Structures and Electron Attachment Properties of Halomethanes (CX n Y m,X = H,F; Y = Cl,Br, I; n = 0,4; m = 4n)

Abstract

Theoretical studies of structures of neutral molecules and their anions as well as dissociative electron attachment properties are presented for the halomethanes of general formula CX _n Y _m ; X = H, F; Y = Cl, Br, I; n = 0,4; m = 4 – n. The dissociative electron attachment seems to be the primary process resulting in toxicity of these species. The halomethane anions containing hydrogens are formed as radical-anion adducts. When H is replaced by F, these species become true σ? radicals. The electron affinities are computed using a variety of computational techniques including the four-order M?ller-Plesset (MP4) technique that included 250 basis functions. It is challenging to compare the computed results with experiment due to dearth of experimental data and uncertainties in the existing experimental data. Thus in certain cases larger differences are found between the computed and experimental values.     

Theoretical prediction of noble gas containing anions FNgO- (Ng = He, Ar, and Kr)

The structures and energies of the noble gas containing anions FNgO- (Ng = He, Ar, and Kr) have been calculated by high-level ab initio calculations. The FNgO- anions were found to be deep-energy minima at the singlet electronic state, and their energies are significantly lower than those at the triplet state. High dissociation energy barriers to Ng + OF- were also predicted. The unexpected stability of the FNgO- was due to the dramatic ion-induced O=Ng bond formation. The calculated results suggested possible experimental identification of the anionic species and even some related "ionic compounds" under cryogenic conditions.     

Noble gas anions: a theoretical investigation of FNgBN- (Ng = He-Xe)

Noble gas anions of general formula FNgBN- (Ng = He-Xe) have been investigated by MP2, coupled-cluster, and multireference-CI calculations with correlation-consistent basis sets. These species reside in deep wells on the singlet potential energy surface and are thermodynamically stable with respect to the loss of F, F-, BN, and BN-. They are unstable with respect to Ng + FBN-, but at least for Ng = Ar, Kr, and Xe, the involved energy barriers are high enough to suggest their conceivable existence as metastable species. The stability of FNgBN- arises from the strong F--stabilization of the elusive NgBN. The character of the boron-noble gas bond passes from purely ionic for FHeBN- and FNeBN- to covalent for FXeBN-.     

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.     

HNgBeF3 (Ng = Ar‐Rn): Superhalogen‐supported noble gas insertion compounds

Ab initio and density functional theory‐based calculations are performed to study the structure, stability, and nature of bonding of superhalogen‐supported noble gas (Ng) compounds of the type HNgY where (Ng = Ar‐Rn; Y = BeF₃). Here, BeF₃ acts as the superhalogen. Calculations show that the HNgBeF₃ spontaneously dissociates into product following the dissociation channels: HNgBeF₃ → HBeF₃ + Ng and HNgBeF₃ → Ng + HF + BeF₂. The transition states are optimized and the energy barriers are computed to show the metastable behavior of HNgBeF₃. HNgBeF₃ molecules are kinetically stable with respect to the first dissociation process having energy barriers of 1.0, 5.0, 10.6, and 13.9 kcal/mol for Ar, Kr, Xe, and Rn analogues, respectively, at CCSD(T)/Aug‐cc‐pVTZ level. These calculations suggest that the HXeBeF₃ and HRnBeF₃ can be shown to be stable up to ∼100 K temperature with a half‐life of ∼10² seconds. The nature of H Ng and two different types of Ng F bonds in HNgBeF₃ molecules is explored through the natural bond orbital and electron density analyses. The large Wiberg bond index (WBI) values for the H Ng bond indicate the formation of almost a single bond in between H‐atoms and Ng‐atoms, whereas small WBI values for the two Ng F bonds indicate a noncovalent interaction in between them. The electron density analysis further supports the covalency of the H Ng bond and noncovalent interaction in the two Ng F bonds in HNgBeF₃.     

Noble‐Gas Complexes: Theoretical Investigation of Multicenter Polynuclear Species

Ab initio calculations at the MP2 level of theory disclose the conceivable existence of neutral complexes containing four or five distinct noble gases (Ng) each bound to a distinct Be‐atom. These multicenter polynuclear Ng molecules are formally obtained by replacing the H‐atoms of CH₄ and but‐2‐yne with ? NBeNg moieties, which behave as independent monovalent ‘functional groups’. Our investigated complexes include the five homotetranuclear [C(NBeNg)₄] complexes 1 – 5 (Ng=He? Xe), the five heterotetranuclear complexes [CN₄Be₄(He)(Ne)(Ar)(Kr)] ( 6 ), [CN₄Be₄(He)(Ne)(Ar)(Xe)] ( 7 ), [CN₄Be₄(He)(Ne)(Kr)(Xe)] ( 8 ), [CN₄Be₄(He)(Ar)(Kr)(Xe)] ( 9 ), and [CN₄Be₄(Ne)(Ar)(Kr)(Xe)] ( 10 ), and the heteropentanuclear complex [HC₄N₅Be₅(He)(Ne)(Ar)(Kr)(Xe)] ( 11 ). We also investigated the five model complexes [H₃CNBeNg] (Ng=He? Xe) containing a single ? NBeNg moiety. The geometries and vibrational frequencies of all these species, invariably characterized as minimum‐energy structures, were computed at the MP2(full)/6‐31G(d,p)/SDD level of theory, and their stability with respect to the loss of the various Ng‐atoms was evaluated by single‐point calculations at the MP2(full)/6‐311G(d)/SDD level of theory. The beryllium‐Ng binding energies range from ca. 17 (Ng=He) to ca. 63 (Ng=Xe) kJ/mol, and the results of natural‐bond‐orbital (NBO) and atoms‐in‐molecules (AIM) analysis reveal that the Be? Ng interaction is essentially electrostatic for helium, neon, argon, and krypton, and has probably a small covalent contribution for xenon.     

Structure and stability of the organo-noble gas molecules XNgCCX and XNgCCNgX (Ng = Kr, Ar; X = F, Cl)

Density functional theory (B3LYP) and ab initio theory [second-order M?ller-Plesset perturbation theory (MP2) and coupled-cluster theory including single, double, and quasiperturbative triple excitations (CCSD(T))] have been used in combination with the standard and augmented correlation consistent basis sets (cc-pVnZ and aug-cc-pVnZ, where n = D, T, and Q) to investigate potential new noble gas compounds. Two classes of molecules were studied: XNgCCNgX and XNgCCX, where Ng = Kr and Ar and X = F and Cl. These molecules were characterized by finding the ground-state structures and calculating the relative energies, charge distributions, and vibrational frequencies. In addition, transition-state structures were also determined and decomposition pathways were identified through intrinsic reaction coordinate calculations.     

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.     

Bonding analysis for NgAuOH (Ng = Kr,Xe)

Noble‐gas‐noble‐metal hydroxides NgAuOH (Ng = Kr, Xe) are investigated at the MP2 theoretical level. All species are found to be in Cs symmetry with an approximate linear Ng? Au? O moiety. The noble‐gas‐noble‐metal bond lengths are comparable with covalent limits, and the corresponding binding energies have been computed to be 59.6 and 83.4 kJ/mol for KrAuOH and XeAuOH, respectively. Except the charge‐induced dipole contribution to the binding energies, the remainder could be ascribed to the higher‐order charge‐induction energies, dispersion energies, the contributions of multipole moments on AuOH and covalent effects. The title species are sufficiently chemical bound and are expected to be stable species theoretically. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Structural peculiarities and the possibility of the existence of small water cluster anions (H2O) n − withn≤4

Structures of (H₂O) _n ⁻ anions withn≤4 were optimized at the UHF/4-31++G^** level and their stability was estimated at the MP2/4-31++G^** level. The trimer anion has a chain-like structure while the tetramer anion can exist either in a chain-like or a cyclic configuration. In the dimer anion and in the chain-like anions, the excess electron density is localized on the terminal water molecule, an acceptor of the H-bond proton. In the cyclic anion, it is uniformly distributed over the free hydrogen atoms. All considered anions have energy values higher than those of the corresponding neutral oligomers. The detachment of an electron from the anions should proceed with the liberation of energy. However, trimer and larger anions are stable against dissociation into individual water molecules and a free electron. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 41–46, January, 1997.     

A noble interaction: An assessment of noble gas binding ability of metal oxides (metal = Cu,Ag, Au)

An in silico study is performed on the structure and the stability of noble gas (Ng) bound MO complexes (M = Cu, Ag, Au). To understand the stability of these Ng bound complexes, dissociation energies, dissociation enthalpy, and dissociation free energy change are computed. The stability of NgMO is also compared with that of the experimentally detected NgMX (X= F, Cl, Br). It is found that MO has lower Ng binding ability than that of MX. All the dissociation processes producing Ng and MO are endothermic in nature and for the Kr‐Rn bound MO (M = Cu, Au), and Xe and Rn bound AgO cases, the corresponding dissociation processes are turned out to be endergonic in nature at standard state. The Wiberg bond indices of Ng? M bonds and Ng→M electron transfer gradually increase from Ar to Rn and for the same Ng they follow the order of NgAuO > NgCuO > NgAgO. Energy decomposition analysis shows that the Ng? M bonds in NgMO are partly covalent and partly electrostatic in nature. Electron density analysis further highlights the partial covalent character in Ng? M bonds. © 2016 Wiley Periodicals, Inc.     

Structures and electron attachment properties of halomethanes (CX(n)Y(m), X=H, F; Y=Cl, Br, I; n=0,4; m=4-n)

Theoretical studies of structures of neutral molecules and their anions as well as dissociative electron attachment properties are presented for the halomethanes of general formula CX(n) Y(m); X=H, F; Y=Cl, Br, I; n=0,4; m=4-n. The dissociative electron attachment seems to be the primary process resulting in toxicity of these species. The halomethane anions containing hydrogens are formed as radical-anion adducts. When H is replaced by F, these species become true sigma* radicals. The electron affinities are computed using a variety of computational techniques including the four-order M?ller-Plesset (MP4) technique that included 250 basis functions. It is challenging to compare the computed results with experiment due to dearth of experimental data and uncertainties in the existing experimental data. Thus in certain cases larger differences are found between the computed and experimental values.     

Syntheses,Structures, and Bonding of NgF2⋅CrOF4, NgF2⋅2CrOF4 (Ng=Kr,Xe), and (CrOF4)∞

The noble-gas difluoride adducts, NgF₂ ⋅ CrOF₄ and NgF₂ ⋅ 2CrOF₄ (Ng=Kr and Xe), have been synthesized and structurally characterized at low temperatures by Raman spectroscopy and single-crystal X-ray diffraction. The low fluoride ion affinity of CrOF₄ renders it incapable of inducing fluoride ion transfer from NgF₂ (Ng=Kr and Xe) to form ion-paired salts of the [NgF]⁺ cations having either the [CrOF₅]⁻ or [Cr₂O₂F₉]⁻ anions. The crystal structures show the NgF₂ ⋅ CrOF₄ adducts are comprised of F_t−Ng−F_b- - -Cr(O)F₄ structural units in which NgF₂ is weakly coordinated to CrOF₄ by means of a fluorine bridge, F_b, in which Ng−F_b is elongated relative to the terminal Ng−F_t bond. In contrast with XeF₂ ⋅ 2MOF₄ (M=Mo or W) and KrF₂ ⋅ 2MoOF₄, in which the Lewis acidic, F₄(O)M- - -F_b- - -M(O)F₃ moiety coordinates to Ng through a single M- - -F_b−Ng bridge, both fluorine ligands of NgF₂ coordinate to CrOF₄ molecules to form F₄(O)Cr- - -F_b−Ng−F_b- - -Cr(O)F₄ adducts in which both Ng−F_b bonds are only marginally elongated relative to the Ng−F bonds of free NgF₂. Quantum-chemical calculations show that the Cr−F_b bonds of NgF₂ ⋅ CrOF₄ and NgF₂ ⋅ 2CrOF₄ are predominantly electrostatic with a small degree of covalent character that accounts for their nonlinear Cr- - -F_b−Ng bridge angles and staggered O−Cr- - -F_b−Ng−F_t dihedral angles. The crystal structures and Raman spectra of two CrOF₄ polymorphs have also been obtained. Both are comprised of fluorine-bridged chains that are cis- and trans-fluorine-bridged with respect to oxygen.     

Theoretical investigation of the noble gas molecular anions XAuNgX? and HAuNgX? (X?=?F, Cl, Br; Ng?=?Xe, Kr, Ar)

The geometries, atomic charge distributions, vibrational frequencies, and relative energies of the noble gas molecular anions XAuNgX^? and HAuNgX^? (X?=?F, Cl, Br; Ng?=?Xe, Kr, Ar) were investigated at the MP2 and CCSD(T) levels of theory. The Au?CNg bond length of X(H)AuNgX^? is mainly dependent on the electronegative fragment bonded to the Au atom rather than on that bonded to the Ng atom. The presence of the right X^? anion stabilizes the Au?CNg bond of X(H)AuNg. Based on the interatomic distances and atomic charge distributions, X(H)AuNgX^? may be better described as X(H)AuNg···X^? rather than as X(H)^?···AuNgX. The MP2 calculations indicate that, for the Xe, Kr, and Ar molecular anion series, (i) X(H)AuNgX^? is less stable than the global minimum X(H)AuX^??+?Ng by ca. 25?C35, 33?C48, and 37?C57?kcal/mol, respectively, (ii) the reaction barriers are ca. 5?C14, 3?C9, and 2?C5?kcal/mol, respectively, when the anion dissociates into X(H)AuX^??+?Ng through the bending transition state, and (iii) X(H)AuNgX^? is more stable than the dissociation limit X(H)AuNg?+?X^? by ca. 14?C38, 11?C30, and 9?C25?kcal/mol, respectively.     

On the Bonding Nature of the Noble Gas Compounds MRg$^+$ and MRgF (M=Co,Rh, Ir; Rg=Ar,Kr, Xe)

The structure and stability of the compounds MRg$^+$ and MRgF (Rg=Ar, Kr, and Xe; M=Co, Rh, and Ir) were investigated using the B3LYP, MP2, MP4(SDQ) and CCSD(T) methods. We reported the geometry, vibrational frequencies and thermodynamics properties of these compounds. A series of theoretical methods on the basis of wavefunction analysis, including natural bond orbitals, atoms in molecules, electron localization function, and energy decomposition analysis, were performed to explore bonding nature of the M$-$Rg and Rg$-$F bonds. These bonds are mainly noncovalent, the metal weakly interacts with Rg in MRg$^+$, but their interaction is much stronger in MRgF. The neutral molecule MRgF can be well described by the Lewis structure [MRg]$^+$F$^-$.     

Resonance Character of Copper/Silver/Gold Bonding in Small Molecule⋅⋅⋅MX (X=F,Cl, Br,CH3, CF3) Complexes

The resonance character of Cu/Ag/Au bonding is investigated in B???M?X (M=Cu, Ag, Au; X=F, Cl, Br, CH₃, CF₃; B=CO, H₂O, H₂S, C₂H₂, C₂H₄) complexes. The natural bond orbital/natural resonance theory results strongly support the general resonance‐type three‐center/four‐electron (3c/4e) picture of Cu/Ag/Au bonding, B:M?X?B⁺?M:X^?, which mainly arises from hyperconjugation interactions. On the basis of such resonance‐type bonding mechanisms, the ligand effects in the more strongly bound OC???M?X series are analyzed, and distinct competition between CO and the axial ligand X is observed. This competitive bonding picture directly explains why CO in OC???Au?CF₃ can be readily replaced by a number of other ligands. Additionally, conservation of the bond order indicates that the idealized relationship b_B???M+b_MX=1 should be suitably generalized for intermolecular bonding, especially if there is additional partial multiple bonding at one end of the 3c/4e hyperbonded triad.     

Stability of Noble‐Gas‐Bound SiH3+ Clusters

The stability of noble gas (Ng)‐bound SiH₃⁺ clusters is explored by ab initio computations. Owing to a high positive charge (+1.53 e^?), the Si center of SiH₃⁺ can bind two Ng atoms. However, the Si?Ng dissociation energy for the first Ng atom is considerably larger than that for the second one. As we go down group 18, the dissociation energy gradually increases, and the largest value is observed for the case of Rn. For NgSiH₃⁺ clusters, the Ar–Rn dissociation processes are endergonic at room temperature. For He and Ne, a much lower temperature is required for it to be viable. The formation of Ng₂SiH₃⁺ clusters is also feasible, particularly for the heavier members and at low temperature. To shed light on the nature of Si?Ng bonding, natural population analysis, Wiberg bond indices computations, electron‐density analysis, and energy‐decomposition analysis were performed. Electron transfer from the Ng centers to the electropositive Si center occurs only to a small extent for the lighter Ng atoms and to a somewhat greater extent for the heavier analogues. The Si?Xe/Rn bonds can be termed covalent bonds, whereas the Si?He/Ne bonds are noncovalent. The Si?Ar/Kr bonds possess some degree of covalent character, as they are borderline cases. Contributions from polarization and charge transfer and exchange are key terms in forming Si?Ng bonds. We also studied the effect of substituting the H atoms of SiH₃⁺ by halide groups (?X) on the Ng binding ability. SiF₃⁺ showed enhanced Ng binding ability, whereas SiCl₃⁺ and SiBr₃⁺ showed a lower ability to bind Ng than SiH₃⁺. A compromise originates from the dual play of the inductive effect of the ?X groups and X→Si π backbonding (p_z–p_z interaction).     

Structures and isomerization of neutral and zwitterion serine–water clusters: Computational study

Calculations are presented for the structure and the isomerization reaction of various conformers of the bare serine, neutral serine–(H₂O)_n and serine zwitterion–(H₂O)_n (n = 1, 2) clusters. The effects of binding water molecules on the relative stability and the isomerization processes are examined. Hydrogen bonding between serine and the water molecule(s) may significantly affect the relative stability of conformers of the neutral serine–(H₂O)_n (n = 1, 2) clusters. The sidechain (OH group) in serine is found to have a profound effect on the structure and isomerization of serine–(H₂O)_n (n = 1, 2) clusters. Conformers with the hydrogen bonding between water and the hydroxyl group of serine are predicted. A detailed analysis is presented of the isomerization (proton transfer) pathways between the neutral serine–(H₂O)₂ and serine zwitterion–(H₂O)₂ clusters by carrying out the intrinsic reaction coordinate analysis. At least two water molecules need to bind to produce the stable serine zwitterion–water cluster in the gas phase. The isomerization for the serine–(H₂O)₂ cluster proceeds by the concerted double and triple proton transfer mechanism occurring via the binding water molecules, or via the hydroxyl group. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005