13C NMR spectroscopy of [4.n.0] bicyclic compounds: Assignment of the ring junction configuration from ring junction configuration from ring junction 13C chemical shifts—application and limits

Several 6-methyl-9-carbamoyltetrahydro-4H-pyrido[1,2-α]pyrimidin-4-ones have been prepared using phosgene iminium chloride. These compounds can exist in equilibrium as the cis (3A) imine ? (3B) enamine ? trans (3C) imine. ¹H, ¹³C and ¹⁵N NMR prove that the cis- and trans-imine isomers are predominant in the equilibrium. ¹H NMR data reveal that the share of the 3B enamine form is negligible at measurable concentrations. The isomeric ratio 3A:3C is time dependent and can be monitored by measuring the CH₃? C-6 and (CH₃)₂N signals. The ¹³C NMR data show that doublets in the range 42–45 ppm for C-9 are only compatible with the imine forms 3A and 3C. The SCS values of the CH₃? C-6 and OCN(CH₃)₂ groups were calculated and used for identification of the cis and trans isomers. ¹⁵N NMR data show that the N-1 chemical shift of the imine is approximately ? 140 ppm for compound 3, whereas that of a fixed enamine is around ? 267.8. This provides additional support for the predominance of the imine tautomers in the equilibrium 3A ? 3B ? 3C. ¹⁵N data allow the stereoisomers 3A and 3C to be distinguished.     

DFT study on the isomerization in vitamin B6

Isomerization and tautomerism reactions of the active form of vitamin B6, pyridoxal phosphate, are studied at B3LYP level of theory using 6-311++G(2df,p) basis set in gas and aqueous phases. Twenty-three transition state (TS) structures for vitamin B6 isomerization are optimized, including 13 TS structures for O–H and C–C rotations, 8 TS structures for imine–enamine tautomerism, and 2 TS structures for keto–enol tautomerism. Activation energy (E _a), imaginary frequency (υ), and Gibbs free energy of activation (ΔG ^#) for the isomerization reactions are calculated. The activation energies of the imine–enamine tautomerism are in the range of 190–280 kJ/mol and of O–H and C–C rotations are mainly less than 60 kJ/mol. Also, our calculation shows that the imine forms of B6 are mainly more stable than the enamine forms. Effect of microhydration on the TS structures and activation energies is also investigated. It is found that the presence of water molecules catalyzes only the imine–enamine tautomerism.     

Reaction of an “Invisible” Frustrated N/B Lewis Pair with Dihydrogen

D ‐(+)‐Camphor forms the enamine 2 with piperidine. Compound 2 adds HB(C₆F₅)₂ at the enamine carbon atom C3 to form a Lewis acid/Lewis base adduct (exo‐/endo‐isomers of 3 ). Exposure of 3 to dihydrogen (2.5 bar, room temperature) leads to heterolytic splitting of H₂ to form the H⁺/H^? addition products ( 4 , two diastereoisomers) of the “invisible” frustrated Lewis pairs ( 5 , two diastereoisomers) that were apparently generated in situ by enamine hydroboration under equilibrium conditions.     

Structure and formation of gaseous [C4H6O]+˙ ions: 1—The enolic ions [CH2C(OH)?CHCH2]+˙ and [CH2CH?CHCH(OH)]+˙ and their relationship with their keto counterparts

The [C₄H₆O]^+˙ ion of structure [CH₂?CHCH?CHOH]^+˙ (a) is generated by loss of C₄H₈ from ionized 6,6-dimethyl-2-cyclohexen-1-ol. The heat of formation ΔH_f of [CH₂?CHCH?CHOH]^+˙ was estimated to be 736 kJ mol^?1. The isomeric ion [CH₂?C(OH)CH?CH₂]^+˙ (b) was shown to have ΔH_f, ? 761 kJ mol^?1, 54 kJ mol^?1 less than that of its keto analogue [CH₃COCH?CH₂]^+˙. Ion [CH₂?C(OH)CH?CH₂]^+˙ may be generated by loss of C₂H₄ from ionized hex-1-en-3-one or by loss of C₄H₈ from ionized 4,4-dimethyl-2-cyclohexen-1-ol. The [C₄H₆O]^+˙ ion generated by loss of C₂H₄ from ionized 2-cyclohexen-1-ol was shown to consist of a mixture of the above enol ions by comparing the metastable ion and collisional activation mass spectra of [CH₂?CHCH?CHOH]^+˙ and [CH₂?C(OH)CH?CH₂]^+˙ ions with that of the above daughter ion. It is further concluded that prior to their major fragmentations by loss of CH₃˙ and CO, [CH₂?CHCH?CHOH]⁺˙ and [CH₂?C(OH)CH?CH₂]^+˙ do not rearrange to their keto counterparts. The metastable ion and collisional activation characteristics of the isomeric allenic [C₄H₆O]^+˙ ion [CH₂?C?CHCH₂OH]^+˙ are also reported.     

Dicyanmethanido- und Cyanamido-oxoanionen. Untersuchungen zum sauerstoffanalogen Charakter der C(CN)2- und NCN-Gruppen

The equivalence of the C(CN)₂- and the NCN-groups with oxygen in the series of homologous ions C(CN), N(CN), OCN^? and NOC(CN), NO causes us to postulate a more general value of this relation. Arguments for the formulation of a pseudochalkogen series C(CN)₂? NCN? O are discussed. Synthesis, structure and reactivity of some dicyanmethanido- and cyanamido-oxoanions RCOY^?, CO₂Y^2?, COY, NOY^?, NO₂Y^?, PO₃Y^3?, PO₂Y and SO₂Y^2? are described. (Y ? C(CN)₂, NCN.) The preparation of some triorganoleadacyles is reported.     

The solubility of magnetite and the hydrolysis and oxidation of Fe2+ in water to 300°C

The solubility of carefully characterized magnetite, Fe₃O₄, in dilute aqueous solutions saturated with H₂ has been measured at temperatures from 100 to 300°C in a flow apparatus. Solution compositions included either HCl or NaOH molalities of up to 1 and 40 mmole-kg^?1, respectively, and H₂ molalities of 0.0779, 0.779, and 8.57 mmole-kg^?1. The dependence of the equilibrium solubility on the pH and reduction potential were fitted to a scheme of soluble ferrous and ferric species consisting of Fe²⁺, FeOH⁺, Fe(OH)₂, Fe(OH) ₃ ^? , Fe(OH)₃, and Fe(OH) ₄ ^? . Solubility products from the fit, corresponding to the reactions $$\tfrac{1}{3}Fe_3 O_4 + (2 - b)H^ + + \tfrac{1}{3}H_2 \rightleftharpoons Fe(OH)_b^{2 - b} + (4/3 - b)H_2 O$$ and $$\tfrac{1}{3}Fe_3 O_4 + (3 - b)H^ + \rightleftharpoons Fe(OH)_b^{3 - b} + \tfrac{1}{6}H_2 + (4/3 - b)H_2 O$$ were used to derive thermodynamic constants for each species. The extrapolared value for the Gibbs energy of formation of Fe²⁺ at 25°C is ?88.92±2.0 kJ-mole^?1, consistent with standard reduction potentials in the range E^o(Fe²⁺)=?0.47±0.01 V. The temperature coefficient of the equilibrium Fe molality, (?m(Fe, sat.)/?T)_{m(H2).m(NaOH)}, changes from negative to positive as the NaOH molality is increased to the point where Fe(OH) ₃ ^? and Fe(OH) ₄ ^? predominate.     

Encapsulation of AGE‐Breaker Alagebrium by Cucurbit[7]uril Improved the Stability of Both Its Carbonyl α‐Hydrogen and Thiazolium C2‐Hydrogen

As determined by both ¹H NMR and UV/Vis spectroscopic titration, ESI‐MS, isothermal titration calorimetry, and DFT molecular modeling, advanced glycation end products (AGE) breaker alagebrium (ALA) formed 1:1 guest–host inclusion complexes with cucurbit[7]uril (CB[7]), with a binding affinity, K_a, in the order of magnitude of 10⁵ m ^?1, thermodynamically driven by both enthalpy (ΔH=?6.79 kcal mol^?1) and entropy (TΔS=1.21 kcal mol^?1). For the first time, a dramatic inhibition of keto–enol tautomerism of the carbonyl α‐hydrogen of ALA has been observed, as evidenced by over an order of magnitude decrease of both the first step rate constant, k₁, and the second step rate constant, k₂, during hydrogen/deuterium exchange in D₂O. Meanwhile, as expected, the reactivity of C2‐hydrogen was also inhibited significantly, with an upshift of 2.09 pK_a units. This discovery will not only provide an emerging host molecule to modulate keto–enol tautomerism, but also potentially lead to a novel supramolecular formulation of AGE‐breaker ALA for improved stability and therapeutic efficacy.     

Preferred Conformations of Stabilized Phosphorus Ylides

The stabilized phosphorus ylides, Ph₃P=C(CO.R′)CO.OR; 1, R=Et, R′=CH₂P⁺Ph₃; 2, R=R′=Me; 3, R=Et, R′=Me; 4, R=Prⁱ; R′=Me; 5, R=Bu^t; R′=Me, adopt a near planar conformation in the crystal which allows extensive electronic delocalization. The keto and alkoxylic oxygens are oriented and align favorably with the cationoid phosphorus. These conformations bring methyl hydrogens in the ester residue into proximity with the face of a phenyl group and lead to π-shielding and upfield shifts of the ¹HNMR signals of 3 over a wide temperature range (-50–95°C) in (CD₃)₂CO, CDCl₃ and DMSO_d-6. Geometries of 2 and 3, optimized by using the HF 3-21 (G^*) or 6-31 (G^*) basis sets, are very similar to those in the crystal, but semiempirical treatments generate structures in which either the ester or keto moiety is twisted out of plane.

     

Rate constants for the elementary reactions between CN radicals and CH4, C2H6, C2H4, C3H6, and C2H2 in the range: 295 ⩽ T/K ⩽ 700

Pulsed laser photolysis, time-resolved laser-induced fluorescence experiments have been carried out on the reactions of CN radicals with CH₄, C₂H₆, C₂H₄, C₃H₆, and C₂H₂. They have yielded rate constants for these five reactions at temperatures between 295 and 700 K. The data for the reactions with methane and ethane have been combined with other recent results and fitted to modified Arrhenius expressions, k(T) = A′(298) (T/298)ⁿ exp(?θ/T), yielding: for CH₄, A′(298) = 7.0 × 10^?13 cm³ molecule^?1 s^?1, n = 2.3, and θ = ?16 K; and for C₂H₆, A′(298) = 5.6 × 10^?12 cm³ molecule^?1 s^?1, n = 1.8, and θ = ?500 K. The rate constants for the reactions with C₂H₄, C₃H₆, and C₂H₂ all decrease monotonically with temperature and have been fitted to expressions of the form, k(T) = k(298) (T/298)ⁿ with k(298) = 2.5 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.24 for CN + C₂H₄; k(298) = 3.4 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.19 for CN + C₃H₆; and k(298) = 2.9 × 10^?10 cm³ molecule^?1 s^?1, n = ?0.53 for CN + C₂H₂. These reactions almost certainly proceed via addition-elimination yielding an unsaturated cyanide and an H-atom. Our kinetic results for reactions of CN are compared with those for reactions of the same hydrocarbons with other simple free radical species. © John Wiley & Sons, Inc.     

Unsaturated dodecahedranes—Synthesis of the highly pyramidalized,highly reactive C20H18 and C20H16 olefins

Methodological alternatives for the preparation of highly strained, highly pyramidalized dodecahedrene 2 (E_str=87.3 kcal mol^?1; ?=43.5°, MM2) and 1,16-dodecahedradiene 3 (E_str=105.3 kcal mol^?1; ?=42.9°, MM2) have been explored, protection/deprotection strategies have been tested—with the eye on their utilization for the generation of higher unsaturated dodecahedranes (e.g. 1,4, 16-triene 4, 1,4,10 (14),16-tetraene 5). For the acquisition of preparative quantities of monoene 2 the “P₂F” catalyzed cis-β-elimination in bromododecahedrane, of diene 3 the FVP fragmentation of a “twofold protected” precursor (bis-furan adduct) have become the protocols of choice, which both profit from the recent synthetic advances along the pagodane → dodecahedrane scheme. Because of unusually effective steric protection the highly tilted C=C double bonds of 2 (λ_max (CH₃CN) = 254 nm, ν _C=C = 1658 cm^?1, δ_C=C = 164.4) and 3 (δ_C=C = 170.5) enter into thermal stabilization pathways (dimerization, oligomerization) only at higher temperatures (for 2 ca. 50% consumption after 5 h at 100°C in a 3·10^?3 molar toluene solution); extreme sensitivity to oxygen is primarily attributed to kinetically and thermodynamically promoted allylic hydrogen abstraction.     

Rb{Pr6C2}I12, an Iodide with a 13‐Electron Isolated Octahedral {Pr6C2} Cluster

Rb{Pr₆(C)₂}I₁₂ was obtained from a mixture of RbI, PrI₃, Pr and C as black single crystals at elevated temperatures. The black crystals are triclinic, (no. 2), a = 960.1(2), b = 957.0(2), c = 1003.4(2) pm, α = 71.74(2), β = 70.69(2), γ = 72.38(2)°, V = 805.6(3) 10⁶ pm³, Z = 1; R₁ = 0.0868 for all 2749 measured independent reflections. Rb{Pr₆(C)₂}I₁₂ contains {Pr₆(C₂)} clusters isolated from each other, surrounded by twelve edge‐bridging and six terminal ligands. The [{Pr₆(C)₂}Iⁱ₁₂I^a₆]^? units are connected via i‐a/a‐i bridges according to {Pr₆C₂}Iⁱ_6/1I^i‐a_6/2I^a‐i_6/2 with rubidium ions occupying twelve‐coordinate interstices.     

Rate constants for aqueous‐phase reactions of SO4− with C2F5C(O)O− and C3F7C(O)O− at 298 K

Perfluorocarboxylic acids and their anions (PFCAs), such as perfluorooctanate (C₇F₁₅C(O)O^?), have been generally recognized to be global pollutants and are believed to persist in the environment. Kinetic data for reactions of sulfate anion radicals (SO₄^?) with PFCAs are needed to evaluate the residence times of PFCAs in the environment, but no kinetic data have been reported, except for the rate constant for the reaction of SO₄^? with trifluoroacetate (CF₃C(O)O^?) (k₁). In this study, using the fact that PFCAs react with SO₄^? to form shorter chain PFCAs, we determined rates relative to k₁ of the reactions of photolytically generated SO₄^? with two short‐chain PFCAs, pentafluoropropionate (C₂F₅C(O)O^?; k₂) and heptafluorobutyrate (C₃F₇C(O)O^?; k₃), along with conversion ratios for conversion of C₂F₅C(O)O^? into CF₃C(O)O^? (α) and conversion of C₃F₇C(O)O^? into C₂F₅C(O)O^? (β) and CF₃C(O)O^? (γ) at 298 K. Values of k₁, k₂, or k₃ might change over the course of reaction with increasing ionic strength. Nevertheless, if the values of k₁/k₂, k₂/k₃, α, β, and γ remain almost constant during the reaction, a simple equation involving relative rates, such as k₁/k₂, can be used to relate the concentrations of C₃F₇C(O)O^?, C₂F₅C(O)O^?, and CF₃C(O)O^?. We compared the relative rates, such as k₁/k₂, and the conversion ratios determined from various experimental runs with different initial conditions to check whether relative rates and conversion ratios remained almost constant during each experimental run. The values of k₁/k₂, k₂/k₃, α, β, and γ seemed to remain almost constant, which facilitated determination of k₂/k₁ = 0.89 ± 0.07, k₃/k₁ = 0.84 ± 0.08, α = 0.88 ± 0.05, β = 0.75 ± 0.05, and γ = 0.17 ± 0.02. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 276–288, 2007     

Cross‐Aldol Reaction of Isatin with Acetone Catalyzed by Leucinol: A Mechanistic Investigation

Comprehensive mechanistic studies on the enantioselective aldol reaction between isatin ( 1 a ) and acetone, catalyzed by L ‐leucinol ( 3 a ), unraveled that isatin, apart from being a substrate, also plays an active catalytic role. Conversion of the intermediate oxazolidine 4 into the reactive syn‐enamine 6 , catalyzed by isatin, was identified as the rate‐determining step by both the calculations (ΔG^≠=26.1 kcal mol^?1 for the analogous L ‐alaninol, 3 b ) and the kinetic isotope effect (k_H/k_D=2.7 observed for the reaction using [D₆]acetone). The subsequent reaction of the syn‐enamine 6 with isatin produces (S)‐ 2 a (calculated ΔG^≠=11.6 kcal mol^?1). The calculations suggest that the overall stereochemistry is controlled by two key events: 1) the isatin‐catalyzed formation of the syn‐enamine 6 , which is thermodynamically favored over its anti‐rotamer 7 by 2.3 kcal mol^?1; and 2) the high preference of the syn‐enamine 6 to produce (S)‐ 2 a on reaction with isatin ( 1 a ) rather than its enantiomer (ΔΔG^≠=2.6 kcal mol^?1).     

An X‐ray crystallographic and density functional theory study of (3Z)‐4‐(5‐ethylsulfonyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one and (3Z)‐4‐(5‐tert‐butyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one

The Schiff base enaminones (3Z)‐4‐(5‐ethylsulfonyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one, C₁₃H₁₇NO₄S, (I), and (3Z)‐4‐(5‐tert‐butyl‐2‐hydroxyanilino)pent‐3‐en‐2‐one, C₁₅H₂₁NO₂, (II), were studied by X‐ray crystallography and density functional theory (DFT). Although the keto tautomer of these compounds is dominant, the O=C—C=C—N bond lengths are consistent with some electron delocalization and partial enol character. Both (I) and (II) are nonplanar, with the amino–phenol group canted relative to the rest of the molecule; the twist about the N(enamine)—C(aryl) bond leads to dihedral angles of 40.5 (2) and −116.7 (1)° for (I) and (II), respectively. Compound (I) has a bifurcated intramolecular hydrogen bond between the N—H group and the flanking carbonyl and hydroxy O atoms, as well as an intermolecular hydrogen bond, leading to an infinite one‐dimensional hydrogen‐bonded chain. Compound (II) has one intramolecular hydrogen bond and one intermolecular C=O...H—O hydrogen bond, and consequently also forms a one‐dimensional hydrogen‐bonded chain. The DFT‐calculated structures [in vacuo, B3LYP/6‐311G(d,p) level] for the keto tautomers compare favourably with the X‐ray crystal structures of (I) and (II), confirming the dominance of the keto tautomer. The simulations indicate that the keto tautomers are 20.55 and 18.86 kJ mol⁻¹ lower in energy than the enol tautomers for (I) and (II), respectively.     

The anomeric effect: Ab-initio studies on molecules of the type X?CH2?O?CH3

The anomeric effect of the functional groups X = C?N, C?CH, COOH, COO^?, O? CH₃, NH₂, and NH⁺₃ has been studied with ab initio techniques. Geometry effects upon rotation around the central C? O bond in X? CH₂? O? CH₃ have been compared in the various compounds. The energy differences between the conformers with a gauche and trans (X? C? O? C) arrangement were calculated at the 6-31G* level in the fully optimized 4-21G geometries. Energy differences calculated at the 4-21G level appeared not to be reliable, especially for the groups X that contain non-sp³ hybridized atoms. The 6-31G* energy differences indicate a normal anomeric effect for X = COO^?, O? CH₃, and NH₂(g⁺) (ca. 13 kJ/mol) and a small anomeric effect for X = COOH, C?N, and C?CH (ca. 6 kJ/mol). For X = NH₂(t) and NH⁺₃ a reverse anomeric effect occurs. These observations are in line with experimental results and evidence is given for a competition among various stereoelectronic interactions that occur at the same anomeric center. Geometry variations can be understood in terms of simple rules associated with anomeric orbital interactions. Trends followed when the group X is varied cannot be related in a straightforward way to the energy differences between the trans and the gauche forms in these compounds. Only the variation in the gauche torsion angle X? C? O? C follows roughly the same trend.     

Optisch aktive übergangsmetall-komplexe: XXXIII. Substituenteneinfluss auf die racemisierungs geschwindigkeit von (+)579und(−)579−C5H5MN(COC6H5)(NO)P(pC6H4Y)3

In the reaction of [C₅H₅Mn(CO)₂(NO)] [X] ([X] = [BF₄], [PF₆]) with p-substituted triarylphosphines P(p-C₆H₄?Y)₃ [Y = CF₃, Cl, F, C₆H₅, CH₃, OCH₃, N(CH₃)₂] the asymmetric monosubstitution products [C₅H₅Mn(CO)(NO)P(p-C₆H₄?Y)₃] [X] are formed, which can be converted into the neutral esters C₅H₅Mn(COOC₁₀H₁₉)(NO)P(p-C₆H₄?Y)₃ by natrium menthoxide. The diastereoisomers (+)₅₇₉^? and (?)₅₇₉^?C₅H₅Mn(COOC₁₀H₁₉)(NO)P(p-C₆H₄?Y)₃ are separated by fractional crystallisation and transformed into the enantiomeric salts (+)₅₇₉^? and (?)₅₇₉-[C₅H₅Mn(CO)(NO)P(p-C₆H₄?Y)₃] [X] by cleavage with HCl and precipitation with NH₄PF₆. The (+)₅₇₉^? and (?)₅₇₉^? rotating salts in the reaction with LiC₆H₅ yield the carbonyl addition products (+)₅₇₉^? and (?)₅₇₉^? C₅H₅Mn(COC₆H₅)(NO)P(p-C₆H₄?Y)₃ and the ring addition products (+)₅₇₉^? and (?)₅₇₉^?(exo-C₆H₅)C₅H₅Mn(CO)(NO)P(p-C₆H₄?Y)₃, which can be separated by chromatography.The salts (+)₅₇₉^? and (?)₅₇₉^?[C₅H₅Mn(CO)(NO)P(p-C₆H₄?Y)₃] [X] and the cyclopentadiene complexes (+)₅₇₉^? and (?)₅₇₉-(exo-C₆H₅)C₅H₅Mn(CO)(NO)P(p-C₆H₄?Y)₃ are configurationally stable, whereas the esters (+)₅₇₉^? and (?)₅₇₉^?C₅H₅Mn(COOC₁₀H₁₉)(NO)P(p-C₆H₄?Y)₃ and the benzoyl complexes (+)₅₇₉^? and (?)₅₇₉^?C₅H₅Mn(COC₆H₅)(NO)P(p-C₆H₄?Y)₃ epimerise or racemise in solution.The rate of racemisation of the benzoyl compounds (+)₅₇₉^? and (?)₅₇₉C₅H₅Mn(COC₆H₅)(NO)P(p-C₆H₄?Y)₃ was measured polarimetrically in the temperature range 0–45° C. It turned out that electron-releasingsubstituents Y in the ligand P(p-C₆H₄?Y)₃ increase the half-lives, whereas electron-attracting substituents decrease the half-lives. There is a linear correlation between the σ-constants of the substituents and the rate constants of the racemisation (reaction constant p = +2.14).     

X–ray Single Crystal Structure and Raman Spectrum of Ammonium Hexafluoroarsenate

The structure of ammonium hexafluoroarsenate, NH₄AsF₆, has been determined by X‐ray diffraction using a single crystal grown from saturated solution in anhydrous HF. NH₄AsF₆ crystallizes rhombohedral with the KOsF₆ structure type, with a = 7.459(3) Å, c = 7.543(3) Å (at 200 K), Z = 3, space group (No. 148). No phase transition was observed in the 100 K–296 K temperature range. The structure is dominated by regular AsF₆ octahedra and disordered NH₄⁺ cations. Raman spectrum of a single crystal of NH₄AsF₆ shows the bands at 372 cm^?1, 572 cm^?1, 687 cm^?1 (AsF₆^?) and at 3240 cm^?1 and 3360 cm^?1 (NH₄⁺).     

Substituent effects on enol nitrosation of 1,3‐diketones

The keto–enol tautomerism of 3‐chloro‐pentane‐2,4‐dione (ClPD) was studied in aqueous micellar solutions of cationic, anionic, and nonionic surfactants. The enol of ClPD tautomerizes rapidly in water to the equilibrium proportions of the keto form, K_E=0.55; whereas the keto–enol conversion of 3‐ethyl‐pentane‐2,4‐dione (EPD) is a much slower reaction than the enol nitrosation. Kinetics of enol –nitrosation of both ClPD and EPD in aqueous acid medium using nitrous acid shows first‐order dependence upon [ketone] and linear or curve relationships of the observed rate constant, k_o, as a function of [nitrite] or [H⁺]; the observed behavior depends on the molecular structure of diketone and varies with the experimental conditions. The reaction is strongly catalyzed by Cl^?, Br^?, or SCN^?, and the observed rate constant shows a curve dependence on [Br^?] or [SCN^?], which is more pronounced at high acidity. The results are consistent with a reaction mechanism in which the nitrosation occurs initially on the enol–oxygen and releasing a proton to form a chelate–nitrosyl complex intermediate in steady state. Fine differences on the mechanistic spectrum of enols nitrosation are considered on the basis of the molecular structure of the diketone. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 668–679, 2012     

Structures and stabilities of [C3H2]+ ˙ and [C3H2]2+ ions

Ab initio molecular orbital theory using basis sets up to 6-311G^{* *}, with electron correlation incorporated via configuration interaction calculations with single and double substitutions, has been used to study the structures and energies of the C₃H₂ monocation and dication. In agreement with recent experimental observations, we find evidence for stable cyclic and linear isomers of [C₃H₂]^{+ ˙}. The cyclic structure (, a) represents the global minimum on the [C₃H₂]^{+ ˙} potential energy surface. The linear isomer (, b) lies somewhat higher in energy, 53 kJ mol^?1 above a. The calculated heat of formation for [HCCCH]^{+ ˙} (1369 kJ mol^?1) is in good agreement with a recent experimental value (1377 kJ mol^?1). For the [C₃H₂]²⁺ dication, the lowest energy isomer corresponds to the linear [HCCCH]²⁺ singlet (h). Other singlet and triplet isomers are found not to be competitive in energy. The [HCCCH]²⁺ dication (h) is calculated to be thermodynamically stable with respect to deprotonation and with respect to C? C cleavage into CCH⁺ + CH⁺. The predicted stability is consistent with the frequent observation of [C₃H₂]²⁺ in mass spectrometric experiments. Comparison of our calculated ionization energies for the process [C₃H₂]^{+ ˙} → [C₃H₂]²⁺ with the Q_min values derived from charge-stripping experiments suggests that the ionization is accompanied by a significant change in structure.     

Electrochemical Responses and Sensitivities of Films Based on Multi-Walled Carbon Nanotubes in Aqueous Solutions

Novel films consisting of multi-walled carbon nanotubes (MWCNTs) were fabricated by means of chemical vapor deposition with decomposition of either acetonitrile (ACN) or benzene (BZ) using ferrocene as catalyst. The electrochemical responses of MWCNT-based films towards the ferrocyanide/ferricyanide, [Fe(CN)₆]^3?/4? redox couple were probed by means of cyclic voltammetry and electrochemical impedance spectroscopy at 25.0?±?0.5?°C. Both MWCNT-based films exhibit Nernstian response towards [Fe(CN)₆]^3?/4? with some slight kinetic differences. Namely, heterogeneous electron transfer rate constants lying in ranges of 2.69?×?10^?2?C1.7?×?10^?3 and 9.0?×?10^?3?C2.6?×?10^?3?cm·s^?1 were obtained at v?=?0.05?V·s^?1 for MWCNT_ACN and MWCNT_BZ, respectively. The detection limit of MWCNT_ACN, estimated to be about 4.70?×?10^?7?mol·L^?1 at v?=?0.05?V·s^?1, tends to become slightly poorer with the increase of the scan rate, namely at v?=?0.10?V·s^?1 the detection limit of 1.70?×?10^?6?mol·L^?1 was determined. Slightly poorer response ability was exhibited by MWCNT_BZ; specifically the detection limits of 1.57?×?10^?6 and 4.35?×?10^?6?mol·L^?1 were determined at v?=?0.05 and v?=?0.10?V·s^?1, respectively. The sensitivities of MWCNT_ACN and MWCNT_BZ towards [Fe(CN)₆]^3?/4? were determined as 1.60?×?10^?7 and 1.51?×?10^?7?A·L·mol^?1·cm^?2, respectively. The excellent electrochemical performance of MWCNT_ACN is attributed to the presence of incorporated nitrogen in the nanotube??s structure.