Josep Maria Bofill

Updated Hessian matrix and the restricted step method for locating transition structures

A family of the updated Hessian matrices for locating transition structures is presented. An analysis and improvement of the restricted step algorithm described by Culot et al. is proposed. The efficiency of the latter method is compared with other well‐established methods for locating transition structures.

The Ozonolysis of Ethylene: A Theoretical Study of the Gas‐Phase Reaction Mechanism

An important source of atmospheric polution, the gas-phase ozonolysis of ethylene, has been submitted to systematic theoretical investigation. Apart from its concerted cleavage to the Criegee intermediates, the ethylene primary ozonide (POZ) decomposes in a stepwise mechanism by the alternative routes shown here.

A reduced‐restricted‐quasi‐Newton–Raphson method for locating and optimizing energy crossing points between two potential energy surfaces

We present a method for the location and optimization of an intersection energy point between two potential energy surfaces. The procedure directly optimizes the excited state energy using a quasi‐Newton–Raphson method coupled with a restricted step algorithm. A linear transformation is also used for the solution of the quasi‐Newton–Raphson equations. The efficiency of the algorithm is analyzed and demonstrated in some examples.

Tropospheric formation of hydroxymethyl hydroperoxide, formic acid, H2O2, and OH from carbonyl oxide in the presence of water vapor: a theoretical study of the reaction mechanism.

We have carried out a theoretical investigation of the gas-phase reaction mechanism of the H2COO+ H2O reaction, which is interesting for atmospheric purposes. The B3LYP method with the 6-31G(d,p) and 6-311 + G(2d,2p) basis sets was employed for the geometry optimization of the stationary points. Additionally, single-point CCSD(T)/6-311 + G(2d,2p) energy calculations have been done for the B3LYP/6-311 + G(2d,2p) optimized structures. The reaction begins with the formation of a hydrogen-bond complex that we have calculated to be 6 kcalmol(-1) more stable than the reactants. Then, the reaction follows two different channels. The first one leads to the formation of hydroxymethyl hydroperoxide (HMHP), for which we have calculated an activation barrier of deltaGa(298) = 11.3 kcalmol(-1), while the second one gives HCO + OH + H2O, with a calculated activation barrier of deltaGa(298) = 20.9 kcalmol(-1). This process corresponds to the water-catalyzed decomposition of H2COO, and its unimolecular decomposition has been previously reported in the literature. Additionally, we have also investigated the HMHP decomposition. We have found two reaction modes that yield HCOOH+H2O; one reaction mode leads to H2CO + H2O2 and a homolytic cleavage, which produces H2COOH + OH radicals. Furthermore, we have also investigated the water-assisted HMHP decomposition, which produces a catalytic effect of about 14 kcalmol(-1) in the process that leads to H2CO + H2O2.

Atmospheric Formation of OH Radicals and H2O2 from Alkene Ozonolysis under Humid Conditions

Detailed mechanistic knowledge about the formation of OH radicals and H2O2 in alkene ±ozone reactions is of enormous interest for tropospheric chemistry, since these molecules are among the most important oxidants in the atmosphere. Hydroxyl radicals oxidize many gaseous trace compounds rapidly and, accordingly, their concentration determines the atmospheric lifetimes of many compounds. Therefore, OH radicals play a key role for the chemistry of the polluted atmosphere. H2O2 contributes to acid precipitation by the conversion of SO2 to H2SO4 and it is also known to damage trees and plants. 5] An important source for OH radicals during daytime represents the photolysis of ozone. During nighttime, OH radicals are most likely generated by reactions between NO3 and aldehydes, or NO3 and alkenes followed by a reaction with O2. In recent years, convincing experimental evidence has been collected to confirm the gas phase formation of OH radicals in the ozonolysis of alkenes, 11±20, 38] both during dayand nighttime. After early controversies concerning the question how hydroxyl radicals are formed from the alkene ±ozone reaction, 11] recent reports on the direct observation of OH radicals provide evidence that OH radicals are produced in the alkene ozonolysis. 13, 19, 20] Quantum chemical investigations have provided convincing evidence that confirm and clarify the mechanism leading to radical formation. The process is highly efficient, in particular for internal alkenes, 16, 26, 27] and hence this source of OH radicals competes with the photolysis of ozone in the daytime and with reactions initiated by NO3 at night. On the other hand, it is well known that hydrogen peroxide is formed in the atmosphere through recombination of two HO2 radicals, but recent experimental evidence indicates that the reaction of ozone with alkenes produces H2O2 in a mechanism which involves water vapor but no HO2 radicals. Hence, alkene ozonolysis plays an important role to explain the formation of both OH radicals and H2O2 from anthropogenic and biogenic alkenes in urban and rural areas 22] as well as in indoor air. This reaction is initiated by the addition of ozone to the double bond of the alkene and the formation of a primary ozonide (POZ; 1,2,3-trioxolane), which is then cleaved to give a carbonyl oxide (Criegee intermediate) and a carbonyl compound, Equation (1).

Point-dipole approximation of the exciton coupling model versus type of bonding and of excitons in porphyrin supramolecular structures.

The application of the exciton coupling model to interacting porphyrin chromophores is discussed. Covalently bonded systems and ionic or electrostatically bonded homoassociates require different orientations of the transition dipole moments in order to explain the experimental results: according to the symmetry of the assembly for covalently bonded porphyrins, and assuming isolated chromophores for ionic bonded porphyrins. Further, for covalently bonded systems, an extended exciton coupling has been demonstrated, but the ionic systems are in agreement with non-extended couplings. The relation of these facts to a molecular description of solid-state Wannier-Mott or Frenkel excitons is briefly discussed.

The reaction path intrinsic reaction coordinate method and the Hamilton-Jacobi theory

The definition and location of an intrinsic reaction coordinate path is of crucial importance in many areas of theoretical chemistry. Differential equations used to define the path hitherto are complemented in this study with a variational principle of Fermat type, as Fukui [Int. J. Quantum Chem., Quantum Chem. Symp. 15, 633 (1981)] reported in a more general form some time ago. This definition is more suitable for problems where initial and final points are given. The variational definition can naturally be recast into a Hamilton-Jacobi equation. The character of the variational solution is studied via the Weierstrass necessary and sufficient conditions. The characterization of the local minima character of the intrinsic reaction coordinate is proved. Such result leads to a numerical algorithm to find intrinsic reaction coordinate paths based on the successive minimizations of the Weierstrass E-function evaluated on a guess curve connecting the initial and final points of the desired path.

Unconventional biradical character of titanium enolates.

Experimental and theoretical studies evidence an unconventional biradical character for the titanium enolates derived from α-alkoxy ketones and TiCl4. EPR experiments and ab initio calculations suggest that these titanium enolates have an open shell electronic ground state.

NEW STRATEGIES TO INCORPORATE THE SOLVENT POLARIZATION IN SELF-CONSISTENT REACTION FIELD AND FREE-ENERGY PERTURBATION SIMULATIONS

A first‐order perturbation treatment of the polarization contribution to the free energy of hydration is presented. Very simple expressions for the computation of the total electrostatic free energy of solvation, the polarization contribution, and its components (distortion and stabilization) are derived. These equations can be used with either continuum (quantum and classical) or discrete approaches. The reliability of these equations is examined by comparison with rigorous expressions derived previously within the framework of the self‐consistent reaction field theory. Indeed, the suitability of the classical expressions for the distortion and stabilization terms used in the context of discrete strategies has been explored by comparison between self‐consistent reaction field and molecular dynamics–free‐energy perturbation results. The excellent agreement found between the two techniques allows us to envisage a procedure to account for the polarization in force field‐derived methods.

Variational nature, integration, and properties of Newton reaction path

The distinguished coordinate path and the reduced gradient following path or its equivalent formulation, the Newton trajectory, are analyzed and unified using the theory of calculus of variations. It is shown that their minimum character is related to the fact that the curve is located in a valley region. In this case, we say that the Newton trajectory is a reaction path with the category of minimum energy path. In addition to these findings a Runge-Kutta-Fehlberg algorithm to integrate these curves is also proposed.