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) A, 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.