Costereosymmetric α-olefin copolymers

Copolymerization of propylene and 1-butene with highly stereospecific three-component coordination catalysts produced multiblock crystalline copolymers having stereo-regular sequences of both propylene and 1-butene. Copolymers containing from 3 to about 80% 1-butene had two DTA melting points which were attributable to polypropylene and poly-1-butene crystallinity. Those containing from 18 to about 70% 1-butene had x-ray diffraction patterns showing peaks characteristic of polypropylene and form I poly-1-butene, but form II poly-1-butene crystallinity was never observed. The multiblock copolymer structure observed is also supported by the fact that the product of the reactivity ratios is greater than unity. The composition distributions of low-conversion and continuously prepared copolymers were similar and relatively broad. For example, copolymers containing an average of 12% 1-butene had species containing from 5–30% 1-butene. High-conversion copolymers had an even broader composition distribution due to the gradual increase of the 1-butene concentration in the comonomer mixture as the copolymerization proceeded. The absence of homopolymers was demonstrated by fractionation. The ability to detect homopolymers was proved by the fact that a mixture of stereoregular polypropylene and poly-1-butene were readily separated. Increasing the amount of 1-butene tended to decrease those properties dependent upon crystallinity such as hardness, tensile strength, stiffness, density, and melting point, but tended to improve significantly the impact strength, low temperature properties, and clarity of molded objects. These duocrystalline copolymers retained a much higher level of properties than that observed for random copolymers prepared with less stereospecific coordination catalysts.     

Substituent control of hydrogen bonding in palladium(II)-pyrazole complexes

Inter- and intramolecular hydrogen bonding of an N-H group in pyrazole complexes was studied using ligands with two different groups at pyrazole C-3 and C-5. At C-5, groups such as methyl, i-propyl, phenyl, or tert-butyl were present. At C-3, side chains L-CH(2)- and L-CH(2)CH(2)- (L = thioether or phosphine) ensured formation of chelates to a cis-dichloropalladium(II) fragment through side-chain atom L and the pyrazole nitrogen closest to the side chain. The significance of the ligands is that by placing a ligating side chain on a ring carbon (C-3), rather than on a ring nitrogen, the ring nitrogen not bound to the metal and its attached proton are available for hydrogen bonding. As desired, seven chelate complexes examined by X-ray diffraction all showed intramolecular hydrogen bonding between the pyrazole N-H and a chloride ligand in the cis position. In addition, however, intermolecular hydrogen bonding could be controlled by the substituent at C-5: complexes with either a methyl at C-5 or no substituent there showed significant intermolecular hydrogen bonding interactions, which were completely avoided by placing a tert-butyl group at C-5. The acidity of two complexes in acetonitrile solutions was estimated to be closer to that of pyridinium ion than those of imidazolium or triethylammonium ions.     

SCF and MC–SCF Study of CO2 with bond angle variation

Multiconfiguration (MC ) SCF calculations are reported for CO₂ for bond angles between 60° and 180°. The ground state configuration is found to be …?5a4b∣b∣a for small bending angles and …?6a3b∣b∣a for large bending angles, the change in ground state character occurring at a bond angle of about 100°. The force constant for bending obtained from the MC –SCF function is about 8.0% lower than the corresponding SCF value, and in considerably better agreement with experiment.     

Semiempirical calculations of internal barriers to rotation and ring puckering: II. Amindo/3 study

The MINDO/3 technique is examined for its applicability to rotational barriers and ring puckering of strained-ring compounds. Comparisons are made with MINDO/2' and INDO on the compounds ethane, acetone, isobutylene, 3-oxetanone, cyclobutane, cyclobutanone, methylenecyclobutane, and 1,1-difluorocyclobutane. The MINDO/3 method improperly predicts all ring compounds to energetically favor the planar conformation as does its predecessor. Some improvement in bond lengths and bond angles is observed by MINDO/3 but the rotational barriers are still underestimated.     

Development of an epi-fluorescence assay for monitoring yeast viability and pretreatment hydrolysate toxicity in the presence of lignocellulosic solids

Applied Biochemistry and Biotechnology -     

Anomalous Energy Minima Surfaces

Previous publications^2–5 have examined the phenomenon of level crossings as predicted by the semiempirical techniques CNDO/2⁶, IMDO⁷, and MINDO/3⁸ for certain molecules when the total energy is viewed as a function of some conformational property. The purpose of this article is to examine this same phenomenon using the extended Hückel method(EHM)⁹, the iterated extended Hückel method(IEHM), and an ab-initio procedure with a minimal STO-3G basis set. The molecule chosen for this study is CO₂ due to its structural simplicity and since previous calculations^2–5 have demonstrated level crossing with this molecule.