Role of the sulfhydryl group on the gas phase fragmentation reactions of protonated cysteine and cysteine containing peptides

The gas phase fragmentation reactions of protonated cysteine and cysteine-containing peptides have been studied using a combination of collisional activation in a tandem mass spectrometer and ab initio calculations [at the MP2(FC)/6-31G*//HF/6-31G* level of theory]. There are two major competing dissociation pathways for protonated cysteine involving: (i) loss of ammonia, and (ii) loss of the elements of [CH₂O₂]. MS/MS, MS/MS of selected ions formed by collisional activation in the electrospray ionization source as well as ab initio calculations have been carried out to determine the mechanisms of these reactions. The ab initio results reveal that the most stable [M + H − NH₃]⁺ isomer is an episulfonium ion (A), whereas the most stable [M + H − CH₂O₂]⁺ isomer is an immonium ion (B). The effect of the position of the cysteine residue on the fragmentation reactions of the [M + H]⁺ ions of all the possible simple dipeptide and tripeptide methyl esters containing one cysteine (where all other residues are glycine) has also been investigated. When cysteine is at the N-terminal position, NH₃ loss is observed, although the relative abundance of the resultant [M + H − NH₃]⁺ ion decreases with increasing peptide size. In contrast, when cysteine is at any other position, water loss is observed. The proposed mechanism for loss of H₂O is in competition with those channels leading to the formation of structurally relevant sequence ions.     

Kinetics and mechanisms of the redox reaction of hexaaquotitanium(III) and the ethylenediamine tetraacetato titanium(III) complex by aqueous solutions of bromine

At 25°C, I = 1.0 M (CF₃SO₃⁻Li⁺+CF₃SO₃H), [H⁺] = 0.034–0.274 M and λ = 453 nm, the rate equation for the oxidation of Ti(H₂O), ₆³⁺ by bromine was found to be: −d/[Br₂]T/dt=kK/[Br₂][Ti^III]/[H⁺]+K+kK/[Br₃⁻][Ti^III]/[H⁺+K, where k = 9.2 × 10⁻³ M ⁻¹ s ⁻¹ and K = 4.5 × 10⁻³ M. At [H⁺] = 1.0 M, [Br⁻] = 0.05–0.4 M, the apparent second-order rate constant decreases as [Br⁻] increases.

The pH-dependence of the oxidation of Ti^III-edta by bromine is interpreted in terms of the change in identity of the Ti^III-edta species as the pH of the reaction medium changes. The second-order rate constants were fitted using a non-linear least-square computer program with (1/k₀^edta)² weighting into an equation of the form: k₀^edta =k₁+k₂K₁[H⁺]⁻¹+k₃K₁K₂[H⁺]^−2/1+K₁[H⁺[H⁺⁻¹+K₁K₂[H⁺]⁻², with K₁ and K₂ fixed as earlier determined at 9.55 × 10⁻³ and 2.29 × 10⁻⁹ M, respectively, for the oxidation of bromine. k₁=k₂=(3.1±0.32)×10³M⁻¹s⁻¹ k₃=(2.3±0.45)×10⁶N⁻¹s⁻¹.

It is proposed that these electron transfer reactions proceed by univalent changes with the production of Br₂^.− as a transient intermediate. An outer-sphere mechanism is proposed for these reactions. The homonuclear exchange rate for Ti^III-edta+Ti^IV-edta is estimated at 32 M⁻¹ s⁻¹.     

Interaction of heterocyclic thioamides with tellurium(II) and tellurium(IV). Syntheses and crystal structures of bis[2-(2-thioxo-1,3- thiazolidin-3-yl)-4,5-dihydro-1,3- thiazolium]hexabromotellurate(IV) and tris(1- methylimidazole-2-(3H)-thione)tellurium(II) bromide

Reaction of 1,3-thiazolidine-2-thione with tellurium(IV) in hydrobromic acid medium gave the hexabromotellurate, [C₆H₉N₂S₃]₂²⁺[Te^IVBr₆]²⁻ (3). Reaction of 1-methylimidazoline-2-(3H)-thione (L″) with tellurium(IV), in hydrobromic acid medium, gave the mixed-ligand tellurium(II) complex [Te^IIL″₃Br]⁺Br⁻ (4). The structures of [C₆H₉N₂S₃]₂²⁺[Te^IVBr₆]²⁻ (3) and [Te^IIL″₃Br]⁺Br⁻ were determined by single crystal X-ray diffraction methods. In 3 the unit cell contains [TeBr₆]²⁻ anions and two [C₆H₉N₂S₃]⁺ cations. There is no direct bonding between the metal atom and the cations. In the anion only slight angular deviations from the perfect octahedral geometry are observed. The lone pair of electrons on tellurium(IV) is found to be stereochemically inert. In the cation the two five-membered heterocyclic rings adopt different conformations. In complex 4, in the solid state, the planar [Te^IIL″₃Br]⁺ cation and Br⁻ anion are held together by ionic interactions. In the cation, L″ is bonded to the central tellurium atom through the sulphur atom.     

Reactivity of tri- and tetra-nuclear ethylidyne cyclopentadienylcobalt clusters towards protonic acids

The bis(μ₃-ethylidyne) tricobalt cluster [(CpCo)₃(μ₃-CCH₃)₂] (1b) is protonated by trifluoroacetic acid to give the dicobalt edge-protonated cation [H(CpCo)₃(μ₃-CCH₃)₂]⁺ [lb + H]⁺. Protonation of the μ₃-ethylidyne tetracobalt cluster hydride [H(CpCo)₄(μ₃-CCH₃)] (3) takes place in two consecutive steps. At low temperature [H₂(CpCo)₄(μ₃-CCH₃)]⁺ [3 + H]⁺ is formed first, and is then slowly converted into [H₃(CpCo)₄(μ₃-CCH₃)]²⁺ [3 + 2H]²⁺ by an excess of acid. As judged by the ¹H NMR data and the crystal structure of [3 + X]⁺[(CF₃COO)₂X]⁻ (X = H or D) the endo hydrogens in [3 + H]⁺ and [3 + 2H]²⁺ occupy μ₃-(Co₃) face capping hydridic positions. The cations [1b + H]⁺ and [3 + H]⁺ show hydride fluxionality in solution, which in the case of [3 + H]⁺ can be frozen out on the NMR timescale at low temperature (ΔG ^≠ (203 K) = 40.8 kJ/mol). The structure of [3 + X]⁺ [(CF₃COO)₂X]⁻ (X = H or D) was determined by X-ray crystallography. One of the hydrides/deuterides is located on the crystallographic mirror plane, capping a tricobalt face of the cluster cation. The other endo hydrogen atom is believed to be disordered between the other two μ₃-(Co₃) sites, which are related by space group symmetry. Deuteronation of 3 shows a strong normal kinetic deuterium isotope effect. From the temperature independence of the ¹H NMR spectrum of [3 + 2D]²⁺ a non-fluxional solution structure can be inferred. In all the systems studied, hydridic (μ₂- or μ₃-) sites are thermodynamically preferred to possible isomeric agostic CoHC or Co₂HC sites for the endo hydrogens. Agostic interactions cannot, however, be ruled out in transient intermediates during the course of the protonations.     

Bis(η-pentamethylcyclopentadienyl)-and(ηcyclopentadienyl) (η-pentamethylcyclopentadienyl)-platinium dications: Pt(IV) metallocenes

Reaction of [Pt₂(η⁵-C₅Me₅)₂(η-Br)₃]³⁺(Br⁻)₃ with C₅R₅H (R = H,Me) in the presence of AgBF₄ gives the first platinocenium dications, [Pt(η⁵-C₅Me₅)(η⁵-C₅R₅)]²⁺(BF₄⁻ )₂. On electrochemical reduction, [pt(η⁵-C₅Me₅)₂]²⁺ yields [Pt(η⁴-C₅Me₅H)(η²-C₅Me₅)]+ BF₄⁻. kw]Cyclopentadienyl; Metallocenes; Platinum; Electrochemistry     

Isotopic exchange reactions between iron-sulphur-nitrosyl clusters and nitrite

The anion [Fe₄S₃(NO)₇]⁻ undergoes slow exchange with labelled nitrite [¹⁵NO₂]⁻ to yield a product [Fe₄S₃(¹⁴NO)(¹⁵NO)₆]⁻ in which complete isotopic exchange has occurred at the basal Fe(NO)₂ groups, but with no exchange at the apical Fe(NO) group. The neutral Fe₄S₄(NO)₄ reacts rapidly with [¹⁵NO₂⁻ to give fully exchanged [Fe₄S₃(¹⁵NO)₇]⁻, and it is proposed that the conversion proceeds by fragmentation, followed by complete isotopic exchange and rapid reassembly. The binuclear anion [Fe₂S₂(NO)₄]²⁻ also yields, with [¹⁵NO₂]²⁻ in CD₂Cl₂ solution, the fully exchanged [Fe₄S₃(¹⁵NO)₇]⁻, and a mechanism involving successive fragmentation, exchange and reassembly steps is proposed; however in aqueous solution, a clean exchange reaction occurs to give [Fe₂S₂(¹⁵NO)₄]²⁻. Neutral binuclear esters Fe₂(SR)₂(NO)₄ (R = Me, Et, or Ph) with [¹⁴NO₂]⁻ yield the mononuclear paramagnetic [Fe(¹⁴NO)₂(¹⁴NO₂)₂]⁻, and with [¹⁵NO₂]⁻ the analogous [Fe(¹⁵NO)₂(₁₅NO₂)₂]⁻.     

Das perfluortriazinium-kation als oxidationsmittel in der metallorganischen synthese—ein neuer weg zur darstellung von [Cp2MCl2](M = Mo

The oxidation of Cp₂MCl₂ (M= Mo, W) with perfluortriazinium tetrafluoroborate, [(FCN)₃F]⁺[BF₄]⁻, in the presence of a flouride ion acceptor (BF₃ or PF₅) in SO₂ solution yielded the cationic metallocene complexes [Cp₂MCl ₂]²⁺[BF₄]⁻ or [C_p2MCl₂] ²⁺[BF₄]⁻[PF₆]⁻ (M = Mo, W), respectively. In these reactions, for the first time the perfluortriazinium cation has proved to be easy to handle and a useful oxidizer in organometallic chemistry. The oxidizer strength of three fluorotriazinium cations, [(XCN)₃F]⁺ (X = F, Cl, H), has been computed ab initio (HF/6 − 31 + G) and calibrated on literature data which were obtained by local density functional calculations. It was anchored to its F⁺ zero point by an experimental value for KrF⁺. ab]Die Oxidation von Cp²MCl₂ mit (M = MO, W) Perfluortriaziniumtetrafluoroborat, [(FCN)₃F]⁺[BF₄]⁻, in Anwesenheit eines Fluoridionenakzeptors (BF₃ oder PF₅) führte in SO₂-Lösung zur Bildung der kationischen Metallocen-Komplexe [Cp₂MCl₂₊]²⁺[BF₄]₂⁻ bzw. [Cp₂MCl₂]²⁺[BF₄]⁻ [PF₆]⁻ (M = Mo, W). In diesen Reaktionen konnte erstmals gezeigt werden, daß Perfluortriazinium-Kationen einfach zu handhabende und nützliche Oxidationsmittel im Bereich der metallorganischen Synthese darstellen. Das (Mdationsvermögen von drei Fluorotriazinium-Kationen, [(XCN)₃F]⁺(X = F, Cl, H), wurde ab initio berechnet (HF/6 − 31 + G) und mit Hilfe von Literaturdaten, die mittels local density functional-Berechnungen erhalten und am experimentellen Wert von KrF ⁺ bezüglich des F⁺ Nullpunktes verankert wurden, kalibriert.     

Flash photolysis/temperature jump study of the vibrational relaxation of N2O(010) by O(P) and NO

This survey begins with the photochemistry at 254 nm and 298 K in the system H₂O₂COO₂RH, the primary objective of which is to determine the rate constants for the reaction OH + RH → H₂O + R relative to the well-known rate constant for the reaction OH + CO → CO₂ + H. Inherent in the scheme is that the reaction HO₂+CO→OH+CO₂ is negligible compared with the OH reaction, and a literature consensus gives k_HO2 < 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹, or some 10⁶ less than k_OH at 298 K. Theoretical calculations establish that the first stage in the HO₂ reaction is the formation of a free radical intermediate HO₂ + CO → HOOCO (perhydroxooxomethyl) which decomposes to yield the products, and that the rate of formation of the intermediate is equal to the rate of formation of the products. The structure of the intermediate and a reaction profile are shown.

High temperature rate data reported subsequent to the data in the consensus and theoretical calculations lead here to a recommendation that, in the range 250–800 K, k_HO2 = 3.45 × 10⁻¹²T^1/2 exp(1.15 × 10⁴/T) cm³ molecule⁻¹ s⁻¹, the hard-sphere-collision Arrhenius modification. This yields k_HO2(298) = 1.0 × 10⁻²⁷ cm³ molecule⁻¹ s⁻¹ or some 10¹⁴ slower than k_OH(298).     

Ab initio study of the two competitive channels HO2 + H → H2 + O2 and HO2 + H → H2O + O

Saddle point geometries and barrier heights have been calculated for the H abstraction reaction HO₂(²A″)+H(²S) → H₂(¹Σ⁺_g)+O₂(³Σ⁻_g) and the concerted H approach-O removing reaction HO₂ (²A″)+H(²S) → H₂O(¹A₁)+O(³P) by using SDCI wavefunctions with a valence double-zeta plus polarization basis set. The saddle points are found to be of C_s symmetry and the barrier heights are respectively 5.3 and 19.8 kcal by including size consistent correction. Moreoever kinetic parameters have been evaluated within the framework of the TST theory. So activation energies and the rate constants are estimated to be respectively 2.3 kcal and 0.4×10⁹ ℓ mol⁻¹ s⁻¹ for the first reaction, 20.0 kcal and 5.4.10⁻⁵ ℓ mol⁻¹ s⁻¹ for the second. Comparison of these results with experimental determinations shows that hydrogen abstraction on HO₂ is an efficient mechanism for the formation of H₂ + O₂, while the concerted mechanism envisaged for the formation of H₂O + O is highly unlikely.     

Synthesis, structure and nuclease activity of copper complexes of disubstituted 2,2′-bipyridine ligands bearing ammonium groups

A series of Cu(II) complexes of disubstituted 2,2′-bipyridine bearing ammonium groups [Cu(L¹⁻⁴)₂Br]⁵⁺ (1–4, L¹ = [5,5′-(Me₂NHCH₂)₂-bpy]²⁺, L² = [5,5′-(Me₃NCH₂)₂-bpy]²⁺, L³ = [4,4′-(Me₂NHCH₂)₂-bpy]²⁺, L⁴ = [4,4′-(Me₃NCH₂)₂-bpy]²⁺ and bpy = 2,2′-bipyridyl) were synthesized, of which complexes 1 and 4 were structurally characterized. Both coordination configurations of Cu(II) ions can be described as distorted trigonal bipyramid. The interaction between all complexes and CT-DNA was evaluated by thermal-denaturation experiments and CD spectroscopy. Results show that the complexes interact with CT-DNA via outside electrostatic interactions and their binding ability follows the order: 1 > 2 > 3 > 4. In the absence of any reducing agents, the cleavage of plasmid pBR322 DNA by these complexes was investigated and the hydrolysis kinetics of DNA was studied in Tris buffer (pH 7.5) at 37 °C. Obtained pseudo-Michaelis–Menten kinetic parameters: 15.0, 13.6, 2.01 and 1.69 h⁻¹ for 1, 2, 3 and 4, respectively, indicate that complexes 1 and 2 exhibit very high DNA cleavage activities. According to their crystal data, the high nuclease activity may be attributed to the strong interaction of the metal moiety and two ammonium groups with phosphate groups of DNA.     

Spectral characterization of a novel near-infrared cyanine dye: a study of its complexation with metal ions

The spectral features of the near-infrared (NIR) dye TG-170 in different solutions and its complexation with several metal ions were investigated. The absorbance maxima of the dye are at λ=819, 805, and 791 nm in dimethyl sulfoxide (DMSO), methanol, and a buffer of pH 5.9, respectively. These values match the output of a commercially available laser diode (780 nm), thus making use of such a source practical for excitation. The emission wavelengths of the dye are at λ_em =822, 812, and 803 nm in DMSO, methanol, and the buffer, respectively. The molar absorptivity and fluorescence quantum yield increase accordingly. The addition of either an Al(III) ion or Be(II) ion resulted in fluorescence quenching of the dye. The Stern–Volmer quenching constant, K_SV, was calculated from the Stern–Volmer plot to be K_SV=3.11×10⁵ M⁻¹ for the Al(III) ion and K_SV=1.17×10⁶ M⁻¹ for the Be(II) ion. The molar ratio of the metal to the dye was established to be 1:1 for both metal ions. The stability constant, K_S, of the metal–dye complex was calculated to be 4.37×10⁴ M⁻¹ for the Al–dye complex and 1.94×10⁶ M⁻¹ for the Be–dye complex.     

3D H-bonding networks self-assembly from pyridinium derivatives and bis(maleonitriledithiolato)zincate(II)

The two ion-pair complexes, [pyH]₂[Zn(mnt)₂] (1) and [4,4′-bipyH₂]-[Zn(mnt)₂] (2), were synthesized, where mnt²⁻ denotes maleonitriledithiolate, and [pyH]⁺, [4,4′-bipyH₂]²⁺ represent pyridinium and diprotonated 4,4′-bipyridinium, respectively. Their single crystal structures show that there are strong bifurcated H-bonding interactions between the cations of the pyridinium derivative and the [Zn(mnt)₂]²⁻ anions in both 1 and 2. The bifurcated H-bonding interactions between the N–H of the pyridiniums and the CN groups of the mnt²⁻ ligands give rise to a 2D layered H-bonding network, the adjacent layers come together in such way as mutual embrace to give a tight pack, thus 2D hydrogen-bonding sheets further develop into 3D H-bonding networks through weak C–HS and ππ stacking interactions in 1. As for 2, the cations and anions connect into several types of H-bonding macrorings ([2+2], [3+3] and [4+4]), these H-bonding macrorings fuse to extend into 2D layered structure, the interpenetration between [3+3] and [4+4] type H-bonding macrorings in the adjacent layers give further rise to novel 3D extended H-bonding networks, in which there are clearly parallel stacks of cations and the chelate rings of anions.     

Reactivity and electronic property of decaphenylmetallocenes of Mo and W atoms

The perphenylmetallocene complexes (η⁵-C₅Ph₅)₂W (1), [(η⁵-C₅Ph₅)₂W]⁺I₃⁻ (1⁺I₃), (η⁵-C₅Ph₅)₂Mo (2) and [(η⁵-C₅Ph₅)₂Mo]⁺I₃⁻ (2⁺I₃) have been prepared. Hydrogenation of 1 in THF produces (η⁵-C₅Ph₅)₂WH₂ (4), while (η⁵-C₅Ph₅)₂WHCl (3) is afforded in 1,2-dichloroethane solvent. Carbonylation of 1 produces (η⁵-C₅Ph₅)₂W(CO) (5). Treatment of 1 with the strong acid CF₃SO₃H leads to the dicationic species [(η⁵-C₅Ph₅)₂W]⁺²[CF₃SO₃]⁻₂ (1⁺²Tf₂) after crystallization. The structures of 2⁺I₃ and 1⁺²Tf₂ have been determined by an X-ray diffraction study. The magnetic susceptibility study indicates a ³E_2g ground-state for 1 and 2, and a ⁴A_2g ground-state for 1⁺ and 2⁺.     

碱金属离子与三肽复合物的气相裂解反应

为了探索侧链R基团对碱金属离子与多肽复合物气相裂解反应的影响, 采用电喷雾电离质谱法研究了碱金属离子Li⁺, Na⁺和K⁺分别与甘氨酸三肽(GGG)、 甘氨酰-苯丙氨酰-甘氨酸三肽(GFG)和甘氨酰-甘氨酰-苯丙氨酸三肽(GGF)形成的复合物的气相裂解反应. 质谱定性实验结果表明, Li⁺, Na⁺和K⁺与GGG, GFG或GGF在气相中可以形成稳定的复合物, 配合比为1∶1或2∶1. 竞争反应质谱图显示, GGG, GFG或GGF与碱金属离子形成的复合物的质谱峰丰度按Li⁺, Na⁺, K⁺顺序依次下降, 表明随着碱金属离子半径的增加, 它们与三肽的结合强度依次减弱. 碰撞诱导解离显示, 母体离子[GGG+Na]⁺, [GGF+Na]⁺和[GFG+Na]⁺ 的质心碰撞能量E(CM)₅₀数值分别为1.94, 1.76和1.63 eV. 通过质谱滴定法测得[GGG+Na]⁺, [GFG+ Na]⁺和[GGF+Na]⁺ 的结合常数lg\begin{array}{c}{K}_{a_1}\end{array}分别为5.30, 5.25和5.17. 质谱法定量结果进一步确认复合物的稳定性顺序为[GGG+Na]⁺>[GGF+Na]⁺>[GFG+Na]⁺, 表明由于空间位阻的影响, 侧链R基团含有苄基的GFG或 GGF与Na⁺的键合强度要小于侧链R全部为H的GGG. 串级质谱分析结果显示, 碱金属化的GGG断裂位点较多, 可解离出丰富的金属化a₂, b和y型碎片离子, 而碱金属化的GGF和GFG解离出的金属化y型离子较多, b型离子其次, 金属化a型离子几乎没有. 此外, 双碱金属化的GGF可解离出较多金属化y型离子. 复合物[GGF+Na]⁺的裂解曲线显示, 当碰撞能量为25 eV时, [y₂+Na-H]⁺ 和[b₂+Na+OH]⁺为主要碎片离子, 当碰撞能量>40 eV时, 只有[b₂+Na+OH]⁺ 碎片离子占有优势数量. 根据质子化三肽裂解机理可以推测, 钠化GFG裂解后生成含噁唑酮的[b₂+Na]⁺离子, 该离子经过一系列过渡态生成[a₂+Na]⁺(2-苄基-4-咪唑酮), 而不是常见的亚胺离子.     

Kinetics of formation of intermediates in silver ion assisted aquations

There is kinetic evidence of the formation of [Co(NH₃)₅NCSAg₃]⁵⁺ in the interaction of [Co(NH₃]⁵NCS]²⁺ with Ag⁺ in aqueous solution, with pseudo-first-order formation rate constant k = 0.158 s⁻¹ for the forward reaction in the following equation at 25°C and [Ag⁺] in the range of 1.23–5.0 × 10⁻² mol dm⁻³ and 0.10 ionic strength (NaClO₄):

Full-size image

Additionally, the formation constant, β₂, for [Co(NH₃)₅NCSAg₂]⁴⁺ has been determined to be log β₂ = 4.717. For the [Rh(NH₃)₅I]²⁺-Ag⁺ reaction there is evidence of an outer-sphere interaction with rate constants of k₂ = 670 dm³ mol⁻¹ s⁻¹ at 25°C and 0.10 ionic strength. This outer-sphere species undergoes further reaction to give the silver ion containing intermediates of the aquation reactions.     

The ion-pair association constant of oxalate with fac-tris(2-aminomethylpyridine)cobalt(III)

The equilibrium constant K for the ion-pair formation fac-[Co(pic)₃]³⁺ + C₂O₂₂₋ fac-[Co(pic)₃]³⁺/C₂O₄²⁻¹ where pic = 2-aminomethylpyridine, has been determined spectrophotometrically at 0.35 M (KCl) ionic strength and 25.0°C, using four different calculation approaches. The best results were obtained when the concentration of the minor component (the cobalt complex ion) was not neglected in comparison with the oxalate ion concentration. The value of K (5.3 M⁻¹) increases when the supporting electrolyte is LiCl (K = 8.2 M⁻¹). The effect of the ionic strength variation from 0.35 to 2.0 M (LiCl) was also investigated.     

The coordinatively unsaturated cluster cations [Pt3{M(CO)3}(μ-Ph2PCH2PPh2)3] (M = Re, Mn)

The coordinatively unsaturated cluster [Pt₃(μ₃-CO)(μ-dppm)₃]²⁺ (1, dppm = Ph₂PCH₂PPh₂) reacts with Na⁺[M(CO)₅]⁻ to give the mixed metal clusters [Pt₃{M(CO)₃}(μ-dppm)₃]⁺ (M = Re, 2; Mn, 3). The new clusters are characterized by spectroscopic methods and, for M = Re, by an X-ray structure determination. The Pt₃Re core in 2 is tetrahedral with particularly short metal-metal distances.     

Copper thioether chemistry: Synthesis and X-ray crystal structures of binuclear copper(I) complexes [Cu2(L)] {L = [24]aneS8, [28]aneS8} incorporating octathia macrocycles

Reaction of L {L = [24]aneS₈, [28]aneS₈} with two molar equivalents of [Cu(NCMe)₄]X (X⁻ = ClO₄⁻, BF₄⁻, PF₆⁻) in MeCN affords the white binuclear copper(I) complexes [Cu₂(L)]²⁺. A single crystal X-ray structure determination of [CU₂([24]aneS₈)](BF₄)₂ shows two tetrahedral copper(I) centres, each of which is coordinated to four thioether sulphur-donors, Cu---S(1) = 2.263(3), Cu---S(4) = 2.363(3), Cu---S(7) = 2.349(3), Cu---S(10) = 2.261(3) Å. The Cu … Cu distance is 5.172(3) Å. A single crystal X-ray structure determination Of [CU₂([28]aneS₈)](ClO₄)₂ shows that this complex also contain two tetrahedral copper(I) centres, each coordinated to four thioether sulphur-donors, Cu---S(1) = 2.278(5), Cu---S(4) = 2.333(5), Cu---S(8) = 2.328(5), CU---S(11) = 2.268(5) Å. The Cu … Cu distance of 6.454(3) Å is greater than in [CU₂([24]aneS₈)]²⁺ , reflecting the greater cavity size in [CU₂([28]aneS₈)]²⁺. Cyclic voltammetry of [CU₂([24]aneS₈)]²⁺ and [CU₂([28]aneS₈)]²⁺ at platinum electrodes in MeCN (0.1 M ⁿBU₄NPF₆) shows irreversible oxidations at E_pa, = +0.88 V, +0.92 V vs Fc/Fc⁺, respectively, at a scan rate of 200 mV s⁻¹. Coulometric measurements in MeCN confirm these oxidations to be two-electron (one electron per copper) processes to give binuclear copper(II) species. Oxidation of the binuclear copper(I) precursors with H₂SO₄ or HNO₃ affords ESR-active copper(II) species which presumably incorporate SO₄²⁻ and NO₃⁻ bridges.     

The chemical production of electronically excited states in the H/NF2 system

A mixture of NF₃ and Ar is passed through an rf discharge in a flow-system to produce, among other species, F and NF₂. When H₂, D₂, or CH₄ are added downstream, reactions with F atoms produce vibrationally excited HF or DF together with H, D, or CH₃. The latter free radicals can react with NF₂, probably by an elimination reaction to produce electronically excited NF: NF₂(²B₁) + H(D, CH₃) → HF^*(DF^* + NF(a¹Δ). A vibrational-to-electronic energy transfer process between the products of this reaction then produces the next higher state of NF: HF(ν 2) + NF(a¹Δ) → HF(ν−2) + NF(b¹Σ⁺). A similar transfer process has also been found between the electronically excited a¹Δ states of O₂ and NF: O₂(a¹Δ) + NF(a¹Δ) → O₂(X³Σ⁻) + NF(b¹Σ⁺). The H or D atoms but not the CH₃ radicals are then found to react with either NF(a¹Δ) or NF(X³Σ⁻) to produce electronically excited N(₂D) atoms, which in turn react with the NF(a¹Δ) molecules to produce N₂(B³Π_g). The observed nitrogen first positive radiation has been demonstrated to be produced entirely by this reaction mechanism rather than by the N(⁴S) recombination that accounts for the Rayleigh afterglow. In addition, the occurrence of the reaction N(₂D) + N₂O → NO(B²Π_r) + N₂ (X¹Σ⁺_g) has been verified. Finally we have observed emission at 3344 Å, which we attribute to the NF(A³Π), which has not been previously reported.     

Novel lithocholaphanes: Syntheses, NMR, MS, and molecular modeling studies

Novel head-to-head lithocholaphanes 6 and 11 have been synthesized via precursors 1–5 and 7–10 with overall good yields, and characterized by ¹H, ¹³C, and ¹⁵N NMR spectroscopy, ESI-TOF mass spectrometry, thermal analysis, and molecular modeling. In addition, the binding abilities of 6 and 11 towards alkali metal cations have been investigated via competitive complexation studies using equimolar mixtures of Li⁺, Na⁺, K⁺, and Rb⁺-cations, and cholaphanes 6 and 11. The formation of cation–cholaphane adducts was detected by ESI-TOF mass spectrometry. The trends in these comparative binding studies are nicely reproduced theoretically with PM3 energetically optimized structures of 6 and 11 and their interaction energies with alkali metal cations calculated by molecular mechanics. Cholaphane 11 possessing a peptoid type structural fragment, –(CH₂CONHCH₂CH₂)₂O–, as a coordination sphere, shows binding tendency towards lithium and sodium cations, whereas 6 possessing an ester type, –(CH₂OCOCH₂)₂O–, moiety and a bigger cavity size than 11, shows merely a tendency towards bigger alkali metal cations, potassium and rubidium.