Statistical Modeling of Gas-Phase Organometallic Reactions Based on Density Functional Theory: Ni+ + C3H8

Imaging the dynamics of ion–molecule reactions

A range of ion–molecule reactions have been studied in the last years using the crossed-beam ion imaging technique, from charge transfer and proton transfer to nucleophilic substitution and elimination.

Activation of propane C-H and C-C bonds by gas-phase Pt atom: a theoretical study

The reaction mechanism of the gas-phase Pt atom with C₃H₈ has been systematically investigated on the singlet and triplet potential energy surfaces at CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz level. Pt atom prefers the attack of primary over secondary C-H bonds in propane. For the Pt + C₃H₈ reaction, the major and minor reaction channels lead to PtC₃H₆ + H₂ and PtCH₂ + C₂H₆, respectively, whereas the possibility to form products PtC₂H₄ + CH₄ is so small that it can be neglected. The minimal energy reaction pathway for the formation of PtC₃H₆ + H₂, involving one spin inversion, prefers to start at the triplet state and afterward proceed along the singlet state. The optimal C-C bond cleavages are assigned to C-H bond activation as the first step, followed by cleavage of a C-C bond. The C-H insertion intermediates are kinetically favored over the C-C insertion intermediates. From C-C to C-H oxidative insertion, the lowering of activation barrier is mainly caused by the more stabilizing transition state interaction ΔE^≠_int, which is the actual interaction energy between the deformed reactants in the transition state.

Reaction rate constants and mechanistic detail of the Ni+ + butanone reaction

The unimolecular decomposition kinetics of the jet-cooled Ni(+)-butanone cluster ion has been monitored over a range of internal energies (16000-18800 cm⁻¹). First-order rate constants are acquired for the precursor ion dissociation into three product channels. The temporal growth of each fragment ion is selectively monitored in a custom instrument and yields similar valued rate constants at a common ion internal energy. The decomposition reaction is proposed to proceed along two parallel reaction coordinates. Each dissociative pathway is rate-limited by the initial Ni(+) oxidative addition into either the C-CH₃ or C-C₂H₅ σ-bond in the butanone molecule. Ratios of integrated product ion intensities as well as the measured rate constants are used to determine values for each σ-bond activation rate constant. The lowest energy measurement presented in this study occurs when the binary complex ion possesses an internal energy of 16000 cm⁻¹. Under this condition, the Ni(+) assisted decomposition of the butanone molecule is rate limited by k(act)(C-C₂H₅) = (0.92 ± 0.08) × 10⁵ s⁻¹ and k(act)(C-CH₃) = (0.37 ± 0.03) × 10⁵ s⁻¹. The relative magnitudes of the two rate constants reflect the greater probability for reaction to occur along the C-C₂H₅ σ-bond insertion pathway, consistent with thermodynamic arguments. DFT calculations at the B3LYP/6-311++G(d,p) level of theory suggest the most likely geometries and relative energies of the reactants, intermediates, and products.

Rate-limiting step in the low-energy unimolecular decomposition reaction of Ni+* acetone into Ni+CO + ethane

Rate constants for the low-energy Ni(+)-assisted C-C bond cleavage reaction of deuterium-labeled acetone have been acquired under jet-cooled conditions in the gas phase. The energies used to initiate the dissociative reactions of the precursor complex ion Ni(+)(d(6)-Ac) are well below that required to cleave C-C sigma-bonds in isolated organic molecules. The rate constants are compared to those acquired previously for the lighter Ni(+)(h(6)-Ac) isotope and result in a substantial kinetic isotope effect (k(H)/k(D) approximately 5.5). Arguments are made that implicate isomerization leading to C-C bond coupling as the rate-limiting step (not C-C sigma-bond activation) in the dissociative reaction.

Activation of CH4 by gas-phase Ni+ and the thermochemistry of Ni-ligand complexes

The kinetic energy dependence of the reaction of Ni+ (2D) with methane has been studied using guided ion beam mass spectrometry. Formation of NiH+, NiCH2+ and NiCH3+ are all observed with thresholds near 2 eV, and NiCH+ is observed at higher kinetic energies. The dehydrogenation reaction is shown to proceed over a barrier in excess of the endothermicity by examining the reverse reaction of NiCH2(+) + D2. Collision-induced dissociation of NiCH2+ and NiCH4+ with Xe provides additional information on the products and reaction intermediates. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in eV) of D0(Ni( +)-H) = 1.60 +/- 0.08, D0(Ni(+)-CH) = 3.12 +/- 0.12, D0(Ni(+)-CH2) = 3.20 +/- 0.08, D0(Ni(+)-CH3) = 1.76 +/- 0.07, and D0(Ni(+)-CH4) = 1.00 +/- 0.05. The experimental thermochemistry is favorably compared with previous experimental results and density functional theory calculations (B3LYP), which also establish the electronic structures of these species and provide insight into the reaction mechanism. The results for Ni(+) are compared with those for the third-row transition metal congener Pt+ and the differences in behavior and mechanism are discussed.

Theoretical survey of the potential energy surface of methyl nitrite + Cu+ reaction

The gas-phase reaction of methyl nitrite with Cu+ has been investigated using density functional theory. The geometries and energies of all the stationary points involved in the reaction have been investigated at the B3LYP/6-311+G(2df,2pd) level. Seven different structures of the encounter complexes could be formed when Cu+ attacking at different electronegative heteroatoms of trans and cis conformational isomers of methyl nitrite, in which the inner oxygen attacks account for the most stable complexes. Extensive conversions could take place for these complexes converting into each other. Various mechanisms leading to the loss of NO and HNO are analyzed in terms of the topology of the potential energy surface. The reaction proceeds exclusively from the inner oxygen attachments, followed by four different mechanisms, i.e., direct dissociation, direct H abstraction, N-O activation, and C-H activation, where the former two provide direct channels for the respective losses of NO and HNO, the third one accounts for both of the losses, and C-H activation is unlikely to be important due to the energetics.

Activation of CH4 by gas-phase Mo+, and the thermochemistry of Mo-ligand complexes

The kinetic-energy dependence of the reactions of Mo(+) ((6)S) with methane has been studied using guided ion beam mass spectrometry. No exothermic reactions are observed in this system, as also found previously, but efficient dehydrogenation occurs at slightly elevated energies. At higher energies, MoH(+) dominates the product spectrum and MoC(+), MoCH(+), and MoCH(3)(+) are also observed. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies (in eV) of D(0)(Mo(+)-C) = 4.55 +/- 0.19, D(0)(Mo(+)-CH) = 5.32 +/- 0.14, D(0)(Mo(+)-CH(2)) = 3.57 +/- 0.10, and D(0)(Mo(+)-CH(3)) = 1.57 +/- 0.09. The results for Mo(+) are compared with those for the first- and third-row transition-metal congeners, Cr(+) and W(+), and the differences in behavior and mechanism are discussed. Theoretical results are used to elucidate the geometric and electronic structures of all product ions as well as the complete potential-energy surface for reaction. The efficiency of the coupling between the sextet and quartet spin surfaces is also quantified.

Thermochemistry, bonding, and reactivity of Ni+ and Ni2+ in the gas phase

In this review, we present a general overview on the studies carried out on Ni(+-)- and Ni(2+)-containing systems in the gas phase since 1996. We have focused our attention in the determination of binding energies in parallel with an analysis of the structure and bonding of the complexes formed by the interaction of Ni(+) with one ligand, or in clusters where this metal ion binds several identical or different ligands. Solvation of Ni(2+) by different ligands is also discussed, together with the theoretical information available of doubly charged Ni-containing species. The final section of this review is devoted to an analysis of the gas-phase uni- and bimolecular reactivity of Ni(+) and Ni(2+) complexes.

Theoretical Survey of the Gas-Phase Reactions of Allylamine with Co+

Density functional theory calculations have been carried out to survey the gas-phase reactions of allylamine with Co+. The geometries and bonding characteristics of all the stationary points involved in the reactions have been investigated at the B3LYP/6-311++G(d,p) level. Final energies are obtained by means of the B3LYP/6-311+G(2df,2pd) single-point calculations. The performance of these theoretical methods is valuated with respect to the available thermochemical data. Co+ strongly binds allylamine by forming a chelated structure in which the metal cation binds concomitantly to the two functional groups of the neutral molecule. Various mechanisms leading to the loss of NH3, NH2, C2H2, and H2 are analyzed in terms of the topology of the potential energy surface. The most favorable mechanism corresponds to the loss of NH3, through a process of C-N activation followed by a concerted beta-H shift. The accompanying NH2 elimination is also discussed. The loss of C2H2 is also favorable, through C-C activation and stepwise beta-H shift, giving Co+(NH2CH3) and Co+H(NH2CH2) as the product ions. Various possible channels for the loss of H2 are considered. The most favorable mechanism of the H2 loss corresponds to a pathway through which the metal acts as a carrier, connecting a hydrogen atom from the methylidyne group of allylamine with a hydrogen atom of the terminal methylene group. The product ion of this pathway has a tricoordinated structure in which Co+ binds to the terminal two Cs and N atoms of the NH2CH2CCH moiety.

Reaction of Acetaldehyde with Ni+: An Extended Theoretical Study of the Decarbonylation Mechanism of Acetaldehyde by First-Row Transition Metal Ions

We report herein a theoretical study of the reaction of acetaldehyde with Ni+ as an extension of our two recent papers on the decarbonylation of acetaldehyde by late first-row transition metal ions [Zhao, Zhang, Guo, Wu, Lu Chem. Phys. Lett. 2005, 414, 28; Zhao, Guo, Zhang, Wu, Lu ChemPhysChem 2006, 7, 1345]. Geometries of all the stationary points involved in the reaction have been fully optimized at the B3LYP/6-311+G(2df,2pd) level and the decarbonylation mechanism is analyzed in terms of the topology of potential energy surface. Combining with the previous studies, it is found that for the Cr+, Co+, and 4Fe+ mediated systems decarbonylation of CH3CHO only takes place via C-C activation, and aldehyde C-H activation is unlikely to be important, whereas both C-C and aldehyde C-H activations by Ni+ and 6Fe+ could result in the decarbonylation of CH3CHO, where hydride-containing species M+(H)(CO)(CH3) is found to be a common minimum along the reaction pathways.

Gas-phase ion chemistry in organometallic systems

This review essentially deals with positive ion/molecule reactions occurring in gas-phase organometallic systems, and encompasses a period of time of approximately 7 years, going from 1997 to early 2004. Following the example of the excellent review by Eller & Schwarz (1991; Chem Rev 91:1121-1177), in the first part, results of reaction of naked ions are presented by grouping them according to the neutral substrate, while in the second part, ligated ions are grouped according to the different ligands. Whenever possible, comparison among similar studies is attempted, and general trends of reactivities are evidenced.

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.

C–H activation of alkanes on Rhn+ (n=1–30) clusters: Size effects on dehydrogenation

The rate coefficients for the dehydrogenation of ethane, propane, and isobutane with cationic rhodium atoms Rh+ and clusters Rh+ n of up to 30 atoms were measured under single-collision conditions in a Fourier-transform ion cyclotron resonance mass spectrometer. The reaction rates are cluster size dependent and parallel for all the three alkanes. While the reactions proceed close to the theoretical collision rates for a large number of clusters, characteristic minima are observed for Rh+ (5/6/9/19/28). The degree of dehydrogenation varies with the cluster size with maxima for 10< or =n< or =15 for the three alkanes and for n=3 and 2-4 in the cases of ethane and propane, respectively. However, complete dehydrogenation is only observed for the reaction of Rh+ 11 with propane. Dehydrogenation is remarkably selective and no other neutral products than H2 are observed. The results are interpreted in terms of likely cluster geometries.

Reaction of Bare VO+ and FeO+ with Ammonia: A Theoretical Point of View

The potential energy surfaces corresponding to the dehydration reaction of NH(3) by VO(+) ((3)Sigma, (1)Delta, (5)Sigma) and FeO(+) ((6)Sigma, (4)Delta) metal oxide cations have been investigated within the framework of the density functional theory in its B3LYP formulation and by employing new optimized basis sets for iron and vanadium. The reaction is proposed to occur through two hydrogen shifts from the nitrogen to the oxygen atom giving rise to multicentered transition states. Possible spin crossing between surfaces at different spin multiplicities has been considered. The energy profiles are compared with the corresponding ones for the insertion of bare cations to investigate the influence on reactivity of the presence of the oxygen ligand. The topological analysis of the gradient field of the electron localization function has been used to characterize the nature of the bonds for all the minima and transition states along the paths.

Theoretical study of ammonia and methane activation by first-row transition metal cations M(+) (M = Ti, V, Cr)

The potential energy surfaces for the reaction of first-row transition metal cations Ti(+)((4)F,(2)F), V(+)((5)D,(3)F), and Cr(+)((6)S,(4)D) with NH(3) and CH(4) have been built up by using density functional theory. In all cases, the high-spin ion-dipole complex, which is the most stable species on the respective potential energy hypersurfaces, is initially formed. In the second step, a hydrogen shift process leads to the formation of the insertion products, which are more stable in a low-spin state. From these intermediates three dissociation channels have been considered. All the results have been compared with existing experimental and theoretical data and our earlier work on the reactivity of Sc(+), to clarify similarities and differences in the behavior of the transition metal ions considered.

Potential energy surface for activation of methane by Pt(+): a combined guided ion beam and DFT study

A guided-ion beam tandem mass spectrometer is used to study the reactions of Pt(+) with methane, PtCH(2)(+) with H(2) and D(2), and collision-induced dissociation of PtCH(4)(+) and PtCH(2)(+) with Xe. These studies experimentally probe the potential energy surface for the activation of methane by Pt(+). For the reaction of Pt(+) with methane, dehydrogenation to form PtCH(2)(+) + H(2) is exothermic, efficient, and the only process observed at low energies. PtH(+), formed in a simple C-H bond cleavage, dominates the product spectrum at high energies. The observation of a PtH(2)(+) product provides evidence that methane activation proceeds via a (H)(2)PtCH(2)(+) intermediate. Modeling of the endothermic reaction cross sections yields the 0 K bond dissociation energies in eV (kJ/mol) of D(0)(Pt(+)-H) = 2.81 +/- 0.05 (271 +/- 5), D(0)(Pt(+)-2H) = 6.00 +/- 0.12 (579 +/- 12), D(0)(Pt(+)-C) = 5.43 +/- 0.05 (524 +/- 5), D(0)(Pt(+)-CH) = 5.56 +/- 0.10 (536 +/- 10), and D(0)(Pt(+)-CH(3)) = 2.67 +/- 0.08 (258 +/- 8). D(0)(Pt(+)-CH(2)) = 4.80 +/- 0.03 eV (463 +/- 3 kJ/mol) is determined by measuring the forward and reverse reaction rates for Pt(+) + CH(4) right harpoon over left harpoon PtCH(2)(+) + H(2) at thermal energy. We find extensive hydrogen scrambling in the reaction of PtCH(2)(+) with D(2). Collision-induced dissociation (CID) of PtCH(4)(+), identified as the H-Pt(+)-CH(3) intermediate, with Xe reveals a bond energy of 1.77 +/- 0.08 eV (171 +/- 8 kJ/mol) relative to Pt(+) + CH(4). The experimental thermochemistry is favorably compared with density functional theory calculations (B3LYP using several basis sets), which also establish the electronic structures of these species and provide insight into the reaction mechanism. Results for the reaction of Pt(+) with methane are compared with those for the analogous palladium system and the differences in reactivity and mechanism are discussed.