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 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]


Related Articles

  • Additivity of the one-third power of the electron density in the hydrogen molecule–ion and hydrogen molecule. Lee, Chengteh; Zdravkovic, Slobodan; Parr, Robert G. // Journal of Chemical Physics;2/1/1990, Vol. 92 Issue 3, p2114 

    It is established that in the hydrogen molecule-ion and hydrogen molec the one-third power of the ground-state electron density is very well represented as a superposition of spherically symmetric densities on the two nuclei.(AIP)

  • Calculation of the electron affinity of TeF6. Rappé, A. K. // Journal of Chemical Physics;12/1/1986, Vol. 85 Issue 11, p6576 

    Ab initio Hartree–Fock calculations at the valence-double-zeta-plus-diffuse functions level are used to calculate the electron affinity of TeF6 as a function of the Te–F distance. We obtain an electron affinity of 3.1 eV. Our results are compared to previous calculations on...

  • Study on the spin-states of cobalt-based double-layer perovskite Sr2Y0.5Ca0.5Co2O7. He, H.; Zhang, W. Y. // European Physical Journal B -- Condensed Matter;Feb2008, Vol. 61 Issue 4, p457 

    The spin-states of cobalt based perovskite compounds depend sensitively on the valence state and local crystal environment of Co ions and the rich physical properties arise from strong coupling among charge, spin, and orbital degrees of freedom. While extensive studies have been carried out in...

  • Application of the multiconfiguration time-dependent Hartree–Fock method to CH+: The auxiliary active space. Jaszunski, Michał; Rizzo, Antonio; Yeager, Danny L. // Journal of Chemical Physics;9/1/1988, Vol. 89 Issue 5, p3063 

    The multiconfigurational time-dependent Hartree–Fock (MCTDHF) method is used to study the excitation energies and transition moments of the CH+ ion. A consistent approach to the selection of configurations based on the complete active space (CAS) model is successfully applied. In this...

  • Structure prediction and targeted synthesis: A new NanN2 diazenide crystalline structure. Zhang, Xiuwen; Zunger, Alex; Trimarchi, Giancarlo // Journal of Chemical Physics;11/21/2010, Vol. 133 Issue 19, p194504 

    Significant progress in theoretical and computational techniques for predicting stable crystal structures has recently begun to stimulate targeted synthesis of such predicted structures. Using a global space-group optimization (GSGO) approach that locates ground-state structures and stable...

  • Scalable implementation of analytic gradients for second-order Z-averaged perturbation theory using the distributed data interface. Aikens, Christine M.; Fletcher, Graham D.; Schmidt, Michael W.; Gordon, Mark S. // Journal of Chemical Physics;1/7/2006, Vol. 124 Issue 1, p014107 

    The analytic gradient expression for second-order Z-averaged perturbation theory is revised and its parallel implementation is described in detail. The distributed data interface is used to access molecular-orbital integral arrays stored in distributed memory. The algorithm is designed to...

  • Localized orbital/local origin method for calculation and analysis of NMR shieldings. Applications to 13C shielding tensors. Hansen, Aage E.; Bouman, Thomas D. // Journal of Chemical Physics;6/1/1985, Vol. 82 Issue 11, p5035 

    A theory of NMR shielding tensors is derived from Ramsey’s expressions, using the framework of the random phase approximation (RPA) and localized molecular orbitals. By expanding angular momentum terms relative to a local origin for each orbital and using properties of the RPA solutions,...

  • A partially restricted Hartree–Fock approach. Kollmar, Christian // Journal of Chemical Physics;3/1/1994, Vol. 100 Issue 5, p3683 

    We present a Hartree–Fock method which gives the molecular orbitals (MOs) of a single determinant wave function consisting of a restricted part (one orbital assigned to each pair of spin-up and spin-down electrons) and an unrestricted part (different orbitals for different spins). The MOs...

  • Theoretical study of GaAs surface passivation with Se. Gayen, Sanjukta; Ermler, Walter C.; Sandroff, Claude J. // Journal of Chemical Physics;1/1/1991, Vol. 94 Issue 1, p729 

    The mechanism for the quenching of surface states of GaAs by adsorbed elemental Se is studied using atomic clusters in ab initio Hartree–Fock calculations. Both the Ga- and the As-terminated surfaces are tied off with Se atoms in a 2×1 reconstruction. The most stable structure is...


Read the Article


Sign out of this library

Other Topics