In contrast to observations with carbohydrates, experiments with 4‐alkoxy‐substituted acetals indicate that an alkoxy group can accelerate acetal hydrolysis by up to 20‐fold compared to substrates without an alkoxy group. The acceleration of ionization in more flexible acetals can be up to 200‐fold when compensated for inductive effects. 相似文献
The effect of tetramethylenedicarboxylic dibenzoylhydrazide (designated here as TMC) on the nonisothermal and isothermal crystallization behavior of PLA was investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD). TMC shows excellent nucleating effect on PLA. With the addition of 0.05 wt% TMC, the crystallization half-time of PLA decreases from 26.06 to 6.13 min at 130 °C. The isothermal crystallization data were further analyzed by the Avrami model. The values of the Avrami exponent of the blends are comparable to that of neat PLA, indicating that the presence of TMC does not change the crystallization mechanism of the matrix. The observation from POM and WAXD measurements showed that the presence of TMC increases significantly the nuclei density of PLA but has no discernible effect on its crystalline structure. 相似文献
Field experiments were devised to mimic the entrapment conditions under the rubble of collapsed buildings aiming to investigate the evolution of volatile organic compounds (VOCs) during the early dead body decomposition stage. Three pig carcasses were placed inside concrete tunnels of a search and rescue (SAR) operational field terrain for simulating the entrapment environment after a building collapse. The experimental campaign employed both laboratory and on-site analytical methods running in parallel. The current work focuses only on the results of the laboratory method using thermal desorption coupled to comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (TD-GC × GC-TOF MS). The flow-modulated TD-GC × GC-TOF MS provided enhanced separation of the VOC profile and served as a reference method for the evaluation of the on-site analytical methods in the current experimental campaign. Bespoke software was used to deconvolve the VOC profile to extract as much information as possible into peak lists. In total, 288 unique VOCs were identified (i.e., not found in blank samples). The majority were aliphatics (172), aromatics (25) and nitrogen compounds (19), followed by ketones (17), esters (13), alcohols (12), aldehydes (11), sulfur (9), miscellaneous (8) and acid compounds (2). The TD-GC × GC-TOF MS proved to be a sensitive and powerful system for resolving the chemical puzzle of above-ground “scent of death”. 相似文献
A series of dinuclear Pt(II) complexes bridged with polyethyleneglycol ether of the type [ClPt(tpy)O(CH2CH2O)n(tpy)PtCl]Cl2 where n = 1 (Ptdteg), 2 (Ptdtdeg), 3 (Ptdtteg), 4 (Ptdttteg), and linker-free complex, (Ptdt) (where tpy = 2,2′:6′,2″-terpyridine), were synthesized and characterized to investigate the role of bridging polyethyleneglycol ether linker on the substitution reactivity of dinuclear Pt(II) complexes. Substitution reactions were studied using thiourea nucleophiles, viz. thiourea (TU), 1,3-dimethyl-2-thiourea (DMTU), 1,1,3,3-tetramethyl-2-thiourea (TMTU) under pseudo-first-order conditions as a function of concentration and temperature by conventional stopped-flow reaction analyzer. The reactions gave single exponential fits following the rate law kobs = k2[Nu]. Introduction of polyethyleneglycol ether linker decreases the electrophilicity of the platinum center and the whole complex. The results obtained indicate that the rate of substitution is controlled by both electronic and steric hindrance which increases with the length of the linker. Experimental results are supported by density functional theory calculations and structures obtained at B3LYP/LANL2DZ level. The order of the reactivity of the nucleophiles is TU > DMTU > TMTU. The magnitude and the size of the enthalpy of activation and entropy of activation support an associative mode of mechanism, where bond formation in the transition state is favored. 相似文献
The hydroperoxo iron(III) intermediate P450camFeIII–OOH, being the true Compound 0 (Cpd 0) involved in the natural catalytic cycle of P450cam, could be transiently observed in the peroxo‐shunt oxidation of the substrate‐free enzyme by hydrogen peroxide under mild basic conditions and low temperature. The prolonged lifetime of Cpd 0 enabled us to kinetically examine the formation and reactivity of P450camFeIII–OOH species as a function of varying reaction conditions, such as pH, and concentration of H2O2, camphor, and potassium ions. The mechanism of hydrogen peroxide binding to the substrate‐free form of P450cam differs completely from that observed for other heme proteins possessing the distal histidine as a general acid–base catalyst and is mainly governed by the ability of H2O2 to undergo deprotonation at the hydroxo ligand coordinated to the iron(III) center under conditions of pH≥p${K{{{\rm P450}\hfill \atop {\rm a}\hfill}}}$ . Notably, no spectroscopic evidence for the formation of either Cpd I or Cpd II as products of heterolytic or homolytic O?O bond cleavage, respectively, in Cpd 0 could be observed under the selected reaction conditions. The kinetic data obtained from the reactivity studies involving (1R)‐camphor, provide, for the first time, experimental evidence for the catalytic activity of the P450FeIII–OOH intermediate in the oxidation of the natural substrate of P450cam. 相似文献
The adsorption of propene on neutral gold clusters is investigated in a collision cell under a few collision conditions. The adsorption reaction is studied by pressure‐dependent kinetic measurements and delayed unimolecular dissociation of the excited Aun?propene complexes. The cluster size (n=9–25) and temperature (T=90–300 K) dependence of the propene adsorption is analyzed. Strong size dependences of the absorption reaction are observed; a larger propene adsorption probability was found for gold clusters composed of an even number of atoms. Propene binding energies are estimated by comparison of the temperature‐dependent unimolecular dissociation rates with rates obtained by using statistical RRKM modeling. The Aun–propene binding energies decrease non‐monotonously with cluster size and are in the range of 1.2–0.85 eV for n=9–25. Finally, the bonding of C3H6 on Aun is qualitatively described and similarities with the absorption of CO molecules on gold clusters are discussed. 相似文献
The femtosecond transient absorption (TA) characterization of a new benzothiadiazole (BT)‐based donor–acceptor conjugated copolymer, poly[(2,6‐dithieno[3,2‐b:2′,3′‐d]thiophene)‐alt‐(4,7‐di(4‐octyldodecylthiopen‐2‐yl)‐2,1,3‐benzo[c][1,2,5]thiadiazole (PBT), as well as its fluorinated derivatives, PFBT and PDFBT, is carried out. Additionally, bulk heterojunction (BHJ) films consisting of the copolymers and [6,6]‐phenyl‐C71‐butylic acid methyl ester (PC70BM) are examined using TA spectroscopy. Both the singlet excited state dynamics in the copolymers and the charge transfer state dynamics in the BHJs are investigated in terms of fluorination dependency; the fluorinated copolymers exhibit less singlet exciton recombination rate than the fluorine‐free copolymer, and the BHJs including the fluorinated copolymers display slower monomolecular recombination than the fluorine‐free analogue. Furthermore, the excitation‐intensity‐dependent TA dynamics of the copolymers and BHJs is investigated, revealing that, when sufficiently high excitation intensity is used to induce annihilation processes, the fluorinated copolymers and BHJs incorporating the fluorinated copolymers show more rapid TA decay ascribable to morphological enhancement. These TA spectroscopic findings are found to correlate with the device characteristics with respect to fluorinated content in the polymer solar cells. In particular, both the short‐circuit current density and fill factor of BHJ solar cells correspond closely with the fast decay parameters of the BHJ films under high excitation intensity.