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).     

Nucleotide/Protein Interaction: Energetic and Structural Features of Na,K-ATPase

Microcalorimetric titrations allow to recognize and investigate high-affinity ligand binding to Na,K-ATPase. Titrations with the cardiac glycoside Ouabain, which acts as a specific inhibitor of the enzyme, have provided not only the thermodynamic parameters of high-affinity binding with a stoichiometric coefficient of about 0.6 but also evidence for low-affinity binding to the lipid. The marked enthalpic contribution of -95 kJ mol^-1 at 298.2 K is partially compensated by a large negative entropy change, attributed to an increased interaction between water and the protein. The calorimetric ADP and ATP titrations at 298.2 K are indicative of high-affinity nucleotide binding either in 3 mM NaCl, 3 mM MgCl₂ or at high ionic strength such as 120 mM choline chloride. However, no binding is detected in the buffer solution alone at low ionic strength. The affinities for ADP and ATP are similar, around 10⁶ M^-1 and the stoichiometric coefficients are close to that of Ouabain binding. The exothermic binding of ADP is characterized by a ΔH and ΔS value of -65 kJ mol^-1 and -100 J mol^-1 K^-1, respectively. TheΔH value for ATP binding is larger than for ADP and is compensated by a larger, unfavorable ΔS value. This leads to an enthalpy/entropy compensation, which could express that H-bond formation represents the major type of interaction. As for Ouabain, the negative ΔS values that are also characteristic of nucleotide binding can indicate an increase of solvate interaction with the protein due to a conformational transition occurring subsequent to the binding process. The resulting binding constants are discussed with regard to the results of other studies employing different techniques. A molecular interaction model for nucleotide binding is suggested. This revised version was published online in July 2006 with corrections to the Cover Date.     

Finite extensibility and orientational effects in rubbers. A physical interpretation of the ‘van der Waals equation’ for the elastic force

We propose a physical interpretation of the so-called van der Waals equation of state for rubbers, which gives a relation between the force and the deformation. On a phenomenological basis this equation takes the finite extensibility and a non-defined interaction into account. Here the fininte extensibility is discussed for the dilute case (no entanglements) and the highly entangled limit. The intramolecular interactions are described by orientational effects. The resulting equation of state for the force shows the same features as the van der Waals equation.Dedicated to Prof. H.-G. Kilian on the occasion of his 60th birthday.     

Interactions of water-soluble polymers with small molecules II: Poly(2-diethylaminoethyl methacrylate)-methyl orange and its homolog systems at different temperatures

In order to investigate the interactions of poly(2-diethylaminoethyl methacrylate) (PDEAEMA) with methyl orange and its homologs in solution, temperature dependence of the complex formation has been examined in detail by the measurements of transmittance and specific conductance for the systems. Furthermore, the binding course of dyes to PDEAEMA has been studied on the basis of thermodynamic parameters obtained from equilibrium dialysis experiments at different temperatures. It was observed that the flocculation process shifted to lower dye concentrations in accordance with increasing hydrophobicity of the dyes in the order, methyl orange < ethyl orange < butyl orange, and the process of complex formation was characterized by three separate regions according to the slope of specific conductivity-mixing ratio curve for mixtures of PDEAEMA and dye. The temperature dependences of F,H and S suggest that, for dyes-PDEAEMA complex formation, the hydrophobic interaction is predominant at a low temperature but the electrostatic interaction becomes important as the temperature increases.     

The Weak Interaction on CO2 …CO van der Waals Complex : A Theoretical Study

The geometries of van der Waals complex CO2…CO were optimized at DFT and second-order Moller-Plesset perturbation(MP2) levels with the large basis set,three stable structures were found.The most stable structure has a T-shape geometry in which the CO lies along the C2 axis of CO2,with the two C atoms direct contact and R(C…C)=0.3227nm.The corresponding energies of the most stable structure were calculated by means of MP2,MP4D,MP4DQ,MP4SDTQ,MP4SDQ,CCSD and CCSD(T) methods,The BSSE (basis set superposition error) wads eliminated by the Boys-Bernardi counterpoise correction(CP) method.According to thermodynamics data.van der Waals complex CO2…CO can found at a low temperature and or a high pressure,There is a little charge transferred between the two interacted subunits.In the most stable structure,CO2 is the acceptor and CO is the donor.     

Interactions between terminally substituted amino acids in an aqueous and a non-aqueous environment. Enthalpic interaction coefficients in water and in N,N-dimethylformamide at 25°C

Enthalpies of dilution of the N-acetyl amides of glycine, L-alanine, L-valine, L-leucine, and L-phenylalanine, dissolved in N,N-dimethylformamide (DMF) as a solvent have been measured at 25°C. The results obtained have been analyzed to give the enthalpic interaction (or virial) coefficients of the solutes and these are compared with information previously obtained in aqueous systems. There are marked differences in the interaction properties in the two solvents and, while the additivity approach of Savage and Wood is applicable to the solutes in water it is not suitable for representing the interactions in DMF. A correlation is presented between the enthalpic second virial coefficients in DMF and the propensity of side-chains to be in proximity in globular proteins.     

Initiation of polymer degradation via transfer of infectious species

A novel dual stage chemiluminescence detection system incorporating individually controlled hot stages has been developed and applied to probe for material interaction effects during polymer degradation. Utilization of this system has resulted in experimental confirmation for the first time that in an oxidizing environment a degrading polymer A (in this case polypropylene, PP) is capable of infecting a different polymer B (in this case polybutadiene, HTPB) over a relatively large distance. In the presence of the infectious degrading polymer A, the thermal degradation of polymer B is observed over a significantly shorter time period. Consistent with infectious volatiles from material A initiating the degradation process in material B it was demonstrated that traces (micrograms) of a thermally sensitive peroxide in the vicinity of PP could induce degradation remotely. This observation documents cross-infectious phenomena between different polymers and has major consequences for polymer interactions, understanding fundamental degradation processes and long-term aging effects under combined material exposures.     

Calculations of concentration dependences of chemical shifts for acrylamide in water—DMSO mixed solvent

The lattice model of associated solutions was used to calculate the concentration dependences of the chemical shifts of¹³C magnetic nuclei in the acrylamide molecule in water—DMSO solvent. Each pair of bonds between the acrylamide group containing a magnetic nucleus and the neighboring solvent molecule was assumed to additively contribute to the change in the chemical shift of the nucleus in question. The new approach affords the same accuracy in describing the experimental data at any ratios of the solution components. It was shown that the chemical shifts and the excess heat of mixing of the water—DMSO binary solution can be simultaneously described using the same set of energy parameters of the model. The model makes it possible to employ experimental data on chemical shifts for studying intermolecular interactions in solutions. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1983–1987, October, 1998.     

Ab initio through‐space/bond interaction analysis of the long C–C bonds in Bi(anthracene‐9,10‐dimethylene) photoisomers

The bi(anthracene‐9,10‐dimethylene) photoisomer has remarkably long C–C single bonds. To examine the lengthening of the C–C bond, we propose a novel procedure for quantitatively analyzing orbital interactions in a molecule at the level of the ab initio molecular orbital method. In this procedure, we can cut off the specific through‐space/bond interactions in a molecule by artificially increasing the absolute magnitude of the exponents in a Gaussian function. Then, the spatial orbital interactions were perfectly cut off, and, each term that makes up the total energy, that is, the nuclear–electron attractions, the electron–electron repulsions, and the nuclear–nuclear repulsions cancel each other. Several model molecules of the photoisomer were analyzed by this procedure. It was found that the orbital interaction between the p orbital on the benzene ring and the σ* orbital on the C–C bond in question, σ→σ* electron transfer through π orbital, weakens the C–C bond efficiently when these orbitals were located in the “periplanar” conformation. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001