Krypton Compounds Research Articles

High-Pressure Reactivity of Kr and F2—Stabilization of Krypton in the +4 Oxidation State

Since the synthesis of the first krypton compound, several other Kr-bearing connections have been obtained. However, in all of them krypton adopts the +2 oxidation state, in contrast to xenon which forms numerous compounds with an oxidation state as high as +8. Motivated by the possibility of thermodynamic stabilization of exotic compounds with the use of high pressure (exceeding 1 GPa = 10 kbar), we present here theoretical investigations into the chemistry of krypton and fluorine at such large compression. In particular we focus on krypton tetrafluoride, KrF4, a molecular crystal in which krypton forms short covalent bonds with neighboring fluorine atoms thus adopting the +4 oxidation state. We find that this hitherto unknown compound can be stabilized at pressures below 50 GPa. Our results indicate also that, at larger compressions, a multitude of other KrmFn fluorides should be stable, among them KrF which exhibits covalent Kr–Kr bonds. Our results set the stage for future high-pressure synthesis of novel krypton compounds.

Krypton oxides under pressure.

Under high pressure, krypton, one of the most inert elements is predicted to become sufficiently reactive to form a new class of krypton compounds; krypton oxides. Using modern ab-initio evolutionary algorithms in combination with Density Functional Theory, we predict the existence of several thermodynamically stable Kr/O species at elevated pressures. In particular, our calculations indicate that at approx. 300 GPa the monoxide, KrO, should form spontaneously and remain thermo- and dynamically stable with respect to constituent elements and higher oxides. The monoxide is predicted to form non-molecular crystals with short Kr-O contacts, typical for genuine chemical bonds.

A theoretical study of stabilities, reactivities and bonding properties of XKrOH (X = F, Cl, Br and I) as potential new krypton compounds using coupled cluster, MP2 and DFT calculations

DFT andAb initiocalculations were employed to disclose the conceivable existence of new noble gas molecules, XKrOH.

Is it possible to synthesize organic Ar compound? a theoretical study

The currently synthesized noble gas (Ng) molecules are mostly xenon or krypton compounds. HArF is the only experimentally prepared compound containing a light Ng atom. In this work, a new argon compound with the formula HArC4CN, was predicted to be theoretically stable (5.66 kcal/mol at ROCCSD(T)/6-311++g(2d,2p) level of theory). Two decomposition transition states were found, i.e. the 3-body and 2-body decomposition, which produces H, Ar and C4CN or HC4CN and Ar respectively. The HArC4CN molecule is meta-stable, but the low energy barrier between the stable molecule and the 3-body transition state makes it possible to synthesize from the three fragments and the high energy barrier between the minimum and the 2-body transition state prevented it from decomposing. Natural bond orbital (NBO) and electron localization function (ELF) analysis show a strong ionic bond between Ar and C atoms whereas covalent between Ar and H. Compared with previous work, it is the conjugation effect of −C4CN as well as the its electronegativity that activates the noble gas atom and results in a stable noble gas compound.

X-ray Crystal Structures of [XeF][MF6] (M = As, Sb, Bi), [XeF][M2F11] (M = Sb, Bi) and Estimated Thermochemical Data and Predicted Stabilities for Noble-Gas Fluorocation Salts using Volume-Based Thermodynamics

The crystal structures of the xenon(II) salts, [XeF][SbF(6)], [XeF][BiF(6)], and [XeF][Bi(2)F(11)], have been determined for the first time, and those of XeF(2), [XeF][AsF(6)], [XeF][Sb(2)F(11)], and [XeF(3)][Sb(2)F(11)] have been redetermined with greater precision at -173 °C. The Bi(2)F(11)(-) anion, which has a structure analogous to those of the As(2)F(11)(-) and Sb(2)F(11)(-) anions, has been structurally characterized by single crystal X-ray diffraction for the first time as its XeF(+) salt. The fluorine bridge between the bismuth atoms is asymmetric with Bi---F(b) bond lengths of 2.092(6) and 2.195(6) A and a Bi---F(b)'---Bi bridge bond angle of 145.3(3)°. The XeF(+) cations interact with their anions by means of Xe---F(b)---M bridges. Consequently, the solid-state Raman spectra of [XeF][MF(6)] (M = As, Sb, Bi) were modeled as the gas-phase ion pairs and assigned with the aid of quantum-chemical calculations. Relationships among the terminal Xe-F(t) and bridge Xe---F(b) bond lengths and stretching frequencies and the gas-phase fluoride ion affinities of the parent Lewis acid that the anion is derived from are considered. The analogous krypton ion pairs, [KrF][MF(6)] (M = As, Sb, Bi) were also calculated and compared with their previously published X-ray crystal structures. The calculated cation-anion charge separations indicate that the [XeF][MF(6)] salts are more ionic than their krypton analogues and that XeF(2) is a stronger fluoride ion donor than KrF(2). The lattice energies, standard enthalpies, and free energies of formation for salts containing the NgF(+), Ng(2)F(3)(+), XeF(3)(+), XeF(5)(+), Xe(2)F(11)(+), and XeOF(3)(+) (Ng = Ar, Kr, Xe) cations were estimated using volume-based thermodynamics (VBT) based on crystallographic and estimated ion volumes. These estimated parameters were then used to predict the stabilities of noble-gas salts. VBT is used to examine and predict the stabilities of, inter alia, the salts [XeF(m)][Sb(n)F(5n+1)] and [XeF(m)][As(n)F(5n+1)] (m = 1, 3; n = 1, 2). VBT also confirms that XeF(+) salts are stable toward redox decomposition to Ng, F(2), and MF(5) (M = As, Sb), whereas the isolable krypton compounds and the unknown ArF(+) salts are predicted to be unstable by VBT with the ArF(+) salts being the least stable.

A stable argon compound

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.

About copper valence and superconductivity

We describe the copper valence in superconductors based on our arguments on La2O3 crystal structure. In order to explain the two oxygen sites in La2O3, it has been supposed that O 1−II ions occupy the tetrahedral site while O 2−II occupy the octahedral site. Oxygen ions in tetrahedral site form a covalent bond with lanthanum, which can be written [LaO]1+. Considering the chemical and crystallographic properties of Tl and Bi compounds, [TlO]1+ and [BiO]1+ groups appear as defined by strong covalent bonds between Bi or Tl and O. This leads to the supposition that the four oxygen ions which coordinate to copper in Tl and Bi copper oxide-based superconductors are O 1−II ions. The bivalent character of copper is then obtained through covalent bonds. For La2−x Sr x O4 compounds, copper is supposed to have valence three, but spectroscopic studies point out bivalent copper. We show that krypton shell of Sr is responsible for the lack of one unit of valence as expected from krypton compounds for example KrF2.

Novel bonding situations in noble-gas chemistry

We are currently in a much better position to understand the factors underlying noble-gas compound formation. This has recently been accomplished by synthesizing over two dozen new xenon and krypton compounds in which the noble gas is bonded to organic fragments through nitrogen. The ligand groups consist of perfluoropyridines, nitriles and s-trifluorotrizine adducted to the lewis acid cations XeF+ and KrF+.The chemistry represents the first time the strongly oxidizing XeF+ and KrF+ cations have been bonded to organic moieties. The formation of HCN-XeF+AsF6- and several organic nitrile cation RCNXeF+) led to successful attempts to bond one of the best fuels known, HCN, to what is one of the strongest oxidants known, KrF+. Although the HCN-KrF+AsF6- salt is a violent detonator in the solid state at ca. -60 °C, it has been fully characterized by multi-NMR (1H, 13C, 15N and 19F) and low-temperature Raman spectroscopy. Three further examples of krypton-nitrogen bonded species in which the KrF+ cation is bonded to an organic base have also been prepared,i.e., RFCNKrF+ (RF = CF3, C2F3, n-C3F7).More recently, this chemistry has been extended to the XeOTeF5+ cation. Representative nitrile, perfluoropyridine and S- trifluorotriazine adduct cations have been prepared. The synthesis of a number of these adduct cations with Sb(OTeF3)6- as the counter ion offers the possibility of synthesizing thermally less stable XeOTeF3+ adduct cations at low temperature in low polarity solvents. This has become possible owing to the high solubility of the novel precursor salt, XeOTeF5+Sb(OTeF5)6-.Our recent discovery of the first krypton-nitrogen bonds and the previous existence of Kr-F bonded species have served to underscore the apparent anomalous non-existence of Kr-O bonds. We have recently put a considerable amount of effort into trying to synthesize the first such example of a Kr-O bond and will present the results of these findings.