Through Space Interactions of Double Bonds by Photoelectron Spectroscopy

Article abstract of DOI:10.1021/ja00284a022

Steric effects between double bonds and remote polar substituents, previously manifested in nuclear magnetic resonance (NMR) spectra, are also evident in the ultraviolet photoelectron spectra (USP).An ether functionality was introduced at the 3-axial position of exo-methylenecyclohexane by means of acetal groups ( dimethyl and ethylene).The 3-axial ether group destabilizes the ? orbital on the double bond by 0.1-0.2 eV by a through space interaction (the 3-equatorial ether group by itself has little or no effect).This interaction apparently is responsible for the decreased proportion of 3-axial methoxyl observed by NMR spectroscopy.In contrast, 4-axial ether functionalities in cyclohexene show a slight stabilizing of the ? orbital by through bond electron withdrawal.These results also agree with the NMR observations, since the endocyclic double bond of cyclohexene permits a much larger proportion of 4-axial methoxyl.Ab initio calculations support the observations by paralleling the observed ?-orbital energies and by providing electron densities.Whereas 3-axial methoxyl clearly polarizes the double bond in methylenecyclohexane, 4-axial methoxyl has little or no effect on the electron densities of cyclohexene, even though methoxyl is closer to the endocyclic than to the exocyclic double bond.The NMR, UPS, and ab initio results provide an initial understanding of the three dimensionality of the ?-electron steric effects.