Photoelectron Spectra and Electronic Structures of Some Acceptor‐Substituted Cyclopropanes: Linear Correlation of Substituent Effects on MO Energies with Molecular Structures

Abstract

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 w S and w 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 CÀC bonds. Although the nitro group in compound 5 causes large stabilizations of both w S and w A , their energy difference Dw remains rather small; this is in agreement with a relatively small difference Dr of the CÀC 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 nitriles 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 calculations on geometrically distorted cyclopropane (1) and from the PE data of 2 ± 8.

Photoelectron Spectra and Electronic Structures of Some Cyanocyclopropanes

“…The vibrational fine structure of the bands supports the assignments based on the B3LYP results to π(CN) and n(N) ionizations of the two nitrile groups, but because of the difficulties in the analysis of composite bands this cannot be regarded as a proof that the different result of the PM3 method is false. For compound 2, strong bands corresponding to π(CN) and n(N) ionizations of the nitrile groups have been found in a similar range (12.8Ϫ13.6 eV) [1] as for 3 and 4. For direct comparison with the latter compounds, the IPs of succinonitrile as an open-chain molecule with two vicinal nitrile groups are most informative.…”

“…The PE spectra of compounds 1 [1,8,9] and 2 [1] have been investigated previously. The measured spectra of compounds 3 and 4 are depicted in Figure 2.…”

“…[1,2] However, the problem of the inclusion of polysubstituted cyclopropanes still needs further elucidation. In the first paper in this series, [1] cyanocyclopropane (1) and 1,1-dicyanocyclopropane (2) were included.…”

“…[1,2] However, the problem of the inclusion of polysubstituted cyclopropanes still needs further elucidation. In the first paper in this series, [1] cyanocyclopropane (1) and 1,1-dicyanocyclopropane (2) were included. We have now investigated two further cyclopropane nitriles, trans-and cis-1,2-dicyanocyclopropane (3,4), by the same technique and analysed the data for all four compounds for the correlation between bond lengths and ionization energies.…”

“…[1Ϫ3] In order to investigate such a relationship, monosubstituted cyclopropanes were first studied, [1,3] but some derivatives with two or three substituents were also found to follow the correlation. [1,2] However, the problem of the inclusion of polysubstituted cyclopropanes still needs further elucidation.…”