Identification and geometry of the pre-reactive complex buta-1,3-diene···ClF by rotational spectroscopy

Abstract

The rotational spectra of the two isotopomers C ₄ H ₆ ··· ³⁵ ClF and C ₄ H ₆ ··· ³⁷ ClF of a complex formed between buta-1,3-diene and chlorine monofluoride were observed by using a fast-mixing nozzle in a Balle–Flygare Fourier-transform microwave spectrometer. Rotational constants, quartic centrifugal distortion constants and Cl nuclear–quadrupole and spin–rotation coupling constants were determined in each case. Diagonalisation of the complete ³⁵ Cl nuclear quadrupole coupling tensor (χ _aa , χ _bb , χ _cc , χ _ab , χ _ac and χ _bc ) led to the principal axis values 〈χ _ii 〉 (i = x, y and z). A detailed interpretation of the observed spectroscopic constants for both isotopomers led to the conclusion that the Cl end of the ClF molecule interacts with the C ₃ C ₄ π bond of buta-1,3-diene to give a complex in which the ClF molecule is perpendicular to the plane of buta-1,3-diene. The distance r(*···Cl) of the Cl atom from the centre (*) of the C ₃ C ₄ π bond is 2.736(4) Å, the angle C ₃ –*–Cl = 95.0(2)° and the system *···Cl–F is nearly collinear (ϑ = 177.4°). A comparison of the properties of the prototype bπ.aσ complexes ethene···ClF, allene···ClF and buta-1,3-diene···ClF is presented. Several similarities are noted but the weaker binding in buta-1,3-diene···ClF (k _σ = 6.2 N m ⁻¹ ) is consistent with a weakening of the formal π bonds C ₁ C ₂ and C ₃ C ₄ by conjugation in buta-1,3-diene.