This review is concerned with the impact of quantum chemistry on chemical reactions. Starting from the mid-sixties it focusses on those developments which have enabled us to predict essential features of simple chemical reactions. Thus model theories and computational methods are presented which provide the tools for these predictions. Then procedures to characterize potential surfaces and search methods for reaction paths are described. It is also attempted to relate these features to the terminology of the experimentalist. Finally a systematic survey of the main types of reaction (rearrangement, addition, elimination, substitution) is given.

Quantum chemical methods and their applications to chemical reactions

Diketene (C4H4O2) monomers were isolated in cryogenic Ar (15 K) and Xe (30 K) matrices. The infrared (IR) spectra of the freshly deposited matrices show that diketene monomers exclusively adopt the 4-methylene-oxetan-2-one form. In situ photochemical transformations of diketene were induced by tunable UV laser light. Diketene was found to be photostable when exposed to near-UV irradiations (λ> 300 nm). Irradiations in the middle-UV domain showed different types of photochemical reactivity occurring upon irradiations with 280 > λ > 240 nm and λ = 225 nm. The photoproducts were characterized by IR spectroscopy supported by B3LYP/6-311++G(d,p) calculations. Upon irradiation in the 280 > λ > 225 nm range, diketene was found to decompose in two ways: (i) with production of two parent ketene molecules (O═C═CH2), and (ii) with production of cyclopropanone (CP) plus carbon monoxide. For irradiations in the 280 > λ > 240 nm range, diketene exhibited two additional reactions: (iii) decomposition to allene (H2C═C═CH...

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UV-Induced unimolecular photochemistry of diketene isolated in cryogenic inert matrices.

Quantum chemical CI calculations with the semiempirical MO method SINDO1 are performed to study the rearrangement reactions of 1,2,6,7-octatetraen, 2,3-dimethylenebicyclo(2.2.0)hexane, 3,4-dimethylene-1,5-hexadiene and bicyclo(4.2.0)octa-1,5-diene. It is shown that the most favorable pathway of each of these six rearrangements involves the 2,3-dimethylene-1,4-cyclohexadiyl diradical as an interceptable intermediate. Two further intermediates, 1,2-divinyl-1-cyclobutene and 1,2-divinylylcyclobutane appear, but the latter with little importance. Energies and geometries of the four reactants resp. products, the three intermediates and twelve transitions states are presented. The mechanism of the rearrangements is discussed.

The role of 2,3-dimethylene-1, 4-cyclohexadiyl diradical as a reactive intermediate in some rearrangement reactions

The potential surface of 1,2-dimethylcyclobutane is investigated with respect to fragmentation and isomerization by the semiempirical MO method SINDO1. Energy and geometry of eight transition states and four intermediates are determined for the nonconcerted fragmentations under optimization of all internal coordinates. We find that symmetric cleavage ofcis-dimethylcyclobutane is the most favored process leading to fragmentation. Whereas the symmetric elimination involves two different barrier heights, the asymmetric elimination involves only two equal barrier heights. The possibility of isomerization ofcis- totrans- dimethylcyclobutane was also studied and revealed two different barrier heights lower than the corresponding heights for fragmentation.

Fragmentation and isomerization of 1,2-dimethylcyclobutane

The semiempirical MO method SENDO1 is developed for the treatment of extended systems. The Large Unit Cell (LUC) approach is incorporated to achieve feasibility of calculations in real space. We present the resulting formulas and discuss convergence problems for the core Hamiltonian with respect to lattice sums. The influence of an increase of the LUC on bond length and binding energy per atom is investigated for several one- and two-dimensional systems.

Development of Sindo1 for Extended Systems

Alternative pathways for the thermolysis of diketene were investigated with a semiempirical SINDO method [1]. Transition states and activation energies for isomerization to cyclobutane-1,3-dione and 2,4-dimethylene-1,3-dioxetane were determined. The dissociation and formation of these three dimers into and from two ketenes was also calculated. The results disclose the possibility of existence of intermediates as well as the fragmentation into further products. Entropy appears as a major factor in this reaction at higher temperature.

Reaction pathways for various ketene dimers

The cycloreversion of the following ring systems was studied: (1) Cyclopentyl anion → allyl anion + ethylene, (2) Cyclohexene → butadiene + ethylene, (3) Cyclohept-4-enyl cation → butadiene + allyl cation. The potential energy surfaces of these reactions were investigated by the semiempirical MO method SINDO1 under inclusion of a 4 × 4 CI. All internal coordinates were varied. We found major differences in the nature and geometry of transition states and intermediates of these reactions. Diradical character is involved only in the fragmentation of the six membered ring.

C. P. D. Dwivedi

Papers

Reaction pathways for various ketene dimers

Cycloreversion reactions for five to seven membered rings

Fragmentation and isomerization of 1,2-dimethylcyclobutane

Fragmentation and isomerization of 1,2-dimethylcyclobutane