Víctor L. Cruz

A theoretical study of the comonomer effect in the ethylene polymerization with zirconocene catalytic systems

The PM3(tm) semiempirical method has been used to optimize the structures for the reactants and transition states of the first and second ethylene insertion processes into zirconocene catalytic systems. The results obtained for these reactions are compared with calculations published in the literature performed at different ab-initio theoretical levels. The agreement between our calculations and those reported in the literature is satisfactory. Taking advantage of the reduced computational effort required in semiempirical calculations two additional processes related with the so-called comonomer effect were also studied: ethylene/1-hexene copolymerization, and chain termination reaction, both in the homopolymerization and in copolymerization of ethylene with 1-hexene comonomer. The calculated activation energies support some experimental findings such as the higher polymerization activities in the presence of comonomers and also the molecular weight reduction of the copolymers due to the more favorable β-elimination reactions.

Water-mediated conformations of the alanine dipeptide as revealed by distributed umbrella sampling simulations, quantum mechanics based calculations, and experimental data.

An exhaustive umbrella sampling simulation of the alanine dipeptide in solution is reported. The presence of stable Y conformations in solution is assessed by both quantum calculations and experimental data from X-ray and NMR protein-solved structures available in the protein coil library. The agreement between experimental and simulation Ramachandran plots of the dipeptide in solution is excellent. A suitable explanation of the stabilization of the Y conformation mediated by water for the alanine dipeptide is discussed on the basis of Car-Parrinello MD calculations of the dipeptide in water.

Ab initio calculation of ethylene insertion in zirconocene catalyst systems: A comparative study between bridged and unbridged complexes

Abstract The ab initio method at the Hartree-Fock level has been used on two zirconocene systems, the unbridged Cp 2 ZrCH 3 + and the bridged (Si(CH 3 ) 2 )Cp 2 ZrCH 3 + compounds. For each complex the insertion of an ethylene molecule has been followed from reactant to product through the corresponding transition states. The calculated activation energies are of the order of magnitude of those obtained experimentally, explaining the higher activity found for the unbridged catalytic complex compared to the bridged. The geometries are in agreement with other studies and comparable with those derived by crystallographic methods. The α-agostic interaction observed in the reactants as well as in the transition states and resulting products confirms that the Brookhart-Green mechanism is followed for the polymerization reaction.

Effect of a second ethylene molecule on the insertion of ethylene in zirconocene catalyst systems: A QM semiempirical study

The PM3(tm) semiempirical method was used to show the effect of a second ethylene molecule in the backside position on the frontside ethylene insertion in the Cp2ZrnPr+ γ-agostic resting state. The same calculations without a companion second ethylene molecule were performed to compare geometrical parameters, energies, and electrostatic charges. The results obtained show that the geometrical parameters for both cases were identical, but differences in the electrostatic charges were observed. The energy profile presented two barriers, the first corresponding to the alkyl-chain rotation along the ZrCα bond and the second relating to the insertion process itself. The presence of a companion second ethylene molecule affected the energetic profile by lowering the energy barrier of the first stage with respect to the process without the companion second ethylene molecule. These results provide some theoretical support to the well-known trigger effect.

Copolymerization of ethylene and styrene by homogeneous metallocene catalysts. 1. Theoretical studies with rac-ethylenebis-(tetrahydroindenyl)MCl2 [M=Ti, Zr] systems

Abstract A computational study of the ethylene–styrene copolymerization with rac -ethylenebis(tetrahydroindenyl)MCl 2 [M=Ti, Zr] systems using DFT methods is presented. The complexation, coordination and insertion energies for ethylene and styrene monomers as well as for the styrene–ethylene copolymerization steps into the catalytic active site models [Et 2 (IndH 4 ) 2 ]MCH 3 + [M=Ti, Zr] were calculated. The goal of this study is to examine the influence of the metal atom [Ti, Zr] on the copolymerization activity. It could be concluded that zirconocene catalyst is much more active than the titanocene based catalyst. This could be explained by the higher steric congestion around the Ti as compared to the Zr complex. Furthermore, it was found that the primary styrene insertion gives rise to complexes in which the active sites are blocked by the phenyl ring in both metal atoms, so that only the secondary insertion of the styrene is possible. These facts might help to clarify the already published experimental results.

DFT study of hydrogenolysis as a chain transfer mechanism in olefin polymerisation catalysed by nickel-diimine-type catalysts

Abstract This report describes a computational DFT study, based on the hybrid density functional B3LYP method, designed to investigate the hydrogenolysis process that occurs in ethylene polymerisation catalysed by a nickel-bulk diimine catalyst (N,N′-bis(2,6-diisopropylphenyl)-2,3-butanediimine nickel(II) methyl cation [{(2,6-(iPr)2Ph)NC(Me)–C(Me)=N(2,6-(iPr)2Ph)}NiCH3]+. Mechanisms of ethylene and hydrogen insertions into the growing polymer chain and ethylene insertion into the hydride complex formed after hydrogenolysis were investigated. The results obtained for these catalysts were then compared to those calculated for metallocene catalysts. Due to a higher difference in energy barriers between ethylene and hydrogen insertion, it may be concluded that controlling the molecular weight of the resultant polymer using nickel diimine catalysts is even more difficult than for the metallocene catalysts. Through statistical thermodynamics and kinetics calculations it was shown that the activity and molecular weights of polymers produced by nickel-diimine catalysts are dramatically decreased in the presence of hydrogen with respect to those of polymers generated by metallocene catalysts. These findings help to clarify the contradictory experimental results published in the scientific and technical literature regarding the effect of hydrogen on the catalytic activity and molecular weight of polymers produced using nickel diimine catalysts.

Computational studies of the Brookhart's type catalysts for ethylene polymerization. 1. Effect of the active site conformations on the catalyst activities

Abstract DFT calculations have been carried out on the cationic species for the two different Brookharts catalyst systems: [{ArNCH–HCNAr}NiR″]+ (3a) and [{ArNCMe–MeCNAr}NiR″]+ (3b) (where Ar={2,6-C6H3(Me)2} and R″=Me). These calculations reveal that the conformation of aryl groups attached to nitrogen atoms could provide a suitable explanation for the large experimental differences found in the ethylene polymerization activity.

Assessment of the intrinsic conformational preferences of dipeptide amino acids in aqueous solution by combined umbrella sampling/MBAR statistics. A comparison with experimental results.

The propensities of 19 amino acid dipeptides have been calculated by a distributed umbrella sampling molecular dynamics simulation procedure using the OPLS-AA force field. The potential of mean force maps was estimated with the multiple Bennett acceptance ratio statistics. The resulting propensities compare satisfactorily well with very recently published experimental data on equivalent systems. In particular, α conformation-probabilities for all of the dipeptides remain much lower than either β or P(II) propensities. This result is in agreement with most experimental data for dipeptides. However, it is also in contrast with most simulation studies performed so far with other force fields, where α conformations result even more probable than P(II) or β ones. We discuss the behavior of the OPLS-AA force field, which can be useful for the improvement of this model in reproducing the recent experimental observations on amino acid dipeptides.

Ethylene-styrene copolymerization with constrained geometry catalysts: A density functional study

A density functional theory (DFT) study of the ethylene-styrene copolymerization process with titanium-based constrained geometry catalyst (CGC) is presented. To establish the difference between simplified CGC or real CGC models, i.e., considering all ligands of the catalyst, we have performed calculations for ethylene and styrene insertions in both models. Thus, we have used two different DFT functional, BP86 and B3L YP along with two basis set, LANL 2DZ (without polarization functions) and DZVP (including polarization functions). We have noted certain differences between theoretical results published by other authors and our theoretical and experimental data.

Diosgenone Synthesis, Anti-Malarial Activity and QSAR of Analogues of This Natural Product

Solanum nudum Dunal steroids have been reported as being antimalarial compounds; however, their concentration in plants is low, meaning that the species could be threatened by over-harvesting for this purpose. Swern oxidation was used for hemisynthesis of diosgenone (one of the most active steroidal sapogenin diosgenin compounds). Eighteen structural analogues were prepared; three of them were found to be more active than diosgenone (IC50 27.9 μM vs. 10.1 μM, 2.9 μM and 11.3 μM). The presence of a 4-en-3-one grouping in the A-ring of the compounds seems to be indispensable for antiplasmodial activity; progesterone (having the same functional group in the steroid A-ring) has also displayed antiplasmodial activity. Quantitative correlations between molecular structure and bioactivity were thus explored in diosgenone and several derivatives using well-established 3D-QSAR techniques. The models showed that combining electrostatic (70%) and steric (30%) fields can explain most variance regarding compound activity. Malarial parasitemia in mice became reduced by oral administration of two diosgenone derivatives.