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[en] A kinetic study on nucleophilic substitution reactions of 4-pyridyl benzoate (2a) and O-4-pyridyl thionobenzoate (2b) with a series of cyclic secondary amines in acetonitrile at 25.0°C is reported. Plots of pseudo-first-order rate constant (k_o_b_s_d) vs. [amine] are linear and pass through the origin for the reactions of 2a but curve upward for those of 2b. The upward curvature observed for the reactions of 2b is typical for reactions that proceed through a stepwise mechanism with a zwitterionic intermediate T"±, which decomposes to the products via uncatalyzed and catalyzed routes competitively. The reaction of 2a has been suggested to proceed through a stepwise mechanism with T±, in which expulsion of the leaving group occurs in the rate-determining step on the basis of a linear Broensted-type plot with β_n_u_c = 0.77. The catalyzed reaction of 2b from T"± has been proposed to proceed through a concerted mechanism with a six-membered cyclic transition state rather than via a stepwise pathway with an anionic intermediate T"−. Factors influencing reactivity and reaction mechanism are discussed in detail
[en] A kinetic study is reported on nucleophilic substitution reactions of 4-nitrophenyl isonicotinate with a series of cyclic secondary amines in MeCN. The plots of kobsd vs. [amine] curve upward for the reactions with weakly basic amines (e. g., morpholine, 1-(2-hydroxyethyl)piperazine, and piperazine) but are linear for those with strongly basic amines (e. g., piperidine and 3-methylpiperidine). The curved plots for the reactions with the weakly basic amines are typical for reactions reported previously to proceed through uncatalyzed and catalyzed routes with two intermediates (e. g., a zwitterionic tetrahedral intermediate T± and its deprotonated form T-). In contrast, the linear plots for the reactions with the strongly basic amines indicate that the catalytic route (i. e., the deprotonation process to yield T- from T± by a second amine molecule) is absent. The Brφnsted-type plots for Kk2 and Kk3 (i. e., the rate constants for the uncatalyzed and catalyzed routes, respectively) exhibit excellent linear correlations with βnuc = 0.99 and 0.69, respectively. The effect of amine basicity on the reaction mechanism is discussed in detail
[en] Nucleophilic addition reactions of benzylamines (BA; XC6H4CH2NH2) to ethyl-α-cyanocinnamates (ECC; YC6H4CH=C(CN)COOEt) have been investigated in acetonitrile at 30.0 .deg. C. The rate is first order with respect to BA and ECC. The rate is slower than that expected from the additive effect of σ- or R- for the activating groups (CN and COOEt). Natural bond orbital πc=c calculations show that the contribution of COOEt group may not be fully effective despite the coplanar molecular structure. The selectivity parameters including the cross-interaction constant (ρXY=-0.22) indicate that the addition occurs in a single step. The kinetic isotope effects (kH/kD=2.5-2.8) involving deuterated BA (XC6H4CH2ND2) nucleophiles and activation parameters (ΔH=4∼6 kcal mol-1; ΔS= -45∼-52 e.u.) suggest a cyclic transition state in which N-Cα and H-Cβ bonds are formed concurrently
[en] The kinetic studies on the reactions of dipropyl chlorophosphate (3O) with substituted anilines (XC6H4NH2) and deuterated anilines (XC6H4ND2) have been carried out in acetonitrile at 55.0 .deg. C. The obtained deuterium kinetic isotope effects (DKIEs: kH/kD) are primary normal (kH/kD = 1.09-1.01) with the strongly basic anilines while secondary inverse (kH/kD = 0.74-0.82) with the weakly basic anilines. The steric effects of the two ligands on the rates are extensively discussed for the anilinolyses of the (R1O)(R2O)P(=O or S)Cl-type chlorophosphates and chlorothiophosphates. A concerted mechanism is proposed with a frontside nucleophilic attack involving a hydrogen-bonded four-center-type transition state for the strongly basic anilines and with a backside attack transition state for the weakly basic anilines on the basis of the DKIEs, primary normal and secondary inverse with the strongly and weakly basic anilines, respectively
[en] Nucleophilic addition reactions of benzylamines (XC_6H_4CH_2NH_2) to β-cyanostilbenes (YC_6H_4CH=C(CN)C_6H_4Y') have been studied in acetonitrile at 30.0 .deg. C. A greater degree of N-C_α bond formation (larger β_X) is obtained with a stronger electron-withdrawing substituent in either α- (δσ_Y > 0) or β-ring (δσ_Y' > 0). A stronger charge development is observed in the TS on C_β (ρ_Y' = 1.06 for X=Y=H) rather than on C_α (ρ_Y = 0.62 for X=Y'=H) indicating the lag in the resonance development into the activating group (CN) on C_β in the transition state. Similarly, the magnitude of ρ_X_Y' (.0.72) is greater than ρ_X_Y (.0.66) due to a stronger interaction of the nucleophile with β-ring than α-ring. The positive sign of ρ_Y_Y' correctly reflects π bond cleavage between the two rings in the TS. Relatively large kinetic isotope effects (k_H/k_D ≥ 2.0) involving deuterated nucleophiles (XC_6H_4CH_2ND_2) suggest a four-membered cyclic TS in which concurrent N-C_α and H(D)-C_β bond formation occurs
[en] The kinetic studies on the reactions of O-methyl (1), O-propyl (3) and O-isopropyl (4) phenyl phosphono-chloridothioates with substituted anilines and deuterated anilines have been carried out in acetonitrile at 55.0 .deg. C. A concerted SN2 mechanism is proposed for the anilinolyses of 1, 3 and 4. The anilinolysis rates of the phosphonochloridothioates are predominantly dependent upon the steric effects over the inductive effects of the two ligands. The deuterium kinetic isotope effects (DKIEs; kH/kD) are primary normal with 1 and 3, while secondary inverse with 4. Primary normal and secondary inverse DKIEs are rationalized by frontside and backside nucleophilic attack transition state, respectively. The DKIEs of the phosphonochloridothioates do not have any consistent correlations with the two ligands
[en] Hazardous and noxious substances (HNS) spill in the marine environment is an issue of growing concern, and it will mostly continue to do so in the future owing to the increase of high chemical traffic. Nevertheless, the effects of HNS spill on marine environment, especially on aquatic organisms are unclear. Consequently, it is emergent to provide valuable information for the toxicities to marine biota caused by HNS spill. Accordingly, the acute toxicity of three preferential HNS and sub-lethal effects of acrylonitrile on Brachionus plicatilis were evaluated. The median lethal concentration (LC50) at 24 h were 47.2 mg acrylonitrile L−1, 276.9 mg styrene L−1, and 488.3 mg p-xylene L−1, respectively. Sub-lethal toxicity effects of acrylonitrile on feeding behavior, development, and reproduction parameters of B. plicatilis were also evaluated. Results demonstrated that rates of filtration and ingestion were significantly reduced at 2.0, 4.0, and 8.0 mg L−1 of acrylonitrile. Additionally, reproductive period, fecundity, and life span were significantly decreased at high acrylonitrile concentrations. Conversely, juvenile period was significantly increased at the highest two doses and no effects were observed on embryonic development and post-reproductive period. Meanwhile, we found that ingestion rate decline could be a good predictor of reproduction toxicity in B. plicatilis and ecologically relevant endpoint for toxicity assessment. These data will be useful to assess and deal with marine HNS spillages.
[en] We present deep high-angular resolution observations of the high-mass protostar NGC 7538 S, which is in the center of a cold dense cloud core with a radius of 0.5 pc and a mass of ∼2000 M sun. These observations show that NGC 7538 S is embedded in a compact elliptical core with a mass of 85-115 M sun. The star is surrounded by a rotating accretion disk, which powers a very young, hot molecular outflow approximately perpendicular to the rotating accretion disk. The accretion rate is very high, ∼(1.4-2.8) x 10-3 M sun yr-1. Evidence for rotation of the disk surrounding the star is seen in all largely optically thin molecular tracers, H13CN J = 1 → 0, HN13C J = 1 → 0, H13CO+ J = 1 → 0, and DCN J = 3 → 2. Many molecules appear to be affected by the hot molecular outflow, including DCN and H13CO+. The emission from CH3CN, which has often been used to trace disk rotation in young high-mass stars, is dominated by the outflow, especially at higher K levels. Our new high angular resolution observations show that the rotationally supported part of the disk is smaller than we previously estimated. The enclosed mass of the inner, rotationally supported part of the disk (D ∼ 5'', i.e., 14,000 AU) is ∼14-24 M sun.
[en] DR21(OH) is a pc-scale massive, ∼7000 Msun clump hosting three massive dense cores (MDCs) at an early stage of their evolution. We present a high angular resolution mosaic, covering ∼70'' x 100'', with the IRAM Plateau de Bure Interferometer at 3 mm to trace the dust continuum emission and the N2H+ (J = 1-0) and CH3CN (J = 5-4) molecular emission. The cold, dense gas traced by the compact emission in N2H+ is associated with the three MDCs and shows several velocity components toward each MDC. These velocity components reveal local shears in the velocity fields which are best interpreted as convergent flows. Moreover, we report the detection of weak extended emission from CH3CN at the position of the N2H+ velocity shears. We propose that this extended CH3CN emission is tracing warm gas associated with the low-velocity shocks expected at the location of convergence of the flows where velocity shears are observed. This is the first detection of low-velocity shocks associated with small (subparsec) scale convergent flows which are proposed to be at the origin of the densest structures and of the formation of (high-mass) stars. In addition, we propose that MDCs may be active sites of star formation for more than a crossing time as they continuously receive material from larger scale flows as suggested by the global picture of dynamical, gravity-driven evolution of massive clumps which is favored by the present observations.