Semiclassical study of collision-induced predissociation: Comparison of the Landau-Zener model with the method of analytic continuation

Semiclassical calculations are carried out by two methods for the problem of collision-induced predissociation of electronically excited I₂. The first method is that of surface-hopping with the Landau-Zener model. The second method is similar to surface-hopping, except that analytic continuation of the adiabatic potential energy surfaces replaces the Landau-Zener model. Results of the calculations by the two methods compare favorably with each other and with experiment. The possible advantages of the second method are discussed.Camille and Henry Dreyfus Teacher-Scholar, Alfred P. Sloan Research Fellow.     

Modification of the Roothaan equations to exclude BSSE from molecular interaction calculations

The Roothaan equations have been modified to compute molecular interactions between weakly bonded systems at the SCF level of theory without the basis set superposition error (BSSE). The increase in complication with respect to the usual SCF algorithm is negligible. Calculation of the SCF energy on large systems, such as nucleic acid pairs, does not pose any computational problem. At the same time, it is shown that a modest change in basis-set quality from 3-21G to 6-31G changes the binding energy by about 50% when computed according to standard SCF “supermolecule” techniques, while remaining practically constant when computed without introducing BSSE. Bader analysis shows that the amount of charge transferred between the interacting units is of the same order of magnitude when performed on standard SCF wave functions and those computed using the new method. The large difference between the corresponding computed energies is thus ascribed to the BSSE. © 1996 John Wiley & Sons, Inc.     

Optimized spin-coupled virtual orbitals

The goal of obtaining accurate potential energy surfaces for the study of intermolecular forces is of fundamental importance. In this article, we propose a new method to generate a set of virtual orbitals optimized in such a way to give very compact valence bond (VB) wave functions with the same accuracy of very large VB calculations. These optimized virtual orbitals, completely free from BSSE, are developed in the framework of spin coupled theory. Test calculations on the He ··· LiH system are presented together with a comparison with previous studies. © 1996 John Wiley & Sons, Inc.     

Convergence of the valence-bond calculation for methane

A minimal Slater basis set valence-bond calculation for the ground state of methane, including all possible structures not involving carbon 1s² excitations is reported. Simplifications for the computation of the matrix elements between Slater determinants are outlined. Comparisons are made with truncated expansions that include only certain valence-bond structures chosen according to physical criteria and with SCF-CI calculations. The result suggests that suitably selected valence-bond functions can be of utility for potential energy surface calculations.     

Novel carbohydrate-based bifunctional organocatalysts for nucleophilic addition to nitroolefins and imines

Glucosamine has been selected as a cheap and readily available chiral scaffold for the synthesis of a series of novel enantiomerically pure bifunctional organocatalysts bearing a tertiary amino group in proximity to a (thio)urea group. The catalytic behaviour of these compounds, both as neutral and N-protonated species, was investigated using the addition of acetylacetone to β-nitrostyrene as a model reaction. Under optimized experimental conditions, chemical yields up to 93% and enantioselectivities up to 89% were obtained. Semiempirical (AM1) computational studies allowed to find a theoretical rationale for the chemical and stereochemical behaviour of the catalyst of choice. These catalysts were also preliminarily investigated as promoters in the addition of diethyl malonate to the N-Boc imine of benzaldehyde, affording the product in up to 81% ee.     

Nanoparticles in energy technology: examples from electrochemistry and catalysis

Nanoparticles are key components in the advancement of future energy technologies, thus, strategies for preparing nanoparticles in large volume by techniques that are cost-effective are required. In the substitution of fossil-fuels by renewable energy resources, nanometer-sized particles play a key role for synthesizing energy vectors from varying and heterogeneous biomass feedstocks. They are extensively used in reformers for the production of hydrogen from solid, liquid, or gaseous energy carriers. Catalyst activities depend critically on their size-dependent properties. Nanoparticles are further indispensable as electrocatalysts in fuel cells and other electrochemical converters. The desire to increase the activity per unit area, and decrease the necessary amount of the expensive catalytic standard, platinum, has spurred innovative approaches for the synthesis of platinum-alloy nanoparticles by wet chemistry, colloidal routes, or physical techniques such as sputtering.     

The electronic structure of nitrilimine: absence of the carbenic form

The electronic structure of nitrilimine HCNNH is shown to essentially be propargylic by CASSCF and Spin-Coupled (modern VB) calculations; in contrast to a recent claim, the carbenic resonance form is absent.