Nessima Salhi-Benachenhou

Rearrangement and hydrogen scrambling pathways of the toluene radical cation : A computational study

A computational study is undertaken to provide a unified picture for various rearrangement reactions and hydrogen scrambling pathways of the toluene radical cation (1). The geometries are optimized with the BHandHLYP density functional, and the energies are computed with the ab initio CCSD(T) method, in conjunction with the 6-311+G(d,p) basis set. In particular, four channels have been located, which may account for hydrogen scrambling, as they are found to have overall barriers lower than the observed threshold for hydrogen dissociation. These are a stepwise norcaradiene walk involved in the Hoffman mechanism, a rearrangement of 1 to the methylenecyclohexadiene radical cation (5) by successive [1,2]-H shifts via isotoluene radical cations, a series of [1,2]-H shifts in the cycloheptatriene radical cation (4), and a concerted norcaradiene walk. In addition, we have also investigated other pathways such as the suggested Dewar-Landman mechanism, which proceeds through 5, via two consecutive [1,2]-H shifts. This pathway is, however, found to be inactive as it involves too high reaction barriers. Moreover, a novel rearrangement pathway that connects 5 to the norcaradiene radical cation (3) has also been located in this work.

Nonlinear Optical Properties of Some Substituted Biphenyls

A series of donor-acceptor substituted biphenyls is examined using semiempirical SCF-MO methods. The geometries, dipole moments and first hyperpolarizabilities are obtained from AM1, MNDO and PM3 calculations. The finite-field method is used to calculate the polarizabilities. The properties of the substituents and the torsion angle between the phenyl rings are of fundamental importance for the hyperpolarizabilities. Among the three methods, AM1 leads to torsion angles closest to the experimental data. For each molecule, the HOMO is found to be localized on the donor part to some extent, whereas the LUMO is more concentrated on the acceptor part. The spectroscopic properties of the molecules are investigated using the ZINDO model. For all the molecules considered, the transition from the HOMO to the LUMO is found to be the most important for charge transfer, and this transition has been used in a two-state model to compute the first hyperpolarizability.

McLafferty rearrangement of the radical cations of butanal and 3‐fluorobutanal: A theoretical investigation of the concerted and stepwise mechanisms

The stepwise and concerted pathways for the McLafferty rearrangement of the radical cations of butanal (Bu+) and 3‐fluorobutanal (3F‐Bu+) are investigated with density functional theory (DFT) and ab initio methods in conjunction with the 6‐311+G(d,p) basis set. A concerted transition structure (TS) for Bu+, (H), is located with a Gibbs barrier height of 37.7 kcal/mol as computed with CCSD(T)//BHandHLYP. Three pathways for the stepwise rearrangement of Bu+ have been located, which are all found to involve different complexes. The barrier height for the Hγ transfer is found to be 2.2 kcal/mol, while the two most favorable TSs for the Cα–Cβ cleavage are located 8.9 and 9.2 kcal/mol higher. The energies of the 3F‐Bu+ system have been calculated with the promising hybrid meta‐GGA MPWKCIS1K functional of DFT. Interestingly, the fluorine substitution yields a barrier height of only 20.5 kcal/mol for the concerted TS, (3F‐H). A smaller computed dipole moment, 12.1 D, for (3F‐H) compared with 103.2 D for (H) might explain the stabilization of the substituted TS. The Hγ transfer, with a barrier height of 4.9 kcal/mol, is found to be rate‐determining for the stepwise McLafferty rearrangement of 3F‐Bu+, in contrast to the unsubstituted case. By inspection of the spin and charge distributions of the stationary points, it is noted that the bond cleavages in the concerted rearrangements are mainly of heterolytic nature, while those in the stepwise channels are found to be homolytic.

Isomerization pathways from the norbornadiene to the cycloheptatriene radical cation by opening a bridgehead-methylene bond: a theoretical investigation

Three skeletal rearrangement channels for the norbornadiene (N*+) to the 1,3,5-cycloheptatriene (CHT*+) radical cation conversion, initialized by opening a bridgehead-methylene bond in N*+, are investigated using the quantum chemical B3LYP, MP2 and CCSD(T) methods in conjunction with the 6-311 +G(d,p) basis set. Two of the isomerizations proceed through the norcaradiene radical cation (NCD*+), either through a concerted path (N*+ - NCD*+), or by a stepwise mechanism via a stable intermediate (N*+ - I1 - NCD*+). At the CCSD(T)/6-311 +G(d,p)//B3LYP/6-311 +G(d,p) level, the lowest activation energy, 28.9 kcal mol(-1), is found for the concerted path whereas the stepwise path is found to be 2.3 kcal mol(-1) higher. On both pathways, NCD*+ rearranges further to CHT*+ with significantly less activation energy. The third channel proceeds from N*+ through I1 and then directly to CHT*+, with an activation energy of 37.1 kcal mol(-1). The multi-step channel reported earlier by our group, which proceeds from N*+ to CHT*+ via the quadricyclane and the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cations, is 4.6 kcal mol(-1) lower than the most favorable path of the present study. If the methylene group is substituted with C(CH3)2, however, the concerted path is estimated to be 5.6 kcal mol(-1) lower than the corresponding substituted multi-step path at the B3LYP/6-311+(d,p) level. This shows that substitution of particular positions can have dramatic effects on altering reaction barriers in the studied rearrangements. We also note that identical energies are computed for CHT*+ and NCD*+ whereas, in earlier theoretical investigations, the former was reported to be 6-17 kcal mol(-1) more stable than the latter. Finally, a bent geometry is obtained for CHT*+ with MP2/6-311 +G(d,p) in contradiction with the planar conformation reported for this cation in earlier computational studies.

A theoretical study of the cationic dimerization and polymerization of isobutene

Abstract The initial steps in the radiation-induced polymerization of isobutene have been studied by quantum chemical ab initio and semiempirical calculations. The addition of an isobutene cation to a neutral isobutene molecule to form a dimer radical cation is found to be a strongly exothermic reaction, by 29–32 kcal mol−1 depending on the computational method. A 17 kcal mol−1 barrier towards a one-hydrogen-shift isomerization reaction, yielding a 2,5-dimethyl-2-hexene radical cation, is obtained at the PMP2/6-31G(d, p) level, which is significantly higher than the value of 6 kcal mol−1 obtained before for the corresponding isomerization of the ethene dimer radical cation. The further steps of the polymerization of isobutene are investigated in terms of addition reactions between a neutral isobutene moiety and the addition complex formed in the step before. The positive charge and the radical centres are found to be located in opposite ends of each of the radical cationic intermediate complexes, the positive charge centre being energetically the most favourable site of attachment. The overall reaction is thermodynamically favourable and has a high spatial selectivity. The polymer chain has a high structural symmetry and no cross-linking.

Symmetry breaking in charge-transfer compounds. The effects of electric fields and substituents on the properties of bipyrazine cations

The effects of an electric field and of various substituents on the symmetry breaking of degenerate near-midgap orbitals and on different properties in bi-N,N′-pyrazine-1,6-hexatriene dications ([C4N2H4—(CH)6—C4N2H4]2+) are investigated by means of semiempirical PM3 and INDO CI methods. The electric field is simulated by applying positive/negative point charges at varying distances from the end-points, and the substitutions are done with single chlorine atoms or with CN, OH or CH3 groups, at various positions along the chain or on one of the pyrazine rings. The results are compared with calculations on the unsubstituted, field-free system. It is found that an electric field (e.g., as applied over a membrane) leads to significant symmetry breaking and also polarizes the HOMO and LUMO, such that electron transfer between these orbitals generates large dipole-moment shifts and non-negligible oscillator strengths. With substituents, no major symmetry breaking is observed for the ground state. Instead, strong modifications of the orbital picture are observed, in particular when using the stronger electron-withdrawing substituents. Placing the substituent in a ring position does, furthermore, lead to the possibility of large charge transfer.

Theoretical Calculations of Kinetic Isotope Effects for a Series of Substituted Aziridines

The nitrogen inversion in aziridine and some parent substituted aziridines is investigated using various computational methods. The stationary points on each potential energy surface are optimized using the B3LYP functional of density functional theory (DFT) in conjunction with the popular 6–31G(d) basis set. Rate constants and thereby kinetic isotope effects (KIEs) are computed at different temperatures using transition state theory (TST) and various techniques of variational transition state theory (VTST). Moreover, corrections are made for tunneling and non-classical reflection, using three semiclassical correction methods: the Wigner tunneling correction as well as the zero-curvature and the small-curvature corrections. The performances of the various methods to compute KIEs are compared and the effects of tunneling are discussed.