Photoelectron Spectra and Electronic Structures of Some Acceptor-Substituted Cyclopropanes: Linear Correlation of Substituent Effects on MO Energies with Molecular Structures
✍ Scribed by Paul Rademacher; Timur Coskun; Klaus Kowski; Oleg Larionov; Armin de Meijere
- Publisher
- John Wiley and Sons
- Year
- 2003
- Tongue
- English
- Weight
- 224 KB
- Volume
- 9
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Abstract
The relationship between electronic and geometrical structures in acceptor‐substituted cyclopropanes has been investigated by B3LYP DFT calculations and photoelectron (PE) spectroscopy. The spectra of cyclopropanecarbaldehyde (2), cyclopropanecarboxylic acid (3), cyclopropanecarboxylic acid methyl ester (4), nitrocyclopropane (5), isothiocyanatocyclopropane (6), cyanocyclopropane (7), and 1,1‐dicyanocyclopropane (8) have been analyzed. The first ionization potential (IP~1~) of compounds 2–5 was found to be 0.1–0.4 eV higher than that of the analogous isopropyl derivatives indicating—contrary to expectation—that in these compounds the cyclopropyl group acts as a weaker electron donor than an isopropyl group. In the other compounds, IP~1~ values are 0.4–1.1 eV lower than in the open‐chain congeners. The Walsh orbitals ω~S~ and ω~A~ 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 CC bonds. Although the nitro group in compound 5 causes large stabilizations of both ω~S~ and ω~A~, their energy difference Δ__ω__ remains rather small; this is in agreement with a relatively small difference Δ__r__ of the CC bond lengths. For the investigated monosubstituted cyclopropanes 2–7, the largest effects with respect to Δ__ω__ and Δ__r__ are caused by the formyl group in carboxaldehyde 2. Comparison of the results for nitriles 7 and 8 indicates that the effects of the cyano groups are additive. A linear relationship between Δ__ω__ and Δ__r__ was established by B3LYP DFT calculations on geometrically distorted cyclopropane (1) and from the PE data of 2–8.