Nonlinear hydrogen bonds O ⋯ HC and the rôle of secondary interactions. The rotational spectrum of the oxirane ⋯ acetylene complex

Rotational constants (A0, B0, C0) and quartic centrifugal distortion constants (ΔJ, ΔJK, δJ) of five isotopomers of a complex (CH2)2O⋯HCCH of oxirane and acetylene were determined by using pulsed-nozzle, Fourier-transform microwave spectroscopy. The complex has Cs symmetry, with the HCCH subunit lying in the plane perpendicular to that of the COC nuclei of oxirane and acting as proton donor in forming a hydrogen bond to the oxygen atom. A weak secondary interaction between the CH2 groups of oxirane and the acetylene π-bond causes a significant nonlinearity of the primary hydrogen bond. This behaviour is compared with that in related complexes.