Dianion Stabilization by (M(C5(CH3)5)2)+: Theoretical Evidence for a Localized Ring in (DDQ)2-

Organic dianions have been stabilized by (M(C(5)(CH(3))(5))(2))(+), where M is iron or cobalt. This has allowed the structural and spectroscopic characterization of these dianions. The structure of (M(C(5)(CH(3))(5))(2))(2)(+) (DDQ)(2-), where DDQ is 2,3-dichloro-5,6-dicyanobenzoquinone, has been determined by x-ray crystallography. The structure of (DDQ)(2-)is consistent with ab initio molecular orbital calculations that suggest a localizd as opposed to a delocalized (aromatic) ring structure.     

Electron solvation in finite systems: femtosecond dynamics of iodide. (Water)n anion clusters

Electron solvation dynamics in photoexcited anion clusters of I-(D2O)n=4-6 and I-(H2O)4-6 were probed by using femtosecond photoelectron spectroscopy (FPES). An ultrafast pump pulse excited the anion to the cluster analog of the charge-transfer-to-solvent state seen for I- in aqueous solution. Evolution of this state was monitored by time-resolved photoelectron spectroscopy using an ultrafast probe pulse. The excited n = 4 clusters showed simple population decay, but in the n = 5 and 6 clusters the solvent molecules rearranged to stabilize and localize the excess electron, showing characteristics associated with electron solvation dynamics in bulk water. Comparison of the FPES of I-(D2O)n with I-(H2O)n indicates more rapid solvation in the H2O clusters.     

An Inorganic Double Helix: Hydrothermal Synthesis, Structure, and Magnetism of Chiral [(CH3)2NH2]K4[V10O10(H2O)2(OH)4(PO4)7]{middle dot}4H2O

Very complicated inorganic solids can be self-assembled from structurally simple precursors as illustrated by the hydrothermal synthesis of the vanadium phosphate, [(CH(3))(2)NH(2)]K(4)[V(10)O(10)(H(2)O)(2)(OH)(4)(PO(4))(7)].4H(2)O, 1, which contains chiral double helices formed from interpenetrating spirals of vanadium oxo pentamers bonded together by P(5+). These double helices are in turn intertwined with each other in a manner that generates unusual tunnels and cavities that are filled with (CH(3))(2)NH(2)(+) and K(+) cations, respectively. The unit cell contents of dark blue phosphate 1, which crystallizes in the enantiomorphic space group P4(3) with lattice constants a = 12.130 and c = 30.555 angstroms, are chiral; only one enantiomorph is present in a given crystal. Magnetization measurements show that 1 is paramagnetic with ten unpaired electrons per formula unit at higher temperatures and that antiferromagnetic interactions develop at lower temperatures.     

Understanding the infrared spectrum of bare CH5+

Protonated methane, CH5+, continues to elude definitive structural assignment, as large-amplitude vibrations and hydrogen scrambling challenge both theory and experiment. Here, the infrared spectrum of bare CH5+ is presented, as detected by reaction with carbon dioxide gas after resonant excitation by the free electron laser at the FELIX facility in the Netherlands. Comparison of the experimental spectrum at approximately 110 kelvin to finite-temperature infrared spectra, calculated by ab initio molecular dynamics, supports fluxionality of bare CH5+ under experimental conditions and provides a dynamical mechanism for exchange of hydrogens between CH3 tripod positions and the three-center bonded H2 moiety, which eventually leads to full hydrogen scrambling. The possibility of artificially freezing out scrambling and internal rotation in the simulations allowed assignment of the infrared spectrum despite this pronounced fluxionality.     

Partial control of an ion-molecule reaction by selection of the internal motion of the polyatomic reagent ion

The ion-molecule reaction NH(3)(+) + ND(3) has been studied at various collision energies (1 to 5 electron volts in the center of mass) with preparation of the NH(3)(+) reagent in two nearly isoenergetic vibrational states. One state corresponds to pure out-of-plane bending of the planar NH(3)(+) ion (0.60 electron volts), whereas the other state is a combination of in-plane and out-of-plane motion (0.63 electron volts). The product branching ratios differ markedly for these two vibrational-state preparations. The differences in reactivity suggest that the in-plane totally symmetric stretching mode is essentially inactive in controlling the branching ratio of this reaction.     

Spectroscopic determination of the OH- solvation shell in the OH-.(H2O)n clusters

There has been long-standing uncertainty about the number of water molecules in the primary coordination environment of the OH- and F- ions in aqueous chemistry. We report the vibrational spectra of the OH-.(H2O)n and F-.(H2O)n clusters and interpret the pattern of OH stretching fundamentals with ab initio calculations. The spectra of the cold complexes are obtained by first attaching weakly bound argon atoms to the clusters and then monitoring the photoinduced evaporation of these atoms when an infrared laser is tuned to a vibrational resonance. The small clusters (n 相似文献   

Synthesis and Characterization of Molybdenum Carbide Clusters MonC4n, (n = 1 to 4)

Laser radiation (XeCl laser, 308-nanometer wavelength) focused into a cell containing Mo(CO)(6) vapor produced ultrafine particles in the extended waist of the laser beam. Negative ion mass spectrometry revealed molybdenum carbide cluster ions with a stoichiometry MonC4n (n = 1 to 4). The MonC4n(-) (n = 2 to 4) ions are completely unreactive with NH(3), H(2)O, and O(2), suggesting structures in which the molybdenum atoms are unavailable for coordination to additional ligands. Collision-induced dissociation studies of these anions show the loss of MoC(4) units as the main fragmentation pathway. This observation, together with the lack of addition reactions, provides a basis for structures in which a planar cluster of two, three, or four molybdenum atoms is surrounded by, and bonded to, carbon dimers.     

Size-Specific Infrared Spectra of Benzene-(H2O)n Clusters (n = 1 through 7): Evidence for Noncyclic (H2O)n Structures

Resonant ion-dip infrared spectroscopy has been used to record size-specific infrared spectra of C(6)H(6)-(H(2)O)n clusters with n = 1 through 7 in the O-H stretch region. The O-H stretch spectra show a dramatic dependence on cluster size. For the n = 3 to 5 clusters, the transitions can be divided into three types-attributable to free, pi hydrogen-bonded, and single donor water-water O-H stretches-consistent with a C(6)H(6)-(H(2)O)n structure in which benzene is on the surface of a cyclic (H(2)O)n cluster. In n = 6 and 7 clusters, the spectra show distinct new transitions in the 3500 to 3600 wave number region. After comparison of these results with the predictions of ab initio calculations on (H(2)O)n clusters, these new transitions have been assigned to double donor O-H stretches associated with the formation of a more compact, noncyclic structure beginning with (H(2)O)(6). This is the same size cluster for which ab initio calculations predict that a changeover to noncyclic (H(2)O)n structures will occur.     

Electrons in finite-sized water cavities: hydration dynamics observed in real time

We directly observed the hydration dynamics of an excess electron in the finite-sized water clusters of (H2O)n- with n = 15, 20, 25, 30, and 35. We initiated the solvent motion by exciting the hydrated electron in the cluster. By resolving the binding energy of the excess electron in real time with femtosecond resolution, we captured the ultrafast dynamics of the electron in the presolvated ("wet") and hydrated states and obtained, as a function of cluster size, the subsequent relaxation times. The solvation time (300 femtoseconds) after the internal conversion [140 femtoseconds for (H2O)35-] was similar to that of bulk water, indicating the dominant role of the local water structure in the dynamics of hydration. In contrast, the relaxation in other nuclear coordinates was on a much longer time scale (2 to 10 picoseconds) and depended critically on cluster size.     

Isomers of Small Carbon Cluster Anions: Linear Chains with up to 20 Atoms

The structure of small carbon cluster anions, Cn(-) (4 相似文献   

小分子气体在聚叔丁基乙炔中扩散溶解行为的分子动力学模拟

采用分子动力学和巨正则Monte Carlo(GCMC)法研究了小分子(H2,O2,CO2,N2,CH4)在聚叔丁基乙炔中的扩散和溶解行为,应用自由体积理论探讨了小分子在聚合物内的扩散机理.结果表明,分子越小,其在聚合物内运动范围越大,扩散系数越大.模拟所得的扩散和溶解系数与实验值吻合较好,同时验证了扩散系数与分子有效直径的关系.计算所得渗透系数,与文献报道的实验数据在同一数量级上,说明分子动力学和蒙特卡洛模拟是研究小分子在聚合物中扩散溶解行为的有效方法.     

A SN2 reaction that avoids its deep potential energy minimum

Chemical dynamics trajectory simulations were used to study the atomic-level mechanisms of the OH- + CH3F --> CH3OH + F- SN2 nucleophilic substitution reaction. The reaction dynamics, from the [OH...CH3...F]- central barrier to the reaction products, are simulated by ab initio direct dynamics. The reaction's potential energy surface has a deep minimum in the product exit channel arising from the CH3OH...F- hydrogen-bonded complex. Statistical theories of unimolecular reaction rates assume that the reactive system becomes trapped in this minimum and forms an intermediate, with random redistribution of its vibrational energy, but the majority of the trajectories (90%) avoided this potential energy minimum and instead dissociated directly to products. This finding is discussed in terms of intramolecular vibrational energy redistribution (IVR) and the relation between IVR and molecular structure. The finding of this study may be applicable to other reactive systems where there is a hierarchy of time scales for intramolecular motions and thus inefficient IVR.     

Ultrafast x-ray diffraction of transient molecular structures in solution

We report direct structural evidence of the bridged radical (CH2ICH2.) in a polar solution, obtained using time-resolved liquid-phase x-ray diffraction. This transient intermediate has long been hypothesized to explain stereo-chemical control in many association and/or dissociation reactions involving haloalkanes. Ultrashort optical pulses were used to dissociate an iodine atom from the haloethane molecule (C2H4I2) dissolved in methanol, and the diffraction of picosecond x-ray pulses from a synchrotron supports the following structural dynamics, with approximately 0.01 angstrom spatial resolution and approximately 100 picosecond time resolution: The loss of one iodine atom from C2H4I2 leads to the C-I-C triangular geometry of CH2ICH2.. This transient C2H4I then binds to an iodine atom to form a new species, the C2H4I-I isomer, which eventually decays into C2H4 + I2. Solvent dynamics were also extracted from the data, revealing a change in the solvent cage geometry, heating, and thermal expansion.     

东北季节性冻融农田土壤CO2、CH4、N2O通量特征研究

为了评估季节性冻融交替对土壤温室气体排放的影响,采用静态暗箱-气相色谱法,监测了东北松嫩平原两种典型农田生态系统(稻田和玉米田)非生长季土壤CO_2、CH_4和N_2O通量变化。研究表明:三种温室气体排放在土壤冻结期、覆雪期、融雪期和解冻期具有明显的季节动态特征。冻结期和融雪期对温室气体排放贡献最大,这两个时期内稻田和玉米田CO_2排放量分别占非生长季总累积排放量的74.9%和68.6%,稻田CH_4排放占非生长季总排放的95.7%,尽管玉米田土壤CH_4以吸收为主,但在融雪过程中存在明显释放峰,短暂的融雪期内N_2O呈集中爆发性释放,稻田和玉米田N_2O通量峰值分别是冻结前的40倍和99倍,排放量占到总累积排放量的73.9%和80.7%,覆雪期土壤CH_4和N_2O存在弱的吸收。另外,土壤温室气体排放存在土地利用方式间的差异,表现在稻田土壤比玉米田(非生长季)具有更高的温室气体排放潜力。稻田土壤CO_2、CH_4和N_2O累积排放量均高于玉米田,表现为净排放(源),而玉米田土壤CH_4通量表现为净吸收(汇);稻田土壤CO_2和CH_4平均排放速率显著高于玉米田;除覆雪期外,其他时期内三种温室气体平均通量在两类农田之间也存在显著差异。总之,在评价季节性冻土区温室气体排放时需要重视土壤冻结和融化过程,同时需要考虑不同土地利用方式间的差异。     

Rotation and solvation of ammonium ion

From nitrogen-15 spin-lattice relaxation times and nuclear Overhauser enhancements, the rotational correlation time (c) for (15)NH(4)(+) was determined in a series of solvents. Values of (c) range from 0.46 to 20 picoseconds. The solvent dependence of (c) cannot be explained in terms of solvent polarity, molecular dipole moment, solvent basicity, solvent dielectric relaxation, or solvent viscosity. The rapid rotation and the variation with solvent can be accounted for by a model that involves hydrogen bonding of an NH proton to more than one solvent molecule in a disordered solvation environment.     

Synthesis of inorganic fullerene-like molecules

The reaction of [Cp*Fe(eta5-P5)] with Cu(I)Cl in solvent mixtures of CH2Cl2/CH3CN leads to the formation of entirely inorganic fullerene-like molecules of the formula [[Cp*Fe(eta5:eta1:eta1:eta1:eta1:eta1-P5)]12[CuCl]10[Cu2Cl3]5[Cu(CH3CN)2]5] (1) possessing 90 inorganic core atoms. This compound represents a structural motif similar to that of C60: cyclo-P5 rings of [Cp*Fe(eta5-P5)] molecules are surrounded by six-membered P4Cu2 rings that result from the coordination of each of the phosphorus lone pairs to CuCl metal centers, which are further coordinated by P atoms of other cyclo-P5 rings. Thus, five- and six-membered rings alternate in a manner comparable to that observed in the fullerene molecules. The so-formed half shells are joined by [Cu2Cl3]- as well as by [Cu(CH3CN)2]+ units. The spherical body has an inside diameter of 1.25 nanometers and an outside diameter of 2.13 nanometers, which is about three times as large as that of C60.     

Escape of hydrogen from venus

Recombination of O(2)(+) represents a source of fast oxygen atoms in Venus' exosphere, and subsequent collisions of oxygen atoms with hydrogen atoms lead to escape of about 10(7) hydrogen atoms per square centimeter per second. Escape of deuterium atoms is negligible, and the ratio of deuterium to hydrogen should increase with time. It is suggested that the mass-2 ion observed by Pioneer Venus is D(+), which implies a ratio of deuterium to hydrogen in the contemporary atmosphere of about 10(-2), an initial ratio of 5 x 10(-5) and an original H(2)O abundance not less than 800 grams per square centimeter.     

杨树2n花粉形态判别方法的比较

采用醋酸洋红染色法,在研究胡杨和毛白杨不同无性系间花粉粒形态差异的基础上,研究不同形态判别标准条件下2n花粉发生频率调查的准确性。研究结果表明,胡杨花粉粒的平均直径为29.7μm,比毛白杨花粉粒的平均直径32.4μm略小。胡杨或毛白杨无性系对花粉粒直径影响显著。通过调查四分体时期四分体、三分体以及二分体的比例,可估算2n花粉的理论频率。胡杨和毛白杨各无性系2n花粉的理论频率与以2n花粉粒直径大于花粉粒平均直径的1.28倍为标准时统计的2n花粉实际频率差异不显著,而与以2n花粉粒直径大于37.0μm为标准时统计的2n花粉实际频率差异极显著。这说明以37.0μm作为判别胡杨或毛白杨2n花粉形态判别标准会导致较大的误差。而以各无性系花粉粒平均直径的1.28倍作为2n花粉形态判别标准更加科学合理。     

杀菌剂离体生物测定的稳定性探讨

研究了药剂稀释方法、药液保存时间、溶解药剂的溶剂对杀菌剂平板法生物测定稳定性的影响.结果表明:药剂以2/3的比例系列稀释时多次测定标准差最小,表现最稳定.戊唑醇溶解于丙酮冰箱保存13 d,对水稻纹枯病菌的活性影响不大,说明这种药剂在溶剂中较稳定.福美双用丙酮溶解后降解速度较快,降解方程为y=0.301 6x+2.167 8,r=0.943 2.戊唑醇溶于丙酮、二甲基甲酰胺、氯仿、二氯甲烷、甲苯等多种溶剂,对水稻纹枯病菌的活性影响不大,多菌灵溶于盐酸、甲酸、乙酸、硫酸后,使水稻纹枯病菌的活性产生较大差异,这可能是多菌灵与这些溶剂形成不同的轭合物后导致活性的变化.