Photoelectron spectra and electronic structures of some acceptor-substituted cyclopropanes: Linear correlation of substituent effects on MO energies with molecular structures.
The relationship between electronic and geometrical structures in acceptor-substituted cyclopropanes has been investigated by B3LYP DFT calcns. and photoelectron (PE) spectroscopy. The spectra of cyclopropanecarbaldehyde (2), cyclopropanecarboxylic acid (3), cyclopropanecarboxylic acid Me ester (4), nitrocyclopropane (5), isothiocyanatocyclopropane (6), cyanocyclopropane (7), and 1,1-dicyanocyclopropane (8) have been analyzed. The first ionization potential (IP1) of compds. 2-5 was found to be 0.1-0.4 eV higher than that of the analogous iso-Pr derivs. indicating-contrary to expectation-that in these compds. the cyclopropyl group acts as a weaker electron donor than an iso-Pr group. In the other compds., IP1 values are 0.4-1.1 eV lower than in the open-chain congeners. The Walsh orbitals wS and wA of the three-membered ring are substantially stabilized to different extents by interactions with substituent orbitals, and this is reflected in shortened distal and elongated vicinal C-C bonds. Although the nitro group in compd. 5 causes large stabilizations of both wS and wA, their energy difference Dw remains rather small; this is in agreement with a relatively small difference Dr of the C-C bond lengths. For the investigated monosubstituted cyclopropanes 2-7, the largest effects with respect to Dw and Dr are caused by the formyl group in carboxaldehyde 2. Comparison of the results for nitrilics 7 and 8 indicates that the effects of the cyano groups are additive. A linear relationship between Dw and Dr was established by B3LYP DFT calcns. on geometrically distorted cyclopropane (1) and from the PE data of 2-8.
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