title: Electron delocalization in polyenes: a semiexperimental equilibrium structure for (3E)-1,3,5-hexatriene and theoretical structures for (3Z,5Z)-, (3E,5E)-, and (3E,5Z)-1,3,5,7-octatetraene.
abstract: Electronic structure theory reveals that π-electron delocalization increases with the chain length in polyenes. To analyze quantitatively this effect a semiexperimental equilibrium structure has been determined for trans-hexatriene by the mixed estimation method. For this fit rotational constants for a number of carbon and hydrogen isotopologues as well as a high-level ab initio structure have been used. The accuracy is 0.001 Å for bond lengths and 0.1° for bond angles. For the three isomers of octatetraene, high-level ab initio calculations have given a comparably accurate structure. These structures have been used in comparison with the structure of s-trans-butadiene to show that "C═C" bonds increase in length and "C-C" bonds decrease in length as the polyene chain lengthens. These structural effects of π-electron delocalization increase toward the center of polyenes. Most likely, π-π conjugation in the molecules studied plays a large part in their planarity that, in turn, forces the hydrogen atoms of cis fragments in bay regions to be in a close contact. Their distance is indeed shorter than the sum of their van der Waals radii, and they seem to participate in a six-membered ring.
authors.affiliation: Department of Chemistry and Biochemistry, Oberlin College , Oberlin, Ohio 44074, United States.
authors.forename: Norman C
authors.initials: NC
authors.lastname: Craig
authors.forename: Jean
authors.initials: J
authors.lastname: Demaison
authors.forename: Peter
authors.initials: P
authors.lastname: Groner
authors.forename: Heinz Dieter
authors.initials: HD
authors.lastname: Rudolph
authors.forename: Natalja
authors.initials: N
authors.lastname: Vogt
mesh_terms: D004583:Electrons; D015394:Molecular Structure; D011090:Polyenes / Q000737:chemistry*; D011789:Quantum Theory; D013237:Stereoisomerism
chemical_list: C000604890:1,3,5,7-octatetraene; D011090:Polyenes; C512191:1,3,5-hexatriene