Jairo Quijano

Homogeneous pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate in the gas phase

The pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate have been examined over the temperature range of 286–330°C and pressure range of 29–108 Torr. In a seasoned vessel and in the presence of the free radical inhibitor cyclohexene or toluene the reaction is homogeneous, unimolecular and obeys a first-order rate law. The elimination products are mainly acetone and ethyl acetate, and very small amounts of ethyl 3-butenoate, acetic acid, ethylene and H2O. The rate coefficient is expressed by the following equation: log k1(s−1)=(12.39±0.46)−(174.5±5.2) kJ mol−1 (2.303RT)−1. The mechanism appears to proceed via a six-membered cyclic transition state, where polarization of the (CH3)C(OH)δ+...δ-CH2COOCH2CH3 bond is rate determining.

Ab initio study on the thermolysis of β-hydroxyolefins in gas phase

Abstract Thermolysis studies of β-hydroxyolefins in gas phase were realized using ab initio MP2 and DFT methods at the 6-31G∗ levels to explore the possibility of determining a possible concerted process with a six-membered cyclic transition state (TS). Vibrational frequency calculations were carried out in order to confirm the stationary states, including TS structures. IRC calculations have been performed in all cases in order to verify that localized TS structures connect with the corresponding minimum stationary points associated with the reactant and products. With the aim of corroborating the postulated mechanism in the experimental study, we present a theoretical study in order to calculate the rate constants and the activation parameters. The results obtained are in accordance with the experimental conclusions.

Hemoglobin precipitation by polyethylene glycols leads to underestimation of membrane pore sizes

The size of pores formed in the plasma membrane by various substances is frequently determined using polyethylene glycols as osmotic protectants. In this work, we have found that the size of pores formed by saponin in the red blood cell membrane determined by hemolysis versus molecular weight of polyethylene glycol was different to that estimated by light dispersion of cell suspensions. After complete swelling of cells induced by saponin in semiisotonic salt media containing 150 mOsm PEG-4000 or PEG-3000, a significant increase in the light absorbance at 640 nm was developed resulting from the formation of hemoglobin precipitates. Easily sedimenting aggregates were also formed when the supernatant of lysed cells was added to the equiosmotic solutions of polyethylene glycols with molecular weight higher than 1000. We suggest that the real size of large pores could be underestimated due to the phenomenon of hemoglobin precipitation by polyethylene glycols.

Understanding the thermal dehydrochlorination reaction of 1-chlorohexane. Revealing the driving bonding pattern at the planar catalytic reaction center

The nature of bonding along the gas-phase thermal decomposition of 1-chlorohexane to produce 1-hexene and hydrogen chloride has been examined at the DFT M05-2X/6-311+G(d,p) level of theory. Based on results both from energetical and topological analysis of the electron localization function (ELF), we propose to rationalize the experimental available results not in terms of a decomposition via a four-membered cyclic transition structure (TS), but properly as a two stage one step reaction mechanism featuring a slightly asynchronous process associated to the catalytic planar reaction center at the TS. In this context, the first electronic stage corresponds to the Cδ+⋯Clδ− bond cleavage, which take place on the activation path earlier the transition structure be reached. There is no evidence of a Cl–C bond at the TS configuration. The second stage, associated to the top of the energy barrier, includes the TS and extends beyond on the deactivation pathway towards the products. The existence of both bonding and nonbonding non covalent interactions (NCI) are also revealed for the first time for the TS configuration.

Alpha-deuterium isotope effects in the thermolysis of β-hydroxy esters

Alpha secondary isotope effects are determined experimentally by thermolysis of β-hydroxy esters. These results are compatible with a change of hybridization of carbon-2 from reactant to transition state (sp3 - sp2).

Understanding the thermal [1s,5s] hydrogen shift isomerization of ocimene

Abstractα-Ocimene, β-ocimenes and alloocimenes are isomeric monoterpenes occurring naturally as oils within several plants and fruits. These thermally unstable compounds are employed in the pharmaceutical and fine-chemicals industries due to their natural plant defense properties and pleasant odors. In this work, and in the context of a recent revival in attention on the subject, we provide new theoretical insights concerning the nature of the electronic reorganization driving the decomposition of cis-β-ocimene to alloocimene. Our findings support the experimental proposal of a rearrangement via a six-membered cyclic transition state in a one-step concerted and highly synchronic process. FigureThermal isomerization of ocimene; two transition structures are feasible considering the relative position (equatorial or axial) of the bulky isobutenyl group at carbon C5

Elimination kinetics of β-hydroxynitriles in the gas phase

The gas-phase elimination kinetics of primary, secondary and tertiary β-hydroxynitriles were examined in static seasoned vessels over the temperature range 360–450 °C and pressure range 47–167 Torr (1 Torr = 133.3 Pa). These reactions are homogeneous, unimolecular and follow a first-order rate law. The rate coefficients are given by the Arrhenius equation: for 3-hydroxypropionitrile log k1 = (14.29 ± 0.47) − (234.9 ± 6.3) kJ mol−1 (2.303 RT)−1; for 3-hydroxybutyronitrile log k1 = (13.76 ± 0.10) − (222.6 ± 0.7) kJ mol−1 (2.303RT)−1; and for 3-hydroxy-3-methylbutyronitrile log k1 (s−1) = (13.68 ± 0.68) − (212.5 ± 8.7) kJ mol−1 (2.303RT)−1. The decomposition rates of the β-hydroxynitriles increase from primary to tertiary carbon containing the OH group. The rates for the β-hydroxynitriles are found to be slower than those for the corresponding β-hydroxyacetylene analogs. The value of log A from 13.7 to 14.4 and the small positive ΔS ≠ indicate a mechanism different from a six-centered cyclic transition state. These data appear to indicate that a four-membered cyclic transition state or a quasi-heterolytic mechanism is conceivable. Copyright

Halodiazirines and halodiazo compounds: a computational study of their thermochemistry and isomerization reaction

A computational study of the isomerization reaction of a series of halodiazirines to halodiazo compounds (cyclic to open-chain RXCN2 species) has been carried out in order to establish the effect of the substituent groups on the isomerization rates and to obtain computational evidence of reaction mechanisms. Fluorine and chlorine were present as the halogen (X) atom, and the groups R=H, CH3, C2H5, n-C3H7, i-C3H7, cyclo-C3H5, phenyl, OCH3 and OH were used. Thermochemical calculations and natural bond orbital analyses were carried out at the B3LYP/6-31+G(d,p) level of theory. The results allowed us to discuss a reaction mechanism that proceeds in two steps: The first is the extrusion of nitrogen and formation of a carbene through a cyclic transition state that promotes the simultaneous breaking of the two C–N bonds, and the second one is described as the rebounding between the carbene and one of the nitrogen atoms of molecular nitrogen, both formed in the first step. The enthalpies of formation of halodiazirines and halodiazoalkanes have been calculated at the G3 level of theory.

Método Melhorado para Calcular a Leitura Infinita de Cinéticas de Primeira Ordem

Infinity readings for first order kinetics can be calculated from any three measurements (triads) of a physical property l at three equally spaced times. Accurate results can be obtained from time intervals as low as 0.4 half-life. Calculation of infinity readings l¥ from several triads at increasing values of time gives an insight into the deviation of the first order kinetics when parallel, consecutive or other spurious reactions occur along with the main first order reaction, not allowing direct measurements or calculation of l¥. The proposed method is more sensitive in distinguishing between first and second order kinetics than the Guggenheim and Kezdy-Swinbourne methods.