Jesus Lezama

Ethylene polymerization by TpRTi(OCH3)3-nCln complexes

Abstract Tp R Ti(OCH 3 ) 3− n Cl n (where Tp=hydrotris(pyrazolyl)borate, R=H, Tp ∗ = hydrotris (3,5-dimethylpyrazolyl)borate, R=3,5-CH 3 ; n =1, 2, 3) complexes have been tested for ethylene polymerization. Tp H Ti(OCH 3 ) 3− n Cl n ( n =1, 2) activated with MAO or MMAO showed higher catalytic activity than dimethyl substituted analogues Tp ∗ Ti(OCH 3 ) 3− n Cl n ( n =1, 2). Steric effects play a major role on the activity of ethylene polymerization. Successive self-nucleation/annealing (SSA) analysis showed that the polymers produced with use of Tp H Ti(OCH 3 ) 3− n Cl n ( n =1, 2) have a higher content of chain branching groups than the polymers deriving from Tp H TiCl 3 . The resulting polymers have melting temperatures characteristic of HDPE.

Kinetics and Mechanisms of the Unimolecular Elimination of 2, 2-Diethoxypropane and 1,1-Diethoxycyclohexane in the Gas Phase: Experimental and Theoretical Study

The gas-phase thermal elimination of 2,2-diethoxypropane was found to give ethanol, acetone, and ethylene, while 1,1-diethoxycyclohexane yielded 1-ethoxycyclohexene and ethanol. The kinetics determinations were carried out, with the reaction vessels deactivated with allyl bromide, and the presence of the free radical suppressor cyclohexene and toluene. Temperature and pressure ranges were 240.1-358.3 °C and 38-102 Torr. The elimination reactions are homogeneous, unimolecular, and follow a first-order rate law. The rate coefficients are given by the following Arrhenius equations: for 2,2-diethoxypropane, log k(1) (s(-1)) = (13.04 ± 0.07) - (186.6 ± 0.8) kJ mol(-1) (2.303RT)(-1); for the intermediate 2-ethoxypropene, log k(1) (s(-1)) = (13.36 ± 0.33) - (188.8 ± 3.4) kJ mol(-1) (2.303RT)(-1); and for 1,1-diethoxycyclohexane, log k = (14.02 ± 0.11) - (176.6 ± 1.1) kJ mol(-1) (2.303RT)(-1). Theoretical calculations of these reactions using DFT methods B3LYP, MPW1PW91, and PBEPBE, with 6-31G(d,p) and 6-31++G(d,p) basis set, demonstrated that the elimination of 2,2-diethoxypropane and 1,1-diethoxycyclohexane proceeds through a concerted nonsynchronous four-membered cyclic transition state type of mechanism. The rate-determining factor in these reactions is the elongation of the C-O bond. The intermediate product of 2,2-diethoxypropane elimination, that is, 2-ethoxypropene, further decomposes through a concerted cyclic six-membered cyclic transition state mechanism.

A new insight on the gas phase retro-Diels–Alder reaction of bicyclic compounds: density functional theory calculations

The mechanisms of the gas-phase thermal decomposition of bicyclo[2.2.1]heptadiene and 3,7,7-trimethylbicyclo[2.2.1]hept-2-ene were examined by density functional theory calculations with the hybrid functionals: B3LYP, CAM-B3LYP, MPW1PW91, and PBEPBE. Reasonable agreements were found between theoretical and experimental values with the B3LYP hybrid functional. Three molecular concerted pathways for bicyclo[2.2.1]heptadiene decomposition are proposed. The retro-Diels–Alder (retro-DA) pathway yields cyclopentadiene and acetylene through a nearly synchronous transition state structure (Sy = 0.97). The other two reaction channels are stepwise with a common step with the formation of the intermediate bicyclo[4.1.0] heptadiene. This reaction is dominated by C–C bond breaking leading to the methylene migration by an early transition state in the reaction coordinate (Sy = 0.91). The rearrangements of the latter intermediate producing toluene were also studied. The retro-DA elimination of 3,7,7-trimethylbicyclo[2.2.1]hept-2-ene gives 1,5,5-trimethyl-cyclopenta-1,3-diene in a less synchronous process (Sy = 0.77). This fact may be due to the electronic effects of the methyl substituent. The latter product is unstable and undergoes methyl migrations to give a more stable isomer 1,2,3-trimethylcyclopenta-1,3-diene. The stepwise mechanism for the retro-DA reaction through a biradical intermediate appears to be unfavourable because the barrier is bigger than that for the concerted reaction.

Theoretical calculations of homogeneous catalysis in the gas phase: elimination kinetics of 2,2-dimethoxypropane in the presence of HCl, F3CCOOH and CH3COOH

ABSTRACT The mechanisms of gas-phase elimination kinetics of 2,2-dimethoxypropane in the presence of hydrogen chloride, trifluoroacetic acid and acetic acid were studied using Moller Plesset, ab initio combined method Complete Basis Set (CBS)-QB3 and various density functional theory methods with 6-311G(d,p) and 6-311++G(d,p) basis sets. The M06-2X/6-311++G(d,p) method provided reasonable agreement with the experimental enthalpy and energy of activation. Formation of 2-methoxypropene and methanol products occurs through six-membered cyclic ring transition state (TS) structure. The TS was characterised by single imaginary frequency, and confirmed through intrinsic reaction coordinate (IRC) calculations. The IRC calculations suggest the development of a van der Waal complex between the 2, 2-dimethoxy propane and the acid catalyst, leading to the TS formation. The process of decomposition in the absence of the acid catalyst requires much higher temperature with an energy of activation above 200 kJ/mol. This fact appears to be a consequence of a four-membered cyclic TS-type of mechanism in the non-catalysed reaction. Structural parameters, analyses of natural bond orbital charges and bond orders of the acid-catalysed elimination reactions in this study suggest that the polarisation of the C–O bond, in the direction Cδ+—Oδ−, is rate-determining in the TS. These reactions are non-synchronous concerted polar in nature.

DFT and ab-initio study on the mechanism of the gas-phase elimination kinetics of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1-ene and their isomerization

The mechanisms of the gas-phase elimination kinetics of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1-ene and their interconversion have been examined at MP2 and DFT levels of theory. These halide substrates yield isoprene and hydrogen chloride. The results MPW1PW91 calculations agree with the experimental kinetic parameters showing the elimination reaction occurs at greater rate for 1-chloro-3-methylbut-2-ene than that for the 3-chloro-3-methylbut-1-ene isomer. The mechanism for the molecular elimination of 1-chloro-3-methylbut-2-ene suggests proceeding through an uncommon six-membered cyclic transition state for alkyl halides in the gas phase, while 3-chloro-3-methylbut-1-ene eliminates through the usual four-membered cyclic transition state. The elongation and subsequent polarization of the C-Cl bond, in the direction of C^{δ+}…Cl^{δ-}, is rate determining step of these reactions. The isomerization of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1-ene was additionally studied. The 1-chloro-3-methylbut-2-ene converts to 3-chloro-3-methylbut-1-ene easier than the reverse reaction. This means that 1-chloro-3-methylbut-2-ene was found thermodynamically more stable than 3-chloro-3-methylbut-1-ene.