Effect of weakening the hydrogen bond on the angular geometry of H2CO⋅⋅⋅HX: Evidence from the rotational spectrum of H2CO⋅⋅⋅HCN

Abstract

The ground-state rotational spectra of three isotopic species (H2CO⋅⋅⋅HC14N, H2CO⋅⋅⋅HC15N, and H2CO⋅⋅⋅DC14N) of the formaldehyde–hydrogen cyanide dimer have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. Analysis of the spectra yielded rotational constants, centrifugal distortion constants, and 14N-nuclear quadrupole coupling constants which for the species H2CO⋅⋅⋅HC14N have the following values: A0=91.2(7.5) GHz, B0=1785.6217(10), C0=1749.7215(10) MHz, ΔJ =4.76(2) kHz, ΔJK=−6.1181(7) MHz, δJ =2.58(1) kHz, χaa=−4.381(3) MHz, and χbb =2.141(4) MHz. The spectroscopic constants have been interpreted to establish a planar geometry for the dimer in which HCN forms a hydrogen bond to O in H2CO. The angle between the C=O bond and HCN symmetry axis is θ=138.1 deg, and the intermolecular distance from O to C is r(O⋅⋅⋅C)=3.271 Å. The difference in the angle θ between H2CO⋅⋅⋅HCN and H2CO⋅⋅⋅HF is discussed on the basis of a simple model for the hydrogen bond in these dimers.