Annia Galano

A new approach to counterpoise correction to BSSE

In the present work the intermolecular BSSE, associated to the A–B interaction, is obtained by subtracting the intramolecular BSSE of the fragments from the intramolecular BSSE of the supermolecule, and considering every atom as a fragment in the calculation of each intramolecular BSSE. This atom by atom scheme (CPaa) is based on the consideration that the proximity of the fragments may affect the intramolecular BSSE of every involved species, and artificially influences the value of the BSSE associated to the supermolecule formation. It drastically decreases the reported counterpoise overcorrection of the A–B interaction, even though it does not deal with all the overcorrection because it includes all the orbitals, and not only the unoccupied ones. This new approach has been tested on the water dimer, some hydrogen fluoride weakly bonded complexes, the conformational analysis of 1,2‐dichloroethane, and the reaction profile of formaldehyde + OH reaction.

Gas phase reactions of C1–C4 alcohols with the OH radical: A quantum mechanical approach

CCSD(T)//BHandHLYP/6-311G(d,p) calculations have been performed to study the OH hydrogen abstraction reaction from C1–C4 aliphatic alcohols. A complex mechanism involving the formation of a stable pre-reactive complex is proposed and the temperature dependence of the rate coefficients is studied over the temperature range of 290–500 K, using conventional transition state theory (CTST). Excellent agreement between calculated and experimental k at 298 K has been obtained. Arrhenius expressions are proposed for 1-propanol and 1-butanol, k1-Propu2006=u20063.06u2006×u200610−12exp(140/T) and k1-Butu2006=u20062.14u2006×u200610−12exp(440/T) cm3 molecule−1·s−1, respectively. The rate coefficient for the formation of the alpha radical is found significantly larger than that of the competing channels for C1–C3 alcohols. The finding that at room temperature the rate constant of 1-butanolγ is the largest one supports some of the previous experimental results.

Theoretical study of the initial reaction between OH and isoprene in tropospheric conditions

The reaction of isoprene with OH radicals has been investigated by ab initio molecular orbital theory. We report the energetics of four different pathways, involving the direct addition of OH to four of the carbon atoms. Calculations have been performed using both density functional theory (BHandHLYP) and Moller–Plesset perturbation theory to the second-order (MP2). Two pre-reactive complexes have been identified, whose stabilization energy with respect to the separated reactants is about 12 kJ mol−1. Their structure is similar to the ones previously reported for OH–ethene and OH–propene adducts: the OH radical is placed over either one of the double bonds at a distance of about 2.1 A, with the H atom pointing towards the C–C bond. The geometries of the transition states corresponding to OH addition at the four different positions have been optimized. The calculated apparent activation energies are negative for addition at the terminal carbon atoms and in excellent agreement with the experimental measurements. Direct addition at the internal carbon atoms involves much higher energy barriers, and these pathways are expected to be negligible at normal temperatures. Thus, the observed formation of 3-methylfuran must occur after radical addition to the terminal carbon atoms, following a pathway such as the one proposed by R. Atkinson, S. M. Aschmann, E. C. Tuazon, J. Arey and B. Zielinska, Int. J. Chem. Kinet., 1989, 21, 594 (). Calculated overall rate constants are obtained, in excellent agreement with experimental values. The two-parameter equation for the calculated overall rate coefficient was found to be (2.12u2006±u20060.42)u2006×u200610−11 exp[(384u2006±u200655)/T] cm3 molecule−1 s−1, while the best fit for the four channels studied here correspond to the following expressions: k1u2006=u2006(2.25u2006±u20060.51)u2006×u200610−11 exp[(253u2006±u200662)/T], k2u2006=u2006(9.60u2006±u20061.18)u2006×u200610−13 exp[(−2871u2006±u200635)/T], k3u2006=u2006(1.81u2006±u20060.22)u2006×u200610−12 exp[(−1567u2006±u200633)/T], and k4u2006=u2006(2.39u2006±u20060.27)u2006×u200610−12 exp[(676u2006±u200633)/T] cm3 molecule−1 s−1.

Mechanism and rate coefficients of the gas phase OH hydrogen abstraction reaction from asparagine: a quantum mechanical approach

Abstract Unrestricted density functional theory (B3LYP) calculations have been performed using the 6-311G(d,p) basis sets, to study the gas phase OH hydrogen abstraction reaction from asparagine. The structures of the different stationary points are discussed. Ring-like structures are found for the transition states. Reaction profiles are modeled including the formation of pre-reactive complexes, and negative net energy barriers are obtained. A complex mechanism involving the formation of a pre-reactive complex is proposed, and the rate coefficients are calculated using Conventional transition state theory over the temperature range 250–350xa0K. ZPE and thermal corrections to the energy for all the species, and BSSE corrections for the pre-reactive complex stabilization and for the energy barriers are included. The rate coefficients are proposed for the first time and it was found that the hydrogen abstraction occurs almost exclusively from the beta site. The following expressions, in Lxa0mol−1xa0s−1, are obtained for the alpha channel, beta channel and the overall temperature dependent rate constants: kα=(1.46±0.04)×108xa0exp[(−1217±7)/T], kβ=(5.48±0.25)×107xa0exp[(1483±13)/T], and k=(5.50±0.25)×107xa0exp[(1482±13)/T], respectively.

On the role of s-cis conformers in the reaction of dienes with OH radicals

The reactions of OH radicals with s-cis and s-trans-butadiene and s-cis-isoprene have been modeled by ab initio Molecular Orbital Theory. Density Functional Theory (BHandHLYP) calculations have been performed for both butadiene and isoprene, and Moller–Plesset Perturbation Theory to the second-order (MP2) has also been used for s-cis-isoprene in order to compare with previous work. Pre-reactive complexes are identified in all cases, with the OH radical placed over either one of the double bonds at a distance of about 2 A and the H atom pointing towards the C–C bond. The geometries of the transition states corresponding to OH addition at all positions have been fully optimized. The calculated apparent activation energies are negative for addition at the terminal carbon atoms and in excellent agreement with the experimental measurements. The possible role of direct additions at the internal carbon atoms in the formation of furan-like compounds is discussed. Energy barriers for the s-cis conformers are found to be smaller than those for the s-trans conformers, especially for addition at the internal carbons, suggesting that the s-cis conformers could play a role in the tropospheric oxidation of dienes. Calculated overall rate constants are in good agreement with experimental values. Partial rate coefficients corresponding to the different channels are reported. The temperature dependence is studied in the 290–500 K range and two-parameter equations are reported for each rate coefficient. The calculated partial rate coefficients of addition to internal carbon atoms are not large enough to account for the observed yield of 3-methylfuran.

Rate coefficients and mechanism of the gas phase OH hydrogen abstraction reaction from serine: a quantum mechanical approach

Abstract Unrestricted density functional theory (B3LYP) calculations have been performed using the 6-311G(d,p) basis sets, to study the hydrogen abstraction from serine by OH in the gas phase. The structures of the different stationary points are discussed. Ring-like structures are found for the transition states. Reaction profiles are modeled including the formation of pre-reactive complexes, and negative net activation energies are obtained, after including basis set superposition error corrections. A complex mechanism involving the formation of a pre-reactive complex is proposed, and the rate coefficients are calculated using conventional transition state theory over the temperature range 250–350 K. The rate coefficients are proposed for the first time. The following expressions, in Lxa0mol −1 xa0s −1 , are found for the alpha channel, beta channel and the overall temperature-dependent rate coefficients: k α =(2.76±0.08)×10 8 exp [(1389±8)/T], k β =(1.26±0.07)×10 8 exp [(1524±17)/T] and k tot =(3.91±0.17)×10 8 exp [(1446±12)/T], respectively.

Kinetics and mechanism of the β-alanine + OH gas phase reaction : A quantum mechanical approach

The OH hydrogen abstraction reaction from beta-alanine has been studied using the BHandHLYP hybrid HF-density functional and 6-311G(d,p) basis sets. The energies have been improved by single point calculations at the CCSD(T)/6-311G(d,p) level of theory. The structures of the different stationary points are discussed. Reaction profiles are modeled including the formation of pre-reactive and product complexes. Negative net activation energy is obtained for the overall reaction. A complex mechanism is proposed, and the rate coefficients are calculated using transition state theory over the temperature range of 250-400 K. The rate coefficients are proposed for the first time and it was found that in the gas phase the hydrogen abstraction occurs mainly from the CH(2) group next to the amino end. The following expressions, in cm(3) mol(-1) s(-1), are obtained for the overall rate constants, at 250-400 and 290-310 K, respectively: k(250-400)= 2.36 x 10(-12) exp(340/T), and k(290-310)= 1.296 x 10(-12) exp(743/T). The three parameter expression that best describes the studied reaction is k(250-400)= 1.01 x 10(-21)T(3.09) exp(1374/T). The beta-alanine + OH reaction was found to be 1.5 times faster than the alpha-alanine + OH reaction.