Correlation of the position of the bond functions in H-X with electronegativity

S type Gaussian bond functions are optimized for HF, H₂O, NH₃, and CH₄. The optimization is carried out with respect to the exponent and position in the H-X bond. The position is found to correlate well with the electronegativity of Pauling and Allred-Rochow.     

Substituent Effects on the C-C Bond Strength, 15[1] Geminal Substituent Effects, 7[2]. Thermochemistry and Thermal Decomposition of Alkyl-substituted Tricyanomethyl Compounds

The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile ( 10a ) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile ( 10b ) decomposed heterolytically into carbenium ions and (CN)₃C⁻ anions, while 9-methyl-9-(tricyanomethyl)fluorene ( 10c ) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH ( 10c ) = 155· 2 kJ mol⁻¹, ΔS ( 10c ) = 58· 5 J mol⁻¹ K⁻¹]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile ( 2 ) (422.45 kJ mol⁻¹), 2,2-dicyanohexanenitrile ( 3 ) (349.74 kJ mol⁻¹), 2,2-dicyano-3-phenylpropionitrile ( 4 ) (540.75 kJ mol⁻¹), 2-butyl-2-methylhexanentrile ( 5 ) (-133.20 kJ mol⁻¹), 2,2-dimethylpentanenitrile ( 6 ) (-45.78 kJ mol⁻¹), and 2-methylbutyronitrile ( 7 ) (2.44 kJ mol⁻¹) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1 RSE ( 1 ) = -18 kJ mol⁻¹ was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol⁻¹) and α,α-dicyanoalkyl (27 kJ mol⁻¹) radicals, which were recalculated from bond homolysis measurements^[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2).     

How bridging ligands and neighbouring groups tune the gold-gold bond strength

Gold-gold interactions in small polynuclear complexes are analysed using extended Hückel calculations. They are influenced by the nature of the ligand donor atoms, by the bridging ligands, but most by the formal oxidation state of the metal. Au---Au bonds are much stronger in complexes of Au(II) and Au(III), but a weak interaction between two d¹⁰ centres exists for Au(I) complexes, owing to mixing of the s and p orbitals with the d orbitals. Phosphines induce stronger metal-metal bonds when coordinated trans to the Au---Au bond in [Au(II)[(CH₂)₂PPh₂]L]₂ (Ph = phenyl), but have the opposite effect when bonded orthogonally to the metal-metal axis in Au(I) binuclear species. When two gold atoms are bridged by a single carbon atom, belonging either to mesityl (Mes = 2,4,6-Me₃C₆H₂) or CR₂, the former produces stronger Au(I)---Au(I) interactions, reflected in shorter distances. Formal oxidation states are proposed for the gold atoms in two mixed-valence clusters, [Au4(C₆F₅)₂((PPh₃)_{2CH})2(PPh3)2](ClO4)2 and [{(2,4,6-C6F3H2)Au(CH2PPh2CH2)2Au{in2}-Au(CH₂PPh₂CH₂)₂Au](ClO₄)₂. The results suggest a higher oxidation state for the outer gold atoms, in both the T-shaped tetranuclear cluster and the Au₆ linear chain.     

New tetra-aryl and bi-aryl porphyrins bearing 5,15-related fluorenyl pendants: the influence of arylation on fluorescence

The new tetra-aryl trans-A₂B₂-porphyrin with two fluoren-2-yl and two phenyl groups at the meso-positions has been synthesized and characterized, together with the analogous bi-aryl trans-A₂-porphyrin incorporating only the fluorenyl groups. The photophysical properties of these new compounds in solution are reported. The two additional phenyl groups in the A₂B₂ porphyrin stabilize the first excited singlet state by approximately 500 cm⁻¹ compared to the A₂ system, as revealed by the red-shift of the absorption Q bands and of the fluorescence emission bands. Both compounds display enhanced emission quantum yields compared to tetraphenylporphyrin: the augmentation is higher for the tetra-aryl than for the bi-aryl system, although not as high as for tetrafluorenylporphyrin. Fluorescence lifetime measurements of the series of four compounds suggest that the trends can be interpreted in terms of the larger influence of fluorenyl over phenyl groups in increasing the fluorescence radiative rate constant of the porphyrin, k_f.     

Thermal stability and bond dissociation energy of fluorinated polymers: A critical evaluation

Intrinsic bond dissociation energies (BDEs) of selected fluorinated polymers are critically evaluated. Two distinctive approaches were followed. In the first one, according to Wu and Rodgers [E.-C. Wu, A.S. Rodgers, J. Am. Chem. Soc. 98 (1976) 6112-6115], starting from the gas-phase enthalpy of polymerization we obtained the polymer backbone BDE through appropriate thermodynamic cycles. Revised experimental results indicate that the CC BDEs of all the polymers taken into account fall within a limited energy interval, comparable to the average experimental uncertainty. Central to the second methodology adopted, is a model compound approach. Thanks to the large number of reliable thermodynamic data available in the scientific literature and to simple end-capping rules, C₂ molecules were chosen as suitable models for infinite linear polymers and alternating copolymers between ethylene and fluorinated olefins. For partially fluorinated polymers, like polyvinylidene fluoride (PVDF), alternating ethylene-tertrafluoroethylene (ETFE) and alternating ethylene-chlorotrifluoroethylene (ECTFE), the weight loss due to HF and HCl evolution during heating experiments was successfully related to the threshold energy E_o(HX) for 1,2-elimination from chemically activated hydrofluoro and hydrofluorochlorocarbons according to the modified Rice-Rampsberger-Kassel (RRKM) unimolecular theory.     

一代树状碳硅烷液晶的光化学研究－－端基含12个4－丁氧基氮苯介晶基元

研究了新的含12个丁氧基偶氮苯介晶基元的五代树状碳硅烷液晶D1及偶氮苯介 晶基元化合物M5在氯仿、四氢呋喃、N，N－二甲基甲酰胺、乙醇和苯等溶剂中的量 子产率、反－顺光异构化、光回复异构、反／顺异构组分比、热回复异构及活化能 。D1和M5的光致变色速率常数为10~(-1)s~(-1)，而含同一偶氮基元的光致变色液 晶聚硅氧烷的光致变色速率常数为10~(-8)s~(-1)，因此，液晶树状物D1的光响应 速度比后者快10~7倍。     

四甲基对联苯二胺光敏化聚苯乙烯砜及其模型化合物二苄基砜分解机理的研究

N、N、 N’、N’-四平基对联苯二胺(NTMB)和 3、3’、5、5’-四甲基对联苯二胺(TMB)可以有效地光敏化二苄基砜(DBS)分解和聚苯乙烯砜(PSS)降解。敏化作用是按电子转移机理进行的,电子转移过程可以由NTMB的单重激发态,也可以由三重激发态发生。由三重态电子转移产生的三重态离子自由基对进一步反应生成产物的效率比单重态离子自由基对的效率高10倍。     

EPM-g-GMA接枝率的测定及其增韧PBT的力学性能和形态结构研究

在同向双螺杆挤出机中通过熔融接枝反应制备了EPM g GMA ,将其与PBT在转矩流变仪中熔融共混可以获得增韧的PBT工程塑料 .实验中EPM g GMA接枝率的测定采用红外工作曲线法 ,选用CCl4 做溶剂以避免溶剂对样品吸收峰的干扰 .随着EPM g GMA接枝率的增加 ,PBT EPM g GMA的缺口冲击强度相应提高 ,共混物中EPM g GMA的粒径尺寸减小 ,当EPM g GMA的接枝率为 4 7mL 1 0 0gEPM时 ,EPM g GMA的粒径尺寸可达 0 5 μm ,PBT EPM g GMA的缺口冲击强度达到 5 1 6kJ m2 ,是纯PBT的 3 1倍     

Effects of steric and hydrophobic forces on the rheological properties of ZrO2 suspensions

 The effect of pH on the flow behavior of ZrO₂ suspensions containing polyacrylic and octanoic acids was evaluated. In the flocculated pH regime, the flow behavior is highly shearthinning and can be described by a power-law model. The shear-thinning behavior increases with increasing degree of flocculation. Maximum shearthinning was observed at the zero zeta potential condition. Hydrophobic interaction arising from adsorbed octanoic acid was found to enhance the shear-thinning behavior. No such enhancement was observed for adsorbed polyacrylic acid. It was also illustrated that the viscosity–pH behavior is a mirror image of the yield stress–pH behavior. A quantitative particle-pair interactions model incorporating steric and hydrophobic interactions was proposed to explain the effects of polyacrylic and octanoic acids on the maximum yield stress. Received: 23 May 1997 Accepted: 4 June 1997