Antisite defects in Sb2-xBixTe3 mixed crystals

A great deal of chemical and spectroscopical processes involve the relative motion of atomic nuclei. For most low-energy processes the Born-Oppenheimer fixed-nuclei approximation is sufficient: the nuclear motion takes place on an effective potential surface which is the sum of the electronic energy and the nuclear repulsion as a function of the nuclear coordinates. One of the main fields of quantum chemical activity is the study of these surfaces. Complete characterization of a multidimensional potential surface is a very complex task. Often, however, the nuclear motion takes place in the vicinity of a reference configuration, and the surface can be adequately characterized by a power series expansion, i.e., by its derivatives with respect to the nuclear coordinates. Traditionally, these derivatives have been evaluated from a pointwise calculation of the energy, followed by a fitting procedure. This method has some serious drawbacks both in efficiency and in numerical accuracy. Indeed, Hartree(1) observes that “the differentiation of a function specified only by a table of values ... is a notoriously unsatisfactory process, particularly if higher derivatives than the first are required” (see Gerratt and Mills(2) for examples).

Direct Use of the Gradient for Investigating Molecular Energy Surfaces

Why is azulene blue and anthracene white? a simple mo picture

Complete triplet—triplet absorption spectra (2000–10000 A) were measured in alcoholic solution at 113°K with naphthalene, anthracene, tetracene, and a few methylated derivatives. Several new bands were observed. Calculations of the higher ππ* triplet states energies with the LCAO—MO—SCF Pariser—Parr—Pople method were improved by the use of large configuration interactions. These include many doubly excited configurations with respect to both the ground singlet state S0 and the lowest triplet state T1, thanks to the systematic use of all monoexcitations with respect to S0 plus all monoexcitations with respect to T1. The comparison of all experimental and theoretical results for the three polyacenes allows one to modify a few previous assignments and to give a general tentative assignment for all observed transitions.

Triplet‐Triplet Spectroscopy of Polyacenes

The structure of chemisorbed CO on the steps of Pt(112) {Pt[3(111)×(001)]} was monitored as a function of coverage by the digital electron stimulated desorption‐ion angular distribution method (ESDIAD), digital low‐energy electron diffraction (LEED), and temperature‐programmed desorption (TPD). The ESDIAD method applied to the desorption of an electronically excited, neutral CO species (the metastable a3 π‐CO state) avoids the influence of image potential effects on the trajectory of desorbing species, yielding true desorption angles, equivalent to Pt‐CO bond orientations. CO adsorbs exclusively on step sites at low coverage with a 20° ‘‘downstairs’’ tilt from the [112] direction (designated ‘‘0°’’). LEED indicates (2×n) order (i.e., two‐fold order along the steps but no order up/down the steps) exists when the steps are half‐filled (about 0.19 ML). At 0.24 ML, CO is still adsorbed only on the step sites but one‐dimensional CO–CO repulsions between nearest neighbors result in CO tilting along the steps. T...

The structure of CO on the Pt(112) stepped surface—a sensitive view of bonding configurations using electron stimulated desorption

The author suggests a procedure offering the possibility to determine, with the use of a double iteration technique, the equilibrium geometry within the framework of a single calculation of the energy function. The method is used for the ground state and the lowest excited singlet state of formaldehyde and for the formyl radical. The individual contributions of the potential energy are discussed in brief.

Study of reaction mechanisms by semiempirical methods I

HMO Energy Characteristics

Experimental and theoretical (HMO and LCI-SCF) study of singlet-triplet transitions in conjugated hydrocarbons and their derivatives

Stationary points of the INDO energy hypersurface for various orientations of ethylene in external electric fields of the strength F=0, 2, 4, 6, 8 and 10 × 1010 V m−1 were found and their characteristics studied by the force constant matrix analysis. Energies, structural parameters, charges, Wiberg indices and dipole moments are presented. The only stable orientation of the ethylene molecule is that for which the CC bond is parallel to the field direction up to F=6 × 1010 V m−1 (orientation (a) in Fig. 1). Above this value the molecule is structurally unstable and it decomposes to the hydride anion and the C2H3+ cation. Rotational instability was found for two perpendicular orientations of the CC bond with respect to the field vector, in which the field vector was parallel and perpendicular to the molecular plane. Pseudorotations with negative eigenvalues of force constant matrices lead to the stable orientation (a). No stationary points were found when the angle between the CC bond and the field vector was between 0 and 90°. The five longest wavelength vibrational bands are presented for selected orientations and field strengths.

Jiří Pancíř

Papers

Study of reaction mechanisms by semiempirical methods I

HMO Energy Characteristics

Experimental and theoretical (HMO and LCI-SCF) study of singlet-triplet transitions in conjugated hydrocarbons and their derivatives

Automatic Geometry Optimization and Vibrational Analysis in External Electric Field: Ethylene

Topological methods of quantum chemistry for a study of chemical reactivity