The rotational spectrum and geometry of the heterodimer oxirane ⋅ ⋅ ⋅ hydrogen cyanide in the vibrational ground state and the vβ(0)=1 state

Abstract

The rotational spectrum of the heterodimer formed by oxirane and hydrogen cyanide has been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. Two sets of rotational transitions of similar intensity have been identified for each of the isotopic species (CH2)2O ⋅⋅⋅ HC14N, (CH2)2O ⋅⋅⋅ HC15N, and (CH2)2O ⋅⋅⋅ DC14N. Each set of rotational transitions was fitted to give rotational constants B and C, centrifugal distortion constants ΔJ, ΔJK, δJ, and HJK, and, where appropriate, 14N–nuclear quadrupole coupling constants χaa and χbb. The rotational constants for each of the two states (labeled A and B) were interpreted in terms of a molecule of Cs symmetry, with a hydrogen bond O ⋅⋅⋅ HCN and a pyramidal geometry at the oxygen atom. For the species (CH2)2O ⋅⋅⋅ HC14N, the angle φ between the bisector of the COC angle and the HCN axis increases from 52.2(3)° in state A to 61.3(3)° in state B, while the distance r(O ⋅⋅⋅C) correspondingly increases from 3.035(4) to 3.130(4)Å. Similar changes are observed in (CH2)2O ⋅⋅⋅ HC15N and (CH2)2O ⋅⋅⋅ DC14N between states A and B. It is concluded that the potential energy barrier to inversion of the configuration at O by means of the low frequency hydrogen bond bending mode νβ(0) is relatively low and that states A and B correspond to the ground state and the state having vβ(0)=1, respectively.