Equilibrium structures for butadiene and ethylene: compelling evidence for pi-electron delocalization in butadiene.
Equilibrium structures have been determined for s-trans-1,3-butadiene and ethylene after adjusting the rotational constants obtained from rotational spectroscopy by vibration-rotation constants calculated from the results of quantum chemical calculations. For butadiene, the formal C=C bond length is 1.338 A, and the formal C-C bond length is 1.454 A. For ethylene, the C=C bond length is 1.3305 A. These values appear to be good to 0.001 A. It is shown for the first time that pi-electron delocalization has the structural consequences of increasing the length of the formal double bond by 0.007 A and decreasing the length of the formal single bond by 0.016 A. Comparisons are made with structures computed with several quantum chemical models. The MP2/cc-pVTZ results agree best with the new re structure.
Craig
Norman C
NC
Department of Chemistry and Biochemistry, Oberlin College, Oberlin, Ohio 44074, USA.
Groner
Peter
P
McKean
Donald C
DC
D002070:Butadienes / Q000737:chemistry*; D004583:Electrons; D005030:Ethylenes / Q000737:chemistry*; D008968:Molecular Conformation; D015394:Molecular Structure; D011789:Quantum Theory
D002070:Butadienes; D005030:Ethylenes; C036216:ethylene