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Rh-catalyzed hydrosilylation of ethylene was theoretically investigated with the DFT, MP4-(SDQ), and CCSD(T) methods, where RhCl(PH 3 ) 3 was adopted as a model catalyst. The ratedetermining step in the Chalk-Harrod mechanism is Si-C reductive elimination, the activation barrier (E a ) of which is 27.4 (28.8) kcal/mol, where the values without parenthesis and in parenthesis are calculated with the DFT and MP4(SDQ) methods, respectively. The rate-determining step in the modified Chalk-Harrod mechanism is either ethylene insertion into the Rh-SiMe 3 bond (E a ) 13.5 (16.9) kcal/mol) at the MP4(SDQ) level or oxidative addition of HSiMe 3 (E a )15.7 (11.3) kcal/mol) at the DFT level. From these results, it should be clearly concluded that the Rh-catalyzed hydrosilylation of ethylene proceeds through the modified Chalk-Harrod mechanism, unlike Pt-catalyzed hydrosilylation of alkene, which takes place through the Chalk-Harrod mechanism. The difference between Rh and Pt catalysts arises from the facts that ethylene is more easily inserted into the Rh-SiMe 3 bond with a moderate E a value than that into the Pt-SiR 3 bond (E a ) 41-60 kcal/mol) and the Si-C reductive elimination of RhCl(CH 3 )(SiMe 3 )(PH 3 ) 2 (C 2 H 4 ) needs a very large E a value. This difference in the ethylene insertion between Pt and Rh catalysts is reasonably interpreted in terms that an alkyl group is formed at a position trans to hydride in the Pt catalyst but formed at a position trans to PH 3 in the Rh catalyst. This is because ethylene can take a position trans to PH 3 in the pseudo-octahedral six-coordinate Rh(III) complex, but ethylene must take a position trans to hydride in the four-coordinate planar Pt(II) complex (remember that Rh(III) and Pt(II) have d 6 and d 8 electron configurations, respectively). The large E a value of the Si-C reductive elimination results from the fact that both sp 3 valence orbitals of SiMe 3 and CH 3 must change their directions from the Rh center toward CH 3 and SiMe 3 , respectively, in the transition state. The present theoretical calculations also show that β-H abstraction by the Rh center easily occurs in RhClH-(CH 2 CH 2 SiMe 3 )(PH 3 ) 2 to yield a Rh(III) vinylsilane complex, RhCl(H) 2 (CH 2 dCHSiMe 3 )(PH 3 ) 2 , with a low activation barrier.

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Ultraviolet photoelectron spectroscopy (UPS) has been applied to the investigation of the electronic structure of oligothiophenes with 4-8 thiophene rings. In a series of a-linked oligomers (an with n being the number of rings), a systematic evolution of the 1T band is observed. Several peaks which correspond to the 1T band are observed in the region of 0.7-3 e V below the Fermi level (E F ), and the bandwidth becomes broader with increasing n. The nonbonding 1Tband is observed at 3.5 eV below EF and its energy is almost independent of the number of thiophene units. UPS spectra of a 7 and a 8 are fairly similar to the spectra of poly thiophene, showing that these oligomers are good model compounds of the polymer. The ionization threshold energy of a 7 and poly thiophene was observed to be 5.3 eV. The effect of irregularity on the 1T-electron system was also studied by using oligomers which contain a (3 iinkage or a vinylene group at the middle of the molecule. The UPS spectra showed that the (3 linkages significantly affect the electronic structure of poly thiophene, while the vinylene group does not. In order to analyze the UPS spectra and to investigate the electronic structures of oligomers, the orbital energies and the geometries of these oligomers are calculated by the semiempirical MNDO-SCF-MO (modified neglect of diatomic overlap self-consistent-field molecular orbital) method. Theoretically simulated spectra of these oligothiophenes derived from the obtained orbital energies by Gaussian broadening are compared with the observed ones. The agreement between the observed and calculated spectra is very good, particularly in the 1T region. It is shown from the optimized geometry that (I) an's have planar structure and 1T electrons are delocalized, (2) the oligomer with (3 linkages has non planar structure leading to limited delocalization of 1T electrons, and (3) the oligomers with a vinylene group are almost planar and the disturbance by the vinylene group on the delocalization is small..) Present address: