氢转移反应HNCO+CH_x(x=1～3)的MP2和QCISD计算研究

在UMP2（FULL）/6-311（d,p）计算水平上,计算并讨论了碳氢自由基(CH_x) 和异氰酸（HNCO）发生氢转移反应位能面上驻点的结构和分子结构变化,并依据 UMP2的优化构型,进一步采用UQCISD（T）方法对反应途径上的驻点进行了单点能 量计算。研究指出,HNCO同CH_2,CH_3自由基反应时,氢转长期过程在分子间以新 的C-H键生成和旧的N-H键的断裂的协同方式进行,反应途径上均存在弱的氢键超分 子复合物;而HNCO和CH分子之间发生的氢转移反应机理是在反应剖面的反应物一边 生成一个较稳定的分子复合物HNJ（CH）CO,而后,在生成物一边又生成一个既有 热力学又有动力学稳定的复合物H_2CNCO。     

Unimolecular dissociation reactions of methyl benzoate radical cation

The blackbody infrared radiation induced dissociation of methyl benzoate (C8H8O2(+*)) radical cation was investigated by using a Fourier transfer ion cyclotron resonance mass spectrometer equipped with a resistively heated (wire temperatures of 400-1070 K) wire ion guide. We observed product ion branching ratios that are strongly dependent upon wire temperature. At low temperatures (670-890 K) the major product ion C7H8 (+*) (m/z 92), which is formed by loss of CO2, and at higher temperatures (above 900 K), loss of methoxy radical ((*)OCH3) competes with loss of CO2. The energies of the various reactant ions and transition states for product ion formation were estimated by using density functional theory molecular orbital calculations, and a proposed mechanism for the dissociation chemistry of C8H8O2 (+*) involving a multistep rearrangement reaction is tested using the Master Equation formalism.     

Chiral recognition by resorcin[4]arene receptors: intrinsic kinetics and dynamics

Molecular recognition of representative amino acids (A) by a chiral amido[4]resorcinarene receptor (1(L)) was investigated in the gas phase by ESI-FT-ICR mass spectrometry. The ligand displacement reaction between noncovalent diastereomeric [1(L).H.A](+) complexes and the 2-aminobutane enantiomers (B) exhibits a distinct enantioselectivity with regard to both the leaving amino acid A and the amine reactant B. The emerging selectivity picture, discussed in the light of molecular mechanics and molecular dynamics calculations, points to chiral recognition by 1(L), as determined by the effects of the host asymmetric frame on the structure, stability, and rearrangement dynamics of the diastereomeric [1(L).H.A](+) complexes and the orientation of the amine reactant B in encounters with [1(L).H.A](+). The results contribute to the development of a dynamic model of chiral recognition of biomolecules by enzyme mimics in the unsolvated state.     

Molecular dynamics simulation of the reaction of hydration of formaldehyde using a potential based on solute-solvent interaction energy components

Molecular dynamics simulations of aqueous solutions at infinite dilution of the reaction of water with formaldehyde, H(2)O + H(2)CO --> H(2)C(OH)(2), were performed using Lennard-Jones 12-6-1 potentials to describe the solute-solvent interactions, and TIP3P to describe the water-water interactions. The Morokuma decomposition scheme of ab initio interaction energies at the SCF level and the dispersion component at MP2 level were used to reproduce the molecular parameters of the solute-water interaction potential. The results show that the functions that use the EX-PL-DIS-ES interaction model to describe the solvation of the reactant and product systems lead to good values of the reaction (DeltaG) and activation (DeltaG(#)) free energy as compared with those from using AMBER-derived parameters, and with the available theoretical and experimental data.     

Synthesis and characterization of perfluoro[1-(2-fluorosulfonyl)ethoxy]ethyl end-capped styrene oligomers

Styrene oligomers with perfluoro[1-(2-fluorosulfonyl)ethoxy]ethyl end-groups have been synthesized with moderate to high yields (52-97%) via radical oligomerization by using perfluoro[2-(2-fluorosulfonyl)ethoxy]propyonyl peroxide (PPP) at various reactant ratios of styrene over PPP (80, 160, 240, 300, 360 and 420) and different temperatures (33, 37 and 42 °C), and characterized by FTIR, ¹H NMR and ¹⁹F NMR. The molecular weight of the oligomers measured by gel permeation chromatography (GPC) is dependent on the reactant ratio and reaction temperature. The polydispersity of the oligomers varies from 1.99 to 3.30. The oligomer obtained at the reactant ratio of 300 has the maximum yield (97%) and much broader polydispersity (3.30). The contact angles of water, θ_H2O, on the oligomer films are much bigger than that of polystyrene (PS). The glass transition temperature of the oligomers, T_g, increases with the increase of molecular weight and is lower than that of the parent polymer.     

Emission properties of dihydropterins in aqueous solutions

Pterins belong to a class of heterocyclic compounds present in a wide range of living systems and accumulate in the skin of patients affected by vitiligo, a depigmentation disorder. The study of the emission of 7,8-dihydropterins is difficult because these compounds are more or less unstable in the presence of O(2) and their solutions are contaminated with oxidized pterins which have much higher fluorescence quantum yields (Φ(F)). In this work, the emission properties of six compounds of the dihydropterin family (6-formyl-7,8-dihydropterin (H(2)Fop), sepiapterin (Sep), 7,8-dihydrobiopterin (H(2)Bip), 7,8-dihydroneopterin (H(2)Nep), 6-hydroxymethyl-7,8-dihydropterin (H(2)Hmp), and 6-methyl-7,8-dihydropterin (H(2)Mep)) have been studied in aqueous solution. The fluorescence characteristics (spectra, Φ(F), lifetimes (τ(F))) of the neutral form of these compounds have been investigated using the single-photon-counting technique. Φ(F) and τ(F) values obtained lie in the ranges 3-9 × 10(-3) and 0.18-0.34 ns, respectively. The results are compared to those previously reported for oxidized pterins.     

On the formation of phenyldiacetylene (C6H5CCCCH) and D5-phenyldiacetylene (C6D5CCCCH) studied under single collision conditions

The crossed molecular beam reactions of the phenyl and D5-phenyl radical with diacetylene (C(4)H(2)) was studied under single collision conditions at a collision energy of 46 kJ mol(-1). The chemical dynamics were found to be indirect and initiated by an addition of the phenyl/D5-phenyl radical with its radical center to the C1-carbon atom of the diacetylene reactant. This process involved an entrance barrier of 4 kJ mol(-1) and lead to a long lived, bound doublet radical intermediate. The latter emitted a hydrogen atom directly or after a few isomerization steps via tight exit transition states placed 20-21 kJ mol(-1) above the separated phenyldiacetylene (C(6)H(5)CCCCH) plus atomic hydrogen products. The overall reaction was determined to be exoergic by about 49 ± 26 kJ mol(-1) and 44 ± 10 kJ mol(-1) as determined experimentally and computationally, thus representing a feasible pathway to the formation of the phenyldiacetylene molecule in combustion flames of hydrocarbon fuel.     

Crossed beam reactions of methylidyne [CH(X2Π)] with D2-acetylene [C2D2(X1Σg(+))] and of D1-methylidyne [CD(X2Π)] with acetylene [C2H2(X1Σg(+))

The crossed molecular beam reactions of ground state methylidyne, CH(X(2)Π), with D2-acetylene, C(2)D(2)(X(1)Σ(g)(+)), and of D1-methylidyne, CD(X(2)Π), with acetylene, C(2)H(2)(X(1)Σ(g)(+)), were conducted under single collision conditions at a collision energy of 17 kJ mol(-1). Four competing reaction channels were identified in each system following atomic 'hydrogen' (H/D) and molecular 'hydrogen' (H(2)/D(2)/HD) losses. The reaction dynamics were found to be indirect via complex formation and were initiated by two barrierless-addition pathways of methylidyne/D1-methylidyne to one and to both carbon atoms of the D2-acetylene/acetylene reactant yielding HCCDCD/DCCHCH and c-C(3)D(2)H/c-C(3)H(2)D collision complexes, respectively. The latter decomposed via atomic hydrogen/deuterium ejection to form the thermodynamically most stable cyclopropenylidene species (c-C(3)H(2), c-C(3)D(2), c-C(3)DH). On the other hand, the HCCDCD/DCCHCH adducts underwent hydrogen/deuterium shifts to form the propargyl radicals (HDCCCD, D(2)CCCH; HDCCCH, H(2)CCCD) followed by molecular 'hydrogen' losses within the rotational plane of the decomposing complex yielding l-C(3)H/l-C(3)D. Quantitatively, our crossed beam studies suggest a dominating atomic compared to molecular 'hydrogen' loss with fractions of 81 ± 23% vs. 19 ± 10% for the CD/C(2)H(2) and 87 ± 30% vs. 13 ± 4% for the CH/C(2)D(2) systems. The role of these reactions in the formation of interstellar isomers of C(3)H(2) and C(3)H is also discussed.     

Mechanism of cysteine oxidation by a hydroxyl radical: a theoretical study.

Cysteine oxidation by HO(.) was studied at a high level of ab initio theory in both gas phase and aqueous solution. Potential energy surface scans in the gas phase performed for the model system methanethiol+HO(.) indicate that the reactants can form two intermediate states: a sulfur-oxygen adduct and a hydrogen bound reactant complex. However these states appear to play a minor role in the reaction mechanism as long as they are fast dissociating states. Thus the main reaction channel predicted at the QCISD(T)/6-311+G(2df,2pd) level of theory is the direct hydrogen atom abstraction. The reaction mechanism is not perturbed by solvation which was found to induce only small variations in the Gibbs free energy of different reactant configurations. The larger size reactant system cysteine+HO* was treated by the integrated molecular orbital+molecular orbital (IMOMO) hybrid method mixing the QCISD(T)/6-311+G(2df,2pd) and the UMP2/6-311+G(d,p) levels of theory. The calculated potential energy, enthalpy, and Gibbs free energy barriers are slightly different from those of methanethiol. The method gave a rate constant for cysteine oxidation in aqueous solution, k=2.4 x 10(9) mol(-1) dm(3) s(-1), which is in good agreement with the experimental rate constant. Further analysis showed that the reaction is not very sensitive to hydrogen bonding and electrical polarity of the molecular environment.     

New insights into the enantioselectivity in the hydrogenation of prochiral ketones

The high enantioselectivity in the hydrogenation of acetophenone catalysed by trans-Ru(H)2(S,S-dpen)(S-xylbinap) is explained in terms of the existence of a stable intermediate formed when the reactant enters the catalyst pocket fixing the molecular orientation.     

Deprotonated N-(2,4-Dinitrophenyl)amino Acids Undergo Cyclization in Solution and the Gas Phase

The collisionally activated mass spectral fragmentations of N-(2,4-dinitrophenyl)alanine and phenylalanine [M - H](-) may be gas-phase analogs of the base-catalyzed cyclization of N-(2,4-dinitrophenyl)amino acids in aqueous dioxane. This latter reaction is one source of the 2-substituted 5-nitro-1H-benzimidazole-3-oxides, which are antibacterial agents. The fragmentation of both compounds, established by tandem mass spectrometric experiments and supported by molecular modeling using DFT methods, indicate that the [M - H](-) ions dissociate via sequential eliminations of CO(2) and H(2)O to produce deprotonated benzimidazole-N-oxide derivatives. The gas-phase cyclization reactions are analogous to the base-catalyzed cyclization in solution, except that in the latter case, the reactant must be a dianion for the reaction to occur on a reasonable time scale. The cyclization of N-(2-nitrophenyl)phenylalanine, which has one less nitro group, requires a stronger base for the cyclization than the compound with a second nitro group at the 4-position. Following losses of CO(2) and H(2)O are expulsions of both neutral molecules and free radicals, the latter being examples of violations of the even-electron ion rule.     

H-Atom abstraction from CH(3)NHNH(2) by NO(2): CCSD(T)/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) and CCSD(T)/6-311+G(2df,p)//CCSD/6-31+G(d,p) calculations

Stationary points of paths for H atom abstraction from CH(3)NHNH(2) (monomethylhydrazine) by NO(2) were characterized via CCSD(T)/6-311++G(3df,2p)//MPWB1K/6-31+G(d,p) and CCSD(T)/6-311+G(2df,p)//CCSD/6-31+G(d,p) calculations. Five transition states connecting CH(3)NHNH(2)-NO(2) complexes to a manifold that includes CH(3)NHNH-HONO, CH(3)NNH(2)-HONO, CH(3)NNH(2)-HNO(2), and CH(3)NHNH-HNO(2) complexes were identified. Transition states that connect CH(3)NHNH-HONO, CH(3)NNH(2)-HONO, CH(3)NNH(2)-HNO(2), and CH(3)NHNH-HNO(2) complexes to each other via H atom exchange and/or hindered internal rotation were also identified. The high point in the minimum energy path from the CH(3)NHNH(2) + NO(2) reactant asymptote to the manifold of HONO-containing product states is a transition state 8.6 kcal/mol above the reactant asymptote. From a kinetics standpoint, this value is considerably higher than the 5.9 kcal/mol value that was estimated for it based on theoretical results for H atom abstraction from NH(3) by NO(2).     

Kinetics of sulfur oxide, sulfur fluoride, and sulfur oxyfluoride anions with atomic species at 298 and 500 K

The rate constants and product-ion branching ratios for the reactions of sulfur dioxide (SO2-), sulfur fluoride (SFn-), and sulfur oxyfluoride anions (SOxFy-) with H, H2, N, N2, NO, and O have been measured in a selected-ion flow tube (SIFT). H atoms were generated through a microwave discharge on a H2/He mixture, whereas O atoms were created via N atoms titrated with NO, where the N had been created by a microwave discharge on N2. None of the ions reacted with H2, N2 or NO; thus, the rate constants are <1 x 10(-12) cm3 s-1. SOxFy- ions react with H by only fluorine-atom abstraction to form HF at 298 and 500 K. Successive F-atom removal does not occur at either temperature, and the rate constants show no temperature dependence over this limited range. SO2- and F- undergo associative detachment with H to form a neutral molecule and an electron. Theoretical calculations of the structures and energetics of HSO2- isomers were performed and showed that structural differences between the ionic and neutral HSO2 species can account for at least part of the reactivity limitations in the SO2- + H reaction. All of the SOxFy- ions react with O; however, only SO2- reacts with both N and O. SOxFy- reactions with N (SO2- excluded) have a rate constant limit of <1 x 10(-11) cm3 s-1. The rate constants for the SOxFy- reactions with H and O are < or =25% of the collision rate constant, as seen previously in the reactions of these ions with O3, consistent with a kinetic bottleneck limiting the reactivity. The only exceptions are the reactions of SO2- with N and O, which are much more efficient. Three pathways were observed with O atoms: F-atom exchange in the reactant ion, F- exchange in the reactant ion, and charge transfer to the O atom. No associative detachment was observed in the N- and O-atom reactions.     

Surface-aligned ion-molecule reaction on the surface of a molecular crystal CD3+ + CD3I --> C2D5+ + DI

An ion-molecule reaction has been observed from a condensed molecular crystal of CD(3)I using the time-of-flight electron-stimulated desorption ion angular distribution technique. The CD(3)I multilayer is produced by growth on an ordered substrate. The reaction occurs between CD(3)(+) ions produced by electron-stimulated desorption and neighbor CD(3)I molecules in the topmost layer of the molecular crystal of CD(3)I, forming product C(2)D(5)(+) ions whose desorption dynamics have been measured. The normal momentum of the product ion is close to that of the reactant ion, suggesting that the reaction is dominated by a two-body collision, i.e., the momentum of the reactant CD(3)(+) ion governs the momentum of the product C(2)D(5)(+) ion. The ion-molecule reaction is of high cross section since the C(2)D(5)(+) yield is comparable to the CD(3)(+) yield. It is found that the yield and directionality of the emission of the C(2)D(5)(+) product ion is governed by the molecular order that is characteristic of the molecular crystal of CD(3)I. Destroying or modifying this order by using a spacer layer of H(2)O diminishes the C(2)D(5)(+) product ion yield relative to the reactant CD(3)(+) yield and broadens the ion emission directions.     

Isoguanine formation from adenine

Several possible mechanisms underlying isoguanine formation when OH radical attacks the C(2) position of adenine (A?C?2) are investigated theoretically for the first time. Two steps are involved in this process. In the first step, one of two low-lying A?C?2???OH reactant complexes is formed, leading to C(2)-H(2) bond cleavage. Between the two reactant complexes there is a small isomerization barrier, which lies well below separated adenine plus OH radical. The complex dissociates to free molecular hydrogen and an isoguanine tautomer (isoG?1 or isoG?2). The local and activation barriers for the two pathways are very similar. This evidence suggests that the two pathways are competitive. After dehydrogenation, there are two possible routes for the second step of the reaction. One is direct hydrogen transfer, via enol-keto tautomerization, which has high local barriers for both tautomers and is not favored. The other option is indirect hydrogen transfer involving microsolvation by one water molecule. The water lowers the reaction barrier by over 20?kcal mol(-1) , indicating that water-mediated hydrogen transfer is much more favorable. Both A+OH(?) →isoG+H(?) reactions are exothermic and spontaneous. Among four isoguanine tautomers, isoG?1 has the lowest energy. Our findings explain why only the N(1)H and O(2)H tautomers of isolated isoguanine and isoguanosine have been observed experimentally.     

Rate coefficients for the reactions of C2(a(3)Pi(u)) and C2(X(1)Sigma(g)(+)) with various hydrocarbons (CH4, C2H2, C2H4, C2H6, and C3H8): a gas-phase experimental study over the temperature range 24-300 K

The kinetics of reactions of C2(a(3)Pi(u)) and C2(X(1)Sigma(g)(+)) with various hydrocarbons (CH4, C2H2, C2H4, C2H6, and C3H8) have been studied in a uniform supersonic flow expansion over the temperature range 24-300 K. Rate coefficients have been obtained by using the pulsed laser photolysis-laser induced fluorescence technique, where both radicals were produced at the same time but detected separately. The reactivity of the triplet state was found to be significantly lower than that of the singlet ground state for all reactants over the whole temperature range of the study. Whereas C2(X(1)Sigma(g)(+)) reacts with a rate coefficient close to the gas kinetic limit with all hydrocarbons studied apart from CH4, C2(a(3)Pi(u)) appears to be more sensitive to the molecular and electronic structure of the reactant partners. The latter reacts at least one order of magnitude faster with unsaturated hydrocarbons than with alkanes, and the rate coefficients increase very significantly with the size of the alkane. Results are briefly discussed in terms of their potential astrophysical impact.     

Coordination environment of aqueous uranyl(VI) ion

Water dissociation from [UO2(OH2)5]2+ is studied with Car-Parrinello molecular dynamics simulations (using the BLYP density functional) in the gas phase and in aqueous solution. Free energies, DeltaA, are estimated from pointwise thermodynamic integration using one U-O(H2) distance as a reaction coordinate. While an isomeric, four-coordinate complex, [UO2(OH2)4]2+.H2O, is more stable than the five-coordinate reactant in the gas phase (DeltaA = -2.2 kcal/mol), the former is strongly disfavored in water (DeltaA = +8.7 kcal/mol).     

室温固相反应制备纳米四水磷酸锌的热化学研究

Nanocrystalline zinc phosphate tetrahydrate was synthesized by solid-state reaction at ambient temperature using Na3PO4·12H2O and ZnSO4·7H2O as regents. The enthalpy of reaction and the standard enthalpy of formation of zinc phosphate tetrahydrate were studied by microcalorimeter. According to Hess s law, a new thermochemical cycle was designed. The dissolution enthalpies of reactant (△H1) and product (△H2) were performed by a RD496 microcalorimeter at 298.15 K using aqueous solution of hydrochloric acid (4mol/L) as a calorimetric solvent. Ultraviolet spectrum, conductivity and refractive index were measured respectively. The results show as follows: △H1=(-47.180±0.084) kJ/mol, △H2=(-7.617±0.096 )kJ/mol. The ultraviolet spectra of the above solution are the same. Conductivity values after 500 times diluted are 2180 and 2181μs/cm respectively. Refractive indexes are 1.3679 and 1.3678 respectively. The standard enthalpy value of the reaction was calculated: △rHm=-39.530 kJ/mol. The standard enthalpy of formation for zinc phosphate tetrahydrate are recommended -4354.004 kJ/mol.     

Dissociation of H2NCH dication in a strong laser field

Ab initio classical molecular dynamics calculations have been used to simulate the dissociation of H(2)NCH(2+) in a strong laser field. The frequencies of the continuous oscillating electric field were chosen to be ω = 0.02, 0.06, and 0.18 au (2280, 760, and 253 nm, respectively). The field had a maximum strength of 0.03 au (3.2 × 10(13) W cm(-2)) and was aligned with the CN bond. Trajectories were started with 100 kcal/mol of vibrational energy above zero point and were integrated for up to 600 fs at the B3LYP/6-311G(d,p) level of theory. A total of 200 trajectories were calculated for each of the three different frequencies and without a field. Two dissociation channels are observed: HNCH(+) + H(+) and H(2)NC(+) + H(+). About one-half to two-thirds of the H(+) dissociations occurred directly, while the remaining indirect dissociations occurred at a slower rate with extensive migration of H(+) between C and N. The laser field increased the initial dissociation rate by a factor of ca. 1.4 and decreased the half-life by a factor of ca. 0.75. The effects were similar at each of the three frequencies. The HNCH(+) to H(2)NC(+) branching ratio decreased from 10.6:1 in the absence of the field to an average of 8.4:1 in the laser field. The changes in the rates and branching ratios can be attributed to the laser field lowering the reaction barriers as a result of a difference in polarizability of the reactant and transition states.     

离子迁移谱定量检测复杂基质中新型炸药六亚甲基三过氧化二胺

六亚甲基三过氧化二胺( Hexamethylene triperoxide diamine, HMTD)是一种新型有机过氧化爆炸物,由于原料易得、制备方法简单,常被用于恐怖袭击和犯罪活动中。本实验基于非放射性电离源真空紫外灯( VUV)发展了一种试剂分子辅助灯电离正离子迁移谱技术,通过优化筛选试剂分子,最终选择丙酮作为HMTD定量检测的试剂分子。利用质谱对丙酮的反应试剂离子和HMTD的产物离子进行了离子归属,确定反应试剂离子为丙酮二聚体离子 m/z 117[( CH3)2 CO ]2 H+, HMTD 的产物离子为其质子化的分子离子m/z 209[ HMTD+H]+。在迁移管和热解析温度120℃的条件下,利用HMTD最大信号强度和第10 s的信号强度对其标准样品进行定量检测,线性范围分别为5~50 ng/μL和5~100 ng/μL,检出限分别可达0.2和0.3 ng/μL。化妆品如香水等常常干扰和抑制离子迁移谱测量,发展在香水基质中HMTD的现场快速筛查和检测方法具有现实意义。将这两种定量方法应用于3种不同品牌香水样品中HMTD的定量检测,对比发现利用HMTD第10 s的信号强度进行定量具有较好的回收率和准确性,该方法适用于复杂基质中HMTD的准确快速定量检测。