Water–hydrocarbon interactions: Structure and internal rotation of the water–ethylene complex

Abstract

The rotational spectra of C2H4–H2O and C2H4–D2O were measured using the molecular beam electric resonance technique. The rotational and centrifugal distortion constants obtained for C2H4–H2O are: B+C=7274.747 (24), B−C=371.103 (8), A=25 858.4 (36), ΔJ=0.0279 (17), ΔJK=1.7352 (66), and δJ=0.002 99 (22) MHz. The dipole moment for both isotopic species is 1.094 (1) D. The structure derived from an analysis of the rotational constants and dipole moment is nonplanar with Cs symmetry. The water molecule is singly hydrogen bonded perpendicular to the plane of the ethylene; i.e., into the π system. The plane of the water bisects the C–C bond. The hydrogen bond length is 2.48 Å. Splittings are observed in the rotational transitions of C2H4–H2O but not in C2H4–D2O. These are assigned to excited torsional levels of the hindered internal rotation of the water with respect to the ethylene. The barrier height is estimated to be V2=1.0±0.2 kcal/mol which is surprisingly high for this weakly bound complex.