Role of ? stabilization in cyclic polyenes

Abstract

ABSTRACT:The Huckel model with bond-length-dependent coupling ␤ , contrarÿ to the original Huckel model with a constant ␤, may be parametrized to give C᎐C bond lengths and some other properties in good agreement with accurate ab initio results. The paradox in seemingly contradictory conclusions of some recent works on whether the electrons promote equal bond lengths or bond alternation in benzene is discussed for the more general group of cyclic polyenes. We find that electrons promote bond alternation but that the interaction between electrons of different bonds always leads to a decrease Ž . of the bond length alternation in cyclic systems except C H compared to the difference 4 4 in bond length between pure single and double bonds.

Elementary addition-substitution reactions. H�ckel approach for the description of aromatic and antiaromatic arylmethyl cations

“…HMO calculations using a bond-length-dependent coupling (β) [3,4] confirm this qualitative explanation: [6]CH + 2 < [10]CH + 2 < 1-naphthyl CH + 2 < [14]CH + 2 ≈ 1-pyrenyl CH + 2 < 9-anthracyl CH + 2 < Deviation from this linear relationship is found for the [4n] cyclicmembered methyl cations ([4n]CH + 2 ).…”

“…They show a sigmoidal character for log k versus E π deloc and E π loc . [3]. b Distances corresponding with the numbering in the 9-anthracyl CH + 2 .…”

“…However, in the case of arylmethyl chlorides as benzyl chloride, there will be opposition against interference of the aromaticity of the phenyl ring by the C + exo site (R C1−Cexo = 1.399 Å; 4 1.387 1.372 C 1 -C 11 1.391 1.371 C 4 -C 12 1.420 1.413 C 9 -C 11 1.442 -C 11 -C 12 1.419 1.412 C 10 -C 12 1 3 1.386 1.367 C 3 -C 4 1.406 1.417 C 4 -C 5 1.395 1.392 C 5 -C 6 1.399 1.403 q exo = 0.44 a The geometries of the corresponding proton complexes are presented as diphenylmethyl-, allyl-, pentadienyl-, heptatrienyl-, and nonatetraenyl cation. [3]. c Ab initio results from Ref.…”

The Role of Quantum Dynamics in Covalent Bonding – A Comparison of the Thomas-Fermi and Hückel Models

“…Thus, Hückel theory predicts in a qualitatively correct manner that there is an alternation of -bond strengths in conjugated polyenes which accounts for the observed variation of CC bond lengths. In fact, the Hückel-SCF model [27], in which the -parameters are defined to be bond length dependent, while the bond lengths in turn are determined on the basis of the corresponding bond orders in a self-consistent way, has been found to be a useful and reliable computational approach in a number of applications [28][29][30][31][32].…”

Hidden-Variable Theorems for Real Experiments

“…Larsson that he has come to similar conclusions using a somewhat related approach [16]. C. S. would like to thank L. Hardy for a useful discussion.…”