Search results for "CONSTANT"
showing 10 items of 1718 documents
Role of Halide Ion in the Mechanism of Protonolysis of the Pt-C Bond in Pt(II) Alkyl and Aryl Complexes.
1985
Abstract A mechanistic study is described for the electrophilic cleavage of the Pt C σ bond in complexes cis -[PtPh 2 (PEtPh 2 ) 2 ] and trans -[PtXR(PEt 3 ) 2 ] (X = Cl and Br, R = Me, Et, n-Pr, n-Bu, CH 2 Ph) by the proton in the presence of halide ions in aqueous methanol (MeOH/H 2 O, 91v/v) which yields cis -[PtClPh(PEtPh 2 ) 2 ] and trans -[PtX 2 (PEt 3 ) 2 ], respectively. The reactions are first-order in substrate for both systems and the general bivariate rate law k obs = [H + ]{ k H + k x K [[X − ]}/(1 + K [[X − ]) is obeyed. The proposed mechanism involves a fast pre-equilibrium formation (K) of a platinum(II) anionic intermediate via interaction of the halide with the square-plan…
Glassy behavior of molecular crystals: A comparison between results from MD-simulation and mode coupling theory
2005
We have investigated the glassy behavior of a molecular crystal built up with chloroadamantane molecules. For a simple model of this molecule and a rigid fcc lattice a MD simulation was performed from which we obtained the dynamical orientational correlators $S_{\lambda \lambda '}({\bf{q}},t)$ and the ``self'' correlators $S_{\lambda \lambda '}^{(s)}(t)$, with $\lambda = (\ell, m)$, $\lambda' = (\ell', m')$. Our investigations are for the diagonal correlators $\lambda = \lambda'$. Since the lattice constant decreases with decreasing temperature which leads to an increase of the steric hindrance of the molecules, we find a strong slowing down of the relaxation. It has a high sensitivity on $…
Substituent effects on13C NMR parameters of chlorinated diphenyl ethers. A multiple linear regression analysis
1995
13C NMR chemical shifts and nJ(C,H) coupling constants of polychlorinated diphenyl ethers (PCDEs) were measured and analysed. The chlorine substituent effects on the chemical shifts and the coupling constants were determined by a multiple linear regression analysis. The 13C NMR chemical shifts depend on the conformational preferences in PCDEs. In addition to single substituent effects, corrective terms reflecting the conformational state of the molecule and the mutual steric interactions of two chlorine atoms had to be taken into account for the reliable prediction of the 13C chemical shifts. In contrast to chemical shifts, conformational effects play a minor role in the substituent effects…
Quantitative Prediction of Concentration Effects in Steric Exclusion Chromatography
1986
Abstract A semiempirical model, based on a previous one quantitatively describing the dependence of the elution volume, V(cA), on the concentration of injected polymer, cA, in exclusion chromatography (SEC) at dilute solutions, has been developed. In the derived equation, concentration effects are mainly governed by the Huggins' coefficient, kA, and by the quadratic coefficient in the polynomial expansion of the reduced specific viscosity, kA. Because of the incertitudes on reliable kA and kA' values, these are respectively removed from the model through she Imai's equation and the empirical correlation kA' + 0.122=kA, here obtained. Thus, A predicted elution volumes besides polymer concent…
Molybdenum(VI)-dioxo complexes with sterically bulky thiocarboxylate ligands. Reactions with aliphatic thiols and electrochemical properties
1991
Abstract Under acid conditions, the reaction of (Bu n 4 N)2[Mo VI O 2 (O 2 CC(S)Ph 2 ) 2 ] with aliphatic thiols yields the monomeric [Mo V O(O 2 CC(S)Ph 2 ) 2 ] − as unique complex product. The experimental pseudo-first order rate constant with respect to the Mo(VI) complex was found to be K =6.1 × 10 −5 s −1 . At neutral pH, however, an unstable Mo(IV) species was formed which was also electrochemically detected in a reversible Mo(V,IV) couple. [Mo V O(O 2 CC(S)Ph 2 ) 2 ] − appears to be obtained by the reaction of [Mo IV O(O 2 CC(S)Ph 2 ) 2 ] 2− with unreacted [Mo VI O 2 (O 2 CC(S)Ph 2 ) 2 ] 2− . Steric features on the ligand (gemdiphenyl groups) explain that the latter reaction does not…
Mononuclear rearrangement of heterocycles in zwitterionic micelles of amine oxide surfactants.
2012
Abstract Rate constants for the mononuclear rearrangement (MRH) of Z -phenylhydrazones of some 5-substituted-3-benzoyl-1,2,4-oxadiazoles in water have been measured in the presence of zwitterionic micelles. The use of micellized N -tetradecyl- N , N -dimethylamineoxide (C 14 DMAO) as the reaction medium allowed to solubilize the otherwise water-insoluble oxadiazoles. Micellar rate effects were analyzed by using a simple pseudo-phase model and compared with those obtained in non-ionic micelles (Triton X-100). Evidence that both the rate of the rearrangement reaction and the binding of the substrates to the micelles are mainly governed by substrate hydrophobicity is obtained. The disagreement…
Macromolecules in ordered media: 4. Poly(2-vinyl pyridine)-liposome association induced by electrostatic interactions
1997
Abstract We have investigated the interaction between vesicles based on dimyristoyl phosphatidic acid (DMPA) and a polymer, such as poly(2-vinyl pyridine) (P2VPy), on the basis of both partition and binding models. The resulting association curves are non monotonous being three regions clearly evidenced. The electrostatic and steric origin of the attractive or repulsive interactions are analyzed as a function of pH, ionic strength and temperature using the fluorescence spectroscopy technique. We emphasize the importance of differentiating the ideal definition of the binding constant from the theoretically evaluated including the activity coefficient, γ, to take into account shifts from the …
Molecular association in water-isomeric pentanol mixtures at 25�C
1982
The results of static dielectric constant and viscosity measurements on solutions of water (concentration range 0 to 0.3 mole fraction) in 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol, and 2-methyl-2-butanol together with previous results for solutions of water in n-pentanol are discussed in terms of the information that they provide on the nature and the extent of molecular association in these solutions. We conclude that in most systems this association leads to the formation of tetrahedral complexes such as H2O(ROH)4. Evidences of the correlation of water-alcohol interactions with molecular parameters (position of OH group, steric hindrance of alkyl chain) of alcohols a…
Azide Binding Controlled by Steric Interactions in Second Sphere. Synthesis, Crystal Structure, and Magnetic Properties of [Ni II 2 (L)(μ 1,1 -N 3 )]…
2016
International audience; The dinuclear Ni-II complex [Ni-2(L-2)][ClO4](2) (3) supported by the 28-membered hexaaza-dithiophenolate macro-cycle (L-2)(2-) binds the N-3(-) ion specifically end-on yielding [Ni-2(L-2)(mu(1,1)-N-3)] [ClO4] (7) or [Ni-2(L-2)(mu(1,1)-N-3)][BPh4] (8), while the previously reported complex [Ni2L1(mu(1,3)-N-3)][ClO4] (2) of the 24-membered macrocycle (L-1)(2-) coordinates it in the end-to-end fashion. A comparison of the X-ray structures of 2, 3, and 7 reveals the form-selective binding of complex 3 to be a consequence of its preorganized, channel-like binding pocket, which accommodates the azide anion via repulsive CH center dot center dot center dot pi interactions …
Higher order matrix differential equations with singular coefficient matrices
2015
In this article, the class of higher order linear matrix differential equations with constant coefficient matrices and stochastic process terms is studied. The coefficient of the highest order is considered to be singular; thus, rendering the response determination of such systems in a straightforward manner a difficult task. In this regard, the notion of the generalized inverse of a singular matrix is used for determining response statistics. Further, an application relevant to engineering dynamics problems is included.