Prediction of donor-acceptor-type novel noble gas complexes in the triplet electronic state

Closed-shell noble gas (Ng) compounds in the singlet electronic state have been extensively studied in the past two decades after the revolutionary discovery of ¹HArF molecule. Motivated by the experimental identification of very strong donor-acceptor-type singlet-state Ng complex ¹ArOH⁺, in the present article, for the first time, we report new donor-acceptor-type noble gas complexes in the triplet electronic state (³NgBeN⁺ (Ng = He-Rn)), where most of the Ng-Be bond lengths are smaller than the corresponding covalent limits. The newly proposed complexes are predicted to be stable by various computational tools, including coupled-cluster and multireference-based methods, with strong Ng-Be bonding (40.4-196.2 kJ mol^-1). We have also investigated ³NgBeP⁺ (Ng = He-Rn) complexes for the purpose of comparison. Various computational results, including the structural parameters, bonding energies, vibrational frequencies, and atoms-in-molecule properties suggest that it may be possible to prepare and characterize these triplet state complexes through suitable experimental technique(s).

Hitting the Bull's Eye: Stable HeBeOH+ Complex

It is now known that the heavier noble gases (Ng=Ar-Rn) show some varying degrees of reactivity with a gradual increase in reactivity along Ar-Rn. However, because of their very small size and very high ionization potential, helium and neon are the hardest targets to crack. Although few neon complexes are isolated at very low temperatures, helium needs very extreme situations like very high pressure. Here, we find that protonated BeO, BeOH⁺ can bind helium and neon spontaneously at room temperature. Therefore, extreme conditions like very low temperature and/or high pressure will not be required for their experimental isolation. The Ng-Be bond strength is very high for their heavier homologs and the bond strength shows a gradual increase from He to Rn. Moreover, the Ng-Be attractive energy is almost exclusively originated from the orbital interaction which is composed of one Ng(s/p_σ )→BeOH⁺ σ-donation and two weaker Ng(p_π )→BeOH⁺ π-donations, except for helium. Helium uses its low-lying vacant 2p orbitals to accept π-electron density from BeOH⁺ . Previously, such electron-accepting ability of helium was used to explain a somewhat stronger helium bond than neon for neutral complexes. However, the present results indicate that such π-back donations are too weak in nature to decide any energetic trend between helium and neon.

Existence of Noble Gas Inserted Phosphorus Fluorides: FNgPF2 and FNgPF4 with Ng-P Covalent Bond (Ng = Ar, Kr, Xe and Rn)

Very limited literature on noble gas (Ng)-phosphorous chemical bonding and our recent theoretical prediction of FNgP molecule motivates us to explore a unique novel class of neutral noble gas inserted...

A new krypton complex – Experimental and computational investigation of the krypton sulphur pentafluoride cation, [KrSF5]+, in the gas phase

The gas-phase reactions of noble gas (Ng) cations, namely Kr+ and Xe+, with SF6 were investigated experimentally by Fourier transform ion cyclotron resonance mass spectrometry and computationally using MP2 and...

XNgNSi (X = HCC, F; Ng = Kr, Xe, Rn): A New Class of Metastable Insertion Compounds Containing Ng-C/F and Ng-N Bonds and Possible Isomerization therein

Recently, astronomically important silaisocyanoacetylene (HCCNSi) possessing a large dipole moment has been detected for the first time with the help of crossed molecular beam experiments. Quantum chemical computations at higher levels of theory have also been performed to characterize the transient species. In this study, we have analyzed the equilibrium geometry, stability, reactivity, and energetics as well as the nature of bonding in the noble gas (Ng) inserted HCCNSi compound. We have also considered its F analogue to understand the influence of the most electronegative atom in the compound. Metastable behavior of the XNgNSi compounds (X = HCC, F; Ng = Kr-Rn) is examined by calculating thermochemical parameters like free energy change (ΔG) and zero-point-energy-corrected dissociation energy (D₀) at 298 K for all possible two-body (2B) and three-body (3B) (both neutral as well as ionic) dissociation channels using coupled-cluster theory [CCSD(T)] in addition to density functional theory (DFT) as well as second order Møller-Plesset perturbation theory (MP2). The set of predicted compounds is found to be endergonic in nature, having high positive free energy change suggesting the thermochemical stability of the compounds except for the 2B Ng-release paths. Though thermodynamically feasible, they are kinetically protected with very high activation free energy barriers. Interestingly, the release of Ng from the parent moiety XNgNSi produces the XSiN isomer, by 180° flipping of the NSi moiety. This can also be seen in the dynamical simulation carried out with the help of atom-centered density matrix propagation (ADMP) technique at 2000K for 1 ps. The bonding in Ng-C, Ng-F, and Ng-N bonds of the studied compounds is analyzed and described with the aid of natural bond orbital (NBO), topological parameters computed using atoms-in-molecules theory (AIM), energy decomposition analysis (EDA), and adaptive natural density partitioning (AdNDP) methods. The natural charge distribution on the constituent atoms suggests that the compounds can be partitioned into both ways of representations, viz., neutral radical as well as ionic fragments. Lastly, the reactivity of the compounds is scrutinized using certain reactivity descriptors calculated within the domain of conceptual density functional theory (CDFT).

On the stability and chemical bond of noble gas halide cations NgX+ (Ng = He - Rn; X = F - I)

Despite the belief that noble gases (Ng) are completely inert and cannot form stable molecules, a variety of Ng compounds have been reported under laboratory conditions and others were recently detected in the interstellar media, raising interest in knowing and studying their bond nature and the physicochemical properties associated with their stability. In the present work, a systematic analysis of the thermodynamic stability of noble gas halide cations (NgX⁺ ) at the CCSD(T)/def2-QZVP level have been performed. In addition, chemical bond was characterized through Natural Bonding Theory (NBO), Quantum Theory of Atoms in Molecules (QTAIM) and Energy Decomposition Analysis (EDA) with relativistic corrections. All methods suggest that NgX⁺ compounds possess a strong covalent bond. However, results show that only compounds containing Ar-Rn atoms are thermodynamic stable with a highly energetic and endergonic dissociation process. For these reasons, it is possible to suggest that several compounds that have not yet been reported could be obtained at the laboratory level or observed in the interstellar medium.

Intriguing structural, bonding and reactivity features in some beryllium containing complexes

We highlighted our contributions to Be chemistry which include bond-stretch isomerism in Be₃²⁻ species, Be complexes bound with noble gas, CO, and N₂, Be based nanorotors, and intriguing bonding situations in some Be complexes.

Noble gas insertion compounds of hydrogenated and lithiated hyperhalogens

Based on density functional theory (DFT) calculations, hydrogenated hyperhalogen HM(BO2)2, lithiated hyperhalogen LiM(BO2)2 (M = Cu, Ag, Au), and their compounds with xenon were studied. Different insertion sites of Xe resulted in various isomers. According to the natural population analysis, the Xe atom donated 0.12-0.77 electrons to HM(BO2)2 and 0.14-0.41 electrons to LiM(BO2)2 when they combined, leading to metastable charge-transfer compounds in most cases. The nature of bonding between xenon and HM(BO2)2/LiM(BO2)2 was found to be related to its location. Covalent bonds were formed when Xe bound with hydrogen atoms, as indicated by the large Wiberg bond indices of the Xe-H bonds. The same was true for most Xe-M bondings. When an Xe-O connection was formed, it was either an ionic or van der Waals force in nature depending on the specific structural feature of the isomer. A parallel study on hyperhalogen-supported Ar and Kr compounds indicated that they were not very stable and were less likely to exist at room temperature, which was in accordance with the high inertness of both Ar and Kr atoms.

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.

Noble gas hydrides in the triplet state: HNgCCO+ (Ng = He, Ne, Ar, Kr, and Xe)

Motivated by the very recent investigations of neutral noble gas compounds in the open-shell configuration, we explored a new series of noble gas hydrides in the triplet state. The possible existence of noble gas-inserted ketenyl cations, HNgCCO+ (Ng = He, Ne, Ar, Kr, and Xe), in their triplet electronic state has been predicted by various ab initio quantum chemical techniques. Density functional theory (DFT), second-order Møller-Plesset perturbation theory (MP2), and coupled-cluster theory (CCSD(T)) based methods have been employed to investigate the structures, energetics, harmonic vibrational frequencies, and charge distribution analysis of these ions. The aforementioned ions have been found to be thermodynamically stable with respect to all plausible 2-body and 3-body dissociation channels, except the 2-body dissociation pathway leading to the formation of global minima products (Ng + HCCO+). Nevertheless, each of the predicted HNgCCO+ ions is connected to the global minima products through a transition state with a finite barrier height on the potential energy surface, which confirms the kinetic stability of the metastable species. Detailed analysis of the optimized structural parameters, energetics, and harmonic vibrational frequencies of the predicted species clearly indicated that a strong covalent bond exists between H and Ng atoms, while a comparatively weak interaction is found between Ng and C atoms. Moreover, charge distribution and atoms-in-molecules (AIM) analysis strongly concurred with the above inferences and also suggested that the predicted metastable ions should exist essentially in the form of [HNg]+[CCO] complex. These results ultimately indicate that these predicted species may be prepared and characterized by suitable experimental technique(s) under a cryogenic environment.

Cyanide-isocyanide isomerization: stability and bonding in noble gas inserted metal cyanides (metal = Cu, Ag, Au)

The internal isomerization, MNC ↔ MCN (M = Cu, Ag, Au), is investigated through quantum chemical computations. CuNC and AgNC are shown to be neither thermochemically nor kinetically stable against transformation to MCN. The free energy barrier (ΔG‡) for AuNC is somewhat considerable (7.1 kcal mol-1), indicating its viability, particularly at low temperature. Further, the Ng inserted analogues, MNgCN (M = Cu, Ag, Au; Ng = Xe, Rn) turn out to be thermochemically stable with respect to all possible dissociation channels but for two two-body dissociation channels, viz., MNgCN → Ng + MCN and MNgCN → Ng + MNC, which are connected to the internal isomerization processes, MNgCN → NgMCN and MNgCN → NgMNC, respectively. However, they are kinetically protected by substantial ΔG‡ values (11.8-15.4 kcal mol-1 for Cu, 9.8-13.6 kcal mol-1 for Ag, and 19.7-24.7 kcal mol-1 for Au). The pathways for such internal conversion are explored in detail. A thorough inspection of the bonding situation of the studied molecules, employing natural bond order, electron density, adaptive natural density partitioning, and energy decomposition analyses indicates that the M-Ng bonds in MNgCN and Ng-C bonds in AuNgCN can be represented as an electron-shared covalent bond. For the other Ng-C bonds, although an ionic description is better suited, the degree of covalent character is also substantial therein.

Stable NCNgNSi (Ng=Kr, Xe, Rn) Compounds with Covalently Bound C-Ng-N Unit: Possible Isomerization of NCNSi through the Release of the Noble Gas Atom

Although the noble gas (Ng) compounds with either Ng-C or Ng-N bonds have been reported in the literature, compounds containing both bonds are not known. The first set of systems having a C-Ng-N bonding unit is predicted herein through the analysis of stability and bonding in the NCNgNSi (Ng=Kr-Rn) family. While the Xe and Rn inserted analogues are thermochemically stable with respect to all dissociation channels, but for the one producing CNSiN and free Ng, NCKrNSi has another additional three-body dissociation channel, NCKrNSi→CN+Kr+NSi, which is exergonic by -9.8 kcal mol^-1 at 298 K. This latter dissociation can be hindered by lowering the temperature. Moreover, the NCNgNSi→Ng+CNSiN dissociation is also kinetically prohibited by a quite high free energy barrier ranging from 25.2 to 39.3 kcal mol^-1 , with a gradual increase in going from Kr to Rn. Therefore, these compounds are appropriate candidates for experimental realization. A detailed bonding analysis by employing natural bond orbital, electron density, energy decomposition, and adaptive natural density partitioning analyses indicates that both Ng-N and C-Ng bonds in the title compounds are covalent in nature. In fact, the latter analysis indicates the presence of delocalized 3c-3e σ-bond within the C-Ng-N moiety and a totally delocalized 5c-2e σ-bond in these compounds. This is an unprecedented bonding characteristic in the sense that the bonding pattern in Ng inserted compounds is generally represented as the presence of covalent bond in one side of Ng, and the ionic interaction in the other side. Further, the dissociation of Ng from NCNgNSi facilitates the formation of a higher energy isomer of NCNSi, CNSiN, which cannot be formed from bare NCNSi as such, because of the very high free energy barrier associated with the isomeric transformation. Therefore, in the presence of Ng atoms it might be possible to detect the high energy isomer.

σ-Aromatic cyclic M3+ (M = Cu, Ag, Au) clusters and their complexation with dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gases and carbon monoxide

The structure, stability, bonding and σ-aromaticity in dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gas and carbon monoxide bound M₃⁺ (M = Cu, Ag, Au) complexes are analyzed.

Noble gas bound beryllium chromate and beryllium hydrogen phosphate: a comparison with noble gas bound beryllium oxide

Two new beryllium based compounds, beryllium hydrogen phosphate and beryllium chromate are found to have remarkable noble gas binding ability, particularly for Ar–Rn atoms.

Noble gas supported B3+ cluster: formation of strong covalent noble gas–boron bonds

Ar to Rn atoms formed exceptionally strong bonds with B₃⁺, where the Ng (HOMO) → B₃Ng₂⁺ (LUMO) σ-donation is the key term to stabilize the complexes.

Theoretical investigation of HNgNH3(+) ions (Ng = He, Ne, Ar, Kr, and Xe)

The equilibrium geometries, harmonic frequencies, and dissociation energies of HNgNH3(+) ions (Ng = He, Ne, Ar, Kr, and Xe) were investigated using the following method: Becke-3-parameter-Lee-Yang-Parr (B3LYP), Boese-Matrin for Kinetics (BMK), second-order Møller-Plesset perturbation theory (MP2), and coupled-cluster with single and double excitations as well as perturbative inclusion of triples (CCSD(T)). The results indicate that HHeNH3(+), HArNH3(+), HKrNH3(+), and HXeNH3(+) ions are metastable species that are protected from decomposition by high energy barriers, whereas the HNeNH3(+) ion is unstable because of its relatively small energy barrier for decomposition. The bonding nature of noble-gas atoms in HNgNH3(+) was also analyzed using the atoms in molecules approach, natural energy decomposition analysis, and natural bond orbital analysis.

Exploring the nature of silicon-noble gas bonds in H3SiNgNSi and HSiNgNSi compounds (Ng = Xe, Rn)

Ab initio and density functional theory-based computations are performed to investigate the structure and stability of H3SiNgNSi and HSiNgNSi compounds (Ng = Xe, Rn). They are thermochemically unstable with respect to the dissociation channel producing Ng and H3SiNSi or HSiNSi. However, they are kinetically stable with respect to this dissociation channel having activation free energy barriers of 19.3 and 23.3 kcal/mol for H3SiXeNSi and H3SiRnNSi, respectively, and 9.2 and 12.8 kcal/mol for HSiXeNSi and HSiRnNSi, respectively. The rest of the possible dissociation channels are endergonic in nature at room temperature for Rn analogues. However, one three-body dissociation channel for H3SiXeNSi and one two-body and one three-body dissociation channels for HSiXeNSi are slightly exergonic in nature at room temperature. They become endergonic at slightly lower temperature. The nature of bonding between Ng and Si/N is analyzed by natural bond order, electron density and energy decomposition analyses. Natural population analysis indicates that they could be best represented as (H3SiNg)+(NSi)- and (HSiNg)+(NSi)-. Energy decomposition analysis further reveals that the contribution from the orbital term (ΔEorb) is dominant (ca. 67%-75%) towards the total attraction energy associated with the Si-Ng bond, whereas the electrostatic term (ΔEelstat) contributes the maximum (ca. 66%-68%) for the same in the Ng-N bond, implying the covalent nature of the former bond and the ionic nature of the latter.

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.

Metastable behavior of noble gas inserted tin and lead fluorides

The metastable FNgEF and FNgEF₃ (E = Sn, Pb; Ng = Kr–Rn) are the first reported neutral compounds possessing Ng–Sn and Ng–Pb covalent bonds.

On the stability of noble gas bound 1-tris(pyrazolyl)borate beryllium and magnesium complexes

1-Tris(pyrazolyl)borate beryllium and magnesium cationic complexes are found to bind Ar–Rn atoms quite effectively.

Review: gas-phase ion chemistry of the noble gases: recent advances and future perspectives

This review article surveys recent experimental and theoretical advances in the gas-phase ion chemistry of the noble gases. Covered issues include the interaction of the noble gases with metal and non-metal cations, the conceivable existence of covalent noble-gas anions, the occurrence of ion-molecule reactions involving singly-charged xenon cations, and the occurrence of bond-forming reactions involving doubly-charged cations. Research themes are also highlighted, that are expected to attract further interest in the future.