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MCD of non‐aromatic cyclic π‐electron systems. Part 6: Pentalenes and heptalenes
Author(s) -
Fleischhauer Jörg,
Raabe Gerhard,
Klingensmith Kenneth A.,
Höweler Udo,
Chatterjee Prabir K.,
Hafner Klaus,
Vogel Emanuel,
Michl Josef
Publication year - 2005
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.20453
Subject(s) - excited state , ground state , pentalene , magnetic circular dichroism , chemistry , excitation , atomic physics , electron , dipole , magnetic dipole , spectral line , sign (mathematics) , physics , molecule , quantum mechanics , organic chemistry , mathematical analysis , mathematics
The magnetic circular dichroism (MCD) spectra of several stable derivatives of pentalene (1) and heptalene (2) have been recorded. The lowest energy transition (from the ground to the S state in perimeter model nomenclature) is extremely weak in MCD and in absorption. The sign patterns of the B terms for the first three strong transitions—N 1 , N 2 , P 1 —in the order of increasing energy, are −−+ for 1 and ++− for 2. These findings are in perfect agreement with numerical results obtained at the SACCI level and with expectations based on the perimeter model of Parts 1–4 of this series, both of which lead to the conclusion that the magnetic mixing of the S excited state with the ground state is dominant. This is an extremely rare situation for an organic molecule, in which B term signs are normally determined by the mutual magnetic mixing of excited states. It can be expected to occur in other conjugated systems derived from 4 N ‐electron perimeters that have a low‐energy first excitation, which is of intrashell nature in the perimeter model and therefore is magnetic‐dipole allowed. In contrast, all low‐energy transitions in the much more common systems derived from (4 N + 2)‐electron perimeters are of intershell nature and are magnetic‐dipole forbidden. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005