Mid-infrared sizes of circumstellar disks around Herbig Ae/Be stars measured with MIDI on the VLTI

We present the first long baseline mid-infrared interferometric observations of the circumstellar disks surrounding Herbig Ae/Be stars. The observations were obtained using the mid-infrared interferometric instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI on Cerro Paranal. The 102 m baseline given by the telescopes UT1 and UT3 was employed, which provides a maximum full spatial resolution of 20 milli-arcsec (mas) at a wavelength of 10 μm. The interferometric signal was spectrally dispersed at a resolution of 30, giving spectrally resolved visibility information from 8 μm to 13.5 μm. We observed seven nearby Herbig Ae/Be stars and resolved all objects. The warm dust disk of HD 100546 could even be resolved in single-telescope imaging. Characteristic dimensions of the emitting regions at 10 μm are found to be from 1 AU to 10 AU. The 10 μm sizes of our sample stars correlate with the slope of the 10–25 μm infrared spectrum in the sense that the reddest objects are the largest ones. Such a correlation would be consistent with a different geometry in terms of flaring or flat (self-shadowed) disks for sources with strong or moderate mid-infrared excess, respectively. We compare the observed spectrally resolved visibilities with predictions based on existing models of passive centrally irradiated hydrostatic disks made to fit the SEDs of the observed stars. We find broad qualitative agreement of the spectral shape of visibilities corresponding to these models with our observations. Quantitatively, there are discrepancies that show the need for a next step in modelling of circumstellar disks, satisfying both the spatial constraints such as are now available from the MIDI observations and the flux constraints from the SEDs in a consistent way

Steric effects of tertiary phosphines on nucleophile-induced insertion of carbon monoxide into transition metal-carbon bonds

Rate constants associated with the tertiary phosphine-induced formation of the metal acyls, [(η5-C5H5)(CO)(PR3)FeCOCH2Cy] and [(η5-C5H5)(CO)2(PR3)MoCOCH2Ph], show no correlation with the electronic properties of the phosphine, but exhibit a marked dependence on steric effects. Rates decrease slightly with increase in phosphine cone angles in the lower ranges, but drop very rapidly at higher cone angles. The behaviour is consistent with the concept of a pocket of limited size around the metal centre, with large intermolecular effects arising when the sizes of the tertiary phosphine and the pocket become comparable

Steric effects in carbon monoxide insertion into manganese-carbon bonds

The reactivity of the manganese-benzyl complexes [(CO)5MnCH2C6H5-nXn] toward carbon monoxide insertion induced by tertiary phosphines in polar solvents has been studied. The steric influences of the alkyl group, solvent, and the phosphine are discussed

ortho-Effects on carbon monoxide insertion in benzyl-molybdenum complexes

The formation of phosphine-substituted acylmolybdenums [(η5-C5H5)-(CO)2(PPh3)Mo(COR)], from reactions between substituted benzylmolybdenum complexes and triphenylphosphine in acetonitrile, is enhanced by the presence of a single bulky substituent in the ortho-position. However di-substitution (for i-Pr, Cl and OMe) causes a pronounced lowering of reactivity. The results are interpreted in terms of the steric demands of the reaction

Solvent and nucleophile effects on the carbonyl insertion reaction into metal-carbon bonds

The rate constant, k3, for the direct insertion of carbon monoxide induced by tertiary phosphines into [(η5-C5H5)(CO)3MoCH 2C6H5] in toluene solution decreases steadily with increasing cone angle of the phosphine. In contrast, for [(CO)5MnCH2C6H5], k3 increases with decrease in the Tolman electronic parameter, v, of the phosphine (i.e., with increasing electron donation) and does not correlate with the cone angle. However, k3 reflects the size of the phosphine to a greater degree as the size of the benzyl substituent on manganese increases. An analysis of k1 values in the solvent-assisted insertion pathway has been made for both the molybdenum and manganese systems in THF, 2-MeTHF and 2,5-Me2THF solutions. The general decrease in k1 with increasing size of the solvent for the cyclopentadienyl-molybdenum system becomes less marked as the size of the benzyl substituent increases. The manganese complexes are relatively less sensitive to solvent size. A comparison has been made between the nucleophilic role of a solvent molecule in the k1 step and that of a tertiary phosphine in the k3 step