Potential Energy Surfaces of Triplet States

The second stage of the study of photochemical reactions, which is the subject of this section, emphasizes the importance of electronic states. This level of description, based on sufficiently accurate ab-initio calculations, can deed with two t5rpes of problems. The first problem is the determination of the potential energy surfaces of the low-lying excited states of a single molecule and it provides us with two families of results  

This question has already been the subject of numerous calculations on diatomic (H2 Heg O2 U2 121), LiH 122.123)  

The second type of problem involves the study of the potential energy surfaces of a reacting system, i.e. a supermolecule involving two molecules, one of which has been primarily excited. This classification will be the skeleton of the following section. 

Before developing it, we ought to mention that these two stages — MO s and electronic states — are static levels of description. They should be followed by a third level which is the dynamical analysis of the behavior of the molecule(s) along the calculated potential energy surfaces. The static surfaces can be expected to predict the nature of many or most possible products. The relative quantum yields, the relative rates, and the possible wavelength dependence of the formation of these products can only be obtained from a dynamical study involving, in particular, a detailed analysis of the radiationless jumps between various surfaces. 

As predicted, the orthogonal triplet is more stable than the planar triplet (by 1.34 eV) and even becomes the ground state of the orthogonal 

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Devaquet, A. Quantum-Mechanical Calculations of the Potential Energy Surface of Triplet States. 54, 1-71 (1974). 

Library of Congress Cataloging in Publication Data (Revised) Main entry under title Triplet states. (Topics in current chemistry 54-55, 66) Includes bibliographies and indexes. CONTENTS 1. Devaquet, A. Quantum-mechanical calculations of the potential energy surfaces of triplet states. Ipaktschi, J., Dauben, W. G., and Lodder, G. Photochemistry of B. y-unsaturated ketones. Maki, A. H. and Zuclich, J. A. Protein triplets states, [etc.] 1. Excited state chemistry-Addresses, essays, lectures. 2.Triplet state-Addrcsses, essays, lectures. I. Series Topics in current chemistry 54 fete.] QD1. F 58 vol. 54-55 [QD461.5] 540 8s [54P.24] 75-1466 ISBN 0-387-07107-5 (v. 1)... 

Our presentation of the wave-mechanical schemes used to determine the potential energy surfaces of triplet states is divided into two parts. In the first, the theoretical framework of the most popular methods is outlined and, to illustrate and compare them, the improvements which result at each step in the description of the excited triplet states of formaldehyde are briefly discussed. In the second part, a number of photophysical and photochemical problems will be analyzed on the basis of calculated potential energy surfaces. 

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