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The Lowest Singlet and Triplet States of o ‐Benzyne: Spin‐Coupled Interpretation of the Electronic Structure at CAS SCF Equilibrium Geometries
Author(s) -
Karadakov Peter B.,
Gerratt Joseph,
Raos Guido,
Cooper David L.,
Raimondi Mario
Publication year - 1993
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.199300032
Subject(s) - chemistry , diradical , singlet state , atomic orbital , wave function , aryne , valence bond theory , molecular orbital , triplet state , valence (chemistry) , ground state , atomic physics , complete active space , valence electron , electronic structure , cndo/2 , electron , computational chemistry , basis set , molecule , physics , quantum mechanics , density functional theory , excited state , organic chemistry
The electronic structure of the X̃ 1 A 1 and ã 3 B 2 states of o ‐benzyne is analyzed by means of spin‐coupled (SC) wave functions involving eight active orbitals: two at the dehydro centers, in the plane of the ring, and six π orbitals, inherited from benzene. The equilibrium geometries of the two states, at which all calculations have been carried out, result from separate full geometry optimizations employing ‘8 in 8’ complete active space self‐consistent field (CAS SCF) wave functions incorporating an analogous core‐valence partitioning. The diradical character of the ground state of o ‐benzyne, as approximated by the singlet SC wave function, is reflected in the low value of the overlap (0.46) between the two in‐plane sp 2 ‐like SC orbitals at the dehydro centers, which are just 1.274 Å apart, and in the nonnegligible extent of triplet coupling between the spins of the electrons in these orbitals — ca. 6.4% of the total spin function for the valence electrons (expressed in the Serber spin basis) is made up of spin eigenfunctions, in which these electrons enter as a triplet pair. The geometry of the ground state is found to have an aryne‐like character, which is further confirmed by the pattern of alternation of the overlaps between neighboring 71 valence orbitals, and by the domination of the total spin function for the valence electrons (expressed in the Rumer spin basis) by the spin‐coupling patterns corresponding to an arynic structure.