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Allowed and forbidden character in the two-photon spectra of perturbed benzenes and napbtbalenes

LIONEL GOODMAN and RICHARD P. RAVA

Department of Chemistry, Rutgers University, New Brunswick, NJ 08903 (U.S.A.)

Probably no approach has revealed more information on the vibrational and electronic wavefunctions of high symmetry polyatomic molecules than the spectroscopic effects of perturbations. Attention in this paper is directed towards the two-photon spectroscopy of the lowest singlet excited states of perturbed benzenes and naphthalenes observed by using photons from the visible region. In particular we discuss (1) kinematic effects on the vibronic mechanisms, (2) assignment of new excited state fundamentals in the isotopic molecules and (3) the effect of chemical substitution. The spectra show that (contrary to what is observed for the one-photon spectrum) off-diagonal kinematic perturbations are important and that inductive substituents are ineffective in inducing allowed character into the two-photon spectrum.

Photoionic states of organic molecules studied by He1 photoelectron spectroscopy

K. KIMURA and Y. ACHIBA

Institute for Molecular Science, Myodaiji, Okuzaki 444 (Japan)

S. KATSUMATA and T. YAMAZAKI

institute of Applied Electricity, Hokkaido University, Sapporo 060 (Japan)

S. IWATA

Department of Chemistry, Keio University, Hiyoshi, Yokohama 222 (Japan)

Molecular ionic states produced by photoionization in the vacuum W region can be studied by means of 584A He1 photoelectron spectroscopy. Re- cently much attention has been devoted to fragmentation processes of the ionic states. In this work we studied ionic states of about 150 fundamental organic molecules from He1 photoelectron spectra in a systematic way in terms of ab initlo self-consistent field molecular orbital calculations (4-3 1G or 6-3 1G) on the basis of Koopmans’ theorem. Furthermore we also studied ionic states of about 50 basic molecules in terms of configuration interaction (CI) calculations of ionic states; these were carried out with a program recently developed by one of us (S.I.). Systematic theoretical assignments with similar accuracy are desir- able. All the He1 photoelectron spectra used here were measured under similar experimental conditions in our laboratory over the last several years. The mole-

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cules studied in the CI calculations were methane, ethane, propane, cyclopropane, ethylene, butadiene, methylene cyclopropane, acetylene, methyl fluoride, meth- anol, methylamine, formaldehyde and 40 other fundamental molecules.

Photochemical three-membered ring cleavage of a-cyclopropyl and a-epoxy ketones: a theoretical study

A. SEVIN and P. CHAQUIN

Laboratoire de Chimie Organique Th&kque, Universitk Park VI, 4 place Jussieu, 75230 Paris Ckkx 05 (France)

Cyclopropylcarboxaldehyde 1 and oxirancarboxaldehyde 2 are used as models for theoretical self-consistent field+zonfiguration interaction ab ini?io simulations of three-membered ring photochemical cleavage.

On vertical n,n* excitation of 1, the opposite C-C bond is surprisingly weak- ened. Nevertheless, the preferred cleavage of the adjacent C-C bond may be interpreted by the number and the efficiency of avoidances of crossings of the surfaces. In contrast, the selectivity of adjacent bond cleavage as a function of the conformation can be interpreted by perturbational concepts within the initial molecule.

For 2 a complex mixing of carbonyl and ring molecular orbitals yields a low-lying triplet state, bearing an important J-C,X* contribution, in close vicinity to the lowest triplet state n,z *. The consequences for reactivity are discussed.

Conformational study of the lowest triplet states of formaldehyde, acetaldehyde, propionaldehyde and acetone

GEORGE R. DE MARe

Laboratoire de Chimie Physique iUoi&culaire, Facutrt? des Sciences, Universirk Libre de Bruxetles CP.160, SO avenue F.D. Roosevelt, B-IO50 Brussels (Belgium)

The study was carried out with a minimal basis set within the unrestricted Hartree-Fock framework_ The energy hypersurfaces E(al, a2, 9,) were gener- ated for the C3H,0 species with energy a function of the methyl rotations (6, and 6,) and C-O out-of-plane bending for acetone and a function of methyl rotation (fil), C2H& rotation (tY2) and CHO out-of-plane deformation (~9~) for propionaldehyde. The analysis of the hypersurface equations revealed the loca-