甘氨酸在乙醇—水混合溶剂中离解常数的测定

本文利用电位滴定法测定了２５℃，ＫＣｌ离子强度Ｉ＝０．１ｍｏｌ．Ｌ＾－１时，甘氨酸在纯水及８个乙醇－水混合溶剂中的二级离解常数ｐＫａ，对应的碱离解常数ｐＫｂ和混合溶剂的质子自递常数ｐＫ。实验结果表明：在所研究的浓度范围内，随着混合溶剂中乙醇含量的增加，甘氨酸的酸离解常数及溶剂的质子自递常数逐渐减小，碱离解常数则逐渐增大。     

乙醇—水混合溶剂中次氮基三乙酸离解平衡的研究

次氮基三乙酸(NTA)在非水或混合溶剂中离解常数的测定尚未见报道,本文用电位法测定了乙醇-水混合溶剂中溶解度较小的NTA的离解常数,用标准盐酸溶液返滴定其钾盐可增加NTA浓度,提高测定准确度,测量系统防止CO_2装置和数据处理方法也比文献简单。     

卤代甲烷的键离解能与取代基效应

将芳环上取代基的电子效应参数引入卤代甲烷, 以卤代甲烷分子Y-CH_nX_{3－n (n＝0～3; Y＝H, F, Cl, Br, I; X＝F, Cl, Br, I)中Y-C键的标准键焓 与中心C原子相键连原子的场/诱导效应之和ΣFi、共轭效应之和ΣRi以及诱导偶极之和Σ(α×F)为参数, 建立了一个定量估算卤代甲烷分子中Y-C键离解能(BDE)的通用模型, BDE(Y-C)＝57.5460＋0.8855 －101.0780ΣRi－64.8390ΣFi－10.1034Σ(α×F). 对35个C-H, C-F, C-Cl, C-Br和C-I键回归分析结果表明, 估算Y-C键离解能的精度在实验误差范围内. 对外部数据集的预测结果表明, 该模型具有较高的预测精度, 可用于预测还没被实验测定的卤甲烷中Y-C键离解能. 还对卤代甲烷中104个C-Y键的键离解能进行了预测. 将芳环上取代基效应用于研究饱和体系化学键性能, 有利于深入理解取代基效应对化学键性能的影响.}     

氨基酸在水/DMF混合溶剂中的焓相互作用

氨基酸是组成蛋白质的基本结构单位。溶质－溶质相互作用是氨基酸溶液热力学性质研究的一个主要方面，氨基酸的稀释焓是与之相应的一个重要热力学性质。目前，研究多集中在水溶液中，研究表明具有非极性侧链氨基酸和具有极性侧链氨基酸的偏摩尔焓是完全不同的，随着氨基酸...     

氨基酸在氯乙醇水溶液中的体积性质

利用精密数字密度计测定了甘氨酸、L-丙氨酸和L-丝氨酸在不同质量分数的氯乙醇水溶液中的密度，计算了这些氨基酸在氯乙醇水溶液中的表观摩尔体积、极限偏摩尔体积、迁移偏摩尔体积、理论水化数和体积作用系数，讨论了迁移偏摩尔体积和理论水化数的变化规律. 结果表明，三种氨基酸在氯乙醇水溶液中的迁移偏摩尔体积均为正值, 且随氯乙醇浓度的增大而增大. 氨基酸侧链对迁移偏摩尔体积的贡献与侧链性质密切相关. 三种氨基酸在氯乙醇水溶液中的理论水化数均随溶液中氯乙醇浓度的增加而减小. 甘氨酸与氯乙醇分子间的相互作用主要以1 : 1和1 : 2形式为主， L-丙氨酸和L-丝氨酸与氯乙醇分子间的相互作用主要以1 : 1形式为主.     

取代基效应对褐煤模型化合物离解焓影响的理论研究

为了探究褐煤热解过程中氧桥键C-O均裂这一重要反应, 选取α-O-4和β-O-4类结构单元作为褐煤模型化合物, 运用不同密度泛函计算了部分模型化合物中C-O的离解焓, 并以CBS-QB3作为理论基准值进行比较, 最后选取M05-2X进行离解焓计算. 结果显示, 对于选定的α-O-4和β-O-4类模型化合物, 其平均离解焓分别为51.0 kcal/mol和66.1 kcal/mol. 周围取代环境能显著影响C-O离解焓, 芳环上存在给电子基团(OH, OCH₃和CH₃)能降低C-O离解焓, 而吸电子基团COOH则能增加其离解焓. 然后深层次分析了取代基效应对C-O离解焓的影响. 此外, 分子内氢键的形成对离解焓也有很大的影响. C-O的离解焓与其键长没有特定的相关性, 不能简单的通过C-O键长来预测其离解焓.     

水-乙醇混合溶剂中弱酸电离的溶剂效应

本文用LKB2277Bioactivity monitor微量流动热量计测定卤代乙酸. 脂肪酸在水-乙醇混合溶剂中的标准电离焓,计算了热力学函数.结合文献数据,计算各酸根离子从水迁移到混合溶剂的迁移自由能,并讨论溶剂效应对弱酸电离的影响1.     

烯丙位化学键均裂离解能及其取代基效应

使用复合从头算方法系统地获得了一批精确的烯丙位化学键的均裂裂解能(BDE). 取代基效应的研究表明, C—H与Si—H的BDE表现出差的Hammett关系, 而N—H, O—H, P—H与S—H的BDE表现出好的Hammett关系. 进一步分析表明烯丙位BDE受共轭效应比诱导/场效应的影响更为明显. 并且还讨论了BDE的基态效应和自由基效应, 其结果与最近报道的有关苄位BDE的结果基本一致.     

薁X-H(X=CH2, NH, O)键离解能取代基效应的理论研究

本文利用UB3LYP/6-311++g(2df,2p)//UB3LYP/6-31+g(d)方法，首次就对位取代薁系列化合物(Y-C10H8X-H)的X-H(X=CH2, NH, O)键离解能进行了理论研究。结果发现，除了6-取代-2-甲基薁，取代基对薁X-H键离解能的效应与苯大致相同。然而，由于薁结构中固有偶极矩与不同取代基的电子效应相互作用，对位取代的羟基薁和氨基薁的反应常数( )变化非常显著。利用GE/RE和SIE理论方法，研究发现虽然基态效应在决定净取代基效应的大小中起了一定作用，但取代基效应主要来源于自由基效应的影响。     

L-丝氨酸在乙醇-水混合溶剂中的稀释焓

氨基酸是重要的生物活性物质,是组成蛋白质的基本结构单位．氨基酸的稀释焓是氨基酸溶液热力学性质研究的一个重要方面［１,２］,即溶质 溶质相互作用．目前氨基酸的稀释焓研究大多集中在纯水溶液中．然而,大多数蛋白质的天然环境并不是单纯的水溶液,而是含有许多有机物质的复杂环境．有机溶剂对蛋白质的溶解度、变性行为、解缔成次一级结构和酶的活性等都有很大影响［３］,蛋白质在非水介质环境条件下的热力学性质与其在水溶液环境条件下的性质是大不相同的．由混合溶剂中稀释焓的研究可以获得溶剂介入的溶质分子间的相互作用信息．…     

Correlation of ground and excited state dissociation constants of trans -para- and ortho-substituted cinnamic acids

The ground and excited state dissociation constants oftrans- para- and ortho-substituted cinnamic acids have been determined in 50% (v/v) dioxan-water mixture at 30°C using the Forster cycle. The measured dissociation constants are analysed in the light of single and dual substituent parameter (DSP) equations. Excited state dissociation constants ofp-substituted cinnamic acids correlate well with the exalted substituent constants. The DSP method of analysis shows that resonance effect is predominant relative to inductive effect in the excited state than in the ground state for the para-substituted acids. The single parameter equation gives poor correlation for the ortho-substituted acids. However, the DSP analysis shows fairly good correlation. The inductive effect is predominant relative to the resonance effect in the excited state than in the ground state     

Solvent effects on the dissociation of weak acids of three charge types in water/N-methylacetamide media

In order to elecidate the role of charge type on the change of dissociation constant with increasing polarity of the solvent medium, the pK values and associated thermodynamic quantities for an uncharged acid (acetic acid), a cationic acid (protonated tris(hydroxymethyl)aminomethane), and two ampholytes (MOPS and Tricine) were determined in mixtures of water andN-methylacetamide (NMA). Electromotive-force measurements of cells without liquid junction containing hydrogen electrodes and silver-silver bromide electrodes were used to determine the pK at nine temperatures from 5 to 45°C. The solvent compositions varied from pure water to a mixture containing a mole fraction of NMA of 0.25 (57.5 mass % NMA). The solvent effects are compared with similar data for water/methanol solvents of decreasing polarity. They reflect both the reduction in interionic energy accompanying the elevation in dielectric constant and also the greater protophilic character of NMA as compared with methanol.     

Solvent effects on acid-base behavior: Five uncharged acids in water-sulfolane solvents

An electrometric titration method utilizing glass electrodes and silver-silver bromide electrodes in a cell without liquid junction has been applied to a determination of the dissociation constants of five uncharged weak acids (HA) in four mixtures of water and sulfolane (tetramethylene sulfone) at 25°C. The acids studied were monochloroacetic, formic, benzoic, and acetic acids andp-nitrophenol, and the mole fractions (x ₂) of sulfolane in the mixed solvents were 0.2, 0.4, 0.6, and 0.8. All measurements were made at a constant ionic strength of 0.02 mole-kg^–1 in a solution containing NaBr at a molality of 0.01 mole-kg^–1. The cell was standardized by measurements of HClO₄ at molalities from 0.002 to 0.01 mole-kg^–1 and, in subsequent measurements, a solution of NaA was titrated with HClO₄. To obtain the thermodynamic pK, an activity correction derived from the Debye-Hückel theory was applied. The pK of all five acids was found to vary linearly withx ₂ in the range 0 to 0.8. By comparison of data for acetic acid in water-methanol, water-dioxane, and water-sulfolane solvents, it was shown that the results are consistent with the known properties of sulfolane (low acidity, basicity, and solvating power), but the linear variation of pK with solvent composition remains unexplained.On leave 1973–1975 from the University of Gdansk, Poland.     

Isotope effect on the solubility of amino acids in water

The solubilities of glycine, alanine, phenylalanine, and proline in H₂O and D₂O from 283 K to 335 K were determined. It was found that glycine and alanine are less soluble in heavy water than in light water but proline is more soluble in heavy water over the whole temperature range studied. Phenylalanine is more soluble in H₂O than D₂O below 310 K but above that temperature heavy water becomes a better solvent. An influence of H/D isotope substitution on the enthalpies of solution is also observed. In the case of glycine and alanine enthalpies of solution in heavy water increase by a small amount and in the same time the solution enthalpy for phenylalanine in D₂O increases markedly. No change in the solution enthalpy for proline was observed. The isotope effects on solubility and the solution enthalpy are qualitatively discussed.     

Substituent effects in the ring-chain tautomerism of 1,3-diaryl-2,3-dihydro-1H-naphth[1,2-e][1,3]oxazines

Condensation of Betti base analogue amino naphthols with substituted benzaldehydes led to 1,3-diaryl-2,3-dihydro-1H-naphth[1,2-e][1,3]oxazines (3-9) which proved to be three-component (r¹-o-r²) tautomeric mixtures in CDCl₃ at 300 K. The electronic effects of the 3-aryl groups on the ratios of the ring-chain tautomeric forms at equilibrium could be described by the equation log K_X=ρσ⁺+log K_X=H. The value of the intercept was found to be strongly influenced by the steric arrangement of the 1,3-diaryl substituents.     

Substituent effects on O–H and S–H bond dissociation enthalpies of disubstituted phenols and thiophenols

The O–H and S–H homolytic bond dissociation enthalpies of a set of disubstituted phenols and thiophenols (NH₂, OH, CH₃, Cl, CF₃, and NO₂) have been computed by a density functional theory procedure with the 6‐311++G(d,p) basis set. A very good agreement between our results and available experimental ones is observed. The effect of substituents on structure, charges and BDEs are investigated and their correlation with Hammett parameters is studied. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Solute-solvent interactions in acid-base dissociation: Seven protonated nitrogen bases in water-N-methyl-2-pyrrolidinone solvents

Acidic dissociation constants of seven protonated nitrogen bases, namely Bis-tris, 2-aminopyridine, Tris, morpholine, 2-amino-2-methyl-1,3-propanediol,n-butylamine, andt-butylamine, have been determined at 15, 25, and 35° C in mixtures of water andN-methyl-2-pyrrolidinone (NM2Py). The mole fractions of cosolvent were 0.2, 0.4, 0.6, and 0.8. Glass and silver-silver chloride electrodes were used in a cell without liquid junction, and both the ionic strength and molality of chloride ion remained substantially constant in all of the measurements. The pK _a of all the protonated bases decreases initially and passes through a minimum as the aqueous solvent is enriched with NM2Py. The order of acidic strengths is the same as in the H₂O-dimethyl sulfoxide (DMSO) solvent system, but the pK _a values are all lower by up to 2 units. NM2Py appears to be a more basic solvent than DMSO or H₂O.On leave 1973–1975 from the University of Gdask, Poland.     

光学活性含硅氨基酸的合成

光学纯的硅取代氨基酸是一类非天然的手性氨基酸合成子，在药物，植物保护 剂和精细化学品的合成具有极为广阔的应用前景，其合成方法包括化学不对称合成 及化学合成外消旋体－生物学拆分两种，综述了该方面的研究进展。     

甘氨酸在DMF/水和乙醇/水混合溶剂中的焓对相互作用

利用LKB2277生物活性检测仪对298.15K时甘;氨酸在纯水,DBF/水和乙醇/水混合溶剂DBF(或乙醇的质量分数为0.05~0.45,以10%递增)中的稀释焓进行了测定,并利用维里展开式法关联得到焓对相互作用系数值.结果表明,焓对相互作用系数h~2与溶剂的结构性质密切相关.在DBF/水混合物溶剂中,h~2随DBF含量的增加而变得更负;而在乙醇/水混合溶剂中,h~2与乙醇含量近似呈抛物线型关系,在乙醇质量分数为0.25时h~2达到一个极小值。