Two different theoretical approaches are used to study the OH radical attack on toluene: the Møller-Plesset perturbation theory and the B3LYP density functional method. The critical points of the potential energy surface for the OH addition to toluene are determined, and rate-equilibrium relationships are discussed. A stable structure corresponding to a prereactive complex which is formed when the OH radical is at about 2.5 Å from toluene is obtained. The existence of this loosely bound system is necessary to explain the experimentally observed negative activation energy. The geometry of transition states and products are determined for addition at different positions in the ring, including the ipso position, which has not been considered in previous works. Energy results at the MP4 and coupled cluster levels calculated at the optimized MP2 and B3LYP geometries confirm that the ipso adduct is more stable than the ortho adduct by about 0.5 kcal/mol. Several routes are proposed for the subsequent reactions of the ipso adduct, which could explain the very high yield of o-cresol with respect to the other cresol isomers.

No abstract

Asphaltene aggregation is a two-step process concerning phase separation and asphaltene particle growth which provoke crude oil destabilization and significant problems during the production, transport, and refining of heavy and extra heavy crude oils. A recent and innovative approach to overcome this problem is the use of ionic liquids (ILs) as inhibitors or stabilizers of asphaltene aggregation. Since the information concerning the properties of the studied ILs is scarce, we characterized some of their electronic properties and critical aggregation concentration (CAC) by quantum chemistry and spectrofluorometry, respectively. We found that the presence of a complex anion such as [AlCl 4 ] -, [BF 4 ] -, and [PF 6 ]incremented the HOMO-LUMO gap (Δ H-L ), electronegativity (χ), absolute hardness (η), and dipole moment (μ) when compared to [Br] --containing ILs. Moreover, the ILs' CAC values showed a linear correlation with the dipole moment. Afterward, we studied the effect of various commercial ILs on the aggregation point (AP) of a heavy crude oil (HCO) due to the increment of (a) its concentration in toluene solutions or (b) the n-heptane volume by means of fluorescence spectroscopy. We have found that the aggregation of HCO occurs at larger crude oil concentration or n-heptane volume in the presence of some ILs. Here, ILs set a polar microenvironment around HCO asphaltenes, which stabilized them against further aggregation and precipitation. The better performance of ILs as inhibitors or stabilizers of asphaltene aggregation was found with those comporting a complex anion, a pyridinium ring, or a shorter alkyl substitution on the cation. Such ILs present the higher values of the calculated electronic properties.

The OH abstraction of a hydrogen atom from both the side chain and the ring of toluene has been studied in the range 275-1000 K using quantum chemistry methods. It is found that the best method of calculation is to perform geometry optimization and frequency calculations at the BHandHLYP/6-311++G(d,p) level, followed by CCSD(T) calculations of the optimized structures with the same basis set. Four different reaction paths are considered, corresponding to the side chain and three possible ring hydrogen abstractions, and the branching ratio is determined as a function of temperature. Although negligible at low temperatures, at 1000 K ring-H abstraction is found to contribute 11% to the total abstraction reaction. The calculated rate coefficients agree very well with experimental results. Side chain abstraction is shown to occur through a complex mechanism that includes the reversible formation of a collisionally stabilized reactant complex.