Ethylene polymerization by bis(salicylaldiminate)nickel(II)/aluminoxane catalysts

The homopolymerization of ethylene by using different catalytic systems based on dinitro‐substituted bis(salicylaldiminate)nickel(II) precursors such as bis[3,5‐dinitro‐N(2,6‐diisopropylphenyl)]nickel(II) and bis[3,5‐dinitro‐N(phenyl)]nickel(II) in combination with organoaluminum compounds was investigated. In particular, the catalytic performances were studied as a function of the main reaction parameters, such as temperature, pressure, Al/Ni molar ratio, and duration. Methylaluminoxane resulted in the best co‐catalyst. Activities up to 200 kg polyethylene/(mol Ni × h) to give a linear high‐molecular‐weight polymer were achieved. The influence of the bulkiness of the substituents on the N‐aryl group of the aldimine ligand was also checked; it resulted in a determinant for catalytic activity rather than for polymer characteristics. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2534–2542, 2004     

Dihydrogen activation promoted by homonuclear ion pairs involving cobalt(II) and cobalt(−I)

Acyltetracarbonyls have been shown to be formed from the reaction of hexene with CO/H₂ (1/1) mixtures (P(CO) = P(H₂) = 300 mmHg) in THF/H₂O at room temperature catalysed by homonuclear Co_II, Co^?I ion pairs.     

Steric and electronic contributions to conformational effects on chemical shifts of acyclic alcohols

Our calculations on bi- and polycyclic alcohols reveal that the Mulliken charge distribution and chemical shift patterns due to hyperconjugation of lone pairs on oxygen with neighboring groups break down or are attenuated for certain spatial relationships of the hydroxyl group. Since in strained ring systems other effects on these parameters may be present, we applied a similar analysis to acyclic alcohols. Calculations at the B3LYP/6-31G^* level on conformers of methanol, ethanol, 1- and 2-propanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 2-butanol, 2-methyl-2-butanol, 1- 2- and 3-pentanol and 2-methyl-3-pentanol, where hyperconjugation may be present, reveal steric effects as modifiers of hyperconjugative patterns affecting carbon-13 chemical shifts in such alcohols. Contrary to what is observed in bi- and policyclic systems, where electrostatic effects interfere with effects due to hyperconjugation, these steric effects may be the main cause for the attenuation of deshielding of nuclei that are subject to hyperconjugation. Electrostatic effects are also present but they do not interfere with hyperconjugation by lone pairs. Conformational effects fall off sharply after the third carbon in the chain.     

Gradient optimization of polarization exponents inab initio MO calculations on H2SO → HSOH and CH3SH → CH2SH2

Summary Analytical gradients were used to optimize the polarization function exponents in the 6-31G(d) and 6-31G(d, p) basis sets for the reactants, transition structures and products in the reactions H₂SO HSOH and CH₃SH CH₂SH₂. The optimizedd exponents on the heavy atoms change by ±10% in the course of the reactions and depend on the bonding of the heavy atoms. Thep exponents on the hydrogens change by as much as a factor of 5 and depend on the element to which the hydrogen is bonded and its valency. The effect of exponent optimization on the relative energies is small (±3 kcal/mol). With the 6-31G(d, p) basis set, optimization of the polarization exponents can make some of the bonds significantly more polar, as judged by the Mulliken charges.     

Conformational preferences in methylsilane and disilane. A quantitative non-empirical analysis of the importance of the hyperconjugative interactions

We have investigated the importance of the hyperconjugative interactions in determining the conformational preferences in methylsilane and disilane, and for comparative purposes also in ethane, using a procedure that provides quantitative information about the energy effects associated with orbital interactions. It is found that (1) these interactions are in all cases destabilizing and less destabilizing in the staggered conformation and that (2) in the absence of these interactions, in ethane the staggered conformer is still more stable, while in methylsilane and disilane the eclipsed conformer becomes more stable.We have also investigated the effects of each type of orbital interaction in terms of a quantitative perturbational analysis.     

Theoretical investigation of the formic acid decomposition kinetics

Decomposition of formic acid (HCO₂H) proceeds via three unimolecular channels: dehydration, decarboxylation, and dissociation, the latter expected to be of minor contribution to the overall kinetics. In addition, despite the similar values reported for the individual activation energies for the dehydration and decarboxylation reactions, experimental works have shown that the former is dominant in the reaction mechanism. These reactions show pressure-dependent rate coefficients, and the high-pressure condition is not yet verified at atmospheric pressure. This work aims to investigate the influence of temperature and pressure on the rate coefficients. Hence, theoretical calculations at the CCSD(T)/CBS level have been performed to accurately describe the unimolecular reaction and Rice-Ramsperger-Kassel-Marcus (RRKM) rate coefficients have been calculated and integrated for the prediction of k(T,P) rate coefficients, adopting both strong and weak collision models, over the intervals 0.5-10 atm and 298-2200 K. Our results suggest that the isomerization path is important and explains the preference for the (CO + H₂O) channel. Rate coefficients for the (CO₂ + H₂) and (CO + H₂O) formations are given, in s⁻¹, as exp(−34404/T) and exp(−33785/T), respectively. The dissociation limit of 107.29 kcal mol^–1, with respect the Z-HCO₂H conformer, leading to OH + HCO, via a barrierless potential curve, with rate coefficients, in s⁻¹, expressed as k_HCO+OH(T) = 1.68 × 10¹⁷ exp(−56018/T). Temperature and pressure dependence for the HCO + OH → CO₂ + H₂ and HCO + OH → CO + H₂O reactions have also been estimated.     

Red to blue tunable upconversion in Tm3+-doped ZrO2 nanocrystals

The effect of dopant concentration on the blue upconversion (UPC) emission of Tm(3+) -doped ZrO(2) nanocrystals under different excitation wavelengths in the red region is reported. The UPC emissions are due to the f-f electronic transitions from excited states (1)G(4) and (1)D(2) of Tm(3+). We observed a chromatic change in the UPC with tuning the excitation wavelength. The UPC emission bands at 475, 488, and 501 nm are observed under excitation at 649 nm, but bands centered at 454 and 460 nm are observed when the excitation wavelength is tuned to 655 nm. The UPC emission could be tuned from 501 to 454 nm ( approximately 47 nm) by changing the excitation wavelength from 649 to 655 nm ( approximately 6 nm). The pump power dependence of the emission bands at 475, 488, and 501 nm were investigated on excitation intensity at 649 nm, and the emission bands at 454 and 460 nm are investigated on excitation intensity at 655 nm, which confirms that all of these UPC emission lines are a two-photon absorption process.     

I sistemi ciclici nello spazio euclideo adn dimensioni

Rendiconti del Circolo Matematico di Palermo Series 1 -     

Symmetric hydrogen bonding. Infrared absorption spectrum of potassium hydrogen succinate single crystal

Polarized spectra of the title compound were recorded and the assignment of the most relevant bands is discussed. Particular attention is paid to the B_u type band at 1680 cm^?1 which is not accounted for in the factor group analysis.