分子内氢键——Ⅱ.邻苯基偶氮芳香酚的内氢键和互变异构

本文通过羟基的化学位移(δ_OH),解离常数(pK_α),质谱外光谱的测量以及PM0和HMO计算,较详细地研究了分子内氢键强度和偶氮-醌腙式互变异构与分子结构的关系.结果表明,内氢键强度(以⊿δ_OH和⊿pK_α来衡量)与PMO和HMO计算的内氢键稳定化能(⊿BE_H)和给予氮原子上的电荷密度(q_N)有正比关系.HMO计算还表明,醌腙式异构体的稳定性(⊿BE_A-H)与⊿BE_H有很好的线性关系.当有内氢键存在时,化合物4～7的醌腙式比偶氮式稳定;而在气体离子态时,HMO计算和质谱结果都表明1～7皆以偶氮式更为稳定,且质谱碎片分布和HMO计算的断裂键的键序(P_xy)有近似的直线关系.所有的实验结果和分子轨道计算结果都证实了1～3具有偶氮式,4～7为偶极偶氮式结构.     

Criegee中间体CH3CHOO与H2O反应机理及酸催化效应

采用CCSD（T）//B3LYP/6-311+G（d,p）方法研究了H₂O及甲酸等6种有机酸对CH₃CHOO与H₂O加成反应的催化作用。结果表明,非催化反应存在双质子迁移和加成反应2条通道,其中加成反应为优势通道。其加成机理为H₂O中OH加到CH₃CHOO的α-C上,同时H₂O中另一个H迁移到CH₃CHOO的端O上。催化剂H₂O及有机酸以氢键复合物的形式参与反应促进了H质子转移,可降低基元反应能垒和表观活化能,且催化效应与有机酸的强度成正比。例如,当分别用H₂O（pK_a=15.7）、甲酸（pK_a=3.75）和草酸（pK_a=1.23）催化时,生成syn-HAHP的基元反应能垒由非催化的69.12 kJ·mol^-1分别降至40.78、18.88和10.61 kJ·mol^-1。非催化反应具有正的表观活化能,而所有催化反应则均具有负的表观活化能。     

不同液体环境下聚丙烯酰胺的单分子力学

利用单分子力谱研究了非离子型聚丙烯酰胺(NPAM)及其水解产物阴离子型聚丙烯酰胺(APAM)在不同液体环境下的单链弹性. 利用改进的自由连接链(M-FJC)模型中的单链弹性模量参数(K₀)描述NPAM在不同pH值水溶液中的单链弹性. 实验结果表明, K₀随着溶液pH值增加而增大, 表明NPAM在碱性溶液中的水解度具有pH依赖性. 由于K₀和高分子链的净电荷正相关, K₀增大表明NPAM链净电荷增多, 结构单元之间的静电排斥作用增强使高分子链呈现高度伸展的构象. 在此基础上研究了APAM在不同pH值溶液中的构象, 单分子力谱数据表明, APAM在酸性水溶液中为柔性链, 在碱性水溶液中呈现较伸展的构象, 从而在分子水平上阐明APAM链构建的水凝胶网络的溶胀机理. 单分子层面的深入研究有望阐明这类高分子的减阻机理.     

Zeeman效应原子吸收分析中元素的吸收线分裂及其对灵敏度影响的研究Ⅱ.精细和超精细结构的作用

应用Voigt函数近似计算公式和光谱线叠加原理,本文描绘了Cu3247.54Å,Bi3067.70Å,Li6707.84和6707.99A双线,Au2427.95Å以及Ag3280.68Å在Zeeman效应原子吸收分析(简称ZAAS)和普通原子吸收分析(简称NAAS)中的吸收线和发射线轮廓.用作图面积积分的方法,估算了ZAAS对NAAS的相对吸光度.计算值与实验值比较相符.从谱线轮廓图和相对吸光度的估算得知:Δν_hfs与Δν_σ-⁺相当,则K_π可能为K_σ-⁺所抵消,灵敏度下降,如Cu3247.54Å和Bi3067.70Å;精细结构双线间距Δν_f与Δν_σ-⁺相当,则K_π也将为K_σ-⁺抵消,灵敏度下降,如Li双线;在一定的磁场强度下,某些谱线所具有的超精细结构间距,却可以使发射线轮廓内K_π得到加强,由Zeeman分裂而导致的灵敏度损失将减少,如Au2427.95A;Lorentz位移和谱线全变宽的大小,都可能造成Zeeman分裂对灵敏度影响大小的变化.     

稳态荧光探针法研究复配乳化剂的聚集行为

采用荧光探针技术,根据芘的第一振动峰I₁(373 nm)与第三振动峰I₃(384 nm)荧光强度的比值(I₁/I₃)随乳化剂浓度的变化,对十二烷基磺酸钠(AS)与壬基酚聚氧乙烯醚(Oπ-10)复配乳化剂的CMC值进行了测定,并与十二烷基磺酸钠(AS),壬基酚聚氧乙烯醚(Oπ-10)的CMC值进行了比较,结果表明复配乳化剂的CMC介于两者之间.以二苯甲酮为猝灭剂,用稳态荧光法测定了不同比例复配乳化剂的胶束聚集数,实验结果表明,复配乳化剂浓度为4—9倍CMC时,测定的胶束聚集数随复配乳化剂浓度的增大而线性增大;且Oπ-10:AS<2:1时,随着复合乳化剂中Oπ-10比例的增大,复配乳化剂胶束聚集数增大.利用芘的I₁/I₃值,结合胶束微环境的极性变化规律,探讨了复配乳化剂的聚集行为对硅丙乳液聚合物的影响.     

γ-巴豆酰内酯激发态动力学的共振拉曼光谱和完全活性空间自洽场(CASSCF)计算研究

采用共振拉曼光谱和完全活性空间自洽场(CASSCF)方法研究了γ-巴豆酰内酯的光吸收S₂态的结构动力学和衰变机制. 采用含时密度泛函理论方法结合光谱实验确认了紫外光谱和振动光谱. 获得了涵盖A-带吸收的4个激发波长下的共振拉曼光谱. 用CASSCF计算得到了S_1,min, S_2,min, T_1,min, T_2,min和T_3,min及其相关势能面交叉点的结构与能量. 研究了A-带共振拉曼光谱强度模式与S_2,min和CI(S₂/S₁)交叉点结构的关系. 借助El-Sayed规则分析了各系间窜跃路径的效率, 提出了γ-巴豆酰内酯从S_2,FC弛豫到基态S₀的2条主要路径: 内转换路径和系间窜跃路径.     

生物质基酸性低共熔溶剂用于高效溶解木质素及溶解机理

设计能高效溶解木质素的溶剂对木质素的高值化利用具有重要意义。本文中设计了基于氯化胆碱、甜菜碱和左旋肉碱作为氢键受体(HBA)和四种氢键供体(HBD)的生物质衍生的酸性低共熔溶剂(DESs),可以溶解包括碱木质素(AL)、脱碱木质素(DAL)、酶解木质素(EHL)和硫酸盐木质素(KL)在内的不同类型的木质素。在大多数所设计的DESs中,EHL比AL、KL和DAL更容易溶解,而不同木质素中羟基的含量对木质素的溶解有显著影响,但是并非在所有DESs中木质素的溶解情况都符合上述规则。氯化胆碱是构建DESs的首选HBA,具有良好的性能并适应于不同类型木质素的溶解,而合适的酸度使苯甲酸和没食子酸乙酯成为对木质素溶解有利的HBDs。研究表明能有效溶解木质素的DESs应具有强的氢键酸度(α值> 0.95)以及与溶解的木质素匹配的合适极性。此外,HBD的pK_a值和DESs的酸度也是评价酸性DESs溶解木质素性能的有效指标。通常具有适中pK_a值的HBDs能够用于构建具有高效的木质素溶解性能的DESs。DESs的粘度对木质素溶解也有一定影响,较低的粘度有助于木质素溶解。     

黄豆黄素与黄豆黄苷吸收光谱和荧光光谱的比较研究

研究了黄豆黄素和黄豆黄苷在不同pH条件下的吸收光谱和荧光光谱, 从分子结构的角度解释了二者呈现不同光谱特征的原因. 黄豆黄素分子基本无荧光. 在弱碱性时, 黄豆黄素分子发生7-OH质子的电离, 导致吸收光谱中320 nm的吸收峰红移至348 nm. 采用pH-光度法测得7-OH质子的电离常数pK_a1=7.08±0.04. 黄豆黄素一价阴离子呈现较强荧光, 最大激发和发射波长λ_ex/λ_em分别为334 nm/464 nm, 荧光量子产率为0.049. 在碱性溶液中, 黄豆黄素4'-OH质子电离, 导致吸收光谱中254 nm的吸收峰红移至260 nm, 电离常数pK_a2=9.96±0.01. 黄豆黄苷分子基本无荧光. 在碱性条件下, 黄豆黄苷分子的4'-OH质子发生电离, 导致吸收光谱中256 nm的吸收峰红移至 280 nm, 电离常数pK_a=9.81±0.03. 黄豆黄苷阴离子基本无荧光, 但在热碱性条件下发生γ-吡喃酮环裂解反应而产生较强荧光, λ_ex/λ_em为288 nm/388 nm, 裂解产物的荧光量子产率为0.056. 虽然, 黄豆黄苷与黄豆黄素是苷与苷元的关系, 但黄豆黄苷不能在热碱性条件下通过糖苷水解转变为黄豆黄素, 二者的荧光增强机理存在本质不同.     

综合利用稻壳制备木糖、电容炭与硅酸钙晶须

利用酸水解稻壳中的半纤维素制备木糖, 并将糖渣经过炭化后分离出碳和硅, 碳采用稀碱溶液活化改性制备电容炭, 硅采用水热法合成了硅酸钙晶须, 从而使稻壳所有组分得到充分利用. 采用循环伏安(CV)和恒流充放电(GCD)研究了电容炭的电化学性能. 通过X射线衍射(XRD)和扫描电子显微镜(SEM)对所得硅酸钙晶须的结构和形貌进行了表征. 实验结果表明, 稻壳酸水解的最优条件为硫酸浓度7%(质量分数)、 固液比(g/mL)为1:8、 反应时间为2.0 h, 在该条件下, 一次水解、 二次水解和三次水解的木糖收率(Y₁/Y₂/Y₃)和浓度(质量分数, C₁/C₂/C₃)都能达到最大值, Y₁=98.5%, C₁=3.6%; Y₂=85.4%, C₂=6.3%; Y₃=76.6%, C₃=9.0%. 采用15 mL 8%(质量分数) NaOH稀碱溶液活化改性制得的电容炭(AC/15)比电容值为77.32 F/g, 而且具有较好的倍率性和循环稳定性; 硅酸钙晶须为扫帚状针钠钙石晶须.     

邻二氯苯激发态动力学

利用飞秒分辨的激光泵浦-探测技术结合飞行时间质谱和光电子速度成像方法研究了邻二氯苯第一电子单重激发态(S₁)的超快动力学.邻二氯苯的S₁态振动基态寿命为(651 ± 10) ps,对应于S₁振动基态向三重态的系间窜越过程.邻二氯苯S₁的高振动激发9a₂18a₂对应两个衰减通道,其中寿命为(458 ± 12) fs的超快过程对应于由处于振动激发的S₁向高振动激发的基态(S₀)发生的内转换过程,而寿命为(90 ± 10) ps过程则对应由S₁态向三重态(T₁)的系间窜越过程,电离产生的光电子能谱中长寿命的谱峰可能与系间窜越过程有关. S₁态高振动态的旋轨耦合程度比低振动态的更强,导致系间窜越过程更快.     

Cell surface acid-base properties of Escherichia coli and Bacillus brevis and variation as a function of growth phase, nitrogen source and C:N ratio

Potentiometric titration has been conducted to systematically examine the acid–base properties of the cell surfaces of Escherichia coli K-12 and Bacillus brevis as a function of growth phase, nitrogen source (ammonium or nitrate), and carbon to nitrogen (C:N) ratio of the growth substrate. The two bacterial species revealed four distinct proton binding sites, with pK_a values in the range of 3.08–4.05 (pK₁), 4.62–5.57 (pK₂), 6.47–7.30 (pK₃), and 9.68–10.89 (pK₄) corresponding to phosphoric/carboxylic, carboxylic, phosphoric, and hydroxyl/amine groups, respectively. Two general observations in the data are that for B. brevis the first site concentration (N₁), corresponding to phosphoric/carboxylic groups (pK₁), varied as a function of nitrogen source, while for E. coli the fourth site concentration (N₄), corresponding to hydroxyl/amine groups (pK₄), varied as a function of C:N ratio. Correspondingly, it was found that N₁ was the highest of the four site concentrations for B. brevis and N₄ was the highest for E. coli. The concentrations of the remaining sites showed little variation. Finally, comparison between the titration data and a number of cell surface compositional studies in the literature indicates one distinct difference between the two bacteria is that pK₄ of the Gram-negative E. coli can be attributed to hydroxyl groups while that of the Gram-positive B. brevis can be attributed to amine groups.     

Use of factorial experimental design for the rapid evaluation of main and interactive factors affecting linearity in calibration curves for sulfate analysis by ion chromatography

Analytical results of anion determination by suppressed ion chromatography are significantly affected by calibration curve calculation. In this paper, as expected, eluent pK_a is shown to influence calibration linearity in the range 1–20 mg/l sulfate, with A carbonate-hydrogencarbonate mixture producing a larger non-linearity than NaOH. Evidence is given for very large errors (about 30–40%) in estimating sample sulfate concentration when linear regression is used instead of a quadratic calibration curve. This study was performed following a 2⁴ run full factorial experimental design, including eluent pK_a, counterion type, solution composition and current level for background suppression as main variables.     

Acid-base dissociation constants of 2,2'-bipyridyl in mixed protic solvents

The acid dissociation constants (K_a), base dissociation constants (K_b) and the autoprotolysis constants (K_s) for 2,2′-bipyridyl in water and in water+alcohol(methanol, ethanol, iso-propanol) mixed solvents have been determined at 25°C and an ionic strength of 0.1 mol l⁻¹, from a direct potentiometric method based on the treatment of the data of a single pH titration. It has been shown that K_a increases, whereas K_b and K_s decrease, with increasing proportion of the alcohol in the mixed solvents. Linear relations between pK_a, pK_b, pK_s and the mole fraction of the alcohol were observed in the composition range investigated. These results are discussed in terms of the properties of solvent and the interactions of the different species existing in dissociation equilibrium with solvents. It is concluded that the higher stabilization of both 2,2′-bipyridyl and its protonated form by dispersion forces and of the proton by its interaction with solvent molecules in the mixed solvents compared with that in water are largely responsible for the observed changes of pK_a with composition. On the other hand, the low stabilization of OH⁻ in the mixed solvents relative to that in water and the electrostatic effect are the main factors in determining the solvent effect on pK_b.     

Critical validation of a new simpler approach to estimate aqueous pKa of drugs sparingly soluble in water

A simple linear approach to estimate the aqueous pK_a of compounds sparingly soluble in water, mainly drugs, from solely one pK_a value determined in any methanol/water mixture is evaluated. The parameters (slope and intercept) of the linear relationships are related to the solvent composition and can be easily calculated according to the acidic or basic functional group of the compound. The method has been tested using the available literature data for phenols, aliphatic carboxylic acids, benzoic acid derivatives, both ortho and non-ortho substituted, amines and imidazole derivatives. The study involves the whole range of solvent composition and about one hundred compounds which show a wide variety of aqueous pK_a, from 1.3 to 12.4. The differences between calculated and previously published aqueous pK_a values are less of 0.2 pK units. Consistent values are obtained whatever the composition of methanol/water mixture employed in the experimental measurements. The results support the usefulness of the tested method as a very simple approach to get reliable aqueous pK_a values for sparingly soluble drugs.     

Photoluminescence of 2-aminofluorene: a relook

The absorption and fluorescence spectra of 2-aminofluorene (2-AF) have been studied as a function of solvent polarity and acid concentration. Using the multiparametric approach of Taft et al., it is clear that 2-AF is a better proton acceptor in the S₀ state and a proton donor in the S₁ state. Excitation of 2-AF to three electronically excited states has shown that fluorescence is always observed from the lowest excited singlet state, but fluorescence quantum yield increases with the increase of λ_exc. The decrease in fluorescence quantum yield with increase in solvent polarity and hydrogen bonding is due to solvent-induced quenching. A correspondence is observed between the decrease and increase in fluorescence intensities of the neutral and monocation, respectively, in the pH range from 6 to 3. Proton-induced fluorescence quenching of neutral 2-AF is noticed in the pH range 3 to 1. pK_a and values were determined for different prototropic equilibria and are discussed.     

Conformational dependence of serotonin theoretical pKa prediction

In the present work we used quantum mechanics calculations to predict the two pK_a’s of 5-hydrotryptamine (5-HT). Proton dissociation reaction succeeded to predict the experimental pK_a1 corresponding to ionization of the protonated amine group but failed for pK_a2 corresponding to ionization of the 5-hydroxyl group. For pK_a2, a cluster-continuum model including three water molecules in the first hydration shell around 5-hydroxyl and 5-hydroxide groups enabled us to reproduce the experimental pK_a2 value. Furthermore, we demonstrated that specific conformations of acid/base pair of 5-HT is critical to predict accurately the experimental pK_a’s of the flexible 5-HT molecule.     

On the properties of Calcein Blue

A satisfactory method for the preparation of Calcein Blue has been devised. Elemental analysis, equivalent weight by neutralization, and the NMR spectrum show the compound to be 4-methylumbelliferone-8-methyleneiminodiacetic acid·0·25H₂O. The ultraviolet absorbance and fluorescence have been studied as a function of pH and, combined with potentiometric titration and solubility date, have yielded for the acid dissociation constants the values pK₁ = 3·0, pK₂ = 6·9, and pK₃ = 11·3. These acid functions are identified respectively as carboxyl, phenol, and ammonium ion, the free Calcein Blue being a zwitter-ion. Calcein Blue fluoresces in both acidic and basic solution when excited at a suitable wavelength. The fluorescence of the doubly-charged anion formed on the neutralization of the phenol group, when excited at 360 nm, reaches a maximum at pH 9, and decreases to zero with the neutralization of the ammonium ion; the wavelength of maximum emission is 455 nm. In the presence of calcium, the fluorescence increases with alkalinity up to pH 9 and then remains constant. The calcium derivative is a 1:1 compound, formation constant 10^7·1. The fluorescence of Calcein Blue at all pH values is quenched by copper(II). The calcium derivative is changed on standing in highly alkaline solution, presumably by ring opening, to another fluorescent material; thus Calcein Blue, although satisfactory as an indicator, is not useful for the direct fluorometric determination of calcium.     

Correlation between hydration and deprotonation in hexaaqua metal complexes

The hydration energy of metallic cations determined with density functional calculations using a double-numerical plus p-polarization basis set, related to the acidity constants of hexaaqua metal complexes, was investigated in the present study. From the results calculated by Vosko-Wilk-Nusair (VWN), Becke-Perdew (BP) and Becke-Lee-Yang-Parr (BLYP) density functionals, a global linear correlation with the observed acidity constants in both main group [Mg(II), Ca(II) and Al(III)] and (post-)transition group [Mn(II), Zn(II), Cd(II), Sc(III), Cr(III), Fe(III), Ga(III) and In(III)] hexaaqua metal complexes has been established:

VWN density functional: pK_a = 16.5760 + 0.0173E_hydr kcal mol⁻¹

BP density functional: pK_a = 15.7329 + 0.0182E_hydr kcal mol⁻¹

BLYP density functional: pK_a = 15.9448 + 0.0185E_hydr kcal mol⁻¹     

Organic photochemistry V: photochemistry of substituted Schiff''s bases—search for luminescent dyes for solar collectors

The pH dependence of the absorption and fluorescence spectra of Schiff's bases derived from 2-amino-4-phenylthiazole and aniline with substitued 2-hydroxybenzaldehydes was investigated. The pK_a values of the Schiff's bases associated with the ground state equilibria were determined spectrophotometrically. The excited state pK_a values were also estimated. Their Stokes shifts were also calculated.     

Ionic equilibria in neutral amphiprotic solvents: Structural effects on dissociation constants of several substituted phenols and mercaptopyrimidines in isopropyl alcohol

The dissociation constants of several families of acids (substituted phenols and mercaptopyrimidines) in isopropyl alcohol medium have been determined by potentiometric titration with tetrabutylammonium hydroxide. Because of ion-pair formation the incomplete dissociation of the tetrabutylammonium salt has been taken into account in the calculation of pK_a. The dissociation constants of the salts were previously measured conductometrically. The resolution of acid strength in isopropyl alcohol relative to that in water has been determined for each series of acids by plotting the pK_a values in isopropyl alcohol vs. those in water. The results show greater resolution in isopropyl alcohol than in water. The resolution of acid strength in tert-butyl alcohol relative to that in isopropyl alcohol has also been determined.