HKrF in solid krypton

Spodium bonding with noble gas atoms

The nature of the bonding between a neutral group 12 member (Zn3, Cd3 and Hg3) ring and a noble gas atom was explored using quantum chemical simulations. Natural bond orbital, quantum theory of atoms in molecules, symmetry-adapted perturbation theory, and molecular electrostatic potential surface analysis were also used to investigate the type of interaction between the noble gas atom and the metal rings (Zn3, Cd3 and Hg3). The Zn3, Cd3 and Hg3 rings are bonded to the noble gas through non-covalent interactions, which was revealed by the non-covalent interaction index. Additionally, energy decomposition analysis reveals that dispersion energy is the key factor in stabilizing these systems.

Beryllium bonding with noble gas atoms

Quantum chemical calculations were carried out to investigate the nature of the bonding between a neutral Be₃ ring and noble gas atom. Electronic structure calculation for these complexes was carried out at different computational levels in association with natural bond orbital, quantum theory of atoms in molecules, electron localization function, symmetry adapted perturbation theory, and molecular electrostatic potential surface analysis of Be₃ complexes. The Be atoms in the Be₃ moiety are chemically bonded to one another, with the BeBe bond dissociation energy being ~125 kJ mol^-1 . The Be₃ ring interacts with the noble gases through non-covalent interactions. The binding energies of the noble gas atoms with the Be₃ ring increases with increase in their atomic number. The non-covalent interaction index, density overlap region indicator and independent gradient model analyses reveal the presence of non-covalent inter-fragment interactions in the complexes. Energy decomposition analysis reveals that dispersion plays the major role towards stabilizing these systems.

Noble Gas in a Ring

We have designed a new type of molecule with a noble gas (Ng = Kr and Xe) atom in a six-membered ring. Their structures and stability have been studied by density functional theory and by correlated electronic structure calculations. The results showed that the six-membered ring is planar with very short Ng–O and Ng–N polar covalent bonds. The calculated energy barriers for all the unimolecular dissociation pathways are higher than 20 and 35 kcal/mol for Ng = Kr and Xe, respectively. The current study suggests that these molecules and their derivatives might be synthesized and observable at cryogenic conditions.

Binding of noble gas atoms by superhalogens

Because of their closed shells, noble gas (Ng) atoms (Ng = Ne, Ar, Kr, and Xe) seldom take part in chemical reactions, yet finding such mechanisms not only is of scientific interest but also has practical significance. Following a recent work by Mayer et al. [Proc. Natl. Acad. Sci. U. S. A. 116, 8167-8172 (2019)] on the room temperature binding of Ar to a superelectrophilic boron site embedded in a negative ion complex, B₁₂(CN)₁₁ ^-, we have systematically studied the effect of cluster size and terminal ligands on the interaction of Ng by focusing on B₁₂X₁₁(Ng) (X = H, CN, and BO) and B₁₂X₁₀(Ng)₂ (X = CN and BO) whose stabilities are governed by the Wade-Mingos rule and on C₅BX₅(Ng) (X = H, F, and CN) and C₄B₂(CN)₄(Ng)₂ whose stabilities are governed by the Huckel's aromaticity rule. Our conclusion, based on density functional theory, is that both the cluster size and the terminal ligands matter-the interaction between the cluster and the Ng atoms becomes stronger with increasing cluster size and the electron affinity of the terminal ligands. Our studies also led to a counter-intuitive finding-removing multiple terminal ligands can enable electrophilic centers to bind multiple Ng atoms simultaneously without compromising their binding strength.

My Trajectory in Molecular Reaction Dynamics and Spectroscopy

This is the story of a career in theoretical chemistry during a time of dramatic changes in the field due to phenomenal growth in the availability of computational power. It is likewise the story of the highly gifted graduate students and postdoctoral fellows that I was fortunate to mentor throughout my career. It includes reminiscences of the great mentors that I had and of the exciting collaborations with both experimentalists and theorists on which I built much of my research. This is an account of the developments of exciting scientific disciplines in which I was involved: vibrational spectroscopy, molecular reaction mechanisms and dynamics, e.g., in atmospheric chemistry, and the prediction of new, exotic molecules, in particular noble gas molecules. From my very first project to my current work, my career in science has brought me the excitement and fascination of research. What a wonderful pursuit!

Unsupported Donor-Acceptor Complexes of Noble Gases with Group 13 Elements

Unsupported donor-acceptor complexes of noble gases (Ng) with group 13 elements have been theoretically studied using density functional theory. Calculations reveal that heavier noble gases form thermodynamically stable compounds. The present study reveals that no rigid framework is necessary to stabilize the donor-acceptor complexes. Rather, prepyramidalization at the Lewis acid center may be an interesting alternative to stabilize these complexes. Detailed bonding analyses reveal the formation of two-center-two-electron dative bonding, where Ng atoms act as a donor.

Theoretical Prediction on the New Types of Noble Gas Containing Anions OBONgO- and OCNNgO- (Ng = He, Ar, Kr and Xe)

The fluorine-less noble gas containing anions OBONgO⁻ and OCNNgO⁻ have been studied by correlated electronic structure calculation and density functional theory. The obtained energetics indicates that for Ng=Kr and Xe, these anions should be kinetically stable at low temperature. The molecular structures and electron density distribution suggests that these anions are stabilized by ion-induced dipole interactions with charges concentrated on the electronegative OBO and OCN groups. The current study shows that in addition to the fluoride ion, polyatomic groups with strong electronic affinities can also form stable noble gas containing anions of the type Y⁻…NgO.

OBCN isomerization and noble gas insertion compounds of identical valence electron number species: stability and bonding

A series of new noble gas (Ng) insertion compounds of the general type XNgX, XNgY and XNgY+ has been theoretically studied using ab initio and DFT methods herein. We first studied the isomerization process of the OBCN compound, and then investigated the bonding properties and stability of the compounds formed by inserting Ng into the single bond of the three low energy isomers by high-level ab initio calculations. The OBNgCN compounds are thermochemically stable with respect to all dissociation channels except for the processes of releasing OBCN/OBNC and free Ng. Furthermore, the two dissociation processes OBNgCN → Ng + OBNC and OBNgNC → Ng + OBCN are kinetically prohibited by the relatively high free energy barrier ranging from 22.7 to 31.7 kcal mol-1 except for the OBKrCN and OBKrNC analogues. And the adaptive natural density partitioning (AdNDP) analysis indicated that chemical bonding in OBNgCN compounds is realized via a delocalized 3-center 2-electron (3c-2e) σ-bond in the B-Ng-C moiety and a totally delocalized 5-center 2-electron (5c-2e) σ-bond in the whole O-B-Ng-C-N. Natural bond orbital (NBO) theory, atoms-in-molecules (AIM) and energy decomposition analysis (EDA) based on the molecular wavefunction revealed that the B-Ng bond and Ng-C bond have some covalent character in OBNgCN. In addition, the calculation and detailed bonding analysis on a large number of neutral and monocationic compounds with identical valence electron numbers to OBNgCN demonstrate that the two bonds directly linked to the Ng atoms have covalent properties in neutral compounds, whereas Ng forms one typical covalent bond and one partial covalent and partial ionic bond with the neighboring atoms in the monocationic compounds.

Exploring the structure, bonding and stability of noble gas compounds promoted by superhalogens. A case study on HNgMX3 (Ng = Ar-Rn, M = Be-Ca, X = F-Br) via combined high-level ab initio and DFT calculations

A series of complexes (HNgMX₃), formed from superhalogen MX₃ (M = Be-Ca, X = F-Br) noble gas (Ar-Rn) and the hydrogen atom, were investigated via combined high-level ab initio and DFT calculations. The high vertical electron detachment energy (VDE) of the superhalogen part will lead to charge transfer from the noble gas hydride to it. This charge transfer gives rise to attractive ionic interaction between the two components and to the existence of these complexes as local minima on the potential energy surface eventually. However, the VDE value of the superhalogen part is not always monotonically correlated with the thermodynamic/kinetic stability of the whole complexes. Therefore the superhalogen itself might not be enough to provide information for the correct prediction of the properties of the whole composites. Although there are exothermic channels of dissociation, the existence of energy barrier might ensure the existence of these Ng hydrides under certain conditions. Our analysis indicates the existence of two important factors, functioning in opposite directions, for the energy barriers along the exothermic channel. To achieve a high energy barrier, the attractive interaction between superhalogen and the H atom in the TS, which lowers the barrier, needs to be suppressed effectively. An understanding of the superhalogen-based composites will provide valuable information on the functional properties and potential application of superhalogens. The details of the interaction between different parts of these composites should be one of the areas of focus in these studies.

Why do higher VDEs of superhalogen not ensure improved stabilities of the noble gas hydrides promoted by them? A high-level ab initio case study

A series of 20 composite structures, consisting of superhalogen and noble gas (Ng) hydrides, was explored via high-level coupled-cluster single, double and perturbative triple excitations calculations in this work. The existence of these composites, as local minima on the potential energy surface, arises from the charge transfer from the Ng hydride part to the superhalogen moiety. Clearly, this transfer could lead to stabilizing the interaction of the ionic type between the two components. The driving force of the charge transfer should be the high vertical electron detachment energy (VDE) of the superhalogen part leading to its enough capability of extracting the electron from the Ng hydride moiety. However, except triggering the ionic attractive interaction, there is nomonotonic correlation between the VDE value and the thermodynamic stability of the whole composite. This counter-intuitive result actually originates from the fact that, irrespective of various superhalogens, only two of their F ligands interact with the Ng atoms directly. Thus, although leading to higher VDE values, the increase in the number of electronegative ligands of the superhalogen moiety does not affect the stabilizing interaction of the composites here directly. In other words, with the necessary charge transfer generated, further increase of the VDE does not ensure the improvement of the thermodynamic stabilities of the whole composite. Moreover, in the transition state of the exothermic dissociation channel, more F atoms will give rise to higher probability of additional attractions between the F and H atoms which should lower the energy barrier. That is to say, increasing VDE, i.e., having more F atoms in many cases, will probably reduce the kinetic stability. Knowing the inevitable existence of the exothermic channel, kinetic stability is crucial to the ultimate goal of experimental observation of these Ng hydrides. Thus, in some cases, only the superhalogen itself may not provide enough information for the correct prediction on the properties of the whole composites. The understanding of the superhalogen-based composites will provide valuable information on the functional properties as well as the application potential of superhalogen clusters. Thus, the corresponding researches should focus on not only the superhalogen itself but also other related aspects, especially the details of the interaction between different parts.

Resonance bonding in XNgY (X = F, Cl, Br, I; Ng = Kr or Xe; Y = CN or NC) molecules: an NBO/NRT investigation

Several noble-gas-containing molecules XNgY were observed experimentally. However, the bonding in such systems is still not understood. Using natural bond orbital and natural resonance theory (NBO/NRT) methods, the present work investigated bonding of the title molecules. The results show that each of the studied XNgY molecules should be better described as a resonance hybrid of ω-bonding and [Formula: see text]-type long-bonding structures: X:^- Ng⁺ - Y, X - Ng⁺: Y^-, and X^{^}Y. The ω-bonding and long-bonding make competing contributions to the composite resonance hybrid due to the accurately preserved bond order conservation principle. We find that the resonance bonding is highly tunable for these noble-gas-containing molecules due to its dependence on the nature of the halogen X or the central noble-gas atoms Ng. When the molecule XNgY consists of a relatively lighter Ng atom, a relatively low-electronegative X atom, and the CN fragment rather than NC, the long-bonding structure X^{^}Y tends to be highlighted. In contrast, the heavy Ng atom and high-electronegative X atom will enhance the ω-bonding structure. Overall, the present work provides electronic principles and chemical insights that help understand the bonding in these XNgY species.

Long-Bonding in HNgCN/NC (Ng = Noble Gas) Molecules: An NBO/NRT Investigation

Recently, Weinhold et al. put forward one new concept of σ̂-type long-bonding and applied it to the comprehension of halogen noble-gas hydrides HNgY (Ng = noble gas; Y = halogen) bonding. The present study extends this new concept into HNgX (X = CN, NC) pseudohalogen molecules. At the B3LYP and CCSD(T) levels of theory, we perform natural bond orbital (NBO) and natural resonance theory (NRT) studies on the HNgX molecules and compare them with the previous results for HNgY molecules. The NBO/NRT results clearly reveal that each of the HNgX, but not the HHeCN, HNeCN, and HNeNC molecules, is composed of three leading resonance structures: two ω-bonding structures H-Ng⁺:X^-, H:^-Ng⁺-X and one long-bonded structure H^∧X. This result indicates that the conventional long-bonding exists in these pseudohalogen molecules, like the long-bonding in HNgY molecules. Unexpectedly, we identify a new type of longer long-bonded structure H^∧C in HNeNC molecule at the B3LYP level, which disappears at the CCSD level. This misleading prediction at the B3LYP level can be traced back to the singlet diradical character, which induces a low-quality geometry. Therefore, the geometry reoptimization of the noble-gas hydrides is indispensable using CASSCF-based methods, if the noble-gas hydrides fail the "Stable" test because of diradical-type instability.

Synthesis, crystal structure and phase transition of a Xe-N2 compound at high pressure: experimental indication of orbital interaction between xenon and nitrogen

The van der Waals compound Xe(N₂)₂ with a C15 Laves structure was successfully synthesised at pressures greater than 4.4 GPa. We found that, at 10 GPa, the structure reversibly transforms from a cubic to a tetragonal phase. Further compression results in changes of Xe-N compound, which could result in the enhancement of orbital interactions between the xenon and nitrogen atoms.

BNg3F3: the first three noble gas atoms inserted into mono-centric neutral compounds - a theoretical study

Following the study of HXeOXeH and HXeCCXeH, in which two Xe atoms were inserted into H2O and C2H2 theoretically and experimentally, the structures and stability of BNg3F3 (Ng = Ar, Kr and Xe), in which three Ng atoms are inserted into BF3, have been explored theoretically using DFT and ab initio calculations. It is shown that BNg3F3 (Ng = Ar, Kr and Xe) with D3h symmetry are local minima with short B-Ng bond lengths of 1.966, 2.027 and 2.214 Å at the CCSD(T)/aug-cc-pVTZ/LJ18 level, which are close to their covalent limits. Note that although BNg3F3 (Ng = Kr and Xe) are energetically higher than the dissociation products 3Ng + BF3, they are still kinetically stable as metastable species with protecting barriers of 13.38 and 17.99 kcal mol(-1) for BKr3F3 and BXe3F3. Moreover, BKr3F3, the tri-Kr-inserted compound, even has comparable kinetic stability to HXeOXeH and HXeOXeF. In addition, upon the formation of BNg3F3, there is a large amount of charge transferred from B to Ng of at least 0.619 e. The calculated Wiberg Bond Indices (WBI) suggest that B-Ng bonds are naturally singly bonded; the large vibrational frequencies of B-Ng and Ng-F stretching modes and the negative Laplacian electron density of B-Ng bonds confirm further that BNg3F3 are stiff molecules with covalent B-Ng bonds. It should be noted that three Ng atoms inserted into mono-centric neutral molecules have not been reported previously. We hope that the present theoretical study may provide important evidence for the experimental synthesis of BNg3F3.

Theoretical Study on the Noble Gas Exchange Reactions of Ng + HNBNg'(+) → Ng' + HNBNg(+) (Ng, Ng' = He, Ne, Ar, Kr, and Xe)

High-level correlated electronic structure calculation and dual-level variational transition state theory with multidimensional tunneling calculation for rate constants have been performed on four noble gas exchange reactions [(1) He + HNBHe'(+) → He' + HNBHe(+), (2) He + HNBNe(+) → Ne + HNBHe(+), (3) Ne + HNBNe'(+) → Ne' + HNBNe(+), and (4) Ar + HNBAr'(+) → Ar' + HNBAr(+)] and on three (3)He isotopic analogues (He + HNB(3)He(+), (3)He + HNBHe(+), and (3)He + HNB(3)He(+)) of the first reaction. The classical barrier heights were predicted to be 8.9, 6.8, 5.7, and 5.5 kcal/mol for the four reactions, respectively. The tunneling effects were found to be important below 250 K for the He reactions and below 150 K for the Ne and Ar reactions. Kinetic helium isotope effects as large as 7.8 at 100 K were predicted for the (3)He + HNB(3)He(+) reaction. Additionally, the structures and energies of the Kr + HNBKr'(+) and Xe + HNBXe'(+) systems have also been studied.

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.

Comparison of hydrogen, halogen, and tetrel bonds in the complexes of HArF with YH3X (X = halogen, Y = C and Si)

Ab initio calculations were performed in order to find equilibrium structures with tetrel, hydrogen or halogen bonds on the potential energy surfaces of the complexes formed between HArF and YH3X (X = halogen, Y = C and Si).

3c/4e -type long-bonding competes with ω-bonding in noble-gas hydrides HNgY (Ng = He, Ne, Ar, Kr, Xe, Rn; Y = F, Cl, Br, I): a NBO/NRT perspective

Novel resonance bonding for the HNgY molecule is demonstrated based on natural resonance theory analyses. Ng/Y affects the ω-bonding vs. long-bonding propensity in each of the HNgY molecules.

Matrix-isolation and ab initio study of HKrCCCl and HXeCCCl

We report on two new noble-gas molecules, HKrCCCl and HXeCCCl, prepared in low-temperature Kr and Xe matrices. These molecules are made by UV photolysis of HCCCl in the matrices and subsequent thermal annealing. The HCCCl precursor is produced by microwave discharge of a mixture of a matrix gas with trichloroethylene (HClC=CCl2). The assignments of the new noble-gas molecules are supported by deuteration experiments and quantum chemical calculations at the MP2(full) and CCSD(T) levels of theory with the def2-TZVPPD basis set. No evidence of ClXeCCH, which is computationally reliably stable, is found in the experiments. ClKrCCH as well as the Ar compounds HArCCCl and ClArCCH are not observed either, which is in agreement with the calculations.

Bonding Motifs of Noble-Gas Compounds As Described by the Local Electron Energy Density

The bonding situation of some exemplary noble-gas (Ng) compounds, including HNg(+), HNgF, FNgO(-), Ng-HF, and NgBeO (Ng = He-Xe) was assayed by examining their local electron energy density H(r). In general, this function partitions the space of atomic species (neutral and ionic) into inner regions of negative values and outer regions of positive values. In the formation of chemical bonds, these atomic regions combine so to form a molecular H(r), Hmol(r), whose plotted form naturally shows the "covalent" and "noncovalent" regions of the molecular species and allows also the recognition of different types of noncovalent interactions such van der Waals, hydrogen, and ionic or partially ionic bonds. The qualitative assignment of the various bonding motifs is corroborated by the topological analysis of Hmol(r), which typically includes several critical points of rank 3 and variable signature. These points are, in particular, characterized here in terms of their bond degree (BD). From a previous definition (Espinosa J. Chem. Phys. 2002, 117, 5529-5542), this quantity is taken as the ratio between the energy density calculated at the critical point of H(r), H(rc), and the corresponding electron density ρ(rc): BD = -H(rc)/ρ(rc). Thus, the BD is positive for covalent interactions (H(rc) < 0) and negative for noncovalent interactions (H(rc) > 0). For structurally related species, the BD result, in general, positively correlated with the binding energies and is, therefore, a semiquantitative index of stability. The present study suggests the general validity of the Hmol(r) to effectively assay the bonding motifs of noble-gas compounds.

Effect of hydration on the organo-noble gas molecule HKrCCH: role of krypton in the stabilization of hydrated HKrCCH complexes

Active role of krypton in the hydration of HKrCCH, a rare gas molecule.

Prediction of Superhalogen-Stabilized Noble Gas Compounds

The discovery of HArF has generated renewed interest in the chemistry of noble gases, particularly their hydrides. Though many weak complexes of noble gases bound by van der Waals interactions are known, the number of halogenated noble gas compounds, HNgX (Ng = noble gas; X = halogen), where the noble gas atom is chemically bound, is limited. These molecules are metastable, and their specialty is that there is substantial ionic bonding between the noble gas atom and the halogen atom. In this Letter, it is shown using density functional theory and second-order Møller-Plesset perturbation theory that by replacing the halogen atoms by superhalogens (Y), whose electron affinities are much larger than those of halogens, more ionic bonds between Ng and Y can be attained. Moreover, the superhalogen-containing noble gas hydrides, HNgY, are more stable compared to their halogenated counterparts.

CNXeCl and CNXeBr species as halogen bond donors: a quantum chemical study on the structure, properties, and nature of halogen···nitrogen interactions

In the present study, strength and characteristic of halogen bond interactions between CNXeY and NCZ molecules are investigated, where Y = Cl, Br and Z = H, CN, F, OH, CH₃, OCH₃, NH₂. MP2/aug-cc-pVTZ calculations indicate that the interaction energies for CNXeY⋯NCZ complexes lie in the range between -1.0 and -3.1 kcal mol⁻¹. Not surprisingly, the calculated interaction energies show a strong correlation with the negative electrostatic potentials on nitrogen atoms. One of the most important results of this study is that, according to energy decomposition analyses, Cl⋯N halogen bonds are largely dependent on dispersion effects, while electrostatic interactions are the major source of the attraction in Br⋯N bonds. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis are used in this study to deepen the nature of the interactions considered. This appears to be the first report on a halogen bond involving halogenated xenon isocyanides.

On the vibrational linear and nonlinear optical properties of compounds involving noble gas atoms: HXeOXeH, HXeOXeF, and FXeOXeF

The vibrational (hyper)polarizabilities of some selected Xe derivatives are studied in the context of Bishop-Kirtman perturbation theory (BKPT) and numerical finite field methodology. It was found that for this set of rare gas compounds, the static vibrational properties are quite large, in comparison to the corresponding electronic ones, especially those of the second hyperpolarizability. This also holds for the dc-Pockels β(-ω;ω,0), Kerr γ(-ω;ω,0,0) and electric field second harmonic generation γ (-2ω;ω,ω,0) effects, although the computed nuclear relaxation (nr) vibrational contributions are smaller in magnitude than the static ones. HXeOXeH was used to study the effects of electron correlation, basis set, and geometry. Geometry effects were found to lead to noticeable changes of the vibrational and electronic second hyperpolarizability. A limited study of the effect of Xe insertion to the nr vibrational properties is also reported. Assessment of the results revealed that Xe insertion has a remarkable effect on the nr (hyper)polarizabilities. In terms of the BKPT, this is associated with a remarkable increase of the electrical and mechanical anharmonicity terms. The latter is consistent with the anharmonic character of several vibrational modes reported for rare gas compounds.

Theoretical prediction of new noble-gas molecules FNgBNR (Ng = Ar, Kr, and Xe; R = H, CH3, CCH, CHCH2, F, and OH)

We have computationally predicted a new class of stable noble-gas molecules FNgBNR (Ng = Ar, Kr, Xe; R = H, CH3, CCH, CHCH2, F, and OH). The FNgBNR were found to have compact structures with F-Ng bond lengths of 1.9-2.2 Å and Ng-B bond lengths of ~1.8 Å. The endoergic three-body dissociation energies of FNgBNH to F + Ng + BNH were calculated to be 12.8, 31.7, and 63.9 kcal mol(-1), for Ng = Ar, Kr, and Xe, respectively at the CCSD(T)/CBS level. The energy barriers of the exoergic two-body dissociation to Ng + FBNH were calculated to be 16.1, 24.0, and 33.2 kcal mol(-1) for Ng = Ar, Kr, and Xe, respectively. Our results showed that the dissociation energetics is relatively insensitive to the identities of the terminal R groups. The current study suggested that a wide variety of noble-gas containing molecules with different types of R groups can be thermally stable at low temperature, and the number of potentially stable noble-gas containing molecules would thus increase very significantly. It is expected some of the FNgBNR molecules could be identified in future experiments under cryogenic conditions in noble-gas matrices or in the gas phase.