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The reaction pathways for 5′-hydroxylation and N-demethylation of nicotine catalyzed by cytochrome P450 were investigated by performing a series of first-principle electronic structure calculations on a catalytic reaction model system. The computational results indicate that 5′-hydroxylation of nicotine occurs through a two-state stepwise process, that is, an initial hydrogen atom transfer from nicotine to Cpd I (i.e., the HAT step) followed by a recombination of the nicotine moiety with the iron-bound hydroxyl group (i.e., the rebound step) on both the high-spin (HS) quartet and low-spin (LS) doublet states. The HAT step is the rate-determining one. This finding represents the first case that exhibits genuine rebound transition state species on both the HS and the LS states for Cα-H hydroxylation of amines. N-Demethylation of nicotine involves a N-methylhydroxylation to form N-(hydroxymethyl)nornicotine, followed by N-(hydroxymethyl)nornicotine decomposition to nornicotine and formaldehyde. The N-methylhydroxylation step is similar to 5′-hydroxylation, namely, a rate-determining HAT step followed by a rebound step. The decomposition process occurs on the deprotonated state of N-(hydroxymethyl)nornicotine assisted by a water molecule, and the energy barrier is significantly lower than that of the N-methylhydroxylation process. Comparison of the rate-determining free energy barriers for the two reaction pathways predicts a preponderance of 5′-hydroxylation over the N-demethylation by roughly a factor of 18:1, which is in excellent agreement with the factor of 19:1 derived from available experimental data. [ABSTRACT FROM AUTHOR]

CO2reforming of CH4on Ni(111) was investigated by using density functional theory. On the basis of thermodynamic analyses, the first step is CH4sequential dissociation into surface CH (CH4→ CH3→ CH2→ CH) and hydrogen, and CO2dissociation into surface CO and O (CO2→ CO O). The second step is CH oxygenation into CHO (CH O → CHO), which is more favored than dissociation into C and hydrogen (CH → C H). The third step is the dissociation of CHO into surface CO and H (CHO → CO H). This can explain the enhanced selectivity toward the formation of CO and H2on Ni catalysts. It is found that surface carbon formation by the Bouduard back reaction (2CO C(ads) CO2) is more favored than by CH4sequential dehydrogenation. The major problem of CO2reforming of CH4is the very strong CO adsorption on Ni(111), which results in the accumulation of CO on the surface and hinders the subsequent reactions and promotes carbon deposition. Therefore, promoting CO desorption should maintain the reactivity and stability of Ni catalysts. The computed energy barriers of the most favorable elementary reaction identify the CH4activation into CH3and H as the rate-determining step of CO2reforming of CH4on Ni(111), in agreement with the isotopic experimental results. [ABSTRACT FROM AUTHOR]