期刊
INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY
卷 102, 期 5, 页码 925-939出版社
WILEY
DOI: 10.1002/qua.20453
关键词
magnetic circular dichroism; pentalene; heptalene; perimeter model
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 4N-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 (4N + 2)-electron perimeters are of intershell nature and are magnetic-dipole forbidden. (c) 2005 Wiley Periodicals, Inc.
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