Substituent effects on cyclonona‐3,5,7‐trienylidenes: a quest for stable carbenes at density functional theory level

Nine boat‐shaped cyclonona‐3,5,7‐trienylidenes are compared and contrasted with respect to their multiplicity, nucleophilicity, electrophilicity, band gap (ΔE_{HOMO ? LUMO}), Natural bond orbital (NBO) atomic charge, force constant, as well as the aptitude for dimerization, and rearrangement through proper isodesmic reactions at B3LYP/AUG‐cc‐pVTZ and B3LYP/6‐311++G**//B3LYP/6‐31+G* levels of theory. The nine cyclic carbenes include unsubstituted (1_CH2) plus eight α‐cyclopropylcyclonona‐3,5,7‐trienylidenes, which are substituted with ?‐SiMe₂, ?‐NMe, ?‐PMe, ?‐O, ?‐S, ?‐CH₂, ?‐cyclopropyl, and ?‐CMe₂ (2_SiMe2, 2_NMe, 2_PMe, 2_O, 2_S, 2_CH2, 2_cyclopropyl, and 2_CMe2, respectively). The latter eight species enjoy the stabilizing interaction of the occupied Walsh orbital of cyclopropyl with the vacant p_π orbital of the carbene center (Walsh_cyclopropyl → p_{π carbene}). Among them, the singlet closed shell 2_NMe appears the most promising for exhibiting the highest relative singlet–triplet energy gap (ΔE_{s ? t} = 27.1 kcal mol^?1). In contrast, the least stable derivative is triplet 2_SiMe2, which exhibits the lowest relative ΔE_{s ? t} of ?5.5 kcal mol^?1. The overall trend of ΔE_s‐t is 2_NMe > 2_PMe > 2_S > 2_O > 2_cyclopropyl > 2_CMe2 > 2_CH2 > 1_CH2 > 2_SiMe2. With one negative force constant, the unsubstituted 1_CH2 turns out to be a transition state, whereas the rest emerge as minima. Copyright © 2015 John Wiley & Sons, Ltd.     

Breathing viability into cyclonona‐3,5,7‐trienylidenes via α‐dimethyl and ά‐moieties at DFT

Cyclonona‐3,5,7‐trienylidene ( 1 ) changes from being a transition state (TS) to minimum states when substituted by α‐methyl groups and ?‐X, where X = CMe₂, NMe, PMe, O, S, cyclopropyl, and SiMe₂ ( 2 , 3 , 4 , 5 , 6 , 7 , 8 , respectively) at density functional theory. Specifically, the parent carbene 1 exhibits a negative vibrational force constant and proves to be an unreachable electrophilic TS while shows C_s symmetry with an NBO atomic charge of +0.70 on its carbenic center. It has a triplet ground state with a rather small singlet‐triplet energy gap (ΔE_s–t = ?4.1 kcal/mol). In contrast, all of its seven scrutinized derivatives enjoy reachable global minima, with C₁ symmetry, desired nucleophilicity, and singlet closed shell (S_cs) ground states (for all but 8 which remains triplet). Stability is indicated by relative ΔE_s–t values: 2 > 3 > 4 > 5 > 6 > 7 > 1 > 8 . The highest ΔE_s–t as well as NBO carbenic atomic negative charge (?0.74) are displayed by 2 . Our carbenes ( 2 , 3 , 4 , 5 , 6 , 7 ) appear more nucleophilic than the synthesized N‐heterocyclic carbenes (imidazol‐2‐ylidenes). Copyright © 2013 John Wiley & Sons, Ltd.     

Zur O2-Dissoziation in der Gleichstromglimmentladung. Teil I Bestimmung des Dissoziationsgrades mittels der O3-Bildung an gekühlten Oberflächen

The synthesis of ozone on the surface S cooled with liquid nitrogen O + O₂·S→O₃·S[1] was used to measure the concentration of atoms in a flow tube downstream of a d.c. discharge. The flow tube was constructed in the form of a U-tube. The ozone was deposited as a blue liquid film in the neighbourhood of the surface of the liquid nitrogen on the walls of the part of the U-tube, which was connected with the discharge tube. The oxygen atom decay in the U-tube was observed by measuring the production of ozone as the distance between the discharge tube (diameter: 5,2 cm) and the surface of the liquid nitrogen was varied. The decay is first order in atom concentration for the pressures p > 1 torr. But for the pressures below p = 1 torr near by the discharge tube we observed deviations from the law of decay In [O]₀/[O] = k·t for the currents of about i = 100 mA. This effect was attributed to the process From the extrapolation of an ozone production curve atom concentrations in the discharge have been obtained. The transition from the ?H”? form to the ?T”? form (striated, low average electric field) of the discharge [2] caused the dissociation to decrease rapidly. The highest dissociation (10%) was obtained in the ?H”? form at high currents (i = 120 mA) and low pressures (p = 0,60 torr). The yield of atomic oxygen per kilowatt hour of energy was measured as functions of pressure and gas flow rate for i = 100 mA and i = 50 mA.     

Organic solar cells with very high fill factor and voltage using tetrapropyl‐tetraphenyl‐diindenoperylene as green donor

We present 2,3,10,11‐tetrapropyl‐1,4,9,12‐tetraphenyl‐diindeno[1,2,3‐cd:1′,2′,3′‐lm]perylene (P4‐Ph4‐DIP) as new green donor for small molecule organic solar cells (SMOSC). P4‐Ph4‐DIP absorbs in the green spectral range of 490–580 nm and thus is a decisive building block towards broad coverage of the sun spectrum. SMOSC with P4‐Ph4‐DIP as donor and C₆₀ as blue‐absorbing acceptor display very high fill factors FF > 76% and open circuit voltages of 0.99 V, reaching η = 1.92% for 12 nm P4‐Ph4‐DIP. Current voltage and external quantum efficiency spectra suggest that an exciton diffusion length E_D < 20 nm is the main limiting factor. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kinetics of alkaline hydrolysis of p‐substituted benzylidenemalononitriles in 50% aqueous acetonitrile: substituent effects and quantification of the electrophilic reactivity

The rate constants of the reaction of p‐X‐substituted benzylidenemalononitriles 1a , 1b , 1c , 1d , 1e , 1f , 1g , 1h with hydroxide ion were measured in 50% water–50% acetonitrile at 20 °C. The experimental kinetic data reveal that the points pertaining to electron donating substituted compounds (X = Me, OMe and NMe₂) exhibit negative deviations from the Hammett plot. However, the Yukawa–Tsuno plot for the same rate constants resulted in a good straight line with an excellent correlation coefficient (r² = 0.9916) and an r value of 1.15. Possible ground‐state stabilization through resonance interactions has been suggested to explain the origin of the nonlinear Hammett plot. On the basis of the relationship between E and σ_p⁺, the electrophilicity parameter E of some benzylidenemalononitriles 1c and 1e , 1f , 1g , 1h has been evaluated. More importantly, the three compounds 1f (E = ?7.90), 1g (E = ?7.80) and 1h (E = ?7.55) exhibit high electrophilicities that compare well with that of 4,6‐dinitrobenzoselenadiazole (E = ?7.40), a compound which has a general behaviour representative for the superelectrophilic dimension. We have shown that the second‐order rate constants calculated from Mayr's approach for the reaction of 1a , 1b , 1c , 1d , 1e , 1f , 1g , 1h with hydroxide ion do not agree with the available experimental data. On the other hand, a good linear correlation between log k_exp and log k_calc has been observed and discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Residual‐density mapping and site‐selective determination of anomalous scattering factors to examine the origin of the Fe K pre‐edge peak of magnetite

The electron‐density distribution and the contribution to anomalous scattering factors for Fe ions in magnetite have been analyzed by X‐ray resonant scattering at the pre‐edge of Fe K absorption. Synchrotron X‐ray experiments were carried out using a conventional four‐circle diffractometer in the right‐handed circular polarization. Difference‐Fourier synthesis was applied with a difference in structure factors measured on and off the pre‐edge (E_on = 7.1082 keV, E_off = 7.1051 keV). Electron‐density peaks due to X‐ray resonant scattering were clearly observed for both A and B sites. The real part of the anomalous scattering factor f′ has been determined site‐independently, based on the crystal‐structure refinements, to minimize the squared residuals at the Fe K pre‐edge. The f′ values obtained at E_on and E_off are ?7.063 and ?6.682 for the A site and ?6.971 and ?6.709 for the B site, which are significantly smaller than the values of ?6.206 and ?5.844, respectively, estimated from the Kramers–Kronig transform. The f′ values at E_on are reasonably smaller than those at E_off. Our results using a symmetry‐based consideration suggest that the origin of the pre‐edge peak is Fe ions occupying both A and B sites, where p–d mixing is needed with hybridized electrons of Fe in both sites overlapping the neighbouring O atoms.     

Variation of electron energy loss spectra of Ni (100) with increasing primary energy

Electron energy loss spectra of clean Ni(1 0 0) show for the first time a 17 eV peak, which is attributed to an interband transtiion. All the observed peaks are shifted to higher energies as the primary electron energy E_p increases from 102 to 2045 eV. This shift is explained by a continuous decay in energy of the primary electrons inside the crystal. At E_p ? 700 eV, the decay takes place in the surface region of the crystal, while at E_p > 700 eV it takes place mainly in the bulk. The rate of decay increases with increasing temperature of the crystal between 300 and 900 K.     

Enols of 2‐nitro‐ and related 2‐substituted malonamides

The structures of 2‐substituted malonamides, YCH(CONR¹R²)CONR³R⁴ (Y = Br, SO₂Me, CONH₂, COMe, and NO₂) were investigated. When Y = Br, R¹R² = R³R⁴ = HEt; Y = SO₂Me, R¹–R⁴ = H and for Y = CONH₂ or CONHPh, R¹–R⁴ = Me, the structure in solution is that of the amide tautomer. X‐ray crystallography shows solid‐state amide structures for Y = SO₂Me or CONH₂, R¹–R⁴ = H. Nitromalonamide displays an enol structure in the solid state with a strong hydrogen bond (O^…O distance = 2.3730 Å at 100 K) and d(OH) ≠ d(O^…H). An apparently symmetric enol was observed in solution, even in appreciable percentages in highly polar solvents such as DMSO‐d₆, but K_enol values decrease on increasing the solvent polarity. The N,N′‐dimethyl derivative is less enolic. Acetylmalonamides display a mixture of enol on the acetyl group and amide in non‐polar solvents, and only the amide in DMSO‐d₆. DFT calculations gave the following order of pK_enol values for Y: H > CONH₂ > COMe ≥ COMe (on acetyl) ≥ MeSO₂ > CN > NO₂ in the gas phase, CHCl₃, and DMSO. The enol on the C?O group is preferred to the aci‐nitro compound, and the N? O? H^…O?C is less favored than the C?O? H^…O?C hydrogen bond. Copyright © 2009 John Wiley & Sons, Ltd.     

Nucleophilicity of metal carbonyl anions in vinylic substitution reactions

Second order rate constants are reported for the reactions of metal carbonyl anions ([M(CO)_nL]^?) with several vinyl halides: PhCCl?C(CN)₂, Z‐ and E‐Ph(CN)C?CHHal (Hal = Cl, Br) which follow the addition–elimination (Ad_NE) substitution mechanism. The obtained data show that the nucleophilic reactivity of [M(CO)_nL]^? anions towards vinyl halides increases in the same order as in aliphatic S_N2 reactions, but much more steeply, by 14 orders of magnitude in the row log{ }: [CpFe(CO)₂]^? (～14), [Re(CO)₅]^? (7.8), [Mn(CO)₅]^? 2.1, [CpW(CO)₃]^? (0.7) > [CpMo(CO)₃]^? (0). A good correlation exists between nucleophilicities of [M(CO)_nL]^? anions towards vinyl (sp²‐carbon) and alkyl halides (sp³‐carbon) with slope 2.7. The reactivity of [M(CO)_nL]^? in a halogen–metal exchange process (with Z‐PhC(CN)?CHI) follows a similar ‘large’ scale as in the Ad_NE process. The nucleophilicity of [M(CO)_nL]^? anions correlates better with their one‐electron oxidation potentials (E_ox) than with their basicity (pK_a of [M(CO)_nL]H). Copyright © 2008 John Wiley & Sons, Ltd.     

Reactions of aryl chlorothionoformates with quinuclidines. A kinetic study

The reactions of quinuclidines with phenyl, 4‐chlorophenyl, 4‐cyanophenyl, and 4‐nitrophenyl chlorothionoformates ( 1 , 2 , 3 , and 4 , respectively) are subjected to a kinetic study in aqueous solution, at 25.0°C, and an ionic strength of 0.2 M (KCl). The reactions are studied by following spectrophotometrically the release of the corresponding phenoxide anion/phenol generated in the parallel hydrolysis of the substrates. Under amine excess, pseudo‐first‐order rate coefficients (k_obs) are found. Plots of k_obs versus [amine] are linear, with slope k_N. The Brønsted‐type plots (log k_N vs. pK_a of aminium ions) are linear, with slopes β = 0.26, 0.22, 0.19, and 0.28 for the reactions with 1 , 2 , 3 , and 4 , respectively. The magnitudes of the slopes indicate that these mechanisms are stepwise, with rate‐determining formation of a zwitterionic tetrahedral intermediate (T^±). A dual parametric equation with the pK_a of the nucleophiles and non‐leaving groups show β_N = 0.26 and β _nlg = ?0.16, also in accordance with the proposed mechanism. On the other hand, the reactivity of these thiocarbonyl substrates and their carbonyl derivatives was studied using their hardness index and compared with their experimental parameters, confirming the proposed mechanisms. By comparison of the title reactions with similar aminolyses, the following conclusions arise: (i) The mechanism of the reactions under investigation is stepwise with rate‐determining formation of T^±. (ii) The reactivity of the substrates toward quinuclidines follows the order 4 > 3 > 2 > 1 . (iii) Quinuclidines are more reactive than isobasic pyridines toward chlorothionoformates. (iv) Chlorothionoformates are less reactive than chloroformates towards quinuclidines in accordance with the HSAB principle. (v) The k_N values for phenyl chloroformate and 4 can be correlated with the pK_a of quinuclidines and also with the hardness values calculated by the HF/3‐21G level of theory. Copyright © 2007 John Wiley & Sons, Ltd.     

Kinetics study of a Diels‐Alder reaction in mixtures of an ionic liquid with molecular solvents

The second‐order rate constants for cycloaddition reaction of cyclopentadiene with naphthoquinone were determined spectrophotometrically in various compositions of 1‐(1‐butyl)‐3‐methylimidazolium terafluoroborate ([bmim]BF₄) with water and methanol at 25 °C. Rate constants of the reaction in pure solvents are in the order of water > [bmim]BF₄ > methanol. Rate constants of the reaction decrease sharply with mole fraction of the ionic liquid in aqueous solutions and increase slightly to a maximum in alcoholic mixtures. Multi‐parameter correlation of logk₂ versus solute–solvent interaction parameters demonstrated that solvophobicity parameter (Sp), hydrogen‐bond donor acidity (α) and hydrogen‐bond acceptor basicity (β) of media are the main factors influencing the reaction rate constant. The proposed three‐parameter model shows that the reaction rate constant increases with Sp, α and β parameters. Copyright © 2008 John Wiley & Sons, Ltd.     

Proton coupled electron transfer reaction of phenols with excited state ruthenium(II) – polypyridyl complexes

The reaction of phenols with the excited state, *[Ru(bpy)₃]²⁺ (E₀ = 0.76 V) and *[Ru(H₂dcbpy)₃]²⁺, (dcbpy = 4,4′‐dicarboxy‐2,2′‐bipyridine) (E₀ = 1.55 V vs. SCE) complexes in CH₃CN has been studied by luminescence quenching technique and the quenching is dynamic. The formation of phenoxyl radical as a transient is confirmed by its characteristic absorption at 400 nm. The k_q value is highly sensitive to the change of pH of the medium and ΔG⁰ of the reaction. Based on the treatment of k_q data in terms of energetics of the reaction and pH of the medium, proton coupled electron transfer (PCET) mechanism has been proposed for the reaction. Copyright © 2010 John Wiley & Sons, Ltd.     

Reactivity of sodium arenesulfinates in the substitution reaction to γ‐functionalized allyl bromides

The kinetics of nucleophilic bimolecular substitution reactions of γ‐functionalized allyl bromides with non‐substituted and p‐substituted sodium arenesulfinates has been studied. Both the structure of allyl bromides and nucleophilicity of arenesulfinate ions exerted a significant effect on the values of the kinetic parameters such as the second‐order rate constants k, activation energy E_A, and changes in the entropy ΔS^≠, enthalpy ΔH^≠, and free energy ΔG^≠ of the formation of the activated complex from reactants. Based on the evaluation of kinetic parameters, the reactants could be arranged, according to their decreasing reactivity in the S_N2‐reactions as follows: p‐toluenesulfinate ion > benzenesulfinate ion > p‐chlorobenzenesulfinate ion and 4‐bromo‐2‐butenenitrile > 1,3‐ dibromopropene, respectively. Comparison was also made between the kinetic data obtained and some delocalization reactivity indexes for both the substrates and nucleophiles. The enthalpy–entropy compensation effect was observed for the reactions of sodium arenesulfinates with γ‐functionalized allyl bromides. Copyright © 2009 John Wiley & Sons, Ltd.     

Photodisintegration of 4He and the Supermultiplet Potential Model of Cluster-Cluster Interactions

p+³H and p+³He elastic collisions are described in terms of a supermultiplet model with [ƒ]-dependent potentials. The phase shifts δ ^[ƒ] _L,t,S (E) with [ƒ] = [4], t = S = 0, L = even are reconstructed from the observable nuclear phase shifts δ _L,S (E) of the above collisions. So, the initial-state interaction V ^[4] _L,0,0 (R) of the ⁴He +Υ→³H +p(³He +n) reaction can be found unambiguously, while the final-state interaction V ^[31] _{L, 1, 0} (R) is defined by the observablesδ _L,0 (E) of p+³He scattering. The data on the proton momentum distribution in ⁴He and on the charge-exchange reaction ³H +p→³He +n confirm the model. In calculating the above photonuclear reactions, in addition to the initial-state and final-state antisymmetrizations, preserving the corresponding symmetry [ƒ], the nucleon-nucleon correlations in the ³H (³He) subsystem were also taken into account. The results are in good agreement both with recent experimental data and theoretical investigations by sofianos, Fiedeldey, and Sandhas, who followed a rather different approach. Received August 5, 1994; revised November 30, 1994; accepted for publication December 30, 1994     

A natural bond orbital analysis of aryl‐substituted polyfluorinated carbanions: negative hyperconjugation

Aryl‐substituted polyfluorinated carbanions, ArCHR_f^? where R_f = CF₃ ( 1 ), C₂F₅ ( 2 ), i‐C₃F₇ ( 3 ), and t‐C₄F₉ ( 4 ), were analyzed by means of the natural bond orbital (NBO) theory at the B3LYP/6‐311+G(d,p) computational level. A lone pair NBO at the formal anionic center carbon (Cα) was not found in the Lewis structure. Instead, significant donor/acceptor NBO interactions between π(Cα‐C1) and σ*(Cβ‐F) or σ*(Cβ‐Cγ) were observed for 1 , 2 , 3a (strong electron‐withdrawing substituent, from p‐CF₃ to p‐NO₂), and 4 . Their second‐order donor/acceptor perturbation interaction energy, E(2), values decreased with the increase of the stability of carbanions. A larger E(2) value corresponds to longer Cβ‐F and Cβ‐Cγ bonds and a shorter Cα‐Cβ bond, indicating that the E(2) values can be associated with the negative hyperconjugation of the Cβ‐F and Cβ‐Cγ bonds. In accordance with this, the E(2) values for π(Cα‐C1) → σ*(Cβ‐F) were linearly correlated with the ΔG^o_β‐F values (an empirical measure of β‐fluorine negative hyperconjugation obtained from an increased acidity). In 3b (weak electron‐withdrawing substituents, from H to m‐NO₂) very large E(2) values for LP(Fβ) → π*(Cα‐Cβ) were obtained. This was attributed to the Cβ‐F bond cleavage and the Cα‐Cβ double bond formation in the Lewis structure that is caused by the extremely strong negative hyperconjugation of the Cβ‐F bond.     

The resonance-like structure observed in the <Superscript>40</Superscript>Ar(pγ)<Superscript>41</Superscript>K reaction

The excitation function of the ⁴⁰Ar(pγ)⁴¹K reaction in the energy range of accelerated protons E _p = 1.0–3.0 MeV was measured. Twelve new resonance states were revealed in the range E _p > 2.6 MeV. The strengths of all measured resonances were determined (for more than 200 states). A resonance-like structure with a center of gravity of 10.2 ± 0.5 MeV was identified; this agrees with the dependence of the center of gravity of the resonance-like structure found earlier on A in the nuclei of the sd shell.     

Kinetic investigation on carbamate formation from the reaction of carbon dioxide with amino acids in homogeneous aqueous solution

The kinetics of carbamate formation from the reaction of carbon dioxide with α‐amino acids in D₂O was first investigated by means of nuclear magnetic resonance spectroscopy. Potassium carbonate was used as the CO₂ source. For each amino acid, the maximum carbamate yield, the apparent rate constant for the carbamate formation k_app, and the rate constants for the formation k₁ and the breakdown k_?1 of the carbamate were estimated. Plots of log k₁ or log k_?1 versus pK_a of amino acids indicated that the formation rate k₁ increased with the basicity (pK_a) of amino acid, while the decomposition rate k_?1 decreased. A Br?nsted β value of 0.39 was obtained from the former plot, being in good agreement with the previously reported ones (0.26–0.43). The observed negative pK_a dependence of log k_?1 (Br?nsted α = 0.34) is reasonable, because the carbamate decomposition is acid‐catalyzed and the steady‐state concentration of H⁺ should be higher for weaker basic amines. The charge (σ) and the lone‐pair energy (E_N) at the nitrogen atom of the amino group were calculated. Although log k₁ correlated with σ and E_N, log k_?1 was unrelated with both of these parameters. Considering that the carbamate formation (k₁) is not only base‐catalyzed but should also be promoted by the nucleophilicity of the amino nitrogen, its correlation with σ and E_N in addition to pK_a is rational. The irrelevance of log k_?1 to σ and E_N is not surprising, because σ and E_N are not a direct measure of [H⁺] of the solution. Copyright © 2011 John Wiley & Sons, Ltd.     

Liouville Type Equations with Singular Data¶and Their Applications to Periodic Multivortices¶for the Electroweak Theory

Motivated by the study of multivortices in the Electroweak Theory of Glashow–Salam–Weinberg [33], we obtain a concentration-compactness principle for the following class of mean field equations: on M, where (M,g) is a compact 2-manifold without boundary, 0 < a≤K(x)≤b, x∈M and λ > 0. We take with α _i > 0, δ _{p i} the Dirac measure with pole at point p _i ∈M, i= 1,…,m and ψ∈L ^∞(M) satisfying the necessary integrability condition for the solvability of (1)_λ. We provide an accurate analysis for solution sequences of (1)_λ, which admit a “blow up” point at a pole p _i of the Dirac measure, in the same spirit of the work of Brezis–Merle [11] and Li–Shafrir [35]. As a consequence, we are able to extend the work of Struwe–Tarantello [49] and Ding–Jost–Li–Wang [21] and derive necessary and sufficient conditions for the existence of periodic N-vortices in the Electroweak Theory. Our result is sharp for N= 1, 2, 3, 4 and was motivated by the work of Spruck–Yang [46], who established an analogous sharp result for N= 1, 2. Received: 24 September 2001 / Accepted: 7 December 2001     

A search for high-energy cosmic gamma-ray bursts

Summary A search has been made for excesses of cosmic-ray counting rates at primary energiesE ₀₁>5 GeV andE ₀₄>2·10⁴ GeV over the time scalet=(1÷100) ms. The measurement was performed by means of a small extensive air shower array operating at mountain altitude (3500 m a.s.l.). During the running time 111 cosmic gamma-ray bursts were detected by satellites; 10 of them certainly (55 probably) above the horizon of the detector. No significant counting rate excess has been recorded out of the statistical fluctuations. Also the search for correlations with satellite events has given a negative result. The upper limit for high-energy cosmic gamma-ray flux in bursts isϕ ₁(E>E ₀₁)<4·10⁻⁵ (t = time scale in ms),ϕ ₄(E>E ₀₄)<1.6·10⁻⁵erg/cm². Paper presented at the 2^o Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984.