TITLE

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

AUTHOR(S)
Shi, Zheng; Brion, C E; Rolke, James; Zheng, Yenyou; Cooper, Glyn; Chong, Delano P; Hu, C Y; Wolfe, Saul
PUB. DATE
March 2002
SOURCE
Canadian Journal of Chemistry;Mar2002, Vol. 80 Issue 3, p222
SOURCE TYPE
Academic Journal
DOC. TYPE
Article
ABSTRACT
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 H�ckel theory, and interpreted in terms of "through bond" and "through space" interactions.Key words: orbitals, lone pairs, orbital interaction, hybridization.Faisant appel � la spectroscopie des moments �lectroniques, une technique qui �tudie la nature de type orbitalaire de la valence (fronti�re) du transfert d'�lectron � partir d'une mol�cule, on a d�termin� les distributions des orbitales mol�culaires haute occup�e (� HOMO �) et pratiquement haute occup�e (� NHOMO �) de la densit� des �lectrons orbitalaires de valence du 1,4-diazabicyclo[2.2.2]octane (DABCO). On a compar� les r�sultats exp�rimentaux � une s�rie de calculs au niveau 6-311++G** afin d'�valuer les m�rites relatifs de trois mod�les orbitalaires diff�rents qui ont �t� couramment utilis�s en chimie. Les orbitales canoniques d�localis�es (avec corr�lation) de Kohn�Sham, calcul�es � l'aide de fonctions B3LYP ou B3PW91 et la th�orie de la densit� fonctionnelle, correspondent pratiquement de fa�on quantitative avec les distributions observ�es pour la densit� du moment de l'�lectron de valence; par ailleurs, les orbitales mol�culaires canoniques d�localis�es de Hartree�Fock (particule ind�pendante) pr�sentent un accord semi-quantitatif. Par opposition, le mod�le de la liaison de valence (hybridation) de Pauling qui est couramment utilis� et enseign� et qui est �quivalent � une description d'orbitales mol�culaires localis�es ne pr�sente aucune corr�lation avec les mesures exp�rimentales. Il en d�coule que, pour des consid�rations de transferts d'�lectrons, les � paires libres � du DABCO ne sont pas localis�es ou hybrid�es, mais qu'elles existent plut�t sous la forme d'orbitales non d�g�n�r�es qui sont d�localis�es de fa�on diff�rente sur le squelette mol�culaire. L'existence, dans le cas du DABCO, de deux ensembles exp�rimentaux diff�rents pour les densit�s des orbitales �lectroniques de valence fournit une confirmation directe du d�doublement d'�nergie HOMO�NHOMO de l'orbitale fronti�re et de la r�organisation qui a �t� pr�dite il y a plusieurs ann�es par Hoffmann et al. � l'aide de la th�orie �largie de H�ckel et interpr�t� en fonction d'interactions � par les liaisons � et � � travers l'espace �.Mots cl�s : orbitales, paires libres, interaction d'orbitales, hybridation.[Traduit par la R�daction]
ACCESSION #
10577116

 

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