(e, 2e) momentum spectroscopic study of the interaction of –CH3 and –CF3 groups with the carbon–carbon triple bond

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Abstract
Relative momentum densities have been measured for the outermost occupied π orbitals of acetylene, propyne, 2‐butyne, and perfluoro‐2‐butyne using the (e, 2e) technique with an improved multiple detector spectrometer. The results, when compared with suitably averaged quantum mechanical calculations, show systematic differences between the molecules which can be interpreted in terms of wave function averages and differences using the ΔB(r) functions. A striking feature of the data is the significant momentum density at zero momentum for all of the molecules, in contrast to the calculations that all show zero momentum density. Ground state molecular distortions are discussed as a possible cause of the effect.