4621cannot effect the N-N bond length. Thus, the character of the 2e M O must control the strength of the M-N and N-N bonds. The Ru dimer has the shortest N-N bond length due to less M -N 2 l a g donation, which results from less N 2 participation in the 2e MO. More electrons are donated into the p-N2 a* system in the mixed-metal Re-Mo dimer, the N b dimer, and the Zr dimer because the two unoccupied d* orbitals located at higher energy than the two occupied d a orbitals stabilize the p N 2 lrg MO. This allows the l a g MO to participate more strongly in the 2e MOs. In the Re-Mo case both high-energy d a orbitals are primarily Mo d?r in character. The Nb and Zr dimers have one occupied and one unoccupied d r orbital localized on each metal center. As a result, M -N2 lag -M electron flow can occur without charge depletion or charge buildup. Synergetic u donation is observed for the Nb dimer, which strengthens the M-N bond and weakens the N-N bond.
Local convective heat transfer coefficients were measured from a smooth NACA 0012 airfoil having a chord length of 0.533 m (21 in.). The measurements were made both in flight on the NASA Lewis Twin Otter icing research aircraft and in the NASA Lewis icing research tunnel (IRT). Flight data were taken for the smooth airfoil at Reynolds numbers based on chord in the range 1.24 X 10 6 to 2.50 x 10 6 and at various angles of attack up to 4 deg. During these flight tests, the freestream velocity turbulence intensity was found to be very low (< 0.1%). Wind tunnel data were acquired in the Reynolds number range 1.20 x 10 6 to 4.52 x 10 6 and at angles of attack from -4 to +8 deg. The turbulence intensity in the IRT was 0.5-0.7% with the cloud-generating sprays off. A direct comparison between the results obtained in flight and in the IRT showed that the higher level of turbulence intensity in the IRT had little effect on the heat transfer for the lower Reynolds numbers but caused a moderate increase in heat transfer at the higher Reynolds numbers. Turning on the cloud-generating spray nozzle atomizing air in the IRT did not alter the heat transfer. The present data were compared with leading-edge cylinder and flat plate heat transfer correlations that are often used to estimate airfoil heat transfer in computer codes.
2573pected on the basis of the structure of molecular sodium ~u1fates.I~ I 4 6 I Figure 11. Structure of the product when sulfate is constrained to be monodentate. on going from a four-membered to a five-membered cyclic structure is 1.9 eV. Our calculations, therefore, suggest that the formation of a four-membered ring is less probable.Finally, we calculate the structure and stability of monodentate sulfate coordinated to Pt(PH,),. The calculated structure is shown in Figure 11. The binding energy of SO2 with the dioxygen complex to form monodentate sulfate is 4.9 eV, showing that it is 0.6 eV less stable than the bidentate sulfate structure, as ex- ConclusionsWe have found sulfate formation from sulfur dioxide and bis(phosphine)platinum proceeds most readily through a five-atom cyclic intermediate, which is similar to the stable pseudo-ozonide that forms when ketones react instead of SOz. Our finding that the sulfur in SO2 attacks one end of coordinated O2 in the initial step suggests the ketone may do the same, rather than first coordinating to an axial Pt site as was suggested.' The four-atom cyclic intermediate is eliminated on the grounds that if it does form it transforms to sulfate only with difficulty and its formation is hindered by the necessity of activating the coordinated O2 ligand.Acknowkdgment. We thank the NASA Lewis Research Center for supporting this work through NASA Grant NAG-3-341.Restiicted Hartree-Fock plus configuration interaction and generalized molecular orbital plus configuration interaction calculations are reported for several structural models of the dioxygen complexes of manganese(I1) porphyrins. An analysis of the results suggests a ground-state structure with a sideon (Griffith) dioxygen and oxygen atoms eclipsing the ring nitrogens. The calculation predicts t h e e singly occupied metal orbitals (t2e1 in pseudocubic symmetry; u'?r'13~ in pseudolinear symmetry) in agreement with the ESR results. In contrast to previous ab initio calculations, the C I results also predict a ground-state end-on (Pauling) structure with three unbaired electrons in metal orbitals that would be consistent with the ESR. On the basis of our calculations alohe, this stfucture'could not be eliminated as a possibility.The preferred conformations and the 'macrocyclic effect" of the cyclic ligands 1,4,7,1O-tetraazacyclododecane and 1,4,7,10tetraoxacyclodecane and their metal complexes are investigated within the framework of the HF-LCAO-MO method. The results show that the general conformation before and after complexation remains the same (only torsion angles change), i.e. the alternate form for the N4 and the maxidentate form for the O4 cyclic ligands. For the factors influencing the "macrocyclic effect", our results indicate the effect of the 'prestraining" of the cyclic ligands to dominate in the case of the O4 ligand. An at least equally important factor for the N4 ligands is the difference in energy gain by metal binding. The coincidence of metal ion size and ligand ring cavity size affects not only en...
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