Transient anions of cis- and trans-cyclooctene studied by electron-impact spectroscopy.

Khrystyna Regeta,Michael Allan,Andreas F M Kilbinger,Amit Nagarkar

doi:10.1039/c4cp04083j

Abstract

The effect which deformation of the double bond in trans-cyclooctene (TCO), compared to cis-cyclooctene (CCO), has on its negative ion - and indirectly on the π* virtual orbital - was studied by electron-impact spectroscopy. Differential elastic and vibrational excitation cross sections were measured at a scattering angle of θ = 135°. The vertical attachment energy (VAE) derived from the vibrational excitation spectra is 1.87 eV in TCO, only 0.09 eV lower than in the unstrained CCO, 1.96 eV. The substantial deformation of the C[double bond, length as m-dash]C bond in TCO thus stabilizes its transient negative ion by a surprisingly small amount and this effect is ascribed in part to the Pauli (steric) destabilization of the TCO π* orbital by the alkyl chain facing the π* lobes. An interesting effect is observed in the elastic cross section which is about 45% larger for TCO at low energies (∼0.4 eV), despite the similar geometrical size of the two molecules. Ramsauer-Townsend minima are observed in the elastic cross section at 0.13 and 0.12 eV for CCO and TCO, respectively. Implications of the findings on enhancement of the dienophile capacity of TCO are discussed.

Highlights

Geometrical distortion is an interesting means of activating chemical bonds, an important example being the deformation of the CQC bond in trans-cyclooctene (TCO) as compared to cis-cyclooctene (CCO)
Measuring vibrational excitation by electron impact is the preferable method of determining resonances because the force field in the molecule is modified during the temporary capture of the incident electron, primarily as a function of the bonding/antibonding character of the temporarily occupied virtual orbital, leading to relaxation, setting the nuclei in motion, and to a vibrationally excited molecule after the departure of the electron
The residence time of the electron in the resonance is generally in the ps range, comparable to the classical vibrational period, but this short time is sufficient to dramatically increase the vibrational excitation cross section, permitting detection of resonances without the problem of an overwhelming background of direct scattering encountered with the elastic cross section

Summary

Introduction

Geometrical distortion is an interesting means of activating chemical bonds, an important example being the deformation of the CQC bond in trans-cyclooctene (TCO) as compared to cis-cyclooctene (CCO). UV-photoelectron spectroscopy removes electrons from the molecules and measures the ionization energies, related to energies of the occupied orbitals by the Koopmans’ theorem.[7] Photoelectron spectra revealed that the p* highest occupied molecular orbital (HOMO) is destabilized by 0.29 eV in TCO relative to CCO.[8] Electron impact spectroscopy inserts electrons into normally empty orbitals, generating transient negative ions (synonymously called ‘resonances’), and measures the corresponding electron attachment energies. The attachment energies are related to energies of virtual orbitals, becoming temporarily occupied in the electron collision, the relation

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Results

Conclusion