Alexander Auer

Name:

Organization: Chemnitz University of Technology

Department:

Title:

Chemicals
References

Orbital instabilities and spin-symmetry breaking in coupled-cluster calculations of indirect nuclear spin–spin coupling constants

Co-reporter:Alexander A. Auer, Jürgen Gauss

Chemical Physics 2009 Volume 356(1–3) pp:7-13

Publication Date(Web):17 February 2009

DOI:10.1016/j.chemphys.2008.10.044

Abstract

The effect of orbital instabilities is investigated for spin-symmetry breaking perturbations, namely the Fermi-contact (FC) and spin–dipole (SD) contributions to the indirect nuclear spin–spin coupling constants. For the CO and N₂ molecules the FC and SD contributions have been calculated and orbital-stability analyses for various interatomic distances have been carried out. This includes calculations at the Hartree–Fock self-consistent field (HF-SCF), coupled-cluster (CC) singles and doubles (CCSD), CC3, CCSD(T), CCSDT-4, CC singles, doubles, and triples (CCSDT) levels, and for the first time also at the CC singles, doubles, triples, and quadruples (CCSDTQ) level of theory. For calculations with relaxation of the reference orbitals in the presence of the perturbation, unphysical results are obtained over a wide range of the potential curve. This is due to a triplet instability of the Hartree–Fock reference determinant which leads to a pronounced pole in the FC and SD contributions. The effect of orbital instabilities in the relaxed methods is most dramatic for perturbative approaches like CCSD(T), while it is less pronounced for methods of the classical CC hierarchy. CC calculations without relaxation of the orbitals, i.e., so-called “unrelaxed” calculations, do not show any of these effects.

Bismuth−Arene π-Interaction: A Combined Experimental and Theoretical Approach

Co-reporter:Alexander A. Auer, Dirk Mansfeld, Christoph Nolde, Wolfgang Schneider, Markus Schürmann and Michael Mehring

Organometallics 2009 Volume 28(Issue 18) pp:5405-5411

Publication Date(Web):August 28, 2009

DOI:10.1021/om900536r

The triorgano bismuth compound [Bi(CH2C6H4Cl-2)3]2 (2) was characterized by an X-ray single-crystal structure analysis, which reveals the formation of a two-dimensional network as a result of bismuth−arene π-coordination and π−π-stacking interactions with distances of 3.659 Å (bismuth−arenecentroid) and 3.869 Å (arene centroids), respectively. In order to elucidate the nature of this bonding situation, a quantum mechanical study was carried out. Additionally, a detailed theoretical study on several model compounds of the type BiX3·C6H6 (X = H, Me, OH, OMe, F, Cl, Br) was carried out at the BSSE-corrected MP2/TZVP level of theory in order to develop a better understanding of the bismuth−arene coordination. The calculated bismuth−arene distances in the model compounds BiMe3·C6H6 (∼3.75 Å) and Bi(OMe)3·C6H6 (∼3.35 Å) compare well with experimental values for [Bi(CH2C6H4Cl-2)3]2 (2) and [Bi(OSiPh2tBu)3]2 (1) of 3.659 and 3.340 Å, respectively. Interaction energies for the model compounds range from 7 kJ/mol for BiMe3·C6H6 to 41 kJ/mol for BiBr3·C6H6.