Coordination mode of nitrate in uranyl(VI) complexes: a first-principles molecular dynamics study

According to Car-Parrinello molecular dynamics simulations for [UO(2)(NO(3))(3)](-), [UO(2)(NO(3))(4)](2-), and [UO(2)(OH(2))(4-)(NO(3))](+) complexes in the gas phase and in aqueous solution, the nitrate coordination mode to uranyl depends on the interplay between ligand-metal attractions, interligand repulsions, and solvation. In the trinitrate, the eta(2)-coordination is clearly favored in water and in the gas phase, leading to a coordination number (CN) of 6. According to pointwise thermodynamic integration involving constrained molecular dynamics simulations, a change in free energy of +6 kcal/mol is predicted for eta(2)- to eta(1)-transition of one of the three nitrate ligands in the gas phase. In the gas phase, the mononitrate-hydrate complex also prefers a eta(2)-binding mode but with a CN of 5, one H(2)O molecule being in the second shell. This contrasts with the aqueous solution where the nitrate binds in a eta(1)-fashion and uranyl coordinates to four H2O ligands. A driving force of ca. -3 kcal/mol is predicted for the eta(2)- to eta(1)- transition in water. This structural preference is interpreted in terms of steric arguments and differential solvation of terminal vs uranyl-coordinated O atoms of the nitrate ligands. The [UO(2)(NO(3))(4)](2-) complex with two eta(2)- and two eta(1)- coordinated nitrates, observed in the solid state, is stable for 1-2 ps in the gas phase and in solution. In the studied series, the modulation of uranyl-ligand distances upon immersion of the complex in water is found to depend on the nature of the ligand and the composition of the complex.     

Development of equations for differential and integral enthalpy change of adsorption for simulation studies

We present equations to calculate the differential and integral enthalpy changes of adsorption for their use in Monte Carlo simulation. Adsorption of a system of N molecules, subject to an external potential energy, is viewed as one of transferring these molecules from a reference gas phase (state 1) to the adsorption system (state 2) at the same temperature and equilibrium pressure (same chemical potential). The excess amount adsorbed is the difference between N and the hypothetical amount of gas occupying the accessible volume of the system at the same density as the reference gas. The enthalpy change is a state function, which is defined as the difference between the enthalpies of state 2 and state 1, and the isosteric heat is defined as the negative of the derivative of this enthalpy change with respect to the excess amount of adsorption. It is suitable to determine how the system behaves for a differential increment in the excess phase adsorbed under subcritical conditions. For supercritical conditions, use of the integral enthalpy of adsorption per particle is recommended since the isosteric heat becomes infinite at the maximum excess concentration. With these unambiguous definitions we derive equations which are applicable for a general case of adsorption and demonstrate how they can be used in a Monte Carlo simulation. We apply the new equations to argon adsorption at various temperatures on a graphite surface to illustrate the need to use the correct equation to describe isosteric heat of adsorption.     

Unique tautomeric properties of isoguanine

The tautomeric properties of isoguanine (also named 2-oxoadenine or 2-hydroxyadenine) have been studied in the gas phase, in different pure solvents, and in the DNA environment using state of the art theoretical methods. Our results show that isoguanine constitutes an unique example of how tautomerism can be modulated by the environment. Compared to the tautomeric preference in the gas phase, both polar solvents and the DNA microenvironment dramatically change the intrinsic tautomeric properties of isoguanine. Tautomers which are important in physiological conditions are less than 1/10(5) of the total population of isoguanine in the gas phase. The impact of the present findings in the understanding of spontaneous mutations and in the design of new nucleobases with multiple recognition properties is discussed.     

Influence of solvent on propagation in cationic polymerization

The influence of solvent on cationic propagation steps-modelled by ethene homopolymerization—is investigated by means of theoretical models. In order to achieve cationation and the first three propagation steps, the influence of solvent was simulated by a continuum model after calculation for the gas phase at the MINDO/3 level. Characteristic changes, caused by the transition from the gas phase to solution (solvent: CH₂Cl₂), were obtained, e.g. an increase of activation barriers and specific alterations of heats of reaction depending on the cationic chain length. The shape of a special potential energy surface for the first propagation step is qualitatively affected by the solvent leading to a change of the character of the activated complex from educt-like (gas phase) to product-like (solution).     

Mechanistic study on flame retardance of polycarbonate with a small amount of potassium perfluorobutane sulfonate by TGA-FTIR/XPS

The work focuses on a mechanistic study of the title system with the aid of both TGA-FTIR (in the gas phase) and XPS (in the condensed phase). It is concluded that: (1) although the change in yield of char for PC/PPFBS is insignificant with respect to PC itself it appears that the majority of the crucial FR activity is in the condensed phase as indicated by the XPS data; (2) TGA-FTIR experiments support the view that the degradation of polycarbonate proceeds by the cleavage of the C_α-O and the C_β-H bonds, leading to the formation of dienes, diols and carbon dioxide; (3) the reverse reaction of the intermediates, like phenols, may occur via the basic catalyst, possibly K-alkyl oxide, a weaker base is generated in the process; (4) voluminous evolution of CO₂ accelerated by PPFBS may give flame retardance by way of intumescent action and its dilution into the gas phase.     

Synthesis and Characterization of TS-1 with Aid of Supercritical CO2

A new route to synthesize TS-1 has been developed using the supercritical carbon dioxide（SCCO2） as a crystallization-assistant agent. SCCO2 plays a dual role： as a reagent changing the alkalinity during the crystallization process and as a medium eliminating mass-transfer limitations（both within the bulk fluid and through liquid/gas, solid/gas or solid/liquid phase boundaries）. In this route, it was shown that the Ti content in TS-I increase compared with that in the TS-1 prepared without SCCO2, but decrease while the SCCO2 pressure increase. The prepared crystal morphology also underwent significant change. The crystallization time of TS-1 can be shorten a lot.     

2',3'-二脱氧-2',3'-二去氢鸟嘌呤核苷构象稳定性的理论研究

采用密度泛函方法在B3LYP/6-31＋G**水平上研究了2',3'-二脱氧-2',3'-二去氢鸟嘌呤核苷分子(D4G)的构象. 分别研究在气相中的孤立分子和一水合物异构体的相对稳定性和异构体之间的相互转变过程, 分析了水分子的参与对D4G异构体的相对稳定性和几何结构参数以及自然电荷的影响. 结果表明, 孤立的D4G分子在气相中存在8种稳定构象, 其中构象d4g-2是所有构象中最稳定的, 气相中D4G主要以d4g-2存在. 气相中各构象的相对稳定性为: d4g-2＞d4g-1＞d4g-5＞d4g-3＞d4g-6＞d4g-4＞d4g-8＞d4g-7. 计算得到的各构象键长和键角数据与实验值接近. 一个水分子的加入对D4G分子的构型参数有所影响, 基本不改变D4G分子各构象的稳定性顺序, 但构象转变的能垒有所提高. 氢键在分子构象中发挥了重要作用.     

2',3'-二脱氧-2',3'-二去氢鸟嘌呤核苷构象稳定性的理论研究

采用密度泛函方法在B3LYP/6-31＋G**水平上研究了2',3'-二脱氧-2',3'-二去氢鸟嘌呤核苷分子(D4G)的构象. 分别研究在气相中的孤立分子和一水合物异构体的相对稳定性和异构体之间的相互转变过程, 分析了水分子的参与对D4G异构体的相对稳定性和几何结构参数以及自然电荷的影响. 结果表明, 孤立的D4G分子在气相中存在8种稳定构象, 其中构象d4g-2是所有构象中最稳定的, 气相中D4G主要以d4g-2存在. 气相中各构象的相对稳定性为: d4g-2＞d4g-1＞d4g-5＞d4g-3＞d4g-6＞d4g-4＞d4g-8＞d4g-7. 计算得到的各构象键长和键角数据与实验值接近. 一个水分子的加入对D4G分子的构型参数有所影响, 基本不改变D4G分子各构象的稳定性顺序, 但构象转变的能垒有所提高. 氢键在分子构象中发挥了重要作用.     

Reactivity of molecular oxygen on the surface of ionic crystals

A possibility of multiplicity change for ground‐state molecular oxygen adsorbed on the surface of regular and doped broad‐gap ionic crystals was considered in the framework of cluster approximation by using SCF MO LCAO quantum chemical methods [semiempirical INDO approximation and ab initio calculation with the 6‐311G** basis set taking into account the correlation effects on the level of second‐order Meller–Plesset perturbation theory (MP2)]. The formation energetics of cyclic products of addition reactions of dioxygen in different multiplet states to furan and cis‐butadiene in the gas phase and on the surface of ionic crystals was considered. (These reactions are typical for the O₂ singlet state in the gas phase.) It is shown that the presence of sites with high effective charge on the crystal surface can result in a situation not requiring, as in the gas phase, multiplicity change in the transition of a system from an initial to the final state, which can significantly affect the kinetic parameters of the reactions. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

金属离子与腺嘌呤异构体复合物的稳定性

在B3LYP/6-311++G**//6-31+G*基组水平上结合PCM模型系统的优化了气相和液相环境中14种腺嘌呤异构体与四种金属离子(Na+,K+,Mg2+,Ca2+)形成的132个稳定复合物。通过能量对比,得到了所有复合物异构体在气液两相中的稳定性顺序及不同金属离子与同一异构体形成的复合物的能量排序,首次给出了液相中这些不同金属离子复合物的最稳定结构。结果发现溶剂效应导致了液相中的复合物稳定性顺序与气相中的相比发生了很大变化;同一异构体与不同金属离子形成的复合物其稳定性在排序中也变化很大。对于这些变化,本文分别从金属离子与腺嘌呤的复合物在气相中的结合能(EBE)及在液相中的溶质溶剂相互作用能(Epol)能等方面进行了系统的阐述     

Analysis of complex samples by solvating gas chromatography (supercritical fluid to gas transition)

The various forms of chromatography are primarily determined by differences in the physical state of the mobile phases. The main chromatographic categories include gas chromatography (GC), liquid chromatography, and supercritical fluid chromatography. Adjusting a temperature and pressure will change the mobile phase from liquid to supercritical fluid to gas, with concomitant changes in their physical properties. In this paper, the technique transition-phase chromatography (TPC) is described. In TPC, different mobile phase conditions exist inside the column. This phase transformation within the column results in huge differences in density, solvating power, viscosity, diffusivity, and, as a consequence, in the chromatographic properties of the mobile phase. TPC experiments using capillary columns packed in our laboratory have shown that when the mobile phase is transformed from supercritical fluid to gas, high column efficiencies can be achieved. The transition from supercritical fluid to gas (also called solvating GC), a particular case of the TPC, is evaluated for the separation of complex real samples (environmental, food, and fuels).     

Comparison of Chirasil-DEX CB as gas chromatographic and ULMO as liquid chromatographic chiral stationary phase for enantioseparation of aryl- and heteroarylcarbinols

For a broad spectrum of simple chiral alcohols, incorporating a (substituted) (het)aryl building block, enantiomer separation characteristics are reported for both gas chromatography on a Chirasil-DEX phase, and liquid chromatography on an (S,S)-ULMO phase. On this chiral Pirkle-type phase, homochiral enantiomers (mostly R) are eluted first without exception. The elution order R before S appears conserved as a rule also for gas chromatographic separations on Chirasil-DEX, though with some remarkable exceptions indicating a change in the dominant discriminative mechanism. This was shown in the homologous series 1-phenylethanol to 1-phenylhexanol having the point of reversal at C4, while the o-methoxy analogues elute from C1 to C4 already in the reversed order.     

Radiation yield of stable radiolysis products of 1-chlorobutane, 1-chlorobutene-2, 1-chloropropane, 2-chloropropane and 1,3-dichloropropane γ-irradiated in an oxygen-free atmosphere

The stable radiolysis products of 1-chlorobutane (1-CB), 1-chlorobutene-2 (1-CB-2), 1-chloropropane (1-CP), 2-chloropropane (2-CP) and 1,3-dichloropropane (1,3-DCP) gamma-irradiated in an oxygen-free atmosphere have been investigated. The pure radiolysis products were separated by preparative gas chromatography and identified by NMR and mass spectroscopy as well as qualitative gas chromatography. The compounds formed were determined by potentiometric analysis and quantitative gas chromatography. From 1-CB we have obtained in the gas phase: HCl, H₂, butane; in the liquid phase: 2-chlorobutane, 1,3-dichlorobutane and a mixture of oligomers of the initial compound (dimer and trimer). We have not recorded H₂ in 1-CB-2. The main gaseous products of radiolysis of 1-CP are HCl and H₂. Radiation yield of isomerization was determined. From 2-CP we have obtained in the gas phase: HCl and H₂; in the liquid phase: 2,2-dichloropropane and a mixture of oligomers of the initial compound (dimers and trimers). From 1,3-DCP we have found in the gas phase: HCl and H₂; in the liquid phase: 1-CP, 2-CP, 1,2-DCP and oligomers. Preliminary schemes for the formation of stable products are proposed.     

Binary ionic liquid mixtures as gas chromatography stationary phases for improving the separation selectivity of alcohols and aromatic compounds

Binary mixtures of two ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride (BMIM-Cl) and 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide (BMIM-NTf2), have been studied for the first time as gas chromatographic stationary phases. The two ILs differ only in the nature of the associated anion. The solvation parameter model was used to examine the change of solvation interactions with the IL stationary phase composition. The hydrogen bond basicity increased linearly as the stationary phase was enriched with the BMIM-Cl IL. The retention factor of short-chained alcohols increased by as much as 1100% when performing the separation on a column containing an IL mixture of 25% BMIM-NTf2/75% BMIM-Cl compared to that of the neat BMIM-NTf2 IL column. By tuning the composition of the IL-stationary phase, the separation selectivity and resolution factors of alcohols and aromatic compounds were improved. A reversal of elution order was observed for specific classes of analytes with enhancements in the stationary phase hydrogen bond basicity.     

极化效应指数与脂肪族胺、醇和醚气相碱性的关系

脂肪族胺、醇和醚的气相质子亲合能(PA)与N, O原子所带电荷(qx)以及烷基的极化效应指数(PEI)的关系可表示为其中a、b、c为系数。回归分析结果表明, 上式较好地表达了脂肪族胺、醇和醚的气相碱性变化规律。     

Nonadiabatic decay dynamics of 9H-guanine in aqueous solution

The nonadiabatic decay of the biologically relevant guanine tautomer (9H-guanine) in aqueous solution has been investigated by trajectory surface hopping simulations in a quantum mechanical-molecular mechanical (QM-MM) framework. The QM part (9H-guanine) was treated at the semiempirical OM2/MRCI level, while the MM part (water) was described by the TIP3P force field. The optimized geometries for the relevant minima and conical intersections are qualitatively similar for 9H-guanine in the gas phase and in aqueous solution, while there are notable solvent-induced shifts in the computed vertical excitation energies (up to about 0.4 eV). Overall, the results from the static OM2/MRCI-based calculations are in reasonable agreement with the available ab initio and experimental data. The dynamics simulations show ultrafast nonradiative decay for 9H-guanine in water that is even slightly faster than in the gas phase, with time constants of 20 fs and around 0.3 ps for the S(2)→ S(1) and S(1)→ S(0) internal conversions, respectively. They predict a change in the S(1)→ S(0) decay mechanism when going from the gas phase to aqueous solution: the major pathway for 9H-guanine in water involves a conical intersection with an out-of-plane distortion of the carbonyl oxygen atom, which does not play any significant role in the gas phase, where the decay mainly proceeds via two other conical intersections characterized by ring distortions and out-of-plane displacement of the amino group, respectively. Possible reasons for this change in the mechanism are analyzed.     

Autoinflammation et stabilité thermique de la poly(vinyl-4-pyridine) quaternisée—IV Methode d'étude des mécanismes d'autoinflammation. Application aux cas du bromure de poly(vinyl-4-pyridinium) et du bromure de poly(N(3-propionyl)4-vinylpyridinium)

We have determined the nature of the combustible gas produced from oxidizing pyrolysis of a polymer (the bromide of poly(vinyl-4-pyridinium) and the bromide of poly(N(3-propionyl)4-vinylpyridinium)). The pyrolysis temperature chosen was just below the self ignition temperature and the pyrolysis was performed in air at atmospheric pressure. We analysed the gaseous products by gas chromatography and mass spectrometry. We found a correlation between the change of the self ignition temperature, the change of the composition of the gas phase and the change of the percentage of residue. We assumed that the gas mixture originates the self ignition of the polymer. The composition of the gas mixture depends on the composition of the polymer.     

Can the ebselen derivatives catalyze the isomerization of peroxynitrite to nitrate?

The reaction of ebselen and its derivatives (1-7) with peroxynitrite anion (ONOO(-); PN) has been studied in gas phase and in aqueous, dichloromethane, benzene, and cyclohexane solutions using B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) and PCM-B3LYP/6-311+G(d,p)//B3LYP/6-311G(d,p) approaches, respectively. It was shown that the reaction of 2 (R=H) with PN proceeds via 2 + PN --> 2-PN --> 2-TS1 (O-O activation) --> 2-O(NO(2)(-)()) --> 2-SeO + NO(2)(-) pathway with a rate-determining barrier of 25.3 (14.8) kcal/mol at the NO(2)(-) dissociation step (numbers presented without parentheses are enthalpies, and those in parentheses are Gibbs free energies). The NO(3)(-) formation process, starting from the complex 2-O(NO(2)(-)()), requires by (7.9) kcal/mol more energy than the NO(2)(-) dissociation process and is unlikely to compete with the latter. Thus, in the gas phase, the peroxynitrite --> nitrate isomerization catalyzed by complex 2 is unlikely to occur. It is shown that the NO(3)(-) formation process is slightly more favorably than the NO(2)(-) dissociation process for complex 4, with a strongest electron-withdrawing ligand R=CF(3). Therefore, complex 4 (as well as complex 6 with R=OH) is predicted to be a good catalyst for peroxynitrite <--> nitrite isomerization in the gas phase. Solvent effects (a) change the rate-determining step of the reaction 2 + PN from NO(2)(-) dissociation in the gas phase to O-O activation, which occurs with barriers of (13.9), (8.4), (8.4), and (8.2) kcal/mol in water, dichloromethane, benzene, and cyclohexane, respectively, and (b) significantly reduce the NO(2)(-) dissociation energy, while only slightly destabilizing the NO(3)(-) formation barrier, and make the peroxynitrite <--> nitrate isomerization process practically impossible, even for complex 4.     

Titanium alkoxide complexes: condensed phase and gas phase comparisons

The complex [Ti(2,4-dimethyl-2,4-pentanediolate)2]2 (1) has been synthesized from [Ti(OiPr)4] by transesterification with a stoichiometric amount of 2,4-dimethyl-2,4-pentanediol. We have characterized complex 1 in the solid state by single-crystal X-ray diffraction and in the gas phase by desorption chemical ionization mass spectrometry (DCI-MS). The structural and mass spectrometric data show complex 1 to be stable as a dimer in both the solid and gas phases. The retention of dimeric nuclearity in the gas phase sets complex 1 apart from other simple titanium alkoxide complexes [Ti(OiPr)4] and [Ti(OMe)4]4 that give rise to respective families of molecular ions in the DCI-MS experiment. The highest mass molecular ions for Ti alkoxide complexes in the gas phase may reveal the highest nuclearity that these complexes achieve in condensed phases. According to this interpretation the complex [Ti(OiPr)4] is principally dimeric in the gas phase and probably also in the pure liquid phase and should be represented by the formula [Ti(OiPr)4]2.     

DFT study on the selective complexation of B12N12 nanocage with alkali metal ions

Density functional theory (DFT) calculations were applied at the M05-2X/6-311++G(d,p) level of the theory to investigate the interaction of the B₁₂N₁₂ nanocage (BN) and alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) in the gas phase and in water. On the basis of the results, BN nanocage is able to form a selective complex with Li⁺. Water, as a solvent, reduces the stability of the metal ion-BN complexes in comparison with the gas phase. Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) analyses, reveal that the electrostatic interaction between the BN and metal ions can be considered as the driving force for complex formation in which the role of water is of significance. Density of states (DOSs) analysis of the BN nanocage structure in the presence of different metal ions showed a noticeable change in the frontier orbitals, especially in the gas phase, and Fermi level shifting toward the lower values.