Conductance of tetraalkylammonium salts in 2-butanone from −45 to 25°C

Interactions of three types of tetraalkylammonium cations (tetrapropyltetrabutyl-and tri-isoamylbutyl- ammonium) with perchlorate and tetraphenylborate anions were studied by the conductivity method in 2-butanone from –45°C to 25°C. Conductance data obtained for diluted solutions (5×10^–5 – 2×10^–3 mol-dm^–3) were used to calculate the limiting molar conductivities and associationconstants. The conductance equation of Fuoss-Hsia including the Chen term and the chemical model assumption were applied. Limiting ion conductivities were calculated assuming equal limiting conductivities of the i-Am₃BuN⁺ and BPh ₄ ^– ions at all temperatures. Gibbs energies and entropies of ion pair formation, calculated from the dependence of association constants on temperature, are presented including the contributions due to short-range forces.     

水+3-甲基吡啶+溴化钠三元溶液临界跨接现象

采用折射率法在较宽温度范围内研究了溴化钠、3-甲基吡啶和水的临界质量分数分别为0.240、0.192和0.568体系的临界性质, 发现在近临界点临界指数为0.365, 与Fishe重整化值一致. 有效临界指数随着温度逐渐远离临界点, 从0.365下降到0.20左右, 但当排除了“正规项”的影响后, 展现出向平均场理论值0.5单调跨接的行为.     

压电石英晶体在电解质水溶液中的振荡性能研究(Ⅱ)——频率、频移特性

根据两面电极浸入液体中的压电晶体等效电路模型和Pierce振荡器振荡方程的解,得到晶体在电解质溶液中的振荡频率(F_1)与溶液电导率(x)的关系式:F_1=(1/2π)(1/C_1+1/C_2)(1/X_e)[(R_e~2+X_e~2)x~2+2R_ex+1]。以自行设计的Pierce晶体振荡器。研究了晶体和主振电路参数、输入电压及检测池外壳屏蔽接地对AT切9MHz石英晶体在KCl、Na_2SO_4水溶液中的频率、相对于纯水频移随溶液电导率变化的影响规律,结果与上式相符。     

X-H (X = C, N, O, Si, P, S) 键离解能计算的密度泛函方法研究

使用了不同密度泛函方法计算X-H (X = C, N, O, Si, P, S) 键离解能，并分析不同密度泛函方法的计算精度。研究发现大多数密度泛函方法包括B3LYP, B3P86, B3PW91, G96LYP, PBE1PBE，和BH&HLYP都明显低估键离解能13-25 kJ/mol。该现象与是否使用无限基组无关，因为即使使用无限基组键离解能仍然被低估。因此密度泛函方法不适合用于键离解能的估算。其中B3P86方法的偏差最小。进一步分析表明，使用限制性开壳层计算并无任何优势，在大多数情况下非限制性开壳层计算实际上比限制性开壳层计算要好。最后，我们发现了密度泛函方法对键离解能的低估是系统的，因此建议利用校准后的UDFT/6-311++G(d, p)方法计算化学键离解能。     

周期性密度泛函理论研究氯气在CuCl(111)表面上的吸附与解离

运用广义梯度密度泛函理论(Generalized Gradient Approximation，GGA)的PBE(Perdew-Burke-Ernzerh)方法结合周期性平板模型，研究了氯气分子和氯原子在CuCl(111)表面上的吸附。通过对不同吸附位和不同单层覆盖度下的吸附能和几何构型参数的计算和比较发现：氯气分子在CuCl(111)表面的吸附为解离吸附；单层覆盖度为0.50时的吸附构型为稳定的吸附构型；氯气分子平行吸附在CuCl(111)表面时最稳定，吸附能最大，达364.5 kJ·mol^-1；伸缩振动频率的计算结果表明，吸附后的氯气分子的伸缩振动频率与自由氯气分子的伸缩振动频率相比，都发生了红移；布居分析结果表明整个吸附体系发生了由Cu原子向氯气分子的电荷转移。氯原子吸附的计算结果显示氯原子以穴位稳定的吸附在CuCl(111)表面。     

Ab initio MO study of potential energy surface of NH2 with CN reaction

An extensive quantum chemical study of the potential energy surfaces (PES) for the association reaction of NH₂ with CN and the subsequent isomerization and dissociation reactions has been carried out using density functional theory (DFT)/B3LYP/6‐311++G(3df,2p) level of theory on both singlet and triplet states. The reaction mechanism on the triplet surface is more complicated than that on the singlet surface. A total of 19 isomers and 46 transition states have been identified and characterized on the triplet PES. Among them, IM2 (IM2a), IM3 (IM3a, IM3b), and IM10 are the lowest‐lying isomers with thermodynamic stability. Twenty available dissociation channels, depending on the different initial isomers, have been identified. On the singlet surface, only 12 isomers and 16 transition states have been found, and among them IM1(S) and IM2(S) are the lowest‐lying isomers. The higher isomerization and dissociation barriers on the singlet surface indicate that the addition and the subsequent reactions of NH₂+CN are most likely to occur on the triplet PES because of the lower barriers. A prediction can be made for the possible mechanism explaining the production of H+HNCN. Besides HNCN, other major products are NH+HCN and NH+HNC, which are produced by direct dissociation reactions from triplet IM2 and IM3, respectively. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Chemical mass shifts in resonance ejection experiments in the quadrupole ion trap

Chemical mass shifts were measured in a Paul ion trap operated in the mass-selective instability scan with resonance ejection using a custom-built instrument. These shifts, which can be as much as 2%, decrease with increasing endcap electrode separation owing to changes in the higher order contributions to the electric field. They also decrease with decreasing helium buffer gas pressure. Both of these effects are analogous to those found with boundary ejection. This suggests that the previously proposed chemical mass shift mechanism based on compound-dependent collisional modification of the ejection delay produced by field faults near the endcap electrode apertures holds true also for resonance ejection. The influence of the resonance frequency on chemical mass shifts was also investigated and it is shown that at certain working points (values of the Mathieu parameter q(z) and a(z)) non-linear resonances greatly reduce the ejection delay for all ions, regardless of their chemical structures, and thus reduce the magnitude of the chemical mass shift. Energetic collisions leading to dissociation can take place at an earlier stage during the ejection process in the mass analysis scan when using resonance ejection compared with boundary ejection. This leads to even larger chemical mass shifts of fragile ions in resonance ejection. Increasing the resonance voltage amplitude can enhance this effect. The chemical mass shifts of fragile ions increase with increase in the resonance voltage amplitude, whereas negligible changes occur for structurally stable ions.     

Agreement between experiment and hybrid DFT calculations for O--H bond dissociation enthalpies in manganese complexes

Information on the accuracy of DFT functionals for redox reactions in transition metal systems is rather limited. To analyze the performance of some popular functionals for redox reactions in manganese systems, calculated O--H bond dissociation enthalpies for Mn-ligands in six different complexes are compared to experimental results. In this benchmark, B3LYP performs well with a mean absolute error of 3.0 kcal/mol. B98 gives similar results to B3LYP (error of 3.8 kcal/mol). B3LYP* gives lower O--H bond strengths than B3LYP and has a mean error of 5.0 kcal/mol. Compared to B98 and B3LYP, B3LYP* has an error trend for the manganese ligands that is more similar to the error for a free water molecule. The nonhybrid functional BLYP consistently and significantly underestimates the O--H bond strengths by approximately 20 kcal/mol. HCTH407 has a rather large mean error of 9.4 kcal/mol and shows no consistent trend. The results support the use of hybrid functionals and the present computational method for large model systems containing manganese. An example is the oxygen evolving complex in photosystem II where hybrid functionals predict the appearance of a Mn(IV)-oxyl radical before the O--O bond formation step.     

Can semi-empirical models describe HCl dissociation in water?

We discuss the failure of commonly used AM1 and PM3 semiempirical methods to correctly describe acid dissociation. We focus our analysis on HCl because of its physicochemical importance and its relevance in atmospheric chemistry. The structure of non-dissociated and dissociated HCl – (H₂O) _n clusters is accounted for. The very bad results obtained with PM3 (and also with AM1) are related to large errors in gas-phase proton affinity of water and gas-phase acidity of HCl. Indeed, estimation of pKa values shows that neither AM1 nor PM3 are able to predict HCl dissociation in liquid water since HCl is found to be a weaker acid than H₃O⁺. We have proposed in previous works a modified PM3 approach (PM3-MAIS) adapted to intermolecular calculations. It is derived from PM3 by reparameterization of the core–core functions using ab initio data. Since parameters for H–Cl and O–Cl core–core interactions were not yet available, we have carried out the corresponding optimization. Application of the PM3-MAIS method to HCl dissociation in HCl–(H₂O) _n clusters leads to a huge improvement over PM3 results. Though the method predicts a slightly overestimated HCl acidity in water environment, the overall agreement with ab initio calculations is very satisfying and justifies efforts to develop new semiempirical methods.     

The dissociation quotients of formic acid in sodium chloride solutions to 200°C

The dissociation quotients of formic acid were measured potentiometrically from 25 to 200°C in NaCl solutions at ionic strengths of 0.1, 0.3 1.0, 3.0, and 5.0 mol-kg^–1. The experiments were carried out in a concentration cell with hydrogen electrodes. The resulting molal acid dissociation quotients for formic acid, as well as a set of infinite dilution literature values and a calorimetrically-determined enthalpy of reaction, were fitted by an empirical equation involving an extended Debye Hückel term and seven adjustable parameters involving functions of temperature and ionic strength. This regressional analysis yielded the following thermodynamic quantities for 25°C: logK=–3.755±0.002, H^o=–0.09±0.15 kJ-mol^–1, S^o=–72.2±0.5 J-K^–1-mol^–1, and C _p ^o =–147±4 J-K^–1-mol^–1. The isocoulombic form of the equilibrium constant is recommended for extrapolation to higher temperatures.