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

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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.

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