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.     

Hypervalent versus nonhypervalent carbon in noble-gas complexes

Silicon in [Cl? SiH₃? Cl]^? is hypervalent, whereas carbon in [Cl? CH₃? Cl]^? is not. We have recently shown how this can be understood in terms of the ball‐in‐a‐box model, according to which silicon fits perfectly into the box that is constituted by the five substituents, whereas carbon is too small and, in a sense, “drops to the bottom” of the box. But how does carbon acquire hypervalency in the isostructural and isoelectronic noble gas (Ng)/methyl cation complexes [Ng? CH₃? Ng]⁺ (Ng=He and Ne), which feature a delocalized D_3h‐symmetric structure with two equivalent C? Ng bonds? From Ng=Ar onwards, the [Ng? CH₃? Ng]⁺ complex again acquires a propensity to localize one of its axial C? Ng bonds and to largely break the other one, and this propensity increases in the order Ng=Ar3Ng⁺ and, for comparison, CH₃Ng⁺, NgHNg⁺, and NgH⁺. It appears that, at variance with [Cl? CH₃? Cl]^?, the carbon atom in [Ng? CH₃? Ng]⁺ can no longer be considered as a ball in a box of the five substituents.     

On the noble-gas-induced intersystem crossing for the CUO molecule: experimental and theoretical investigations of CUO(Ng)n (Ng = Ar, Kr, Xe; n = 1, 2, 3, 4) complexes in solid neon

Uranium atoms excited by laser ablation react with CO in excess neon to produce the novel CUO molecule, which forms distinct Ng complexes (Ng = Ar, Kr, Xe) when the heavier noble gases are added. The CUO(Ng) complexes are identified through CO isotopic and Ng substitution on the neon matrix infrared spectra and by comparison to DFT frequency calculations. The U-C and U-O stretching frequencies of CUO(Ng) complexes are slightly red-shifted from frequencies for the (1)Sigma(+) CUO ground state, which identifies singlet ground state CUO(Ng) complexes. In solid neon the CUO molecule is also a complex CUO(Ne)(n), and the CUO(Ne)(n-1)(Ng) complexes are likewise specified. The next singlet CUO(Ne)(x)(Ng)(2) complexes in excess neon follow in like manner. However, the higher CUO(Ne)(x)(Ng)(n) complex (n = 3, 4) stretching modes approach pure argon matrix CUO(Ar)(n) values and isotopic behavior, which are characterized as triplet ground state complexes by DFT frequency calculations. This work suggests that the singlet-triplet crossing occurs with 3 Ar, 3 Kr, or 4 Xe and a balance of Ne atoms coordinated to CUO in the neon matrix host.     

Hydrogen sulphide H2S and noble gases (Ng = He,Ne, Ar,Kr, Xe,Rn) complexes: A theoretical study of their dynamics,spectroscopy, and interactions

In this work, some basic features of the intermolecular bond in gas phase H₂S-Ng complexes (Ng = He, Ne, Ar, Kr, Xe, and Rn) have been investigated in detail, coupling information from scattering experiments with results of quantum chemical calculations at the CCSD(T)/aug-cc-pVTZ level. Spectroscopic constants, rotovibrational energies, and lifetime as a function of temperature have been evaluated for the complete family of H₂S-Ng systems, and an extensive study of involved intermolecular interactions has been performed. In particular, their nature has been characterized by exploiting Atoms-In-Molecules (AIM), Non-Covalent Interactions (NCI), Symmetry-Adapted Perturbation Theory (SAPT), and Charge Displacement (CD) methods, and it was found that all complexes are bound essentially by near-isotropic van der Waals forces, perturbed by weak-stabilizing charge (electron) transfer contributions. Obtained results also show that these additional contributions increase from He up to Rn, providing an appreciable chemical-stabilizing effect of the noncovalent intermolecular bond for H₂S-heavier Ng systems.     

Bonding of multiple noble-gas atoms to CUO in solid neon: CUO(Ng)n (Ng=Ar, Kr, Xe; n=1, 2, 3, 4) complexes and the singlet-triplet crossover point

Laser-ablated U atoms co-deposited with CO in excess neon produce the novel CUO molecule, which forms distinct Ng complexes (Ng=Ar, Kr, Xe) with the heavier noble gases. The CUO(Ng) complexes are identified through CO isotopic and Ng reagent substitution and comparison to results of DFT frequency calculations. The U[bond]C and U[bond]O stretching frequencies of CUO(Ng) complexes are slightly red-shifted from neon matrix (1)Sigma(+) CUO values, which indicates a (1)A' ground state for the CUO(Ng) complexes. The CUO(Ng)(2) complexes in excess neon are likewise singlet molecules. However, the CUO(Ng)(3) and CUO(Ng)(4) complexes exhibit very different stretching frequencies and isotopic behaviors that are similar to those of CUO(Ar)(n) in a pure argon matrix, which has a (3)A" ground state based on DFT vibrational frequency calculations. This work suggests a coordination sphere model in which CUO in solid neon is initially solvated by four or more Ne atoms. Up to four heavier Ng atoms successively displace the Ne atoms leading ultimately to CUO(Ng)(4) complexes. The major changes in the CUO stretching frequencies from CUO(Ng)(2) to CUO(Ng)(3) provides evidence for the crossover from a singlet ground state to a triplet ground state.     

Detection of tritium and alpha decaying radionuclides in L/ILW by measurements of helium isotopes

As decay products, helium isotopes can clearly indicate the presence of tritium and alpha decaying isotopes in a closed system. This study presents the helium and neon measurements and their interpretation of long-term headspace gas investigations in L/ILW waste drums from Paks Nuclear Power Plant and closed vaults of the Radioactive Waste Treatment and Disposal Facility, Püspökszilágy, Hungary. Development of special sampling methods and preparation lines as well as isotope-analytical measurements of the headspace gas samples were done in the Hertelendi Laboratory of Environmental Studies in the ATOMKI. In the gas samples helium isotopes as well as neon isotopes have been determined mass spectrometrically. While neon content can be of atmospheric origin only, helium can be produced either by alpha decay (⁴He) or decay of tritium (³He). ³H/⁴He and He/Ne ratios have been used to determine the different origin of the helium isotopes. Helium isotope ratios always represented ³He enrichment in the headspace gases produced by the decay of the tritium in the waste. Using the recent ³He concentration in headspace gas the total amount of ³H restored in L/ILW vaults was estimated. The investigated seven different vaults were closed between 1979 and 1995 when they had been full with L/ILW. The calculated tritium activities based on the He measurements showed good agreement with the documented isotope inventory of the vaults. Typical tritium activity concentrations were between 0.1 and 10 Bq/L gas in the drums and between 10 and 1000 Bq/L gas in the vaults. Additionally, one drum showed a higher He/Ne ratio compared to air, which clearly indicates ⁴He excess, thus the presence an alpha source in the waste.     

Ion-etching of discharge-produced tetrafluoroethylene film with hydrogen,oxygen and noble gases. A mass spectrometric analysis

Freshly deposited discharge-produced tetrafluoroethylene films were ion-etched with either helium, neon, argon, oxygen or hydrogen. The ions C⁺, CF⁺, CF₂⁺ and CF₃⁺ comprised most of the positive ions in rare gas discharges, with CF⁺ always dominant. Sputtered fragments containing two or more carbon atoms were rare. These findings are compatible with the ion-etching of a highly crosslinked polymer film. Residual background gases were contributed to 1–3% of the total ion flux even though their actual partial pressures were very low. The concentration of neutral species corresponding to the ions observed was less than one part in ten thousand of the etching gas. With pure hydrogen, very little etching occurred and the degree of ionization relative to the rare gases was low. The principal reaction was the abstraction of fluorine from the polymer to give hydrogen fluoride and a more highly crosslinked film. Oxygen containing discharges produced the largest total yield of all the systems studied and the most evidence of chemical attack on the polymer. The ions observed were CO⁺, CO₂⁺, COF⁺, COF₂⁺ as well as C⁺, CF⁺, CF₂⁺ and CF₃⁺. Thus oxygen etches the polymer by preferentially attacking the carbon-carbon framework.     

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).     

Fragmentation of ionized doped helium nanodroplets: theoretical evidence for a dopant ejection mechanism

We report a theoretical study of the effect induced by a helium nanodroplet environment on the fragmentation dynamics of a dopant. The dopant is an ionized neon cluster Ne(n) (+) (n=4-6) surrounded by a helium nanodroplet composed of 100 atoms. A newly designed mixed quantum/classical approach is used to take into account both the large helium cluster zero-point energy due to the light mass of the helium atoms and all the nonadiabatic couplings between the Ne(n) (+) potential-energy surfaces. The results reveal that the intermediate ionic dopant can be ejected from the droplet, possibly with some helium atoms still attached, thereby reducing the cooling power of the droplet. Energy relaxation by helium atom evaporation and dissociation, the other mechanism which has been used in most interpretations of doped helium cluster dynamics, also exhibits new features. The kinetic energy distribution of the neutral monomer fragments can be fitted to the sum of two Boltzmann distributions, one with a low kinetic energy and the other with a higher kinetic energy. This indicates that cooling by helium atom evaporation is more efficient than was believed so far, as suggested by recent experiments. The results also reveal the predominance of Ne(2) (+) and He(q)Ne(2) (+) fragments and the absence of bare Ne(+) fragments, in agreement with available experimental data (obtained for larger helium nanodroplets). Moreover, the abundance in fragments with a trimeric neon core is found to increase with the increase in dopant size. Most of the fragmentation is achieved within 10 ps and the only subsequent dynamical process is the relaxation of hot intermediate He(q)Ne(2) (+) species to Ne(2) (+) by helium atom evaporation. The dependence of the ionic fragment distribution on the parent ion electronic state reached by ionization is also investigated. It reveals that He(q)Ne(+) fragments are produced only from the highest electronic state, whereas He(q)Ne(2) (+) fragments originate from all the electronic states. Surprisingly, the highest electronic states also lead to fragments that still contain the original ionic dopant species. A mechanism is conjectured to explain this fragmentation inhibition.     

Electron impact ionization of CCl4 and SF6 embedded in superfluid helium droplets

Electron impact ionization of helium nano-droplets containing several 10⁴ He atoms and doped with CCl₄ or SF₆ molecules is studied with high-mass resolution. The mass spectra show significant clustering of CCl₄ molecules, less so for SF₆ under our experimental conditions. Positive ion efficiency curves as a function of electron energy indicate complete immersion of the molecules inside the helium droplets in both cases. For CCl₄ we observe the molecular parent cation CCl₄⁺ that preferentially is formed via Penning ionization upon collisions with He*. In contrast, no parent cation SF₆⁺ is seen for He droplets doped with SF₆. The fragmentation patterns for both molecules embedded in He are compared with gas phase studies. Ionization via electron transfer to He⁺ forms highly excited ions that cannot be stabilized by the surrounding He droplet. Besides the atomic fragments F⁺ and Cl⁺ several molecular fragment cations are observed with He atoms attached.     

3He NMR studies on helium–pyrrole,helium–indole,and helium–carbazole systems: a new tool for following chemistry of heterocyclic compounds

The ³He nuclear magnetic shieldings were calculated for free helium atom and He–pyrrole, He–indole, and He–carbazole complexes. Several levels of theory, including Hartree–Fock (HF), Second‐order Møller‐Plesset Perturbation Theory (MP2), and Density Functional Theory (DFT) (VSXC, M062X, APFD, BHandHLYP, and mPW1PW91), combined with polarization‐consistent pcS‐2 and aug‐pcS‐2 basis sets were employed. Gauge‐including atomic orbital (GIAO) calculated ³He nuclear magnetic shieldings reproduced accurately previously reported theoretical values for helium gas. ³He nuclear magnetic shieldings and energy changes as result of single helium atom approaching to the five‐membered ring of pyrrole, indole, and carbazole were tested. It was observed that ³He NMR parameters of single helium atom, calculated at various levels of theory (HF, MP2, and DFT) are sensitive to the presence of heteroatomic rings. The helium atom was insensitive to the studied molecules at distances above 5 Å. Our results, obtained with BHandHLYP method, predicted fairly accurately the He–pyrrole plane separation of 3.15 Å (close to 3.24 Å, calculated by MP2) and yielded a sizable ³He NMR chemical shift (about ?1.5 ppm). The changes of calculated nucleus‐independent chemical shifts (NICS) with the distance above the rings showed a very similar pattern to helium‐3 NMR chemical shift. The ring currents above the five‐membered rings were seen by helium magnetic probe to about 5 Å above the ring planes verified by the calculated NICS index. Copyright © 2014 John Wiley & Sons, Ltd.     

Searching blue-shifted O–H···Y hydrogen bond in CH<Subscript>3</Subscript>OH complexes with CF<Subscript>4</Subscript>, C<Subscript>2</Subscript>F<Subscript>2</Subscript>, OC,Ne, and He: a theoretical study

The hydrogen-bonded structures of the CH₃OH complexes with CF₄, C₂F₂, OC, Ne, and He are designated as the starting points for geometry optimizations without and with counterpoise (CP) correction at MP2 level of theory with the basis sets 6-31+G*, 6-31++G**, and 6-311++G**, respectively. Tight convergence criteria are applied throughout all geometry optimizations in order to reduce the computational errors. According to the optimizations without CP correction, a blue-shifted O–H···Y (where Y = F, O, Ne, or He) hydrogen bond exists in all these five complexes. The magnitudes of blue shifts of ν(O–H) of the former four complexes with respect to that of CH₃OH are reduced greatly when the polarization and diffuse functions of the hydrogen atoms are added (results from 6-31+G* versus those from 6-31++G**). However, for the complexes CH₃OH–CF₄ and CH₃OH–C₂F₂, our optimizations using the CP corrections did not find the hydrogen-bonded structure to be a stationary point. The energy minimum of both the complexes corresponds to a non-hydrogen-bonded structure.     

XPS and atomic force microscopy of plasma-treated polysulfone

Surface treatment of polysulfone by O₂, H₂, He, Ne, Ar, and CF₄ nonisothermal glow discharges has been investigated by x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The chemical and topographical modification of the surface is found to be strongly influenced by the type of feed gas employed. © 1996 John Wiley & Sons, Inc.     

The Role of Ate Complexes in the Copper‐Mediated Trifluoromethylation of Alkynes

Trifluoromethylation reactions have recently received increased attention because of the beneficial effect of the trifluoromethyl group on the pharmacological properties of numerous substances. A common method to introduce the trifluoromethyl group employs the Ruppert–Prakash reagent, that is, Si(CH₃)₃CF₃, together with a copper(I) halide. We have applied this method to the trifluoromethylation of aromatic alkynes and used electrospray‐ionization mass spectrometry to investigate the mechanism of these reactions in tetrahydrofuran, dichloromethane, and acetonitrile as well as with and without added 1,10‐phenanthroline. In the absence of the alkyne component, the homoleptic ate complexes [Cu(CF₃)₂]^? and [Cu(CF₃)₄]^? were observed. In the presence of the alkynes RH, the heteroleptic complexes [Cu(CF₃)₃R]^? were detected as well. Upon gas‐phase fragmentation, these key intermediates released the cross‐coupling products R?CF₃ with perfect selectivity. Apparently, the [Cu(CF₃)₃R]^? complexes did not originate from homoleptic cuprate anions, but from unobservable neutral precursors. The present results moreover point to the involvement of oxygen as the oxidizing agent.     

Spin‐orbit ZORA and four‐component Dirac–Coulomb estimation of relativistic corrections to isotropic nuclear shieldings and chemical shifts of noble gas dimers

Hartree–Fock and density functional theory with the hybrid B3LYP and general gradient KT2 exchange‐correlation functionals were used for nonrelativistic and relativistic nuclear magnetic shielding calculations of helium, neon, argon, krypton, and xenon dimers and free atoms. Relativistic corrections were calculated with the scalar and spin‐orbit zeroth‐order regular approximation Hamiltonian in combination with the large Slater‐type basis set QZ4P as well as with the four‐component Dirac–Coulomb Hamiltonian using Dyall's acv4z basis sets. The relativistic corrections to the nuclear magnetic shieldings and chemical shifts are combined with nonrelativistic coupled cluster singles and doubles with noniterative triple excitations [CCSD(T)] calculations using the very large polarization‐consistent basis sets aug‐pcSseg‐4 for He, Ne and Ar, aug‐pcSseg‐3 for Kr, and the AQZP basis set for Xe. For the dimers also, zero‐point vibrational (ZPV) corrections are obtained at the CCSD(T) level with the same basis sets were added. Best estimates of the dimer chemical shifts are generated from these nuclear magnetic shieldings and the relative importance of electron correlation, ZPV, and relativistic corrections for the shieldings and chemical shifts is analyzed. © 2015 Wiley Periodicals, Inc.     

Modeling 21Ne NMR parameters for carbon nanosystems

The potential of nuclear magnetic resonance (NMR) technique in probing the structure of porous systems including carbon nanostructures filled with inert gases is analysed theoretically using accurate calculations of neon (²¹Ne) nuclear magnetic shieldings. The CBS estimates of ²¹Ne NMR parameters were performed for single atom, its dimer and neon interacting with acetylene, ethylene and 1,3‐cyclopentadiene. Several levels of theory including restricted Hartree‐Fock (RHF), Møller‐Plesset perturbation theory to the second order (MP2), density functional theory (DFT) with van Voorhis and Scuseria's t‐dependent gradient‐corrected correlation functional (VSXC), coupled cluster with single and doubles excitations (CCSD), with single, doubles and triples included in a perturbative way (CCSD(T)) and single, doubles and tripes excitations (CCSDT) combined with polarization‐consistent aug‐pcS‐n series of basis sets were employed. The impact of neon confinement inside selected fullerene cages used as an NMR probe was studied at the RHF/pcS‐2 level of theory. A sensitivity of neon probe to the proximity of multiple CC bonds in C₂H₂, C₂H₄, C₅H₆ and inside C₂₈, C₃₀, C₃₂, C₃₄ and C₆₀ fullerenes was predicted from ²¹Ne NMR parameters' changes. Copyright © 2013 John Wiley & Sons, Ltd.     

Flash photolysis study of the recombination of chlorine atoms in the presence of various inert gases and NO

The recombination of chlorine atoms has been investigated by flash photolysis in the inert gases He, Ne, Ar, N₂, CO₂, CF₄, SiF₄, SF₆, and C₂F₆. The pressure dependence of the reaction has been measured between 0.5 and about 100 atm for He, N₂, and CO₂. Experiments on the NO-catalyzed recombination of chlorine in the presence of He (0.5–100 atm) permitted a determination of the falloff curve of the reaction Cl+NO(+He)→ClNO(+He).     

Kinetics and mechanism of the pyrolysis of 1-chloro-1,1-difluoroethane in the presence of additives

The thermal dehydrochlorination CF₂ClCH₃→CF₂(DOUBLEBOND)CH₂+HCl has been studied in a static system between 597 and 664 K in the presence of CCl₄, C₂Cl₆, CF₂(DOUBLEBOND)CH₂, HCl, and CF₃CH₃. A kinetic radical and molecular reaction model has been developed. In addition to describing earlier results on the acceleration of the pyrolysis by CCl₄ and the further acceleration by HCl, this model describes quantitatively up to conversions of 20% (i) the dependence of the catalytic effect of CCl₄ at low concentrations, (ii) the stronger catalytic effect of C₂Cl₆, and (iii) the inhibitory effect of added CF₂CH₂ and CF₃CH₃ when CCl₄ is used as a catalyst. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 359–366, 1998     

Kinetics and mechanism of the thermal gas-phase reaction between trifluoromethylhypofluorite,CF3OF,and tetrachloroethene

The thermal gas-phase reaction of CF₃OF with CCl₂CCl₂ has been studied between 313.8 and 343.8 K. The initial pressure of CF₃OF was varied between 10.8 and 77.5 torr and that of CCl₂CCl₂ between 3.7 and 26.8 torr. CF₃OF was always present in excess, varying the initial ratio of CF₃OF to that of CCl₂CCl₂ from 1.3 to 10. Three products were formed: CF₃OCCl₂CCl₂F, CCl₂FCCl₂F, and CF₃O(CCl₂CCl₂) ₂OCF₃. The yields of CF₃OCCl₂CCl₂F were 98–99.5%, based on the sum of the products. The reaction was a homogeneous chain reaction not affected by the total pressure. In presence of O₂ the oxidation of CCl₂CCl₂ to CCl₃C(O)Cl and COCl₂ occurred. The proposed basic reaction steps are: generation of the radicals CF₃O˙ and CCl₂FCCl₂˙ (κ₁) in a biomolecular process between CF₃OF and CCl₂CCl₂, formation of the radical CF₃OCCl₂CCl₂˙ by addition of CF₃O˙ to CCl₂CCl₂, chain generation of CF₃O˙ by abstraction of fluorine atom from CF₃OF by CF₃OCCl₂CCl₂˙ (κ₄), and chain termination by recombination of the radicals CF₃OCCl₂CCl₂˙. The expressions obtained for the constants κ₁ and κ₄ are κ₁ = 3.16 ± 0.6 × 10⁷ exp(−15.2 ± 1.7 Kcal mol⁻¹/RT) dm³ mol⁻¹ s⁻¹, κ₄ = 3.7 ± 0.5 × 10⁹ exp(−6.0 ± 1.1 Kcal mol⁻¹/RT) dm³ mol⁻¹ s⁻¹. © 1996 John Wiley & Sons, Inc.     

Electrochemical and Chemical Syntheses of Trifluoromethylating Reagents and Trifluoromethyl Substituted Compounds

The electroreduction of the halofluoromethanes CF₃Br, CF₂Br₂ and CF₂BrCl has been studied in high‐pressure stainless steel autoclaves at different cathodes [Pt, steel (V2A, V4A), glassy carbon (GC)] and in various solvent‐supporting electrolyte systems (SSE), e.g. DMF/[Bu₄N]Br, NMP/[Bu₄N]BF₄ etc. The reduction potentials for CF₃Br increase from Pt (–1.6 V) < V2A (–1.8 V) < GC (–2.1 V) and are lower for CF₂Br₂ and CF₂BrCl suggesting a reductive cleavage of C‐X bonds as the first step. CF₂Br₂ and CF₂BrCl show a two‐step reduction in accord with the C–X bond energies (C–F > C–Cl > C–Br) and the “Perfluoro‐effect”. The electrolysis of CF₃Br in different SSE‐systems with sacrificial zinc or cadmium anodes has been reinvestigated with our experimental set‐up to elucidate the influence of the experimental conditions on the type and ratio of the products. The observed products CF₃MBr·42L and (CF₃)₂M·42L (M = Zn, Cd; L = DMF or AN) are the same as in the previous investigations, but are obtained in different ratios, as a rule caused by a parallel chemical corrosion of the respective anodes. By using aluminium as sacrificial anode no CF₃Al compounds are formed. The CF₃ species generated by electroreduction of CF₃Br react with the solvents via hydrogen abstraction and formation of CF₃H. The current yield with respect to the dissolution of the Al anode reaches 120 % indicating a considerable chemical corrosion in addition to the anodic oxidation. This result enabled a one‐pot trifluoromethylation reaction of NMP as organic carbonyl substrate and solvent with CF₃Br and aluminium powder (ratio 3 : 2) at higher temperatures (> 70 °C). The complete reaction of CF₃Br to give CF₃H and 1‐methyl‐2‐trifluoromethyl‐4,5‐dihydropyrrol allowed the isolation of the latter by vacuum condensation and distillation in 45 % yield, rel. to the CF₃Br used. Gallium and indium were also applied as sacrificial anodes in combination with CF₃Br as substrate. In both cases, anodic current yields of about 280 % indicated an extreme chemical corrosion together with cathodic metal depositions corresponding to the cathodic current yield. These deposits – in contrast to those of Zn and Cd – do not react with CF₃Br in Grignard‐type conversions to CF₃Ga and CF₃In compounds. So, the observed products (CF₃)_nMBr_3–n·L (M = Ga, In; n 1‐3; L = DMF, NMP) are obviously formed by chemical corrosion of the electro‐activated anodes. Finally, electrochemical and chemical trifluoromethylations were successfully carried out, using R₃SiCl (R = Me, Vi, Ph), Me₃M′Cl (M′ = Ge, Sn) and aluminium anodes or Al‐powder. The products were characterized either after isolation or in the product solutions by NMR‐spectroscopic investigations.