Electron momentum spectroscopy of the frontier electrons of DABCO does not support an sp[sup 3] hybrid lone-pair description

Shi, Zheng; Brion, C E; Rolke, James; Zheng, Yenyou; Cooper, Glyn; Chong, Delano P; Hu, C Y; Wolfe, Saul
March 2002
Canadian Journal of Chemistry;Mar2002, Vol. 80 Issue 3, p222
Academic Journal
The highest occupied molecular orbital (HOMO) and next-highest occupied molecular orbital (NHOMO) valence orbital electron density distributions of 1,4-diazabicyclo[2.2.2]octane (DABCO) have been investigated by electron momentum spectroscopy, a technique that probes the orbital-like nature of valence (frontier) electron transfer out of a molecule. The experimental results are compared to a range of 6-311++G** calculations to assess the relative merits of three different orbital models that have commonly been used in chemistry. The delocalized (correlated) canonical Kohn–Sham orbitals calculated using the B3LYP or B3PW91 functionals and density functional theory provide near quantitative agreement with the observed valence electron momentum density distributions, and the delocalized canonical molecular orbitals of Hartree–Fock (independent particle) theory are in semiquantitative agreement. In contrast, Pauling's widely used and taught valence bond (hybridization) model, which is equivalent to a localized molecular orbital description, does not correspond at all to the experimental measurements. It follows that, for considerations of electron transfer, the "lone pairs" of DABCO are not localized or hybridized, but rather exist as nondegenerate orbitals that are delocalized differently over the molecular framework. The existence of two different experimental valence orbital electron densities of DABCO provides direct confirmation of the frontier orbital HOMO–NHOMO energy splitting and reordering predicted many years ago by Hoffmann et al. using extended Hückel theory, and interpreted in terms of "through bond" and "through space" interactions.Key words: orbitals, lone pairs, orbital interaction, hybridization.Faisant appel à la spectroscopie des moments électroniques, une technique qui étudie la nature de type orbitalaire de la valence (frontière) du transfert d'électron à partir d'une molécule, on a déterminé les distributions des orbitales moléculaires haute occupée (« HOMO ») et pratiquement haute occupée (« NHOMO ») de la densité des électrons orbitalaires de valence du 1,4-diazabicyclo[2.2.2]octane (DABCO). On a comparé les résultats expérimentaux à une série de calculs au niveau 6-311++G** afin d'évaluer les mérites relatifs de trois modèles orbitalaires différents qui ont été couramment utilisés en chimie. Les orbitales canoniques délocalisées (avec corrélation) de Kohn–Sham, calculées à l'aide de fonctions B3LYP ou B3PW91 et la théorie de la densité fonctionnelle, correspondent pratiquement de façon quantitative avec les distributions observées pour la densité du moment de l'électron de valence; par ailleurs, les orbitales moléculaires canoniques délocalisées de Hartree–Fock (particule indépendante) présentent un accord semi-quantitatif. Par opposition, le modèle de la liaison de valence (hybridation) de Pauling qui est couramment utilisé et enseigné et qui est équivalent à une description d'orbitales moléculaires localisées ne présente aucune corrélation avec les mesures expérimentales. Il en découle que, pour des considérations de transferts d'électrons, les « paires libres » du DABCO ne sont pas localisées ou hybridées, mais qu'elles existent plutôt sous la forme d'orbitales non dégénérées qui sont délocalisées de façon différente sur le squelette moléculaire. L'existence, dans le cas du DABCO, de deux ensembles expérimentaux différents pour les densités des orbitales électroniques de valence fournit une confirmation directe du dédoublement d'énergie HOMO–NHOMO de l'orbitale frontière et de la réorganisation qui a été prédite il y a plusieurs années par Hoffmann et al. à l'aide de la théorie élargie de Hückel et interprété en fonction d'interactions « par les liaisons » et « à travers l'espace ».Mots clés : orbitales, paires libres, interaction d'orbitales, hybridation.[Traduit par la Rédaction]


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