Stefan Fau

Electronic structure of CO--an exercise in modern chemical bonding theory.

This paper discusses recent progress that has been made in the understanding of the electronic structure and bonding situation of carbon monoxide which was analyzed using modern quantum chemical methods. The new results are compared with standard models of chemical bonding. The electronic charge distribution and the dipole moment, the nature of the HOMO and the bond dissociation energy are discussed in detail.

Theoretical investigation of the weakly bonded donor—acceptor complexes X3B—H2, X3B—C2H4, and X3B—C2H2 (X = H, F, Cl)

Quantum mechanical calculations at the MP2/6-311G(2df,2pd)//MP2/TZ2P level of theory are reported for the compounds X3B—D with X = H, F, Cl, and D = H2, C2H4, and C2H2. The calculated binding energies show the trend BH3 >> BF3 > BCl3 for the Lewis acids and C2H4 > C2H2 > H2 for the Lewis bases. BCl3 and BF3 form weakly bonded van der Waals complexes with C2H4 and C2H2 with similar binding energies De between 4.2 kcal mol-1 for F3B—C2H4 and 3.2 kcal mol-1 for Cl3B—C2H2. The dihydrogen complexes have lower bond dissociation energies of 0.9 kcal mol-1 for Cl3B—H2 and 0.7 kcal mol-1 for Cl3B—H2. The BH3 complexes are significantly more strongly bonded and show the onset of true chemical bonding. The NBO method finds a polarized 2-electron 3-centre bond for the three H3B—D compounds. The most strongly bonded complex is H3B—C2H4 (De = 15.0 kcal mol-1), while H3B—C2H2 (De = 8.5 kcal mol-1) and H3B—H2 (De = 5.7 kcal mol-1) are more weakly bonded species. The contribution of the zero-point vibrational energy shows...

A crossed molecular beam and ab-initio investigation of the reaction of boron monoxide (BO; X2Σ+) with methylacetylene (CH3CCH; X1A1): competing atomic hydrogen and methyl loss pathways.

The gas-phase reaction of boron monoxide ((11)BO; X(2)Σ(+)) with methylacetylene (CH3CCH; X(1)A1) was investigated experimentally using crossed molecular beam technique at a collision energy of 22.7 kJ mol(-1) and theoretically using state of the art electronic structure calculation, for the first time. The scattering dynamics were found to be indirect (complex forming reaction) and the reaction proceeded through the barrier-less formation of a van-der-Waals complex ((11)BOC3H4) followed by isomerization via the addition of (11)BO(X(2)Σ(+)) to the C1 and/or C2 carbon atom of methylacetylene through submerged barriers. The resulting (11)BOC3H4 doublet radical intermediates underwent unimolecular decomposition involving three competing reaction mechanisms via two distinct atomic hydrogen losses and a methyl group elimination. Utilizing partially deuterated methylacetylene reactants (CD3CCH; CH3CCD), we revealed that the initial addition of (11)BO(X(2)Σ(+)) to the C1 carbon atom of methylacetylene was followed by hydrogen loss from the acetylenic carbon atom (C1) and from the methyl group (C3) leading to 1-propynyl boron monoxide (CH3CC(11)BO) and propadienyl boron monoxide (CH2CCH(11)BO), respectively. Addition of (11)BO(X(2)Σ(+)) to the C1 of methylacetylene followed by the migration of the boronyl group to the C2 carbon atom and/or an initial addition of (11)BO(X(2)Σ(+)) to the sterically less accessible C2 carbon atom of methylacetylene was followed by loss of a methyl group leading to the ethynyl boron monoxide product (HCC(11)BO) in an overall exoergic reaction (78 ± 23 kJ mol(-1)). The branching ratios of these channels forming CH2CCH(11)BO, CH3CC(11)BO, and HCC(11)BO were derived to be 4 ± 3%, 40 ± 5%, and 56 ± 15%, respectively; these data are in excellent agreement with the calculated branching ratios using statistical RRKM theory yielding 1%, 38%, and 61%, respectively.

Gas-Phase Synthesis of Boronylallene (H2CCCH(BO)) under Single Collision Conditions: A Crossed Molecular Beams and Computational Study.

The gas phase reaction between the boron monoxide radical (11BO; X2Σ+) and allene (H2CCCH2; X1A1) was investigated experimentally under single collision conditions using the crossed molecular beam technique and theoretically exploiting ab initio electronic structure and statistical (RRKM) calculations. The reaction was found to follow indirect (complex forming) scattering dynamics and proceeded via the formation of a van der Waals complex (11BOC3H4). This complex isomerized via addition of the boron monoxide radical (11BO; X2Σ+) with the radical center located at the boron atom to the terminal carbon atom of the allene molecule forming a H2CCCH211BO intermediate on the doublet surface. The chemically activated H2CCCH211BO intermediate underwent unimolecular decomposition via atomic hydrogen elimination from the terminal carbon atom holding the boronyl group through a tight exit transition state to synthesize the boronylallene product (H2CCCH11BO) in a slightly exoergic reaction (55 ± 11 kJ mol-1). Statistical (RRKM) calculations suggest that minor reaction channels lead to the products 3-propynyloxoborane (CH2(11BO)CCH) and 1-propynyloxoborane (CH3CC11BO) with fractions of 1.5% and 0.2%, respectively. The title reaction was also compared with the cyano (CN; X2Σ+)-allene and boronyl-methylacetylene reactions to probe similarities, but also differences of these isoelectronic systems. Our investigation presents a novel gas phase synthesis and characterization of a hitherto elusive organyloxoborane (RBO) monomer-boronylallene-which is inherently tricky to isolate in the condensed phase except in matrix studies; our work further demonstrates that the crossed molecular beams approach presents a useful tool in investigating the chemistry and synthesis of highly reactive organyloxoboranes.

ANTI VAN'T HOFF/LE BEL GEOMETRIES OF BORON COMPOUNDS. A THEORETICAL STUDY OF CLASSICAL AND NONCLASSICAL ISOMERS OF B2CH4, B2NH3 AND B2OH2

Abstract The potential energy surfaces of B 2 CH 4 , B 2 NH 3 and B 2 OH 2 have been studied using quantum chemical ab initio methods at the MP2/6-31G(d) level of theory. Improved energies are calculated at MP4/6-311G(2df,2pd). Four energy minimum forms are predicted for B 2 CH 4 . Six isomers are calculated as energy minima for B 2 NH 3 , while three equilibrium structures are predicted for B 2 OH 2 . The electronic structure of the molecules is investigated using the topological analysis of the electron density distribution and its associated gradients and Laplacian.