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phase, the following procedures were adopted: (i) the sample in this water-content region is heated at a scanning rate as low as 0.1 OC/min; (ii) the sample is annealed at temperatures just below the T, and T,* transition points for at least 6 h, respectively. On the basis of these procedures, any exothermic peak characteristic of transformation of the metastable phases into a more stable one was not observed in the heating direction and two endothermic peaks shown in run I of Figure 1 b were not at all affected. This fact indicates that nuclear growth of the coagel phase requires a cooling procedure to crystallize the coexisting water, as is evident in run I1 of Figure 1 b.Finally, we want to discuss the present mode of phase transition in comparison with homologous systems of different counterions and different chain numbers previously investigated by us.The octadecyltrimethylammonium bromide (OTAB) system,2.'2 which has a single chain, shows the same type of G-T curves as those of the present system shown in Figure 5. On the other hand, the dioctadecyldimethylammonium chloride (DODAC),3 which also has a double chain, exhibits a different type of the GTcurves shown in Figure 7 , similar to the octadecyltrimethylammonium chloride (OTAC) system.' The remarkable difference in two types of the G T curves shown in Figures 5 and 7 is the thermodynamic stability of the gel phase; the gel phase of chloride counterion exists in the stable state in the specified temperature region between the T, and Tgel transitions, while the gel phase of bromide counterion exists in the metastable state over all temperatures below the T, transition. However, the simultaneous appearance of the T, and T,* transitions observed in the intermediatewater-content region of the present system is not observed in the OTAB system,2 although the mode of the GTcurves is the same for both systems. Presumably, this may be concerned with a smaller T, transition entropy (AS, = 205 J/(K-mol)) of the OTAB system over all water contents and, consequently, a higher stability of the gel phase, compared with that of the gel phase of the DODAB system. Furthermore, when the T, transition entropy of the DODAB system is compared with a total entropy change associated with the transformation from the coagel to liquid crystal phases of the DODAC system, these double-chain systems exhibit nearly the same value as shown in Figures 5 and 7 . The corresponding entropy change is also nearly the same between the single-chain systems of OTAB and OTAC. However, the T, transition entropy of the double-chain systems is only 1.4 times larger than that of the single-chain systems and is smaller than that expected. As is well-known, a drastic change at the T, transition is attributed to a conformational change of the hydrocarbon chain. The small value of the T, transition entropy of the double-chain systems indicates that the molecular motion of the hydrocarbon chain of these systems is fairly restricted, even at temperatures above the T, transition, which is away from the gen...

The interaction of 1,3-butadiene with atomically adsorbed oxygen on a A g ( l l 0 ) surface is studied. Three different approaches of 1,3-butadiene to a n oxygen atom that is adsorbed in the two-fold bridging site of the grooves on the Ag( 110) surface are studied: a cheletropic 1,4-cycloaddition, a n interaction with the terminal carbon, and a n interaction with the internal carbon of the 1,3-butadiene. Extended Hiickel tight-binding calculations show that the interaction of one of the terminal carbons of 1,3-butadiene with the surface oxygen is favored. This intermediate shows a preference for a 1,4 ring closure reaction rather than a 1,2 ring closure reaction, leading to 2,5-dihydrofurane instead of vinyl epoxide. The interaction between the suggested intermediates and the Ag( 1 10) surface as well as the bonding of different adsorbed products are analyzed and discussed in relation to the experimental results.

4213 7). To date, the results indicate that carborane ligands tend to stabilize electron-deficient complexes of high oxidation state much more so than do cyclopentadienides. However, there is not as yet sufficient experimental or theoretical results to provide a basis for understanding of the factors that dictate the stabilities and properties of these complexes. Further studies are currently underway in our laboratories.The addition of formaldehyde to Cp2Ta(CH3)(CH2) is studied from a molecular orbital point of view.Two possible isomers of the product, O-anti-Cp2(CH3)Ta(OCH2CH2) and O-syn-Cp2(CH3)Ta(OCH2CH2), are analyzed. The anti isomer is suggested to be thermodynamically more stable because it has stronger tantalum-carbon bonds. Steric hindrance of the Cp rings may result in the initial formation of the syn isomer only. Different routes for the following rearrangement to the anti isomer are compared. Berry pseudorotation and turnstile rotations are both high-energy processes. Methyl migration via one of the Cp rings is a possible pathway from energetic and bonding considerations. This reaction path is found to be base catalyzed. The possibility of an acid-catalyzed heterolysis of the tantalum-oxygen bond is discussed. The reduction of ethylene oxide by a Cp2Ta(CH3) complex is also studied. A direct oxygen abstraction pathway is compared to one involving the formation of the tantalaoxetanes Cp2(CH3)Ta-(OCH2CH2). An initial weak cOOrdination of the epoxide through the oxygen atom is suggested. A concerted elimination of the alkene from this precursor is a nonactivated process, opposed to a path that involves formation of tantalaoxetanes by an insertion of tantalum into a carbon-oxygen bond of the epoxide. . -7 J. L. Collman, J. P.; Brauman, J. I.; Meunier, B.; Raybuck, S. A.; Kodadek, T. h o c . Natl. Acad. Sci. U.S.A. 1984, 81, 3245. Collman, J. P.; Brauman, J. I.; Meunier, B.; Hayashi, T.; Kodadek, T.; Raybuck, S. A. J. Am. Chem. SOC. 1985,107, 2000. Collman, J. P.; Kodadek, T.; Raybuck, S. A.; Brauman, J. I.; Papazin, L. M. J. Am. Chem. SOC. 1985, 107, 4343. Collmann, J. P.; Kodadek, T.; Brauman, J. I. (2) (a) Sharpless, K. B.; Umbreit, M. A,; Nieh, M. T.; Flood, T. C. J. Am. Chem. SOC. 1972,94,6538. (b) Berry, M.; Davies, S. G.; Green, M. L. H. Cooper, C.; b i b , S. J.; Rheingold, A. L.; Mayer, J. M. J. Am. Chem. SOC. 1986, 108, 3545. Atagi, L. M.; Over, D. E.; McAlister, D. R.; Mayer, J. M. J. Am. Chem. SOC. 1991, 113, 870. (0 Yaclav, J. S.; Shekharam, T.; Gadgil, V. R. J. Chem. Soc., Chem. Commun. 1990, 843. AnalogyHe I and He I1 photoelectron spectra are reported for Os(q-C6MedNBut, os(rl-1,4(CHMe,)MeC6H,)NBut, and Ir(tl-C5Me6)NBut. Photoelectron bands are assigned using simple molecular orbital models for the complexes. The bonding of the cemium and iridium compounds shows similarities to that of the metallocenes but the metal a1 orbital is more destabilized in the case of the imido complexes.

face area is ca. twice, even after heating at 973 K, the value for the sample where the vanadium has been incorporated by melting, having the same vanadium content (3.4%) and having been calcined at the same temperature (973 K).Acknowledgment. Thanks are given to CICYT (Grant MAT88-0556) and Junta de Castilla y León for finantial support. Gift of the oxide samples is also acknowledged.