A new algorithm is presented for obtaining points on a steepest descent path from the transition state of the reactants and products. In mass‐weighted coordinates, this path corresponds to the intrinsic reaction coordinate. Points on the reaction path are found by constrained optimizations involving all internal degrees of freedom of the molecule. The points are optimized so that the segment of the reaction path between any two adjacent points is given by an arc of a circle, and so that the gradient at each point is tangent to the path. Only the transition vector and the energy gradients are needed to construct the path. The resulting path is continuous, differentiable and piecewise quadratic. In the limit of small step size, the present algorithm is shown to take a step with the correct tangent vector and curvature vector; hence, it is a second order algorithm. The method has been tested on the following reactions: HCN→CNH, SiH2+H2→SiH4, CH4+H→CH3+H2, F−+CH3F→FCH3+F−, and C2H5F→C2H4+HF. Reaction paths calculated with a step size of 0.4 a.u. are almost identical to those computed with a step size of 0.1 a.u. or smaller.

An improved algorithm for reaction path following

A modified conjugate gradient algorithm for geometry optimization is outlined for use with ab initioMO methods. Since the computation time for analytical energy gradients is approximately the same as for the energy, the optimization algorithm evaluates and utilizes the gradients each time the energy is computed. The second derivative matrix, rather than its inverse, is updated employing the gradients. At each step, a one-dimensional minimization using a quartic polynomial is carried out, followed by an n-dimensional search using the second derivative matrix. By suitably controlling the number of negative eigenvalues of the second derivative matrix, the algorithm can also be used to locate transition structures. Representative timing data for optimizations of equilibrium geometries and transition structures are reported for ab initioSCF–MO calculations.

Optimization of equilibrium geometries and transition structures

Ammonia-borane and related compounds as dihydrogen sources.

Ammonia–borane, H3NBH3, is an intriguing molecule for chemical hydrogen storage applications. With both protic N–H and hydridic B–H bonds, three H atoms per main group element, and a low molecular weight, H3NBH3 has the potential to meet the stringent gravimetric and volumetric hydrogen storage capacity targets needed for transportation applications. Furthermore, devising an energy-efficient chemical process to regenerate H3NBH3 from dehydrogenated BNHx material is an important step towards realization of a sustainable transportation fuel. In this perspective we discuss current progress in catalysis research to control the rate and extent of hydrogen release and preliminary efforts at regeneration of H3NBH3.

Ammonia-borane: the hydrogen source par excellence?

Organic molecules as well as metal complexes may exist as several geometric isomers1 which display distinct physical properties and chemical reactivities. A molecule containing two atoms (in general, two carbons) attached together by a double bond with substituents W-Z may be found as two isomeric † C.D. dedicates this review to Professor Andrée Marquet as a mark of his admiration and gratitude. * To whom correspondence should be addressed: Tel: (33) 169 08 52 25. Fax: (33) 169 08 90 71. E-mail: christophe.dugave@cea.fr. ‡ Present address: Département de Chimie, Institut de Pharmacologie, Université de Sherbrooke, 3001, 12e Avenue nord, Sherbrooke, Québec, J1H 5N4 Canada. 2475 Chem. Rev. 2003, 103, 2475−2532

Cis-trans isomerization of organic molecules and biomolecules: implications and applications.

Information theory can be used to evaluate the quality of a wave function by considering its ability to give values for some observables as close as possible to the experimental ones. A new method for improving the quality of a wave function is proposed. This paper deals exclusively with the HF (/b X//sup 1/ Sigma /sup +/) molecule.

Molecular properties and basis set quality: An approach based on information theory

Theoretical study of the azido-tetrazole isomerization

Hydrogen abstraction reactions with R, X≡H, CH3, NH2,OH, and F have been studied at the abinitio 6-31G – UHF level. However, energetic properties were computed at the CI level. Rate constants and A...

Étude théorique des réactions d'abstraction d'hydrogène, avec R, X≡H, CH3, NH2,OH et F

In this work, the heats of formation of some small molecules containing one or more of the following groups. CH3, CF3, NH2, NHF, NF2, NO2, OH, F are studied. The calculations have been carried out theoretically, using the isodesmic procedure of Pople et al. In most cases, the thermal corrections were introduced using experimental (when available) or theoretical (at the SCF 6–31G level) vibrational frequencies. It was found that this correction is relatively negligible (compared to the accuracy of the heats of formation determined); nevertheless, the vibrational frequencies remain useful for determining the heat capacity and the entropy. We observe that the heats of formation, in families such as XCH3-nFn, XNH2-nFn, and CH4-nXn, are related by quasilinear relations (ΔHf ≈ a + bn). Finally, the NF2 and NO2 substituents have large and comparable destabilization effects on the heat of formation. All the other substituents are stabilizing; the importance of the stabilization effect depends on the number of fluorine atoms.

Michel Sana

Papers

Molecular properties and basis set quality: An approach based on information theory

Theoretical study of the azido-tetrazole isomerization

Étude théorique des réactions d'abstraction d'hydrogène, avec R, X≡H, CH3, NH2,OH et F

The theoretical study of the heats of formation of organic compounds containing the substituents CH3, CF3, NH2, NF2, NO2, OH and F

Etude theorique de la cycloaddition dipolaire 1–3