The molecular structure of 1,1-dichlorocyclopropane by gaseous electron diffraction using rotational constants as constraints and by a b i n i t i o gradient computation

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Abstract
The molecular structure of gaseous 1,1‐dichlorocyclopropane has been investigated using room‐temperature electron‐diffraction data with ground state rotational constants from published work introduced as constraints. The models of the structure were specified by parameters generating geometrically consistent distance sets of the type rz = rα0. The vibrational corrections necessary for the conversion of these to rg and to the ra set suitable to the diffraction data, and for the conversion of the observed B0 rotational constants to Bz, were calculated from a quadratic force field very similar to that deduced by others. The results leave no doubt that the unique C–C bond is significantly longer than the other C–C bonds as predicted from theoretical calculations. The important distances (rg), angles (&α), and rms amplitudes of vibration (l) with associated uncertainties estimated as 2σ are r(C–H) = 1.109(8) Å, 〈r(C–C)〉 = 1.511(3) Å, Δr(C–C) = 0.041(11) Å, r(C–Cl) = 1.759(2) Å, &Cl–C–Cl = 112.6(2)°, &H–C–H = 118.7(20)°, l(C–H) = 0.077(8) Å, l(C–C)opp = l(C–C)adj+0.001 = 0.055(3) Å, and l(C–Cl) = 0.053(3) Å. Features of this structure and of similar molecules are discussed. In addition, the geometry has been calculated ab initio by Hartree–Fock level gradient calculations with double‐zeta basis sets augmented with polarization functions on the heavy atoms. The computed structure is very similar to the one derived from the electron‐diffraction experiment except that Δr(C–C) is found to be only 0.020 Å and the computed &H–C–H is slightly smaller than the experimental value.