IL11. A Direct Measure of Metal-Ligand Bonding Replacing the Tolman Electronic Parameter

The prediction of the catalytic activity of transition metal complexes is a prerequisite for homogeneous catalysis. The Tolman Electronic Parameter (TEP) was derived to provide this information. It is based on the CO stretching frequencies of metal-tricarbonyl complexes LM(CO)3 with varying ligands L. It has been used in hundreds of cases as is documented by as many publications. We show [1] that the TEP is misleading as i) it is not based on mode-decoupled CO stretching frequencies and ii) a quantitatively correct or at least qualitatively reasonable relationship between the TEP and the metal-ligand bond strength does not exist. This is demonstrated for a set of 181 nickel-tricarbonyl complexes using both experimental and calculated TEP values. Even the use of mode-mode decoupled CO stretching frequencies does not lead to a reasonable description of the metal-ligand bond strength. A reliable descriptor replacing the TEP is obtained with the help of the metal-ligand local stretching force constant. For the test set of 181 Ni-complexes, a direct metal-ligand electronic parameter (MLEP) in the form of a bond strength order is derived, which reveals that phosphines and related ligands (amines, arsines, stibines, bismuthines) are bonded to Ni both by σ-donation and π−back donation. The strongest Ni-L bonds are identified for carbenes and cationic ligands. The new MLEP quantitatively assesses electronic and steric factors and it can be determined for any metal or transition metal complex, whether it contains CO ligands or not. Dieter Cremer, Southern Methodist University Elfi Kraka, Southern Methodist Universit

A Description of the Chemical Bond in Terms of Local Properties of Electron Density and Energy

Chemical bonding is described in terms of the properties of the one-electron density f2 (r) and the local energy density H (r) = G (r) + V (r). Analysis of a variety of different bonds suggests that covalent bonding requires the existence of ao saddle point rP of f2 (r) in the internuclear region (necessary condition) and a predominance of the local potential energy V (r) at rP : J VP J > GP and, hence, HP< O. A covalent bond can be characterized by the position of r P, the value and the aonisotropy of f!p· These properties of f2 (r) can be used to define polarity, order and n-character of the bond. Information about concentration and depletion of electronic charge at rP is provided by the Laplacian of f2p• 1 2 f!p· Investigation of 1 2 (! (r) does not suffice to detect weak covalent bonds, an observation which is allways valid if accumulation of electronic charge in the internuclear region is taken as the sole indicator for bonding. Interactions between closed shell systems as experienced in ionic, hydrogen bonded or van der Waals systems lead to a positive value of HP. In this case, shared electron density causes destabilization rather than stabilizaotion of the molecule

Environmental effects on molecular conformation: Bicalutamide analogs

a b s t r a c t Two bicalutamide analogs (N-[4-nitro-3-(trifluoromethyl)phenyl]-3-(4-fluorophenyl)sulfinyl-2-hydroxy-2-methyl-propane-amide 2 and its 4-cyano derivative 3) with an R-configured asymmetric carbon atom and a chiral sulfoxide group are described quantum chemically to determine their properties in dependence of their conformation and their (R,S)-configuration at the sulfoxide S atom. Compounds 2 and 3 are known to be novel androgen receptor antagonists with biological activities that depend significantly on the configuration of their stereogenic centers. For the purpose of a rapid differentiation between active and less active diastereomers of 2 and 3, relative energies, conformational preferences in different media, NMR chemical shift values, vibrational spectra, and vibrational circular dichroism (VCD) spectra are calculated for up to 12 different conformers. It is demonstrated that both 2 and 3 prefer strongly different conformations in dependence of the surrounding medium and as a consequence of the change from intrato intermolecular H-bonding. The different diastereomers can be easily distinguished by specific NMR chemical shifts, infrared bands, or VCD rotational strengths

Pancake Bonding Seen through the Eyes of Spectroscopy

From local mode stretching force constants and topological electron density analysis, computed at either the UM06/6-311G(d,p), UM06/SDD, or UM05-2X/6–31++G(d,p) level of theory, we elucidate on the nature/strength of the parallel π-stacking interactions (i.e. pancake bonding) of the 1,2-dithia-3,5-diazolyl dimer, 1,2-diselena-3,5-diazolyl dimer, 1,2-tellura-3,5-diazolyl dimer, phenalenyl dimer, 2,5,8-tri-methylphenalenyl dimer, and the 2,5,8-tri-t-butylphenalenyl dimer. We use local mode stretching force constants to derive an aromaticity delocalization index (AI) for the phenalenyl-based dimers and their monomers as to determine the effect of substitution and dimerization on aromaticity, as well as determining what bond property governs alterations in aromaticity. Our results reveal the strength of the C⋯C contacts and of the rings of the di-chalcodiazoyl dimers investigated decrease in parallel with decreasing chalcogen⋯chalcogen bond strength. Energy density values Hb suggest the S⋯S and Se⋯Se pancake bonds of 1,2-dithia-3,5-diazolyl dimer and the 1,2-diselena-3,5-diazolyl dimer are covalent in nature. We observe the pancake bonds, of all phenalenyl-based dimers investigated, to be electrostatic in nature. In contrast to their monomer counterparts, phenalenyl-based dimers increase in aromaticity primarily due to CC bond strengthening. For phenalenyl-based dimers we observed that the addition of bulky substituents steadily decreased the system aromaticity predominately due to CC bond weakening

Structure of the chlorobenzene–argon dimer: Microwave spectrum and ab initio analysis

The rotational spectra of the 35Cl35Cl and 37Cl37Cl isotopes of the chlorobenzene–argon van der Waals dimer have been assigned using Fourier transform microwave spectroscopy techniques. Rotational constants and chlorine nuclear quadrupole coupling constants were determined which confirm that the complex has CsCs symmetry. The argon is over the aromatic ring, shifted from a position above the geometrical ring center towards the substituted carbon atom, and at a distance of about 3.68 Å from it. This distance is 0.1–0.2 Å shorter than the similar distance in the benzene–argon and fluorobenzene–argon complexes. Experimental results are confirmed and explained with the help of second-order Møller–Plesset perturbation calculations using a VDZP+diffVDZP+diff basis set. The complex binding energy of the chlorobenzene–argon complex is 1.28 kcal/mol (fluorobenzene–argon, 1.17; benzene–argon, 1.12 kcal/mol) reflecting an increase in stability caused by larger dispersion interactions when replacing one benzene H atom by F or by Cl. The structure and stability of Ar⋅C6H5–XAr⋅C6H5–X complexes are explained in terms of a balance between stabilizing dispersion and destabilizing exchange repulsion interactions between the monomers. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70251/2/JCPSA6-113-20-9051-1.pd