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The ionization energies of bridged [14]annulenes and of dicyclohepta[ cd , gh ] pentalene
Author(s) -
Batich Christopher,
Heilbronner Edgar,
Vogel Emanuel
Publication year - 1974
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19740570746
Subject(s) - chemistry , annulene , pentalene , antibonding molecular orbital , natural bond orbital , crystallography , molecular orbital diagram , atomic orbital , atomic physics , molecular physics , computational chemistry , basis set , physics , linear combination of atomic orbitals , quantum mechanics , electron , molecule , density functional theory , organic chemistry
The ionization energies J J , of 1,6;8,13‐alkanediylidene‐[14]annulenes ( 2 to 5 ) and of dicyclohepta[ cd , gh ]pentalene ( 1 ) have been determined by photoelectron spectroscopy, using HeI radiation. The data are interpreted in terms of Koopmans ' theorem ( J J = −ε J ) on the basis of correlation diagrams and with the help of simple molecular orbital models. If the bridge is an ethane‐, propane‐ or butane‐diylidene group, the π‐orbital sequence, in descending order of orbital energies, is (in C 2v ): b 1 , b 2 , a 2 , a 1 . The sequence is due to a complicated and not uniquely definable interplay of inductive, conjugative and homoconjugative effects. A detailed analysis of these effects suggests that the effective angle of twist between two consecutive basis‐AOs 2p μ , 2p ν of the peripheral π‐system should be smaller than the twist angles θ μν determined by X‐ray analysis, i.e. that the pi;‐ribbon adjusts elastically and is no longer locally orthogonal to the σ‐frame. In the non‐alternant hydrocarbon 1 of symmetry D 2 h , the sequence is 2b 2g , 3b 1u , 2b 3g , 1a u , 2b 1u . The sequence 3b 1u above 2b 3g , i.e. the reverse of b 2 above a 1 in the bridged [14]annulenes, is explained as being due to the interaction of the semilocalized perimeter orbitals b 1u and b 3g with the bonding (π(B 1u ))and antibonding (π*(B 3g )) orbital of the central double bond. In 2 the replacement of the two latter orbitals by the Walsh ‐orbitals of the cyclopropane moiety leads to the sequence b 1 , b 2 , a 1 , a 2 . From the data observed for 1 to 5 and for 1,6‐methano‐[10]annulene [11], a crude estimate for the orbital energies of the hypothetical all ‐ cis D 10 h ‐[10]‐ and D 14 h ‐[14]annulenes can be derived.