Recent Advances in Noble-Gas Chemistry

Structure, stability, reactivity and bonding in noble gas compounds

Noble gases (Ngs) are recognized as the least reactive elements due to their fully filled valence electronic configuration. Their reluctance to engage in chemical bond formation necessitates extreme conditions such as low temperatures, high pressures, and reagents with high reactivity. In this Perspective, we discuss our endeavours in the theoretical prediction of viable Ng complexes, emphasizing the pursuit of synthesizing them under nearly ambient conditions. Our research encompasses various bonding categories of Ng complexes and our primary aim is to comprehend the bonding mechanisms within these complexes, utilizing state-of-the-art theoretical tools such as natural bond orbital, energy decomposition, and electron density analyses. These complex types manifest distinct bonding scenarios. In the non-insertion type, the donor-acceptor interaction strength hinges on the polarizing ability of the binding atom, drawing the electron density of the Ng towards itself. In certain instances, especially with heavier Ng elements, this interaction reaches a magnitude where it can be considered a covalent bond. Conversely, in most insertion cases, the Ng prefers to share electrons to form a covalent bond on one side while interacting electrostatically on the other side. In rare cases, both bonds may be portrayed as electron-shared covalent bonds. Furthermore, a host cage serves as an excellent platform to explore the limits of achieving Ng-Ng bonds (even for helium), under high pressure.

On the Nature of the Partial Covalent Bond between Noble Gas Elements and Noble Metal Atoms

This article provides a discussion on the nature of bonding between noble gases (Ng) and noble metals (M) from a quantum chemical perspective by investigating compounds such as NgMY (Y=CN, O, NO₃, SO₄, CO₃), [NgM-(bipy)]+, NgMCCH, and MCCNgH complexes, where M=Cu, Ag, Au and Ng=Kr-Rn, with some complexes containing the lighter noble gas atoms as well. Despite having very low chemical reactivity, noble gases have been observed to form weak bonds with noble metals such as copper, gold, and silver. In this study, we explore the factors that contribute to this unusual bonding behavior, including the electronic structure of the atoms involved and the geometric configuration of the concerned fragments. We also investigate the metastable nature of the resulting complexes by studying the energetics of their possible dissociation and internal isomerization channels. The noble gas-binding ability of the bare metal cyanides are higher than most of their bromide counterparts, with CuCN and AgCN showing higher affinity than their chloride analogues as well. In contrast, the oxides seem to have lower binding power than their corresponding halides. In the oxide and the bipyridyl complexes, the Ng-binding ability follows the order Au > Cu > Ag. The dissociation energies calculated, considering the zero-point energy correction for possible dissociation channels, increase as we move down the noble gas group. The bond between the noble gases and the noble metals in the complexes are found to have comparable weightage of orbital and electrostatic interactions, suggestive of a partial covalent nature. The same is validated from the topological analysis of electron density.

Changes in Structure and Reactivity of Ng2 Encapsulated in Fullerenes: A Density Functional Theory Study

Noble gas can be no noble in certain situations from the perspective of structure, bonding, and reactivity. These situations could be extreme experimental conditions or others. In this contribution, we systematically investigate the impact of fullerene encapsulation on molecular structure and chemical reactivity of noble gas dimers (Ng2) in a few fullerene molecules. To that end, we consider He2, Ne2, and Ar2 dimers encapsulated in C50, C60, and C70 fullerenes. We unveil that bond distances of Ng2 inside fullerene become substantially smaller and noble gas atoms become more electrophilic. In return, these noble gas dimers make fullerene molecules more nucleophilic. Using analytical tools from density functional theory, conceptual density functional theory, and information-theoretic approach, we appreciate the nature and origin of these structure and reactivity changes. The results and conclusions from this work should provide more new insights from the viewpoint of changing the perspectives of noble gas reactivity.

How Far Can One Push the Noble Gases Towards Bonding?: A Personal Account

Noble gases (Ngs) are the least reactive elements in the periodic table towards chemical bond formation when compared with other elements because of their completely filled valence electronic configuration. Very often, extreme conditions like low temperatures, high pressures and very reactive reagents are required for them to form meaningful chemical bonds with other elements. In this personal account, we summarize our works to date on Ng complexes where we attempted to theoretically predict viable Ng complexes having strong bonding to synthesize them under close to ambient conditions. Our works cover three different types of Ng complexes, viz., non-insertion of NgXY type, insertion of XNgY type and Ng encapsulated cage complexes where X and Y can represent any atom or group of atoms. While the first category of Ng complexes can be thermochemically stable at a certain temperature depending on the strength of the Ng-X bond, the latter two categories are kinetically stable, and therefore, their viability and the corresponding conditions depend on the size of the activation barrier associated with the release of Ng atom(s). Our major focus was devoted to understand the bonding situation in these complexes by employing the available state-of-the-art theoretic tools like natural bond orbital, electron density, and energy decomposition analyses in combination with the natural orbital for chemical valence theory. Intriguingly, these three types of complexes represent three different types of bonding scenarios. In NgXY, the strength of the donor-acceptor Ng→XY interaction depends on the polarizing power of binding the X center to draw the rather rigid electron density of Ng towards itself, and sometimes involvement of such orbitals becomes large enough, particularly for heavier Ng elements, to consider them as covalent bonds. On the other hand, in most of the XNgY cases, Ng forms an electron-shared covalent bond with X while interacting electrostatically with Y representing itself as [XNg]+Y-. Nevertheless, in some of the rare cases like NCNgNSi, both the C-Ng and Ng-N bonds can be represented as electron-shared covalent bonds. On the other hand, a cage host is an excellent moiety to examine the limits that can be pushed to attain bonding between two Ng atoms (even for He) at high pressure. The confinement effect by a small cage-like B12N12 can even induce some covalent interaction within two He atoms in the He2@B12N12 complex.

Noble-Noble Strong Union: Gold at Its Best to Make a Bond with a Noble Gas Atom

This Review presents the current status of the noble gas (Ng)‐noble metal chemistry, which began in 1977 with the detection of AuNe⁺ through mass spectroscopy and then grew from 2000 onwards; currently, the field is in a somewhat matured state. On one side, modern quantum chemistry is very effective in providing important insights into the structure, stability, and barrier for the decomposition of Ng compounds and, as a result, a plethora of viable Ng compounds have been predicted. On the other hand. experimental achievement also goes beyond microscopic detection and characterization through spectroscopic techniques and crystal structures at ambient temperature; for example, (AuXe₄)²⁺(Sb₂F₁₁⁻)₂ have also been obtained. The bonding between two noble elements of the periodic table can even reach the covalent limit. The relativistic effect makes gold a very special candidate to form a strong bond with Ng in comparison to copper and silver. Insertion compounds, which are metastable in nature, depending on their kinetic stability, display an even more fascinating bonding situation. The degree of covalency in Ng–M (M=noble metal) bonds of insertion compounds is far larger than that in non‐insertion compounds. In fact, in MNgCN (M=Cu, Ag, Au) molecules, the M−Ng and Ng−C bonds might be represented as classical 2c–2e σ bonds. Therefore, noble metals, particularly gold, provide the opportunity for experimental chemists to obtain sufficiently stable complexes with Ng at room temperature in order to characterize them by using experimental techniques and, with the intriguing bonding situation, to explore them with various computational tools from a theoretical perspective. This field is relatively young and, in the coming years, a lot of advancement is expected experimentally as well as theoretically.

How and Why Does Helium Permeate Nonporous Arsenolite Under High Pressure?

Investigations into the helium permeation of arsenolite, the cubic, molecular arsenic(III) oxide polymorph As₄ O₆ , were carried out to understand how and why arsenolite helium clathrate As₄ O₆ ⋅2 He is formed. High-pressure synchrotron X-ray diffraction experiments on arsenolite single crystals revealed that the permeation of helium into nonporous arsenolite depends on the time for which the crystal is subjected to high pressure and on the crystal history. The single crystal was totally transformed into As₄ O₆ ⋅2 He within 45 h under 5 GPa. After release of the pressure, arsenolite was recovered and a repeated increase in pressure up to 3 GPa led to practically instant As₄ O₆ ⋅2 He formation. However, when a pristine arsenolite single crystal was quickly subjected to a pressure of 13 GPa, no helium permeation was observed at all. No neon permeation was observed in analogous experiments. Quantum mechanical computations indicate that there are no specific attractive interactions between He atoms and As₄ O₆ molecules at the distances observed in the As₄ O₆ ⋅2 He crystal structure. Detailed analysis of As₄ O₆ molecular structure changes has shown that the introduction of He into the arsenolite crystal lattice significantly reduces molecular deformations by decreasing the anisotropy of stress exerted on the As₄ O₆ molecules. This effect and the pΔV term, rather than any specific As⋅⋅⋅He binding, are the driving forces for the formation As₄ O₆ ⋅2 He.

The classification and representation of main group element compounds that feature three-center four-electron interactions

This article provides a means to classify and represent compounds that feature 3-center 4-electron (3c-4e) interactions in terms of the number of electrons that each atom contributes to the interaction. Specifically, Class I 3c-4e interactions are classified as those in which two atoms provide one electron each and the third atom provides a pair of electrons (i.e. LX₂), while Class II 3c-4e interactions are classified as those in which two atoms each provide a pair of electrons and the third atom contributes none (i.e. L₂Z). These classes can be subcategorized according to the nature of the central atom. Thus, Class I interactions can be categorized according to whether the central atom provides one (i.e.μ-X) or two (i.e.μ-L) electrons, while Class II interactions can be categorized according to whether the central atom provides none (i.e.μ-Z) or two (i.e.μ-L) electrons. The use of appropriate structure-bonding representations for these various interactions provides a means to determine the covalent bond classification of the element of interest.

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.

An element of atomic number zero?

The element zero or “neutronium” was a suggestion proposed by the German chemist Andreas von Antropoff in 1925. Can it really be thought of as an element at the present time?

Perspective: crystal structure prediction at high pressures

Crystal structure prediction at high pressures unbiased by any prior known structure information has recently become a topic of considerable interest. We here present a short overview of recently developed structure prediction methods and propose current challenges for crystal structure prediction. We focus on first-principles crystal structure prediction at high pressures, paying particular attention to novel high pressure structures uncovered by efficient structure prediction methods. Finally, a brief perspective on the outstanding issues that remain to be solved and some directions for future structure prediction researches at high pressure are presented and discussed.

In quest of a superhalogen supported covalent bond involving a noble gas atom

The possibility of having neutral Xe-bound compounds mediated by some representative transition metal fluorides of general formula MX3 (where M=Ru, Os, Rh, Ir, Pd, Pt, Ag, Au and X=F) has been investigated through density functional theory based calculations. Nature of interaction between MX3 and Xe moieties has been characterized through detailed electron density, charge density and bond energy decomposition analyses. The feasibility of having compounds of general formula XeMX3 at 298 K has been predicted through thermodynamic considerations. The nature of interaction in between Xe and M atoms is partly covalent in nature and the orbital interaction is the dominant contributor toward these interactions as suggested by energy decomposition analysis.

Predicted organic compounds derived from rare gas atoms and formic acid

Organic insertion compounds of rare gas atoms into formic acid were investigated at the MP2(full)/aug-cc-pVTZ level. There are two configuration isomers for each molecule based on the location of H atoms: trans- and cis-HCOORgH (Rg = Ar, Kr, Xe). Their structures, harmonic frequencies, and decomposition energies have been calculated using the above ab initio method. Using trans-HCOOXeH as an example, natural bond orbital (NBO) and atom-in-molecules (AIM) analyses were also carried out to explore the binding nature of the rare gas atoms. The formation mechanism of molecular orbitals is also presented in this paper. The presented results indicate that HCOOXeH and HCOOKrH are potential candidates for experimental observation.

In quest of strong Be-Ng bonds among the neutral Ng-Be complexes

The global minimum geometries of BeCN2 and BeNBO are linear BeN-CN and BeN-BO, respectively. The Be center of BeCN2 binds He with the highest Be-He dissociation energy among the studied neutral He-Be complexes. In addition, BeCN2 can be further tuned as a better noble gas trapper by attaching it with any electron-withdrawing group. Taking BeO, BeS, BeNH, BeNBO, and BeCN2 systems, the study at the CCSD(T)/def2-TZVP level of theory also shows that both BeCN2 and BeNBO systems have higher noble gas binding ability than those related reported systems. ΔG values for the formation of NgBeCN2/NgBeNBO (Ng = Ar-Rn) are negative at room temperature (298 K), whereas the same becomes negative at low temperature for Ng = He and Ne. The polarization plus the charge transfer is the dominating term in the interaction energy.

Ab initio study of the organic xenon insertion compound into ethylene and ethane

This paper studies Xe-insertion ethylene and ethane compounds, i.e., HXeC2H3 and HXeC2H5. The structures, harmonic frequencies, and energetics for both molecules have been calculated at the MP2(full)/6-311++G(2d,2p) level. Our theoretical results predict the existence of HXeC2H3 and the instability of HXeC2H5. Natural bond orbital (NBO) analysis shows a strong ionic bond between the xenon atom and hydrocarbon radical. In addition, the interaction between the donor (Xe lone pair) and acceptor (the C-C antibonding orbital, i.e., π*(C-C)) increases the stability of HXeC2H3.

Infrared spectra of NgBeS (Ng = Ne, Ar, Kr, Xe) and BeS2 in noble-gas matrices

Laser-ablated beryllium atom has been codeposited at 4 K with hydrogen sulfide in excess noble gas matrices. Four noble-gas compounds NgBeS (Ng = Ne, Ar, Kr, Xe) and the BeS(2) molecule are identified on the basis of the S-34 isotopic substitution, DFT and CCSD(T) theoretical predictions, and a comparison of noble-gas substitution. The agreement between the experimental and calculated vibrational frequencies supports the identification of these molecules. The dissociation energies are calculated at 1.6, 12.6, 10.7, and 13.4 kcal/mol for NeBeS, ArBeS, KrBeS, and XeBeS, respectively, at the CCSD(T) level. The BeS Lewis acid molecule favors strong chemical binding between the Be and Ng atoms.

Freezing in resonance structures for better packing: XeF2 becomes (XeF+)(F-) at large compression

Recent high-pressure experiments conducted on xenon difluoride (XeF(2)) suggested that this compound undergoes several phase transitions up to 100 GPa, becoming metallic above 70 GPa. In this theoretical study, in contrast to experiment, we find that the ambient pressure molecular structure of xenon difluoride, of I4/mmm symmetry, remains the most stable one up to 105 GPa. In our computations, the structures suggested from experiment have either much higher enthalpies than the I4/mmm structure or converge to that structure upon geometry optimization. We discuss these discrepancies between experiment and calculation and point to an alternative interpretation of the measured cell vectors of XeF(2) at high pressure. At pressures exceeding those studied experimentally, above 105 GPa, the I4/mmm structure transforms to one of Pnma symmetry. The Pnma phase contains bent FXeF molecules, with unequal Xe-F distances, and begins to bring other fluorines into the coordination sphere of the Xe. Further compression of this structure up to 200 GPa essentially results in self-dissociation of XeF(2) into an ionic solid (i.e., [XeF](+)F(-)), similar to what is observed for nitrous oxide (N(2)O) at high pressure.

Heats of formation of XeF(3)(+), XeF(3)(-), XeF(5)(+), XeF(7)(+), XeF(7)(-), and XeF(8) from high level electronic structure calculations

Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for XeF(3)(+), XeF(3)(-), XeF(5)(+), XeF(7)(+), XeF(7)(-), and XeF(8) from coupled cluster theory (CCSD(T)) calculations with effective core potential correlation-consistent basis sets for Xe and including correlation of the nearest core electrons. Additional corrections are included to achieve near chemical accuracy of +/-1 kcal/mol. Vibrational zero point energies were computed at the MP2 level of theory. Unlike the other neutral xenon fluorides, XeF(8) is predicted to be thermodynamically unstable with respect to loss of F(2) with the reaction calculated to be exothermic by 22.3 kcal/mol at 0 K. XeF(7)(+) is also predicted to be thermodynamically unstable with respect to the loss of F(2) by 24.1 kcal/mol at 0 K. For XeF(3)(+), XeF(5)(+), XeF(3)(-), XeF(5)(-), and XeF(7)(-), the reactions for loss of F(2) are endothermic by 14.8, 37.8, 38.2, 59.6, and 31.9 kcal/mol at 0 K, respectively. The F(+) affinities of Xe, XeF(2), XeF(4), and XeF(6) are predicted to be 165.1, 155.3, 172.7, and 132.5 kcal/mol, and the corresponding F(-) affinities are 6.3, 19.9, 59.1, and 75.0 kcal/mol at 0 K, respectively.

Atypical compounds of gases, which have been called ‘noble’

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 XeF(2) 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).

Matrix Isolation Infrared Spectroscopic and Theoretical Study of NgMO (Ng = Ar, Kr, Xe; M = Cr, Mn, Fe, Co, Ni) Complexes

The matrix isolation infrared spectroscopic and quantum chemical calculation results indicate that late transition metal monoxides CrO through NiO coordinate one noble gas atom in forming the NgMO complexes (Ng = Ar, Kr, Xe; M = Cr, Mn, Fe, Co, Ni) in solid noble gas matrixes. Hence, the late transition metal monoxides previously characterized in solid noble gas matrixes should be regarded as the NgMO complexes, which were predicted to be linear. The M-Ng bond distances decrease, while the M-Ng binding energies increase from NgCrO to NgNiO. In contrast, the early transition metal monoxides, ScO, TiO, and VO, are not able to form similar noble gas atom complexes.

High coordination chemically bound compounds of noble gases with hydrocarbons: Ng(CCH)4 and Ng(CCH)6, (Ng=Xe or Kr)

Ab initio calculations predict the existence of the compounds Ng(-C[triple bond]CH)4 and Ng(-C[triple bond]CH)6, where Ng=Xe or Kr. Presently known organic noble gas compounds have a coordination number of two at most. The Ng(-C[triple bond]CH)(4) molecules have D(4h) symmetry, and Ng(-C[triple bond]CH)(6) molecules have O(h) symmetry. The bonding in all these compounds is partly ionic and partly covalent, with significant contributions from both types of bonding. The relatively high vibrational frequencies and the substantial Ng-(C[triple bond]CH) binding energy in these species indicate that these compounds should be fairly stable, at least in cryogenic conditions. These compounds could be a very interesting addition to the range of known organic noble gas compounds. Suggestions are made on possible approaches to their preparation.

Structure and stability of xenon insertion compounds of hypohalous acids, HXeOX [X=F, Cl, and Br]: An ab initio investigation

The structure and stability of xenon-inserted hypohalous acids HXeOX (X=F, Cl, and Br) have been investigated theoretically using ab initio molecular orbital calculations. All these molecules are found to consist of a nearly linear HXeO moiety and a bend XeOX fragment. Geometrical parameters of HXeOX are comparable with that of experimentally observed HXeOH species. The dissociation energies corresponding to the lowest-energy fragmentation products, HOX+Xe have been computed to be -398.1, -385.5, and -386.7 kJmol for HXeOF, HXeOCl, and HXeOBr, respectively, at the MP2 level of theory. The respective barrier heights corresponding to the bent transition states (H-Xe-O bending mode) have been calculated to be 138.1, 138.4, and 138.2 kJmol with respect to HXeOX minimum. These species are found to be metastable in their respective potential-energy surface, and the dissociation energies corresponding to the H+Xe+OX products are found to be 56.8, 66.0, and 80.8 kJmol for HXeOF, HXeOCl, and HXeOBr, respectively. The energies corresponding to the H+Xe+O+X dissociation channel have been computed to be 272.0, 309.3, and 299.7 kJmol for HXeOF, HXeOCl, and HXeOBr, respectively, at the same level of theory. Energetics as well as geometrical considerations suggests that it may be possible to prepare these species experimentally similar to that of HXeOH species at low-temperature laser photolysis experiments.

Formation and Characterization of the XeOO+ Cation in Solid Argon

This report presents the preparation and characterization of a xenon-containing cationic radical species, XeOO+. The XeOO+ cation was produced either by co-deposition of the reactive species generated by laser ablation of different transition metals with dioxygen and xenon mixtures in excess argon or by condensation of high-frequency discharged O2/Xe/Ar mixtures at 12 K and is identified by infrared absorptions. High-level quantum chemical calculations indicate that XeOO+ has a bent structure with direct xenon-oxygen dative bonding, and the doublet ground state is much more stable than the Xe + O2+ reactants.

Prediction of a linear polymer made of xenon and carbon

Electronic structure calculations predict the existence of a novel type of a chemically bound noble gas compound. The predicted species is an extended linear and periodic polymer, made of the repeat unit -(XeCC)-, where CC is the acetylenic group. The polymer has a strong partly ionic nature, with positive partial charge on the xenon atoms and a negative one on the CC groups. High energy barriers are found for the removal of a Xe atom from the chain, indicating high stability. This is the first polymer with a noble-gas-containing building block.

Synthesis, structural characterization, and computational study of the strong oxidant salt [XeOTeF5][Sb(OTeF5)6].SO2ClF

The strong oxidant salt [XeOTeF(5)][Sb(OTeF(5))(6)].SO(2)ClF has been synthesized by reaction of stoichiometric amounts of Xe(OTeF(5))(2) and Sb(OTeF(5))(3) in SO(2)ClF solution at -78 degrees C and characterized in SO(2)ClF solution by low-temperature (17)O, (19)F, (121)Sb, (125)Te, and (129)Xe NMR spectroscopy, showing the Xe...O donor-acceptor bond XeOTeF(5)(+).SO(2)ClF adduct-cation to be labile at temperatures as low as -80 degrees C. The salt crystallizes from SO(2)ClF as [XeOTeF(5)][Sb(OTeF(5))(6)].SO(2)ClF, and the low-temperature crystal structure was obtained: triclinic, P, a = 9.7665(5) A, b = 9.9799(4) A, c = 18.5088(7) A, alpha = 89.293(2) degrees , beta = 82.726(2) degrees , gamma = 87.433(3) degrees , V = 1787.67(13) A(3), Z = 2, and R(1) = 0.0.0451 at -173 degrees C. Unlike MF(6)(-) in [XeF][MF(6)] (e.g., M = As, Sb, Bi) and [XeOTeF(5)][AsF(6)], the Sb(OTeF(5))(6)(-) anion is significantly less basic and does not interact with the coordinately unsaturated xenon(II) cation. Rather, the XeOTeF(5)(+) cation and weak Lewis base, SO(2)ClF, interact by coordination of an oxygen atom of SO(2)ClF to xenon [Xe...O, 2.471(5) A]. The XeOTeF(5)(+).SO(2)ClF adduct-cation has also been studied by low-temperature Raman spectroscopy, providing frequencies that have been assigned to adducted SO(2)ClF. The solid-state Raman spectra of XeOTeF(5)(+).SO(2)ClF and Sb(OTeF(5))(6)(-) have been assigned with the aid of electronic structure calculations. In addition to optimized geometries and vibrational frequencies, theoretical data, including gas-phase donor-acceptor bond energies, natural bond orbital (NBO) analyses, and topological analyses based on electron localization functions (ELF), provide descriptions of the bonding in XeOTeF(5)(+).SO(2)ClF and related systems. The quantum mechanical calculations provided consistent trends for the relative strengths of the Xe...O donor-acceptor bond in XeOTeF(5)(+).SO(2)ClF and ion-pair bonds in [XeL][MF(6)] (L = F, OTeF(5); M = As, Sb), with the Xe...O bond of XeOTeF(5)(+).SO(2)ClF being the weakest in the series.

Formation of novel rare-gas molecules in low-temperature matrices

Progress in the study of a new class of chemically bound compounds of noble-gas atoms is reviewed. The focus is on rare-gas molecules of the form HNgY, where Ng is a noble-gas atom and Y is an electronegative group, prepared by photolysis of HY in the rare-gas matrix. Other related types of new molecules of noble-gas atoms are discussed as well. Topics discussed in this review include: (a) The nature of bonding and the energetic stability of the compounds. (b) The vibrational spectroscopy of the molecules, and its role in identification of the species. (c) The mechanism and dynamics of photochemical formation of HNgY in the matrix, and the pathways for thermal and infrared (IR)-induced decomposition. Specifically, attention is given to the issue of "direct" formation following photolysis of HY versus "delayed" formation involving H atom diffusion. (d) Molecules of the lighter rare gases Ar, Ne, and He, focusing on the experimentally prepared HArF and on theoretical predictions suggesting the existence of other molecules. (e) The most-recently discovered photochemically induced insertion compounds of Ng into hydrocarbons, such as HXeCCH. (f) Clusters of HNgY with other molecules. The possible existence of neat aggregates and crystals of HNgY. The reviewed state-of-the-art suggests this field is at an early stage of development with major open questions bearing on the surprising properties of the molecules and on the formation mechanisms. These are part of the challenge for the future.

Theoretical study of Ng–NiN2 (Ng=Ar,Ne,He)

It is shown that NiN(2) and noble gas atoms, Ar, Ne, and He, combine with the binding energy of 11.52, 4.06, and 7.37 kcal/mol, respectively, by the multireference perturbational (CASPT2) method. By the density functional theory calculations using MPWPW91 functionals, the Ni-N-N bending frequency in NiN(2) and Ar-NiN(2) is estimated as 310.7 and 358.7 cm(-1), respectively, the latter of which is in good agreement with the corresponding experimental frequency, 357.0 cm(-1), determined for NiN(2) isolated in solid argon.

An ab initio potential energy surface and predissociative resonances of HArF

A three-dimensional potential energy surface of the ground electronic state HArF is constructed from more than 2000 ab initio points at the multireference averaged quadratic coupled-cluster level employing an augmented large basis set. The calculations indicate that the linear HArF molecule is metastable with a barrier of 0.643 eV in the atomization (HArF --> H + Ar + F) channel and a barrier of 1.017 eV in the dissociation (HArF --> Ar + HF) channel. Variational calculations of low-lying predissociative resonances of both HArF and DArF are performed on the three-dimensional potential energy surface using a complex-symmetric Lanczos propagation method, which yields both positions and widths of the resonance states. The resonance lifetime generally decreases with energy, but strong mode selectivity exists. Reasonably good agreement with experiment confirms the accuracy of our potential. These calculations provide valuable information on the stability and dynamics of HArF/DArF in its ground electronic state.

Chemical Compounds Formed from Diacetylene and Rare-Gas Atoms: HKrC4H and HXeC4H

New organic rare-gas compounds, HRgC4H (Rg = Kr or Xe), are identified in matrix-isolation experiments supported by ab initio calculations. These compounds are the largest molecules among the known rare-gas hydrides. They are prepared in low-temperature rare-gas matrixes via UV photolysis of diacetylene and subsequent thermal mobilization of H atoms at approximately 30 and 45 K for Kr and Xe, respectively. The strongest IR absorption bands of the HRgC4H molecules are the H-Rg stretches with the most intense components at 1290 cm(-1) for HKrC4H and at 1532 cm(-1) for HXeC4H, and a number of weaker absorptions (C-H stretching, C-C-C bending, and C-C-H bending modes) are also found in agreement with the theoretical predictions. As probably the most important result, the IR absorption spectra indicate some further stabilization of the HRgC4H molecules as compared with the corresponding HRgC2H species identified recently (Khriachtchev et al. J. Am. Chem. Soc. 2003, 125, 4696 and Khriachtchev et al. J. Am. Chem. Soc. 2003, 125, 6876). The computational energetic results support this trend. HXeC4H is predicted to be 2.5 eV lower in energy than H + Xe + C4H, which is approximately 1 eV larger than the corresponding value for HXeC2H. We expect that the larger molecules HRgC6H and HRgC8H are even more stable and the HRgC2nH species are good candidates for bulk organic rare-gas material.

Trifluoropropynylxenon(ii) tetrafluoroborate [CF3CCXe] [BF4] – isolation of an alkynylxenon(ii) compound for the first time

The salt [CF3C[triple bond]CXe] [BF4] was prepared as neat compound by the reaction of the hitherto unknown alkynyldifluoroborane CF3C[triple bond]CBF2 with XeF2 in 1,1,1,3,3-pentafluoropropane (PFP) at -45 degrees C in 59% yield. [CF3C[triple bond]CXe] [BF4] was unambiguously characterised by multinuclear NMR spectroscopy in anhydrous HF (aHF) solution.