The rotational spectrum and heavy-atom-planar structure of propargyl benzene (3-phenyl-1-propyne)

The microwave rotational spectrum of propargyl benzene has been studied and its stable conformation has coplanar carbon atoms. This planar structure is confirmed independently by the value of its Pcc second moment consistent with only a pair of H atoms out of the plane, by its rotational spectrum obeying a- and b-type and not c-type selection rules, by its display of spectra of nine rather than six distinguishable monosubstituted 13C isotopomers, by the absence of tunneling splittings, and by the insensitivity of Pcc to 13C isotopic substitution. This conformation is also observed in its isoelectronic analogue, benzyl cyanide. The structure is stabilized by an effective hydrogen bond between an ortho C–H and the π electrons of the triple bond.