DFT calculations have been carried out to study the detailed mechanism of Rh(III)-catalyzed C–H activation/cyclization of 2-acetyl-1-arylhydrazines with alkynes leading to the formation of indoles, in which the hydrazine moiety is used as the internal oxidant. The energy profiles associated with the catalytic cycle, involving N–H deprotonation, C–H activation (a concerted metalation–deprotonation (CMD) process), alkyne insertion, ring rearrangement/isomerization, and finally N–N bond cleavage/reductive elimination to regenerate the active species, are presented and analyzed. Through analysis of the calculation results, we found that the combined processes of the CMD and alkyne insertion contribute to the overall rate-determining step. The N–N bond cleavage step was examined in detail to understand how the internal oxidant interacts with the metal center to facilitate the catalytic reactions. The factor influencing regioselectivity was also investigated. How different types of substrates (alkynes versus alkenes) and internal oxidants (−NH(NHAc) versus −NH(OAc)) influence the reaction mechanisms, Rh(III)/Rh(I) versus Rh(III)/Rh(V) catalytic cycles, was discussed.

The electronic and structural properties of a series of triniobium oxide clusters, Nb 3 O n -and Nb 3 O n (n ) 3-8), are investigated using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. PES spectra are obtained for

We investigated the structures and bonding of two series of early transition-metal oxide clusters, M(2)O(n)(-) and M(2)O(n) (M = Nb, Ta; n = 5-7) using photoelectron spectroscopy (PES) and density-functional theory (DFT). The stoichiometric M(2)O(5) clusters are found to be closed shell with large HOMO-LUMO gaps, and their electron affinities (EAs) are measured to be 3.33 and 3.71 eV for M = Nb and Ta, respectively; whereas EAs for the oxygen-rich clusters are found to be much higher: 5.35, 5.25, 5.28, and 5.15 eV for Nb(2)O(6), Nb(2)O(7), Ta(2)O(6), and Ta(2)O(7), respectively. Structural searches at the B3LYP level yield triplet and doublet ground states for the oxygen-rich neutral and anionic clusters, respectively. Spin density analyses reveal oxygen radical, diradical, and superoxide characters in the oxygen-rich clusters. The M(2)O(7)(-) and M(2)O(7) clusters, which can be viewed to be formed by M(2)O(5)(-/0) + O(2), are utilized as molecular models to understand dioxygen activation on M(2)O(5)(-) and M(2)O(5) clusters. The O(2) adsorption energies on the stoichiometric M(2)O(5) neutrals are shown to be surprisingly high (1.3-1.9 eV), suggesting strong capabilities to activate O(2) by structural defects in Nb and Ta oxides. The PES data also provides valuable benchmarks for various density functionals (B3LYP, BP86, and PW91) for the Nb and Ta oxides.