ELECTRON-DIFFRACTION INVESTIGATION OF THE MOLECULAR-STRUCTURE OF GASEOUS 2,3-BUTANEDIONE (BIACETYL) AT 228-DEGREES-C AND 525-DEGREES-C

An earlier electron-diffraction investigation of biacetyl at a nozzle-tip temperature of 228 °C revealed only a trans conformer, in contrast to certain similar conjugated systems which have substantial amounts of gauche forms as well. New experiments at 525 °C have been carried out with the same result: there is no evidence for the presence of any but the trans conformer in gaseous biacetyl. The important distance (ra) and angle values for the molecule at 228 (older data reanalyzed in terms of the present, more sophisticated model including shrinkages) and 525 °C are respectively r(C=0) = 1.215 (2), 1.216 (2) Å; (r(C—C)) = 1.524 (2), 1.522 (2) Å; r(C—C)conj-r(C—C)Me = -0.014 (21), -0.028 (26) Å; r(C—H) = 1.108 (4), 1.101 (4) Å; ∠CCO =119.5 (6), 118.7 (8)°; ∠CCC = 116.2 (2), 117.1 (4)°; ∠CCH = 109.6 (12), 110.4 (13)°; 5 (root mean square amplitude of C—Cconjtorsion) = 24.1 (34), 30.5 (69)°. The absence of any but the trans form, remarkable in view of the long C-Cconj bond which is suggestive of a very little conjugation stabilization, is discussed. © 1979, American Chemical Society. All rights reserved.