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Multireference as well as single-reference wave functions were adopted to study the surface reaction mechanisms of hydrazine. The initial surface mechanisms resemble those of ammonia and its methyl derivatives. MRMP2 values indicate that the lifetime of initial molecularly adsorbed species should be longer than previously suggested. High energy path as well as low energy path of subsequent surface reactions were found. The theoretical initial surface product of low energy path is consistent with the experimentally suggested structure. Both paths eventually lead to very stable surface products, which are also consistent with the experimentally suggested structures. The reaction channels of the experimentally observed NH3 desorptions were also revealed. It was shown that the high reactivity of hydrazine as compared to ammonia and its methyl derivatives is due to the high nucleophilic ability of the additional nitrogen atom of hydrazine., ((c) 2004 American Institute of Physics)

Multi-reference as well as single-reference quantum mechanical methods were adopted to study the potential energy surface along three possible surface reaction mechanisms of acrylonitrile on the Si(100)-2 x 1 surface. All three reactions occur via stepwise radical mechanisms. According to the computed potential energy surfaces, both [4+2] and [2+2](CN) cycloaddition products resulting from the reactions of surface dimers with the C[triple bond]N of acrylonitrile are expected, due to the negligible activation barriers at the surface. Another possible surface product, [2+2](CC), requires a 16.7 kcal/mol activation energy barrier. The large barrier makes this route much less favorable kinetically, even though this route produces the thermodynamically most stable products. Isomerization reactions among the surface products are very unlikely due to the predicted large activation barriers preventing thermal redistributions of the surface products. As a result, the distribution of the final surface products is kinetically controlled leading to a reinterpretation of recent experiments. An intermediate Lewis acid-base type complex appears in both the [4+2] and [2+2](CN) cycloadditions entrance channels, indicating that the surface may act as an electrophile/Lewis acid toward a strong Lewis base substrate.

The non-relativistic and relativistic potential energy surfaces (PESs) of the symmetric and asymmetric reaction paths of Si(100)-2×1 oxidations by atomic oxygen were theoretically explored. Although only the singlet PES turned out to exist as a major channel leading to 'on-dimer' product, both the singlet and triplet PESs leading to 'on-top' products are attractive. The singlet PESs leading to the two surface products were found to be the singlet combinations (open-shell singlet) of the low-lying triplet state of surface silicon dimer and the ground 3P state of atomic oxygen. The triplet state of the 'on-top' product can also be formed by the ground singlet state of the surface silicon dimer and the same 3P oxygen. The attractive singlet PESs leading to the 'on-dimer' and 'on-top' products made neither the intersystem crossings from triplet to singlet PES nor high energy 1D of atomic oxygen necessary. Rather, the low-lying triplet state of surface silicon dimer plays an important role in the initial oxidations of silicon surface. [ABSTRACT FROM AUTHOR]

The surface reaction pathways of isoxazole and oxazole on Si(100)-2 × 1 surface were theoretically investigated. They both form a weakly bound Si-N dative bond adduct on Si(100)-2 × 1 surface. In the case of isoxazole, the barrierlessly formed Si-N adduct is the most important surface product, that cannot be easily converted into other species. On the other hand, a facile concerted [4+2]CC cycloaddition without involving the initial Si-N dative bond adduct was also found in the case of oxazole adsorption. The existence of Diels-Alder reactions is attributed to the particular arrangement of the two heteroatoms of oxazole in such a way that the two Si-C σ-bonds can be formed in a [4+2] fashion. In short, the unique geometric arrangements and electronegativity of these similar heteroatomic molecules yielded distinctively different surface reaction characteristics. [ABSTRACT FROM AUTHOR]

The thermal decomposition reactions of methylamine, ethylamine, and 1-propylamine absorbed on Si(100)-2 × 1 surface were theoretically investigated. Eight decomposition channels were found leading to desorption products of imine, H2, alkyl cyanide, ammonia, aziridine, alkene, azetidine, and cyclopropane, which supports the experimental assignments. Our mechanistic studies strongly suggest that the alkyl cyanide (hydrogen cyanide in the case of methylamine) channel is coupled with the hydrogen desorption step. The β-hydrogen of ethylamine and 1-propylamine was found to undergo additional decomposition reactions producing aziridine and alkene, which were classified as γ- and β-eliminations, respectively. It was also found that the γ-hydrogen of 1-propylamine undergoes azetidine and cyclopropane producing decompositions, which were classified as δ- and γ-eliminations. In general, γ- and δ-hydrogen involved decomposition reactions are kinetically less favorable than β-hydrogen involved ones. Consequently, it is expected that the thermal decompositions of the primary alkyl amines with longer alkyl chains would not add additional favorable decomposition channels. Except alkyl cyanide and ammonia desorption channels, the decompositions occur in a concerted fashion. [ABSTRACT FROM AUTHOR]