Zhi-Yuan Geng

CH4 activation by W atom in the gas phase: a case of two-state reactivity process.

Gas-phase methane activation by tungsten (W) atoms was studied at the density functional level of theory using the hybrid exchange correlation functional B3LYP. Four reaction profiles corresponding to the septet, quintet, triplet, and singlet multiplicities were investigated in order to ascertain the presence of some spin inversion during the methane activation. Methane activation mediated by W atoms was found to be a spin-forbidden process resulting from the crossing among the multistate energetic profiles. On the basis of the Hammond postulate, this is a typical two-state reactivity (TSR) reaction. The minimum energy crossing points lead to decrease in the barrier heights of TS01, TS12, TS23, and TS24 that correspond to the first, second, and third hydrogen transfer and the reductive elimination step of H(2), respectively. The spin-orbit coupling is calculated between electronic states of different multiplicities at the crossing points (MECPs) to estimate the intersystem crossing probabilities, and the probability of hopping from one surface to the other in the vicinity of the crossing region is calculated by the Landau-Zener type model.

Methane activation by MH+ (M = Os, Ir, and Pt) and comparisons to the congeners of MH+ (M = Fe, Co, Ni and Ru, Rh, Pd).

The mechanism of ligated-transition-metal- [MH(+) (M = Os, Ir, and Pt)] catalyzed methane activation has been computed at the B3LYP level of density functional theory. The B3LYP energies of important species on the potential energy surfaces were compared to CCSD(T) single-point energy calculations. Newer kinetic and dispersion-corrected methods such as M05-2X provide significantly better descriptions of the bonding interactions. The reactions take place more easily along the low-spin potential energy surface. The minimum-energy pathway proceeds as MH(+) + CH(4) → M(H)(2)(CH(3))(+) → TS → MH(CH(2))(H(2))(+) → MH(CH(2))(+) + H(2). The ground states are (5)Π, (4)Σ(-), and (1)Σ(+) for OsH(+), IrH(+), and PtH(+), respectively. The energy level differences of the reactants between the high- and low-spin states gradually become smaller from OsH(+) to PtH(+), being 30.66, 9.17, and 0.09 kcal/mol, respectively. The C-H bond can be readily activated by MH(+) (M = Os, Ir, and Pt) with a negligible barrier in the low-spin state; thus, OsH(+), IrH(+), and PtH(+) are likely to be excellent mediators for the activition of the C-H bond of methane. H(2) elimination is quite facile without barriers in the presence of excess reactants. The products of the reactions of MH(+) (M = Os, Ir, and Pt) + methane are all carbene complexes MH(CH(2))(+). The exothermicities of the reactions are 3.99, 15.66, and 12.14 kcal/mol, respectively. The results for MH(+) (M = Os, Ir, and Pt) are compared with those for the first- and second-row congeners, and the differences in behavior and mechanism are discussed.

Density functional study of SN2 substitution reactions for CH3Cl + CX1X2•− (X1X2 = HH, HF, HCl, HBr, HI, FF, ClCl, BrBr, and II)†

A systematic investigation on the SN2 displacement reactions of nine carbene radical anions toward the substrate CH3Cl has been theoretically carried out using the popular density functional theory functional BHandHLYP level with different basis sets 6‐31+G (d, p)/relativistic effective core potential (RECP), 6‐311++G (d, p)/RECP, and aug‐cc‐pVTZ/RECP. The studied models are CX1X2•− + CH3Cl → X2X1CH3C• + Cl−, with CX1X2•− = CH2•−, CHF•−, CHCl•−, CHBr•−, CHI•−, CF2•−, CCl2•−, CBr2•−, and CI2•−. The main results are proposed as follows: (a) Based on natural bond orbital (NBO), proton affinity (PA), and ionization energy (IE) analysis, reactant CH2•− should be a strongest base among the anion‐containing species (CX1X2•−) and so more favorable nucleophile. (b) Regardless of frontside attacking pathway or backside one, the SN2 reaction starts at an identical precomplex whose formation with no barrier. (c) The back‐SN2 pathway is much more preferred than the front‐SN2 one in terms of the energy gaps [ΔE  cent≠ (front)−ΔE  cent≠ (back)], steric demand, NBO population analysis. Thus, the back‐SN2 reaction was discussed in detail. On the one hand, based on the energy barriers (ΔE  cent≠ and ΔE  ovr≠ ) analysis, we have strongly affirmed that the stabilization of back attacking transition states (b‐TSs) presents increase in the order: b‐TS‐CI2 < b‐TS‐CBr2 < b‐TS‐CCl2 < b‐TS‐CHI < b‐TS‐CHBr < b‐TS‐CHCl < b‐TS‐CF2 < b‐TS‐CHF < b‐TS‐CH2. On the other hand, depended on discussions of the correlations of ΔE  ovr≠ with influence factors (PA, IE, bond order, and ΔE  def≠ ), we have explored how and to what extent they affect the reactions. Moreover, we have predicted that the less size of substitution (α‐atom) required for the gas‐phase reaction with α‐nucleophile is related to the α‐effect and estimated that the reaction with the stronger PA nucleophile, holding the lighter substituted atom, corresponds to the greater exothermicity given out from reactants to products.

Density Functional Theory Studies of Thermal Activation of Methane by MH+ (M = Ru, Rh, and Pd)

The dehydrogenation reaction mechanisms of methane catalyzed by a ligated transition metal MH(+) (M = Ru, Rh, and Pd) have been investigated theoretically. Activation of methane by MH(+) complexes is proposed to proceed in a one-step manner via one transition state: MH(+) + CH(4) --> MH(+)CH(4) --> [TS] --> (MCH(3)(+))H(2) -->MCH(3)(+) + H(2). Both high-spin and low-spin potential energy surfaces are characterized in detail. Our calculations indicate that the ground-states species have low electron spin and a dominant 4d(n) configuration for RuH(+), RhH(+), and PdH(+), and the whole reaction proceeds on the ground-states potential energy surfaces with a spin-allowed manner. The MH(+) (M = Ru, Rh, and Pd) complexes are expected from the general energy profiles of the reaction pathways to efficiently convert methane to metal methyl, thus RuH(+), RhH(+), and PdH(+) are likely to be excellent mediators for the activity of methane. In the reactions of MH(+) with methane, the H(2) elimination from the dihydrogen complex is quite facile without barriers. The exothermicities of the reactions are close for Ru, Rh, and Pd: 11.1, 1.2, and 5.2 kcal/mol, respectively.

Reaction mechanism and chemoselectivity of gold(I)-catalyzed cycloaddition of 1-(1-alkynyl) cyclopropyl ketones with nucleophiles to yield substituted furans

The mechanisms of gold(I)-catalyzed cycloaddition of 1-(1-alkynyl) cyclopropyl ketones with nucleophiles have been investigated using density functional theory calculations at the B3LYP/6-31G (d, p) level of theory. A polarizable continuum model (PCM) has been established in order to evaluate the effects of solvents on the reactions. The results of the calculations indicate that the first step of the catalytic cycle is the cyclization of the carbonyl oxygen onto the triple bond which forms a new and stable resonance structure of an oxonium ion and a carbocation intermediate. The subsequent ring expansion step results in the formation of the final product and regeneration of the catalyst. Furthermore, the regioselectivity and effect of substituents has been discussed, including an analysis of energy, bond length, and natural bond orbital (NBO) charge distributions in the rate-determining step. Our computational results are consistent with earlier experimental observations.