Density Functional Study of Ammonia Activation by Late First-Row Transition Metal Cations

Exploring the potential of the ammine complexes M(NH3)n+ (M = Zr, Re) to activate NH3

NH3 is activated by the complex Zr(NH3)7+ through a mechanism involving radical species.

Curly arrows meet electron density transfers in chemical reaction mechanisms: from electron localization function (ELF) analysis to valence-shell electron-pair repulsion (VSEPR) inspired interpretation

The displacement of the nuclei along the reaction path provides an explanatory interpretation of the electron density transfers making possible to understand chemical reactions.

Topological analysis of the reaction of uranium ions (U+, U2+) with N2O in the gas phase

Density functional theory calculations were performed to study the ability of uranium cations, U(+) and U(2+), to activate the N-N and N-O bonds of N(2)O. A close description of the reaction pathways leading to different reaction products is presented. The obtained results are compared with previous experimental works. The nature of the bonding of all the involved species and the bonding evolution along the reaction pathways was studied by means of the topological analysis of the ELF function.

Mechanisms of the reactions of W AND W+ with H2O: computational studies

The mechanism of the reactions of W and W(+) with the water molecule have been studied for several lower-lying electronic states of tungsten centers at the CCSD(T)/6-311G(d,p)+SDD and B3LYP/6-31G(d,p)+SDD levels of theory. It is shown that these reactions are essentially multistate processes, during which lower-lying electronic states of the systems cross several times. They start with the formation of initial prereaction M(H(2)O) complexes with M-H(2)O bonding energies of 9.6 and 48.2 kcal/mol for M = W and W(+), followed by insertion of the metal center into an O-H bond with 20.0 and 53.3 kcal/mol barriers for neutral and cationic systems, respectively. The overall process of M + H(2)O --> t-HM(OH) is calculated to be highly exothermic, 48.4 and 48.8 kcal/mol for M = W and W(+). From the HM(OH) intermediate the reaction may proceed via several different channels, among which the stepwise HM(OH) --> HMO + H --> (H)(2)MO and concerted HM(OH) --> (H)(2)MO pathways are more favorable and can compete (energetically) with each other. For the neutral system (M = W), the concerted process is the most favorable, whereas for the charged system (M = W(+)), the stepwise pathway is slightly more favorable. From the energetically most favorable intermediate (H)(2)MO the reactions proceed via H(2)-molecule formation with a 53.1 kcal/mol activation barrier for the neutral system. For the cationic system, H-H formation and dissociation is an almost barrierless process. The overall reaction of W and W(+) with the water molecule leading to H(2) + MO formation is found to be exothermic by 48.2 and 39.8 kcal/mol, respectively. In the gas phase with the collision-less conditions the reactions W((7)S) + H(2)O --> H(2) + WO((3)Sigma(+)), and W(+)((6)D) + H(2)O --> H(2) + WO(+)((4)Sigma(+)) are expected to proceed via a 10.4 and 5.1 kcal/mol overall energy barrier corresponding to the first O-H dissociation at the TS1. On the basis of these PESs, we predict kinetic rate constants for the reactions of W and W(+) with H(2)O.

The mechanisms of the reactions of W and W+ with COx (x=1, 2): a computational study

The mechanisms of the reactions of W and W+ with COx (x=1, 2) were studied at the CCSD(T)/[SDD+6-311G(d)]//B3LYP/[SDD+6-31G(d)] level of theory. It was shown that the gas-phase reaction of W with CO2 proceeds with a negligible barrier via an insertion pathway, W(7S)+CO2(1A1)-->W(eta2-OCO)(6A')-->OW(eta1-CO)(1A)-->WO (3Sigma+)+CO(1Sigma). This oxidation process is calculated to be exothermic by 32.4 kcal/mol. Possible intermediates of this reaction are the W(eta2-OCO) and OWCO complexes, among which the latter is 37.4 kcal/mol more stable and lies 39.7 and 7.3 kcal/mol lower than the reactants, W(7S)+CO2(1A1), and the products, WO (3Sigma+)+CO(1Sigma), respectively. The barrier separating W(eta2-OCO) from OWCO is 8.0 kcal/mol (relative to the W(eta2-OCO) complex), which may be characterized as a W+delta-(CO2)-delta charge-transfer complex. Ionization of W does not change the character of the reaction of W with CO2: the reaction of W+ with CO2, like its neutral analog, proceeds via an insertion pathway and leads to oxidation of the W-center. The overall reaction W+(6D) + CO2(1A1)-->W(eta1-OCO)+(6A)-->OW(eta1-CO)+(4A)-->WO+(4Sigma+)+CO(1Sigma) is calculated to be exothermic by 25.4 kcal/mol. The cationic reaction proceeds with a somewhat large (9.9 kcal/mol) barrier and produces two intermediates, W(eta1-OCO)+(6A) and OW(eta1-CO)+(4A). Intermediate W(eta1-OCO)+(6A) is 20.0 kcal/mol less stable than OW(eta1-CO)+(4A), and separated from the latter by a 35.2 kcal/mol barrier. Complex W(eta1-OCO)+(6A) is characterized as an ion-molecular complex type of W+-(CO2). Gas-phase reactions of M=W/W+ with CO lead to the formation of a W-carbonyl complex M(eta1-CO) for both M=W and W+. The C-O insertion product, OMC, lies by 5.2 and 69.3 kcal/mol higher than the corresponding M(eta1-CO) isomer, for M=W and W+, respectively, and is separated from the latter by a large energy barrier.

Mechanisms of the Reactions of W and W+ with NOx (x = 1, 2): A Computational Study

The mechanisms of the reactions of W and W+ with NOx (x=1, 2) were studied at the CCSD(T)/[SDD+6-311G(d)]//B3LYP/[SDD+6-31G(d)] level of theory. It was shown that the insertion pathway of the reaction W(7S)+NO2(2A1) is a multistate process, which involves several lower lying electronic states of numerous intermediates and transition states, and leads to oxidation, WO(3Sigma)+NO(2Pi), and/or nitration, WN(4Sigma)+O2(3Sigmag-), of the W-center. Oxidation products WO(3Sigma)+NO(2Pi) lie 87.6 kcal/mol below the reactants, while the nitration channel is only 31.0 kcal/mol exothermic. Furthermore, it was shown that nitration of W with NO2 is kinetically less favorable than its oxidation. The addition-dissociation pathway of the reaction W(7S)+NO2(2A1) proceeds via the octet (ground) state potential energy surface of the reaction, requires 3.3 kcal/mol barrier, and leads exclusively to oxidation products. Calculations show that oxidation of the W+ cation by NO2 is a barrierless process in the gas phase, proceeds exclusively via the insertion pathway, and is exothermic by 82.9 kcal/mol. The nitration of W+ by NO2 is only 14.1 kcal/mol exothermic and could be accessible only under high-temperature conditions. Reactions of M=W/W+ with NO are also barrierless processes in the gas phase and lead to the N-O insertion product NMO, which are 105.4 and 77.4 kcal/mol lower than the reactants for W and W+, respectively.

Electronic Structure of the [MNH2]+ (M = Sc−Cu) Complexes

B3LYP geometry optimizations for the [MNH2]+ complexes of the first-row transition metal cations (Sc+-Cu+) were performed. Without any exception the ground states of these unsaturated amide complexes were calculated to possess planar geometries. CASPT2 binding energies that were corrected for zero-point energies and including relativistic effects show a qualitative trend across the series that closely resembles the experimental observations. The electronic structures for the complexes of the early and middle transition metal cations (Sc+-Co+) differ from the electronic structures derived for the complexes of the late transition metal cations (Ni+ and Cu+). For the former complexes the relative higher position of the 3d orbitals above the singly occupied 2p(pi) HOMO of the uncoordinated NH2 induces an electron transfer from the 3d shell to 2p(pi). The stabilization of the 3d orbitals from the left to the right along the first-row transition metal series causes these orbitals to become situated below the HOMO of the NH2 ligand for Ni+ and Cu+, preventing a transfer from occurring in the [MNH2]+ complexes of these metal cations. Analysis of the low-lying states of the amide complexes revealed a rather unique characteristic of their electronic structures that was found across the entire series. Rather exceptionally for the whole of chemistry, pi-type interactions were calculated to be stronger than the corresponding sigma-type interactions. The origin of this extraordinary behavior can be ascribed to the low-lying sp2 lone pair orbital of the NH2 ligand with respect to the 3d level.

Activation of Methane by the Iron Dimer Cation. A Theoretical Study

A detailed investigation of the reaction mechanisms underlying the observed reactivity of the iron dimer cation with respect to methane has been performed by density functional hybrid (B3LYP) and nonhybrid (BPW91) calculations. Minima and transition states have been fully optimized and characterized along the potential energy surfaces leading to three different exit channels for both the ground and the first excited states of the dimer. A comparison with our previous work covering the reactivity of the Fe(+) monomer was made to underline similarities and differences of the reaction mechanisms. Results show that geometric arrangements corresponding to bridged positions of the ligands with respect to iron atoms are always favored and stabilize intermediates, transition states and products, facilitating their formation. Binding energies of reaction products have been computed and compared with experimental measurements, and ELF analysis of the bond has been performed to rationalize trends as a function of the structure.

Methane activation by chromium oxide cations in the gas phase: A theoretical study

Density Functional Theory, in its B3LYP formulation, was used to explore quantitative details of the potential energy hypersurfaces for the C-H bond activation reaction of methane by chromium dioxide cation. Both doublet ground and quartet excited states of the cation were considered, and all the minima and transition states localized along the paths leading to the formation of the experimentally observed products were characterized. All the calculated paths involve spin inversions that decrease the barrier heights of the involved transition states but do not play a significant role. Reaction pathways were also studied employing the nonhybrid BP86 functional, the reparametrized B3LYP* functional, and the CCSD(T) approach. Because other examples in the literature indicate that sequential ligation enhances the reactivity of bare transition metals cations, the state-selective reactivity of the chromium monoxide cation with respect to methane was also investigated and compared with that of the bare cation.

Acetylene Cyclotrimerization by Early Second-Row Transition Metals in the Gas Phase. A Theoretical Study

The acetylene cyclotrimerization reaction mediated by the left-hand-side bare transition metal atoms Y, Zr, Nb, and Mo has been studied theoretically, employing DFT in its B3LYP formulation. The complete reaction mechanism has been analyzed, identifying intermediates and transition states. Both the ground spin state and at least one low-lying excited state have been considered to establish whether possible spin crossings between surfaces of different multiplicity can occur. Our results show that the overall reaction is highly favorable from a thermodynamic point of view and ground state transition states lie always below the energy limit represented by ground state reactants. After the activation of two acetylene molecules and formation of a bis-ligated complex, the reaction proceeds to give a metallacycle intermediate, as the alternative formation of a cyclobutadiene complex is energetically disfavored. All the examined reaction paths involve formation of a metallacycloheptatriene intermediate that in turn generates a metal-benzene adduct from which finally benzene is released. Similarities and differences in the behaviors of the considered four metal atoms have been examined.