The rotational constant A0 and planarity of the hydrogen-bonded complex H2CO⋯HCl

Abstract

A fit of µ_a, R-branch transitions of the nearly prolate asymmetric rotor H₂CO⋯HCl (κ≈–0.99) demonstrated that even a relatively large A₀ rotational constant can be determined with useful accuracy from such a limited set of data, provided the frequencies are of sufficient accuracy. Thus the set of spectroscopic constants A₀, B₀, C₀, Δ_J, Δ_JK, δ_J, χ_aa, (χ_bb–χ_cc), χ_ab and ½{M_bb+M_cc} are reported for H₂CO⋯H³⁵Cl. The subsequent observation of two µ_b, R-branch transitions led to a much more accurate value of A₀, but this did not lie outside the error of the preliminary value, confirming the usefulness of the initial approach. The inertial defect Δ₀= 0.5724(1) u Ų demonstrates that the nuclei in H₂CO⋯H³⁵Cl are copolanar. Fits of limited numbers of µ_a, R-branch transition frequencies for each of H₂CO⋯H³⁷Cl, D₂CO⋯H³⁵Cl and D₂CO⋯H³⁷Cl led to Δ₀ values for the isotopomers identical within experimental error to that of H₂CO⋯H³⁵Cl.