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

The rotational spectra of the two isotopomers C4H6...(ClF)-Cl-35 and C4H6...(ClF)-Cl-37 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 Cl-35 nuclear quadrupole coupling tensor (chi(aa), chi(bb), chi(cc), chi(ab), chi(ac) and chi(bc)) led to the principal axis values [chi(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 pi bond of buta-1,3-diene to give a complex in which the CIF 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 pi bond is 2.736(4) Angstrom, the angle C-3-*-Cl=95.0(2)degrees and the system *...Cl-F is nearly collinear (theta=177.4 degrees). A comparison of the properties of the prototype b pi.a sigma 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(6)=6.2 N m(-1))is consistent with a weakening of the formal pi bonds C-1=C-2 and C-3=C-4 by conjugation in buta-1,3-diene.