Rotational spectrum of a weakly bound dimer of formaldehyde and acetylene: Identification and characterization of a bridged planar form involving two nonlinear hydrogen bonds

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Abstract
Pulsed-nozzle, Fourier-transform microwave spectroscopy has been used to observe and measure the rotational spectra of the following five isotopomers of a weakly bound dimer formed between formaldehyde and acetylene: (H2CO,HCCH), (H2CO,DCCD), (H2CO,HCCD), (H2CO,DCCH), and (D2CO,HCCH). Rotational constants Av, Bv, Cv, and centrifugal distortion constants ΔJ, ΔJK, and δJ have been determined for the ground state (v=0) for all species and for a vibrationally excited state (v=1) for the first two. The values for the v=0 state in (H2CO,HCCH) are: A0=30 400.567(3) MHz, B0=2355.8304(3) MHz, C0=2180.3335(5) MHz, ΔJ =13.159(8) kHz, ΔJK =−311.4(2) kHz, and δJ =1.526(5) kHz. A detailed analysis of the rotational constants leads to the conclusion that the dimer has an unusual, bridged planar geometry, with the two subunits bound through two nonlinear hydrogen bonds. One bond has acetylene as the proton donor and the oxygen atom of formaldehyde as the acceptor while in the other a formaldehyde proton interacts with the acetylene π bond. The geometry is characterized by the following distances and angles: r=3.86(2) Å is the distance from the C atom of H2CO to the center of C≡C, θ=80.3(16)° is the angle made by the C2 axis of H2CO with r while φ=40.1(17)° is the corresponding angle made by the acetylene axis. The hydrogen bond distances are r(O⋅⋅⋅H)=2.39 Å and r(center⋅⋅⋅H)=3.12 Å. The vibrationally excited state has been identified with a motion involving the torsional oscillation of the H2CO subunit about its C2 axis. A relatively high barrier to this motion at 90° to the molecular plane is postulated.