon chemistry, preparation of new bonds with noble- gas atoms, and studies of radon compounds.
Abstract: 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 hydrid...
TL;DR: It is shown that the single-reference MP2 calculations can produce a rather inaccurate energy diagram for the formation of noble-gas hydrides, and it is suggested that the computational dissociation energy of the HY precursors should always be compared with the experimental values as a checkpoint for the computational accuracy.
Abstract: We discuss the present status and reliability of theoretical predictions of noble-gas hydride molecules. It is shown that the single-reference MP2 calculations can produce a rather inaccurate energy diagram for the formation of noble-gas hydrides, and this may mislead the theoretical predictions. We suggest that the computational dissociation energy of the HY precursors should always be compared with the experimental values as a checkpoint for the computational accuracy. The computational inaccuracy probably explains why some compounds that are stable with the single-reference MP2 method (HArC4H, HArC3N, and HArCN) did not appear in matrix-isolation experiments, whereas the corresponding compounds with Kr and Xe are known.
TL;DR: A computational and experimental matrix isolation study of insertion of noble gas atoms into cyanoacetylene (HCCCN) is presented, and data obtained in long-term decay of KrHKr+ cations suggest a tentative assignment for the CCCN radical.
Abstract: A computational and experimental matrix isolation study of insertion of noble gas atoms into cyanoacetylene (HCCCN) is presented. Twelve novel noble gas insertion compounds are found to be kinetically stable at the MP2 level of theory, including four molecules with argon. The first group of the computationally studied molecules belongs to noble gas hydrides (HNgCCCN and HNgCCNC), and we found their stability for Ng = Ar, Kr, and Xe. The HNgCCCN compounds with Kr and Xe have similar stability to that of previously reported HKrCN and HXeCN. The HArCCCN molecule seems to have a weaker H-Ar bond than in the previously identified HArF molecule. The HNgCCNC molecules are less stable than the HNgCCCN isomers for all noble gas atoms. The second group of the computational insertion compounds, HCCNgCN and HCCNgNC, are of a different type, and they also are kinetically stable for Ng = Ar, Kr, and Xe. Our photolysis and annealing experiments with low-temperature cyanoacetylene/Ng (Ng = Ar, Kr, and Xe) matrixes evidence the formation of two noble gas hydrides for Ng = Kr and Xe, with the strongest IR absorption bands at 1492.1 and 1624.5 cm -1 , and two additional absorption modes for each species are found. The computational spectra of HKrCCCN and HXeCCCN fit most closely the experimental data, which is the basis for our assignment. The obtained species absorb at quite similar frequencies as the known HKrCN and HXeCN molecules, which is in agreement with the theoretical predictions. No strong candidates for an Ar compound are observed in the IR absorption spectra. As an important side product of this work, the data obtained in long-term decay of KrHKr + cations suggest a tentative assignment for the CCCN radical.
TL;DR: Quantum chemical calculations at the MP2/aug-cc-pVTZ and CCSD(T) levels have been carried out for the title compounds and suggest a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg](+) and to the halogen atoms in[XHX](-) .
Abstract: Quantum chemical calculations at the MP2/aug-cc-pVTZ and CCSD(T)/aug-cc-pVTZ levels have been carried out for the title compounds. The electronic structures were analyzed with a variety of charge and energy partitioning methods. All molecules possess linear equilibrium structures with D∞h symmetry. The total bond dissociation energies (BDEs) of the strongly bonded halogen anions [XHX](-) and [XAuX](-) decrease from [FHF](-) to [IHI](-) and from [FAuF](-) to [IAuI](-) . The BDEs of the noble gas compounds [NgHNg](+) and [NgAuNg](+) become larger for the heavier atoms. The central hydrogen and gold atoms carry partial positive charges in the cations and even in the anions, except for [IAuI](-) , in which case the gold atom has a small negative charge of -0.03 e. The molecular electrostatic potentials reveal that the regions of the most positive or negative charges may not agree with the partial charges of the atoms, because the spatial distribution of the electronic charge needs to be considered. The bonding analysis with the QTAIM method suggests a significant covalent character for the hydrogen bonds to the noble gas atoms in [NgHNg](+) and to the halogen atoms in [XHX](-) . The covalent character of the bonding in the gold systems [NgAuNg](+) and [XAuX](-) is smaller than in the hydrogen compound. The energy decomposition analysis suggests that the lighter hydrogen systems possess dative bonds X(-) →H(+) ←X(-) or Ng→H(+) ←Ng while the heavier homologues exhibit electron sharing through two-electron, three-center bonds. Dative bonds X(-) →Au(+) ←X(-) and Ng→Au(+) ←Ng are also diagnosed for the lighter gold systems, but the heavier compounds possess electron-shared bonds.
Abstract: A general quantum mechanical description of exothermic electron transfer reactions is formulated by treating such reactions as the nonradiative decay of a ’’supermolecule’’ consisting of the electron donor, the electron acceptor, and the polar solvent. In particular, the role of the high‐frequency intramolecular degrees of feedom on the free energy relationship for series of closely related reactions was investigated for various model systems involving displacement of potential energy surfaces, frequency shift, and anharmonicity effects. The free energy plots are generally found to pass through a maximum and to be asymmetric with a slower decrease in the transition probability with increasing energy of reaction. For high‐frequency intramolecular modes this provides a rationalization of the experimental observation of ’’activationless’’ regions. Isotope effects are discussed as also are the oscillatory free energy relationships, predicted for low temperatures and high frequencies, and which are analogous t...
TL;DR: It is reported that the photolysis of hydrogen fluoride in a solid argon matrix leads to the formation of argon fluorohydride (HArF), which is identified by probing the shift in the position of vibrational bands on isotopic substitution using infrared spectroscopy and indicates that HArF is intrinsically stable, owing to significant ionic and covalent contributions to its bonding.
Abstract: 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 predicted1 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 atom2,3. Since then, a range of different compounds containing radon, xenon and krypton have been theoretically anticipated and prepared4,5,6,7,8. Although the lighter noble gases neon, helium and argon are also expected to be reactive under suitable conditions9,10, 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 predictions11,12,13 that argon should form a stable hydride species with properties similar to those of the analogous xenon and krypton compounds reported before14,15,16,17,18.
TL;DR: Photon-Induced Harpooning, Channeled Migration, and Forced and Delayed Cage Exit 1503 A. Cl2 in XesDissociation without cage Exit 1502 IX.
Abstract: III. Sample Preparation and Morphology 1492 IV. Photon-Induced Dissociation 1493 V. Perfect Caging 1494 A. Direct Observation of the Caging Process 1494 B. Stabilization of Fragments and Isomerization 1495 1. OClO 1495 2. ICN 1496 VI. Predissociation 1497 A. Electronic Caging by Solvent Symmetry 1497 B. Differential Solvation 1497 VII. Dissociation through Sudden Cage Exit 1499 A. F(2P) 1500 B. O(1D) 1500 C. S(1D) 1500 VIII. Delayed versus Sudden Exit 1500 A. HI, HCl 1500 B. H2O, D2O 1502 C. H2S, D2S 1502 D. Cl2 in XesDissociation without Cage Exit 1502 IX. Forced and Delayed Cage Exit 1503 A. Cl2 in Ar 1503 B. Cl2 in Kr and Xe 1504 X. Channeled Migration 1504 A. F Atoms 1504 B. O Atoms 1505 C. H Atoms 1506 XI. Photon-Induced Harpooning 1508 XII. Acknowledgments 1510 XIII. References 1510
TL;DR: The chapters in this book describe many of the contributions of matrix-isolation spectroscopy to chemistry and physics in the last decade, and predict the continued evolution of this technique over the next decade.
Abstract: Matrix-isolation spectroscopy, as a technique for studying unstable species, has had tremendous lasting powers. Since 1954, matrix-isolation has been able to assimilate new technology rather than being replaced by it. The matrix-isolation technique for producing and trapping new chemical species has been applied to an ever increasing range of chemical and physical problems since its inception. The last 12 years have seen a substantial number of new developments and applications of the matrix technique. The chapters in this book describe many of the contributions of matrix-isolation spectroscopy to chemistry and physics in the last decade, and as such predict the continued evolution of this technique over the next decade. Experimental techniques are generally closely related to the development of new apparatus, and each chapter has a section describing innovations in instrumentation such as the closed-cycle cryogenic cooler that has revolutionized the technique. In addition, each chapter describes many of the specialized methods used to prepare, trap and study particular new subject species.
TL;DR: An additional solid-state configuration of HArF with higher thermal stability is reported, which proves that the doublet at ∼2020 cm-1 originates from the * Address correspondence to this author: (e-mail) Leonid.Fi
Abstract: During the past decade a number of HRgY molecules (H ) hydrogen; Rg ) Ar, Kr, Xe; Y ) an electronegative fragment) have been characterized experimentally in rare-gas solids and computationally by using ab initio methods.1,2 These species are formed from neutral fragments,3 and experiments support their intrinsic stability.4 These molecules constitute an important intermediate during UV photolysis of HY molecules in rare-gas hosts demonstrating the locality of the primary photolysis.5 One of the HRgY molecules, HXeI, has recently been observed in Xe clusters.6 A stable Ar-containing molecule, HArF, was identified in an Ar matrix.2 The high-level ab initio calculations on HArF confirmed its intrinsic stability.7,8 An unclear experimental fact on HArF is its decrease upon annealing above 27 K,2 which contradicts the calculated decomposition barrier of 0.33 eV.7 In this communication, we report an additional solid-state configuration of HArF with higher thermal stability. The HF/Ar solid mixtures were studied in a closed-cycle helium cryostat (APD, DE 202A) at temperatures down to 7.5 K. The samples were deposited onto a cold CsI substrate by passing Ar gas (40Ar from AGA and 36Ar from ICON Services) over an HFpyridine polymer (Fluka) at room temperature. Photolysis of HF was performed with a Kr continuum lamp (Opthos) emitting in the 127-160 nm spectral interval. Our HF/Ar matrixes are quite monomeric as indicated by the IR absorption bands at 3962.5 and 3953.8 cm-1.9 HArF molecules are prepared in the following. First, HF is photodissociated, which stabilizes H and F atoms in solid Ar and leads to some formation of HArF. Next, the photolyzed sample is annealed, which mobilizes the atoms and leads to an increase in the HArF concentration below 20 K.2 The formation of HArF molecules is demonstrated by strong absorption bands at 1965.7, 1969.4, 1972.3 (νH-Ar), 686.9 (δH-Ar-F), and 435.7 cm-1 (νAr-F). The H-Ar stretching region is presented in Figure 1 (see the upper spectrum). The observed bands agree well with the calculated values and show proper shifts upon H and Ar isotopic substitutions. Surprisingly, annealing above 27 K destroys all bands listed above, which was tentatively explained by secondary reactions of mobile matrix species with HArF molecules.2 It was not noticed in the original paper on HArF2 that the decrease of “unstable” HArF molecules is accompanied with an increase of other bands in the H-Ar stretching (2016.3 and 2020.8 cm-1) and bending (693.5 and 697.0 cm-1) regions. This central observation of the present work is illustrated by the lower spectrum in Figure 1. A sign of the novel bands is visible already after annealing at 20 K. We suggest that this novel set of bands belong to HArF in a different solid-state configuration. This additional configuration is thermally rather “stable” and the corresponding bands decrease only with evaporation of the sample. The H-Ar stretching doublet shifts to 1494.0 and 1496.9 cm-1 upon deuteration, and to 2018.5 and 2023.1 cm-1 upon 36Ar/40Ar substitution [see Figure 2a], i.e., in accord with the calculations and experiment on “unstable” HArF.2 These observations prove that the doublet at ∼2020 cm-1 originates from the * Address correspondence to this author: (e-mail) Leonid.Khriachtchev@ Helsinki.Fi. (1) Lundell, J.; Khriachtchev, L.; Pettersson, M.; Rasanen, M. Low Temp. Phys. 2000, 26, 680. (2) Khriachtchev, L.; Pettersson, M.; Runeberg, N.; Lundell, J.; Rasanen, M. Nature (London) 2000, 406, 874. (3) Pettersson, M.; Nieminen, J.; Khriachtchev, L.; Rasanen, M. J. Chem. Phys. 1997, 107, 8423. (4) Lorenz, M.; Rasanen, M.; Bondybey, V. E. J. Phys. Chem. A 2000, 104, 3770. (5) Khriachtchev, L.; Pettersson, M.; Lundell, J.; Rasanen, M. J. Chem. Phys. 2001, 114, 7727. (6) Baumfalk, R.; Nahler, N. H.; Buck, U. J. Chem. Phys. 2001, 114, 4755 (7) Runeberg, N.; Pettersson, M.; Khriachtchev, L.; Lundell, J.; Rasanen, M. J. Chem. Phys. 2001, 114, 836. (8) Lundell, L.; Chaban, G. M.; Gerber, R. B. Chem. Phys. Lett. 2000, 331, 308. (9) Anderson, D. T.; Winn, J. S. Chem. Phys. 1994, 189, 171. Figure 1. Formation of stable HArF in solid Ar. The HF/Ar matrix is deposited and photolyzed at 7.5 K and annealed at 20 and 35 K. Annealing above 27 K destroys unstable HArF (on the right) and forms stable HArF (on the left). The spectra are measured at 7.5 K.