A gas electron diffraction and ab initio quantum-mechanical investigation into the molecular structure of tricyclo-(3.1.002,4)-hexane

The structure of tricyclo-(3.1.00^2,4)exane has been determined by gas phase electron diffraction. The molecule has an inversion centre. The mean carbon—carbon bond length, averaged over both three- and four-membered rings is 1.508 A. A model with equal C-C bond lengths fits to the measured diffraction intensities. The four-membered ring is planar with valency angles of 90°, while the carbon atoms of the three-membered rings form isosceles triangles. Ab initio quantum mechanical calculations at the STO-3G level support this geometry. The valency angle CCC (between three- and four-membered rings) for the equilateral four-membered ring model has been found experimentally to be 109.9°. The average C-H bond distance (1.080 Å) is small as a result of increased s-character in these bonds in agreement with reported INDO—LMO calculations.     

Pyrolysis-gas chromatography of polymers obtained by Friedel-Crafts reactions of poly(vinyl chloride) with aromatic compounds

Pyrolysis, in combination with gas-chromatography technique, was used in the determination of the structure and study of the thermal degradation mechanism of the condensation polymers obtained by Friedel-Crafts reactions of poly(vinyl chloride) with benzene, toluene, and naphthalene. The separation of the pyrolysis products was made using a column packed with chromosorb W (80–100 mesh) coated with 15% silicone SE-52. The identification of the pyrolysis products resulted as a consequence of the thermal decomposition of condensation polymers and their semiquantitative estimation led to the final conclusion that the initial normal chlorine substitution in the macromolecular chain of poly(vinyl chloride) is followed by an important intramolecular cyclization reaction yielding 1,3-methyleneindan units.     

Etude du parametre d'interaction polymere-polymere pour le systeme ternaire solvant-polydimethylsiloxane-polystyrene. Influence de la concentration et de la masse moleculaire

This paper presents the results of a study of the thermodynamic properties of a ternary system viz. solvent (ethylacetate or benzene)-polydimethylsiloxane-polystyrene. The interaction parameter between the two polymers χ₂₃, determined both from light scattering and studies of the critical conditions of phase separation, has been studied as a function of the molecular weights of the two polymers. Using a polystyrene of fixed molecular weight, we observe a large decrease in χ₂₃ for increasing molecular weight of polydimethylsiloxane. When the molecular weights of the two polymers are very high, the parameter χ₂₃ becomes very small (of the order of 0.01). The values of χ₂₃ obtained for these two polymers are compared to those found for comparable systems, viz. polystyrene-polyisobutene and polydimethylsiloxane-polyisobutene.     

Microstructure determination of poly-(1,3-pentadiene) by a combination of infrared, 60-MHz NMR, 300-MHz NMR,and X-ray diffraction spectroscopy

A quantitative procedure has been developed for characterizing the complete microstructure of polymers of 1,3-pentadiene, including the tacticity of any crystalline component. This can be accomplished by a combination of infrared spectroscopy, X-ray crystallinity, and 300-MHz NMR spectroscopy. A series of high structural purity polymers were synthesized with a series of previously unreported mixed microstructures. These samples were characterized by using the three techniques mentioned, including the previously unreported 300-MHz NMR data. With those results a 60-MHz NMR/IR method of spectroscopy was developed to determine the composition of poly(1,3-pentadiene)s in terms of percent cis-1,2-, cis-1,4-, trans-1,4-, and 3,4-pentadiene units.     

An NQR study of the effects on the torsional oscillations of a molecular crystal due to uniaxial pressures

This work reports the experimental results of the uniaxial pressure dependence at various temperatures of the spin-lattice relaxation time, T₁, in paradichlorobenzene single crystals.The directionality of the pressure dependence, together the temperature dependence ot T₁ near room temperature, give us some information about the dependence of the molecular torsional frequencies and lifetimes of the oscillations ground levels with the intermolecular crystalline potential.     

Analytical and radioanalytical quality control of purity and stability of radiopharmaceutical compound [186gRe]Re-HEDP for bone metastases pain palliation

Summary The nuclear properties of ^186gRe make it a useful agent for radionuclide therapy and imaging. The coordination compound [^186gRe]Re-HEDP has proved to be a successful bone seeking agent for palliation of metastatic bone pain. Chemical, radiochemical and radionuclidic purity of commercial radiopharmaceutical [^186gRe]Re-HEDP have been checked by means by γ- and β-spectrometries, INAA and paper radio-chromatography. The results indicate a good radionuclidic purity, with levels of contamination from the short-lived ¹⁸⁸Re well below the required specifications. After injection of the radiopharmaceutical, the radiochemical measurements conducted in vivo, on biological matrices, blood, plasma and urine, have shown that, entering the systemic circulation, ^186gRe dissociates from the bis-phosphonate complex as hydrosoluble [^186gRe]ReO₄-, and the two chemical species follow different biokinetics.     

High accuracy ab initio studies of Li6+, Li6-, and three isomers of Li6

The structures and energetics of Li(6) (+), Li(6) (-) and three isomers of Li(6) are investigated using the coupled-cluster singles, doubles and perturbative triples [CCSD(T)] method with valence and core-valence correlation consistent basis sets of double- to quadruple-zeta quality (cc-pVXZ and cc-pCVXZ, where X=D-Q). These results are compared with qualitatively different predictions by less reliable methods. Our results conclusively show that the D(4h) isomer is the global minimum structure for Li(6). It is energetically favored over the C(5v) and D(3h) structures by about 5.1 and 7.1 kcal mol(-1), respectively, after the inclusion of the zero-point vibrational energy (ZPVE) correction. Our most accurate total atomization energies are 123.2, 117.6, and 115.7 kcal mol(-1) for the D(4h), C(5v), and D(3h) isomers, respectively. Comparison of experimental optical absorption spectra with our computed electronic spectra also indicate that the D(4h) isomer is indeed the most stable structure. The cation, anion, and some higher spin states are investigated using the less expensive cc-pCVDZ basis set. Adiabatic ionization energies and electron affinities are reported and compared with experimental values. Predictions of molecular properties are found to be sensitive to the basis set used and to the treatment of electron correlation.     

Thermal decomposition of dicyclopentadienylarylvanadium compounds

The thermolysis of compounds of the type Cp₂VR (R = aryl) in the solid state has been studied. A distinct increase in thermal stability is observed upon substitution of the ortho-position of the aryl group. Thermal decomposition occurs with formation of RH, Cp₂ V, a vanadocene homologue with the group R substituted in one of the Cp rings and, probably, a vanadocene homologue with two substituted Cp rings. It is shown that the abstraction of the hydrogen atom from the cyclopentadienyl ring, necessary for the formation of RH, is an intermolecular process, whereas the substitution of the aryl group in the Cp ring is intramolecular. A decomposition mechanism is proposed in which the group R is transferred from the vanadium atom to the C₅H₅ ring of the same molecule by interaction with an aryl group of another molecule. The thermal decomposition of Cp₂VR is compared with that of the analogous titanium compounds.     

Z/E PHOTOISOMERIZATION OF UROCANIC ACID*

The E ? Z photoisomerization of the title compound (UA) (a naturally occurring sunscreen) has been studied in aqueous solution. At a UA concentration of 6mM and using 313nm excitation, φ_E→z= 0.52, φ_Z→E= 0.47 and the photostationary state is 34% E. Under these conditions, loss of UA is minimal. Low energy triplet quenchers fail to impede the isomerization, but the reaction can be induced by several triplet sensitizers. The E_T for UA is estimated to be approximately 55 kcal/mol.     

SN2 reactions in the gas phase. Structure of the transition state

The alkylhalide-halide association ions, [RX₂]^? that are observed in the negative chemical ionization mass spectra of alkyl halides appear to be directly related to the corresponding S_N2 transition states in solution. ‘Frontside’ association of halide ions with bridgehead alkyl halides does not occur in our system. The Change in heats and entropies of association for the chloromethane series is consistent with delocalization in the [RX]₂^? ions.