Multiconfiguration Self-Consistent Field and Multireference Configuration Interaction Methods and Applications

Molecular Reaction Dynamics: Frontmatter

Technical details of a new global mapping technique for finding equilibrium (EQ) and transition structures (TS) on potential energy surfaces (PES), the scaled hypersphere search (SHS) method (Ohno, K.; Maeda, S. Chem. Phys. Lett. 2004, 384, 277), are presented. On the basis of a simple principle that reaction pathways are found as anharmonic downward distortions of PES around an EQ point, the reaction pathways can be obtained as energy minima on the scaled hypersphere surface, which would have a constant energy when the potentials are harmonic. Connections of SHS paths between each EQ are very similar to corresponding intrinsic reaction coordinate (IRC) connections. The energy maximum along the SHS path reaches a region in close proximity to the TS of the reaction pathway, and the subsequent geometry optimization from the SHS maximum structure easily converges to the TS. The SHS method, using the one-after-another algorithm connecting EQ and TS, considerably reduces the multidimensional space to be searched to certain limited regions around the pathways connecting each EQ with the neighboring TS. Applications of the SHS method have been made to ab initio surfaces of formaldehyde and propyne molecules to obtain systematically five EQ and nine TS for formaldehyde and seven EQ and 32 TS for propyne.

Global mapping of equilibrium and transition structures on potential energy surfaces by the scaled hypersphere search method: applications to ab initio surfaces of formaldehyde and propyne molecules.

▪ Abstract Crossed-molecular-beam and laser techniques have enabled experimentalists to measure the state-resolved differential cross sections of elementary chemical reactions. This article reviews recent progress in this area. Particular emphasis is placed on some intriguing physical phenomena associated with a few benchmark reactions and how these measurements help in answering fundamental questions about reaction dynamics. We examine specifically the geometric phase effects in the reaction H + D2, the dynamical resonance phenomenon in F + HD, the unusually large spin-orbit reactivity in Cl(2P) + H2, the insertion reaction O(1D) + H2, and the mode-specific reactivity in Cl + CH4(ν). The give-and-take between experiment and theory in unraveling the physical picture of the dynamics is illustrated throughout this review.

Crossed-beam studies of neutral reactions: state-specific differential cross sections.

Recent studies of state-resolved angular distributions show the participation of reactive scattering resonances in the simplest chemical reaction. This review is intended for those who wish to learn about the state-of-the-art in the study of the H + H2 reaction family that has made this breakthrough possible. This review is also intended for those who wish to gain insight into the nature of reactive scattering resonances. Following a tour across several fields of physics and chemistry where the concept of resonance has been crucial for the understanding of new phenomena, we offer an operational definition and taxonomy of reactive scattering resonances. We introduce simple intuitive models to illustrate each resonance type. We focus next on the last decade of H + H2 reaction dynamics. Emphasis is placed on the various experimental approaches that have been applied to the search for resonance behavior in the H + H2 reaction family. We conclude by sketching the road ahead in the study of H + H2 reactive scattering resonances.

Scattering resonances in the simplest chemical reaction.

Transition States and Rate Constants for Unimolecular Reactions

Publisher Summary This chapter discusses the molecular eigenstate spectra at the threshold for chemical bond breaking. It illustrates and presents the analysis of fully resolved spectra of the vibrational states of formaldehyde and formyl fluoride at the threshold for dissociation. The spectra of D2CO show a complete randomization of vibrational energy. The vibrational levels of the ground electronic state (S0) of formaldehyde are investigated by Stark level-crossing spectroscopy. A frequency-doubled pulse-amplified cw dye laser provides transform limited pulses to excite the single rovibronic levels of the first excited singlet state (S1) of D2CO in a cooled pulsed molecular beam. The statistical distributions of spectral intensities and dissociation rate constants provide information on the mechanism and dynamics of nonradiative decay. The preliminary spectra of HFCO exhibit considerable local mode character in eigenstates even for energies well above the dissociation threshold. The potential surfaces of HFCO are similar to those of formaldehyde except that the S1 state is hjgher in energy and the barrier is a factor of two lower dissociation energies. Studies near the dissociation threshold show that there is a strong systematic dependence of dissociation rate on both rotational and vibrational quantum number. The substantial vibrational mode dependence of intramolecular vibrational energy redistribution and dissociation rates can be anticipated when stimulated emission pumping spectra and dissociation product state dynamics are fully explored.

C. B. Moore

Papers

Transition States and Rate Constants for Unimolecular Reactions

Molecular Eigenstate Spectra at the Threshold for Chemical Bond Breaking: Formaldehyde and Formyl Fluoride

A Transition State Theory-Based Statistical Distribution of Unimolecular Decay Rates with Application to Unimolecular Decomposition of Formaldehyde.