Zur Struktur der Moleküle (AlF)2, OAlF und (OAlF)2 Matrix-IR-Untersuchungen und ab initio SCF-Rechnungen

Structures of the Molecules (AlF)₂, OAlF, and (OAlF)₂. Matrix-I.R. Investigations and Ab Initio SCF Calculations The formation of (AlF)₂ is observed if AlF is trapped in a rare gas matrix. A cyclic D_2h structure is deduced from matrix i.r. and ab initio investigations. Furthermore, (OAlF)₂ is formed in a matrix reaction with O₂. The i.r. frequency of the antisymmetric AlF mode and its ¹⁶O/¹⁸O shift in combination with ab initio calculations lead to the following structure: (D_2h symmetry). We further report results of ab initio calculations for OAlF which confirm previous conclusions derived from i.r. spectra.     

Improvements on the direct SCF method

Three improvements on the direct self-consistent field method are proposed and tested which together increase CPU-efficiency by about 50%: (i) selective storage of costly integral batches; (ii) improved integral bond for prescreening; (iii) decomposition of the current density matrix into a linear combination of previous density matrices—for which the two-electron contributions to the Fock matrix are available—and a remainder ΔD, which is minimized; construction of the current Fock matrix only requires processing of the small ΔD which enhances prescreening.     

Theoretical investigation of structure and stability of oligomers of LiH,NaH, LiF,and NaF

The molecules Li_nH_n, Na_nH_n, Li_nF_n, n=1,..., 4, and NaF and Na₂F₂ are investigated by means of extended basis set SCF and CEPA-PNO computations. In analogy to the D _2h structure of dimers, it is found that trimers have a planar cyclic D _3h equilibrium geometry. For the tetramer of LiH and NaH, the D _4h structure has about the same energy as the 3-dimensional T _d structure, whereas the latter is definitely favoured for Li₄F₄. Correlation effects are investigated for the oligomerization of LiH and the dimerization of LiF. The effect of electron correlation on corresponding E turns out to be small (<4 kJ/mol), except for the case that the T _d tetramer is involved which has a rather large correlation energy.     

Angular Distortions at Tetracoordinate Carbon Planoid Configurations in Substituted Spiro[4.4]nonanes

The configuration of the spiro centre in polycyclic compounds containing a spiro[4.4]nonane substructure is analyzed in terms of symmetry coordinates. As revealed by X-ray structures of such compounds, the local distortions around the quaternary C-atom are dominated by a decrease of the ring-bond angles at the spiro centre and a twist of one ring relative to the other. The planoid deformations given by the planarization index P_C are comparatively small for spiro[4.4]nonanes, which contain either no additional bridges between the rings or only α,β′–or β,β′-polymethylene bridges. This indicates that the local strain in the spiro C(C)₄ fragments is rather small.     

Planarization of Tetracoordinate Carbon Atom. Synthesis of 13-oxa-14-oxo-pentacyclo [5.5.2.1.04,15010,15]pentadecane,a bridged ‘tetraquinacane’ preliminary communication

Structural aspects of planoid deformation of tetracoordinate carbon atom, particularly those in annulenes with central carbon atom are discussed. [12] Annulene 11 with central carbon atom is proposed as compound with strong planoid deformation around the internal carbon centre. The synthesis of a saturated precursor 15 is described.     

Isomerisierung der Bis(triphenylphosphin)platin(0)-Komplexe von Bicyclo[4.2.1]non-1-en und Bicyclo[4.2.1]non-1(8)-en

Isomerization of the Bis(triphenylphosphine)platin(0) Complexes of Bicyclo[4.2.1]non-1-ene and Bicyclo[4.2.1]non-1(8)-ene Isomrisation of the Bredt-Olefins 3 and 7 as the bis (triphenylphosphine)-Pt(0) complexes 1 and 8 , respectively, has been observed. The bridgehead-olefin 3 as well as the bicyclic olefins 4 , 5 and 6 have also been isomerized with catalytical amounts of (ethylene)bis(triphenylphosphine)platinum(0) ( 2 ).     

Computation of the complexation energies of BH3NH3 and BH3PH3

Extended basis set computations on SCF and CEPA level were performed for BH₃NH₃ and BH₃PH₃ to determine the complexation energy ΔE and the equilibrium distance r(BX) between the “heavy” atoms. Our CEPA results (SCF in parentheses): ΔE(BH₃NH) = ?27(?21.3) kcal/mol, ΔE(BH₃PH₃) = ?17(?11.8) kcal/mol, r(BN) = 1.65(1.68) Å, r(BP) = 1.95(1.99) Å indicate a marked influence of electron correlation on these properties.     

The Radical Anions and Trianions of 1,8-Diphenylnaphthalene and of Some of its Derivatives Containing a Cyclophane Substructure

The radical anions of 1,8-diphenylnaphthalene ( 1 ) and its decadeuterio-(D₁₀- 1 ) and dimethyl-( 2 ) derivatives, as well as those of [2.0.0] (1,4)benzeno(1,8)naphthaleno(1,4)benzenophane ( 3 ) and its olefinic analogue ( 4 ) have been studied by ESR and ENDOR spectroscopy, At a variance with a previous report, the spin population in \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {1}^{-\kern-4pt {.}} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {2}^{-\kern-4pt {.}} $\end{document} is to a great extent localized in the naphthalene moiety. A similar spin distribution is found for \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {3}^{-\kern-4pt {.}} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {4}^{-\kern-4pt {.}} $\end{document}. The ground conformations of \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {1}^{-\kern-4pt {.}} $\end{document}-\documentclass{article}\pagestyle{empty}\begin{document}$ \rm {4}^{-\kern-4pt {.}} $\end{document} are chiral of C₂ symmetry. For \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {1}^{-\kern-4pt {.}} $\end{document}, an energy barrier between these conformations and the angle of twist about the bonds linking the naphthalene moiety with the phenyl substituents were estimated as ca. 50 kJ/mol and ca. 45°, respectively. The radical trianions of 1 , D₁₀- 1 , and 2 , have also been characterized by their hyperfine data. In \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {1}^{3-\kern-4pt {.}} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {2}^{3-\kern-4pt {.}} $\end{document}, the bulk of the spin population resides in the two benzene rings so that these radical trianions can be regarded as the radical anions of ‘open-chain cyclophanes’ with a fused naphthalene π-system bearing almost two negative charges. The main features of the spin distribution in both \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {1}^{-\kern-4pt {.}} $\end{document} and \documentclass{article}\pagestyle{empty}\begin{document}$ \rm {1}^{3-\kern-4pt {.}} $\end{document} are correctly predicted by an HMO model of 1 .     

Density functional theory calculations and exploration of a possible mechanism of N2 reduction by nitrogenase

Density functional theory (DFT) calculations have been performed on the nitrogenase cofactor, FeMoco. Issues that have been addressed concern the nature of M-M interactions and the identity and origin of the central light atom, revealed in a recent crystallographic study of the FeMo protein of nitrogenase (Einsle, O.; et al. Science 2002, 297, 871). Introduction of Se in place of the S atoms in the cofactor and energy minimization results in an optimized structure very similar to that in the native enzyme. The nearly identical, short, lengths of the Fe-Fe distances in the Se and S analogues are interpreted in terms of M-M weak bonding interactions. DFT calculations with O or N as the central atoms in the FeMoco marginally support the assignment of the central atom as N rather than O. The assumption was made that the central atom is the N atom, and steps of a catalytic cycle were calculated starting with either of two possible states for the cofactor and maintaining the same charge throughout (by addition of equal numbers of H(+) and e(-)) between steps. The states were [(Cl)Fe(II)(6)Fe(III)Mo(IV)S(9)(H(+))(3)N(3-)(Gl)(Im)](2-), [I-N-3H](2-), and [(Cl)Fe(II)(4)Fe(III)(3)Mo(IV)S(9)(H(+))(3)N(3-)(Gl)(Im)], [I-N-3H](0) (Gl = deprotonated glycol; Im = imidazole). These are the triply protonated ENDOR/ESEEM [I-N](5-) and M?ssbauer [I-N](3-) models, respectively. The proposed mechanism explores the possibilities that (a) redox-induced distortions facilitate insertion of N(2) and derivative substrates into the Fe(6) central unit of the cofactor, (b) the central atom in the cofactor is an exchangeable nitrogen, and (c) the individual steps are related by H(+)/e(-) additions (and reduction of substrate) or aquation/dehydration (and distortion of the Fe(6) center). The Delta E's associated with the individual steps of the proposed mechanism are small and either positive or negative. The largest positive Delta E is +121 kJ/mol. The largest negative Delta E of -333 kJ/mol is for the FeMoco with a N(3-) in the center (the isolated form) and an intermediate in the proposed mechanism.