Two-State Reactivity in Organometallic Gas-Phase Ion Chemistry

Theoretical investigation of the gas-phase Mn(+)- and Co(+)-catalyzed oxidation of benzene by N(2)O

The gas-phase Mn(+)- and Co(+)-mediated oxidation of benzene by N(2)O has been theoretically investigated using density functional theory. The geometries and energies of all the stationary points involved are located. Two different oxidation mechanisms, i.e., mediated by M(+)(benzene) and MO(+), are taken into account. In the former catalytic cycle, benzene initially coordinates to the metal ion affording the M(+)(C(6)H(6)) adduct (M = Mn or Co), then N(2)O coordinates to the nascent benzene complex and gets activated by the metal to yield (C(6)H(6))M(+)O(N(2)). After releasing a molecular nitrogen, through the non-radical and/or O-insertion pathways, the system would be oxidized to phenol and regenerates the active catalyst M(+). This catalytic mechanism is energetically favourable, explaining the efficient Mn(+)- and Co(+)-catalyzed benzene hydroxylation observed in ion cyclotron resonance (ICR) experiments [J. Am. Chem. Soc., 1994, 116, 9565-9570]. For the alternative MO(+)-mediated oxidation mechanism, spin inversion as well as high energy barrier in the course of the N-O activation imply low reaction efficiency of the ground-state reactants, according with the ICR experiment finding that MO(+) was formed from exited M(+)*, thus both Mn(+) and Co(+) are unable to work as a catalyst in this case.

Theoretical study of cyclohexane hydroxylation by three possible isomers of [FeIV(O)(R-TPEN)] 2+: does the pentadentate ligand wrapping around the metal center differently lead to the different stability and reactivity?

Density functional theory calculations have been carried out to elucidate the mechanism of cyclohexane hydroxylation by three possible isomers of [Fe(IV)(O)(N-R-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine)](2+) (R is methyl or benzyl) (Klinker et al. in Angew Chem Int Ed 44:3690-3694, 2005). The calculations offer a mechanistic view and reveal the following features: (a) all the three isomers possess triplet ground states and low-lying quintet excited states, (b) the relative stability follows the order isomer A > isomer B > isomer C, in agreement with the conclusions of Klinker et al., (c) the theoretical investigations provide a rationale to explain the interconversion of the three isomers, (d) the reaction pathways of the C-H hydroxylation are initiated by a hydrogen-abstraction step, and (e) the three isomers react with cyclohexane via two-state-reactivity patterns on competing triplet and quintet spin-state surfaces. As such, in the gas phase, the relative reactivity exhibits the trend isomer B > isomer A, while at the highest level, B2//B1 with zero point energy and solvation corrections, the relative reactivity follows the order isomer B > isomer A > isomer C. Thus, the calculated reaction pathway shows that pyridine rings perpendicular to the Fe-O axis result in more reactive species, and a pyridine ring coordinated trans to the oxygen atom leads to the least reactive isomer.

The dramatic effect of NH3 co-ligation on the Fe+-assisted activation of carbon dioxide in the gas phase: from bare metal ions to complexes

The catalytic efficiency of Fe(+) ion over the CO(2) decomposition in the gas phase has been extensively investigated with the help of electronic structure calculation methods. Potential-energy profiles for the activation process Fe(+) + CO(2) --> CO + FeO(+) along two rival potential reaction paths, namely the insertion and addition pathways, originating from the end-on kappa(1)-O and kappa(2)-O,O coordination modes of CO(2) with the metal ion, respectively, have been explored by DFT calculations. For each pathway the potential energy surfaces of the high-spin sextet (S = 5/2) and the intermediate-spin quartet (S = 3/2) spin-states have been explored. The complete energy reaction profile calculated by a combination of ab initio and density functional theory (DFT) computational techniques reveals a two-state reactivity, involving two spin inversions, for the decomposition process and accounts well for the experimentally observed inertness of bare Fe(+) ions towards CO(2) activation. Furthermore, the coordination of up to three extra ancillary NH(3) ligands with the Fe(+) metal ion has been explored and the geometric and energetic reaction profiles of the CO(2) activation processes Fe(+) + n x NH(3) + CO(2) --> [Fe(NH(3))(n)(CO(2))](+) --> [Fe(NH(3))(n)(O)(CO)](+) --> CO + [Fe(O)(NH(3))(n)](+) (n = 1, 2 or 3) have thoroughly been scrutinized for both the insertion and the addition mechanisms. Inter alia, the geometries and energies of the various states of the [Fe(NH(3))(n)(CO(2))](+) and [Fe(NH(3))(n)(O)(CO)](+) complexes are explored and compared. Finally, a detailed analysis of the coordination modes of CO(2) in the cationic [Fe(NH(3))(n)(CO(2))](+) (n = 0, 1, 2 and 3) complexes is presented.

Copper(II)-Mediated Aromaticortho-Hydroxylation: A Hybrid DFT and Ab Initio Exploration

Mechanistic pathways for the aromatic hydroxylation by [CuII(L1)(TMAO)(O)](-) (L1=hippuric acid, TMAO=trimethylamine N-oxide), derived from the O--N bond homolysis of its [CuII(L1)(TMAO)2] precursor, were explored by using hybrid density functional theory (B3LYP) and highly correlated ab initio methods (QCISD and CCSD). Published experimental studies suggest that the catalytic reaction is triggered by a terminal copper-oxo species, and a detailed study of electronic structures, bonding, and energetics of the corresponding electromers is presented. Two pathways, a stepwise and a concerted reaction, were considered for the hydroxylation process. The results reveal a clear preference for the concerted pathway, in which the terminal oxygen atom directly attacks the carbon atom of the benzene ring, leading to the ortho-selectively hydroxylated product. Solvent effects were probed by using the PCM and CPCM solvation models, and the PCM model was found to perform better in the present case. Excellent agreement between the experimental and computational results was found, in particular also for changes in reactivity with derivatives of L1.

All-Metal Aromatic Complexes Show High Reactivity in the Oxidation Reaction of Methane and Some Hydrocarbons

The C-H activations of methane, ethane, propane, and propene catalyzed by all-metal aromatic complexes Al(4)Fe were investigated. The results reveal that the rate-determining barrier of methane activation reaction with Al(4)Fe is lower than that of both some well-known inorganic catalysts and some metal organic catalysts. It was found that the all-metal aromatic complexes have high reactivity for the C-H activation of ethane, propane, and propene. Further research showed that the ability of all-metal aromatic complex to accept an electron and the degree of electron delocalization on its aromatic plane had obvious influences on the reactivity of Al(4)Fe. The present work predicts a new kind of catalyst for the alkyl C-H activation reaction: all-metal aromatic catalyst.

NR Transfer Reactivity of Azo-Compound I of P450. How Does the Nitrogen Substituent Tune the Reactivity of the Species toward CH and CC Activation?

We studied electronic structures and reactivity patterns of azo-compound I species (RN-Cpd I) by comparison to O-Cpd I of, e.g., cytochrome P450. The study shows that the RN-Cpd I species are capable of C=C aziridination and C-H amidation, in a two-state mechanism similar to that of O-Cpd I. However, unlike O-Cpd I, here the nitrogen substituent (R) exerts a major impact on structure and reactivity. Thus, it is demonstrated that Fe=NR bonds of RN-Cpd I will generally be substantially longer than Fe=O bonds; electron-withdrawing R groups will generate a very long Fe=N bond, whereas electron-releasing R groups should have the opposite effect and hence a shorter Fe=N bond. The R substituent controls also the reactivity of RN-Cpd I toward C=C and C-H bonds by exerting steric and electronic effects. Our analysis shows that an electron-releasing substituent will lower the barriers for both bond activation reactions, since the electronic factor makes the reactions highly exothermic, while an electron-withdrawing one should raise both barriers. The steric bulk of the substituent is predicted to inhibit more strongly the aziridination reactions. It is predicted that electron-releasing substituents with small bulk will create powerful aziridination reagents, whereas electron-withdrawing substituents like MeSO(2) will prefer C-H bond activation with preference that increases with steric bulk. Finally, the study predicts (i) that the reactions of RN-Cpd I will be less stereospecific than those of O-Cpd I and (ii) that aziridination will be more stereoselective than amidation.

On the Origin of the Different Performance of Iron and Manganese Monocations in Catalyzing the Nitrous Oxide Reduction by Carbon Oxide

The potential energy profiles for the Fe+- and Mn+-assisted reduction of N2O by CO were studied at the B3LYP density functional level in order to get the differences in the reaction mechanisms determining the efficiency of iron and the inactivity of manganese as ionic catalysts. Both ground and excited states of cations were taken into account in view of a possible participation of the highest multiplicities to the reduction process. Results indicated that a spin inversion occurs in the rate-determining step of iron ion-catalyzed reaction that improves the performance of the cation. However, also in the absence of the two-state reactivity phenomenon, contrary to manganese ion, iron is active in catalyzing the reaction.

Principle and mechanism of direct porphyrin metalation: joint experimental and theoretical investigation

The direct metalation of tetraphenylporphyrin with bare metal atoms (Co and Zn) was studied with X-ray photoelectron spectroscopy, scanning tunneling microscopy, and temperature-programmed reaction measurements on ordered monolayer films of the molecules adsorbed on a Ag(111) surface. The mechanism of this novel type of surface reaction was investigated using density functional theory (DFT) calculations for the related gas-phase reactions of the unsubstituted porphyrin with the metals Fe, Co, Ni, Cu, and Zn. The reaction starts with the formation of an initial complex, in which the metal atom is coordinated by the intact unreduced porphyrin. This complex resembles the sitting-atop complex proposed for porphyrin metalation with metal ions in solution. In two subsequent steps, the pyrrolic hydrogen atoms are transferred to the metal atom, forming H2, which is eventually released. The activation barriers of the H-transfer steps vary for the different metal atoms. DFT calculations suggest that metalations with Fe, Co, and Ni show two-state reactivity, while those with Cu and Zn proceed on a single potential energy surface. For metalation with Zn, we calculated a barrier of the first hydrogen transfer step of 32.6 kcal mol(-1), in good agreement with the overall experimental activation energy of 31 kcal mol(-1).

Reactivity of high-valent iron-oxo species in enzymes and synthetic reagents: a tale of many states

The Account discusses the phenomenon of two-state reactivity (TSR) or multistate reactivity (MSR) in high-valent metal-oxo reagents, projecting its wide-ranging applicability starting from the bare species, through the reagents made by Que, Nam, and collaborators, to the Mn(V)-oxo substituted polyoxometalate, all the way to Compound I species of heme enzymes. The Account shows how the behaviors of all these variegated species derive from a simple set of electronic structure principles. Experimental trends that demonstrate TSR and MSR are discussed. Diagnostic mechanistic probes are proposed for the TSR/MSR scenario, based on kinetic isotope effect, stereochemical studies, and magnetic- and electric-field effects.

An Efficient Proton-Coupled Electron-Transfer Process during Oxidation of Ferulic Acid by Horseradish Peroxidase: Coming Full Cycle

Quantum mechanics/molecular mechanics calculations were utilized to study the process of oxidation of a native substrate (ferulic acid) by the active species of horseradish peroxidase (Dunford, H. B. Heme Peroxidases; Wiley-VCH: New York, 1999), Compound I and Compound II, and the manner by which the enzyme returns to its resting state. The results match experimental findings and reveal additional novel features. The calculations demonstrate that both oxidation processes are initiated by a proton-coupled electron-transfer (PCET) step, in which the active species of the enzyme participate only as electron-transfer partners, while the entire proton-transfer event is being relayed from the substrate to and from the His42 residue by a water molecule (W402). The reason for the observed (Henriksen, A; Smith, A. T.; Gajhede, M. J. Biol. Chem. 1999, 274, 35005-35011) similar reactivities of Compound I and Compound II toward ferulic acid is that the reactive isomer of Compound II is the, hitherto unobserved, Por(*)(+)Fe(III)OH isomer that resembles Compound I. The PCET mechanism reveals that His42 and W402 are crucial moieties and they determine the function of the HRP enzyme and account for its ability to perform substrate oxidation (Poulos, T. L. Peroxidases and Cytochrome P450. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 4, pp 189). In view of the results, the possibility of manipulating substrate oxidation by magnetic fields is an intriguing possibility.

Catalysts for Monooxygenations Made from Polyoxometalate: An Iron(V)−Oxo Derivative of the Lindqvist Anion

This work uses density functional calculations to design a new high-valent Fe(V)=O catalyst [Mo5O18Fe=O]3-, which is based on the Lindqvist polyoxometalate (Mo6O19(2-)). Because the parent species is stable to oxidative conditions, one may assume that the newly proposed iron-oxo species will be stable, too. The calculated Mössbauer spectroscopic data may be helpful toward an eventual identification of the species. The calculations of C-H hydroxylation and C=C epoxidation of propene show that, if made, [Mo5O18Fe=O]3- should be a potent oxidant that will be subject to strong solvent effect. Moreover, the Lindqvist catalyst leads to an intriguing result; the reaction that starts along an epoxidation pathway with C=C activation ends with a C-H hydroxylation product ((4)6) due to rearrangement on the catalyst. The origins of this result are analyzed in terms of the structure of the catalyst and the electronic requirements for conversion of an epoxidation intermediate to a hydroxylation product. Thus, if made, the [Mo5O18Fe=O]3 will be a selective C-H hydroxylation reagent.

Conversion of Methane to Methanol at the Mononuclear and Dinuclear Copper Sites of Particulate Methane Monooxygenase (pMMO): A DFT and QM/MM Study

Methane hydroxylation at the mononuclear and dinuclear copper sites of pMMO is discussed using quantum mechanical and QM/MM calculations. Possible mechanisms are proposed with respect to the formation of reactive copper-oxo and how they activate methane. Dioxygen is incorporated into the Cu(I) species to give a Cu(II)-superoxo species, followed by an H-atom transfer from a tyrosine residue near the monocopper active site. A resultant Cu(II)-hydroperoxo species is next transformed into a Cu(III)-oxo species and a water molecule by the abstraction of an H-atom from another tyrosine residue. This process is accessible in energy under physiological conditions. Dioxygen is also incorporated into the dicopper site to form a (mu-eta(2):eta(2)-peroxo)dicopper species, which is then transformed into a bis(mu-oxo)dicopper species. The formation of this species is more favorable in energy than that of the monocopper-oxo species. The reactivity of the Cu(III)-oxo species is sufficient for the conversion of methane to methanol if it is formed in the protein environment. Since the sigma orbital localized in the Cu-O bond region is singly occupied in the triplet state, this orbital plays a role in the homolytic cleavage of a C-H bond of methane. The reactivity of the bis(mu-oxo)dicopper species is also sufficient for the conversion of methane to methanol. The mixed-valent bis(mu-oxo)Cu(II)Cu(III) species is reactive to methane because the amplitude of the sigma singly occupied MO localized on the bridging oxo moieties plays an essential role in C-H activation.

Two-State Reactivity in Alkane Hydroxylation by Non-Heme Iron−Oxo Complexes

Density functional theory is used to explore the mechanisms of alkane hydroxylation for four synthetic non-heme iron(IV)-oxo complexes with three target substrates (Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde; J.-U.; Song, W. J.; Stubna, A.; Kim, J.; Münck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc. 2004, 126, 472-473; Rohde, J.-U.; Que, L., Jr. Angew. Chem. Int. Ed. 2005, 44, 2255-2258.). The iron-oxo reagents possess triplet ground states and low-lying quintet excited states. The set of experimental and theoretical reactivity trends can be understood if the reactions proceed on the two spin states, namely two-state reactivity (TSR); an appropriate new model is presented. The TSR model makes testable predictions: (a) If crossing to the quintet state occurs, the hydroxylation will be effectively concerted; however, if the process transpires only on the triplet surface, stepwise hydroxylation will occur, and side products derived from radical intermediates would be observed (e.g., loss of stereochemistry). (b) In cases of crossing en route to the quintet transition state, one expects kinetic isotope effects (KIEs) typical of tunneling. (c) In situations where the two surfaces contribute to the rate, one expects intermediate KIEs and radical scrambling patterns that reflect the two processes. (d) Solvent effects on these reactions are expected to be very large.

Theoretical Investigation of the Decarbonylation of Acetaldehyde by Fe+ and Cr+

We report a comprehensive theoretical study on the decarbonylation of acetaldehyde by Fe+ and Cr+. Various intermediates, transition states, and products involved in the decarbonylation reactions are fully optimized at the B3LYP/6-311+G(2df,2pd) level of theory. The potential energy surfaces (PESs) corresponding to [M,O,C2,H4]+(M=Cr and Fe) are examined in detail using B3LYP and CCSD(T) methods, respectively. The validity of these theoretical methods is calibrated with respect to the available thermochemical data. Calculations suggest that the Cr+ mediated decarbonylation of acetaldehyde takes place in four steps on the sextet surface: encounter complexation, C-C activation, aldehyde H-shift, and nonreactive dissociation, in good accordance with the Co+ mediated decarbonylation of acetaldehyde [Zhao, Zhang, Guo, Wu, Lu, Chem. Phys. Lett. 2005, 414, 28], while for the Fe+/acetaldehyde system decarbonylation can occur on both the quartet and the sextet PESs. The quartet pathway, which experiences spin-orbit coupling between the two surfaces, is energetically more favorable; whereas along the sextet decarbonylation coordinate several high-energy barriers are revealed. The theoretical results are compared with the experimental product kinetic energy and angular distributions of decarbonylation of acetaldehyde by Fe+ and Cr+ measured using a crossed-beam technique [Sonnenfroh, Farrar, J. Am. Chem. Soc. 1986, 108, 3521].

Nonradical mechanism for methane hydroxylation by iron-oxo complexes

A nonradical mechanism for methane hydroxylation by the bare FeO+ complex, Fe-ZSM-5 zeolite, and soluble methane monooxygenase is proposed from quantum chemical calculations. This mechanism is applicable when a metal-oxo species is coordinatively unsaturated. Direct interaction between methane and a metal active center can form a weakly bound methane complex in the initial stages of this reaction. Subsequent C-H bond cleavage to form an intermediate with an HO-Fe-CH3 moiety in a nonradical manner and recombination of the resultant OH and CH3 ligands take place at a metal active center to form a final methanol complex. Thus, this is a nonradical, two-step reaction. The fact that methyl radical is 10-20 kcal/mol less stable than secondary and tertiary carbon radicals and benzyl radicals leads us to propose this mechanism.

Catalytic Mechanism of Dopamine β-Monooxygenase Mediated by Cu(III)−Oxo

Mechanisms of dopamine hydroxylation by the Cu(II)-superoxo species and the Cu(III)-oxo species of dopamine beta-monooxygenase (DBM) are discussed using QM/MM calculations for a whole-enzyme model of 4700 atoms. A calculated activation barrier for the hydrogen-atom abstraction by the Cu(II)-superoxo species is 23.1 kcal/mol, while that of the Cu(III)-oxo, which can be viewed as Cu(II)-O*, is 5.4 kcal/mol. Energies of the optimized radical intermediate in the superoxo- and oxo-mediated pathways are 18.4 and -14.2 kcal/mol, relative to the corresponding reactant complexes, respectively. These results demonstrate that the Cu(III)-oxo species can better mediate dopamine hydroxylation in the protein environment of DBM. The side chains of three amino acid residues (His415, His417, and Met490) coordinate to the Cu(B) atom, one of the copper sites in the catalytic core that plays a role for the catalytic function. The hydrogen-bonding network between dopamine and the three amino acid residues (Glu268, Glu369, and Tyr494) plays an essential role in substrate binding and the stereospecific hydroxylation of dopamine to norepinephrine. The dopamine hydroxylation by the Cu(III)-oxo species is a downhill and lower-barrier process toward the product direction with the aid of the protein environment of DBM. This enzyme is likely to use the high reactivity of the Cu(III)-oxo species to activate the benzylic C-H bond of dopamine; the enzymatic reaction can be explained by the so-called oxygen rebound mechanism.

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.

A Two-State Computational Investigation of Methane CH and Ethane CC Oxidative Addition to [CpM(PH3)]n+ (M=Co, Rh, Ir;n=0, 1)

Reductive elimination of methane from methyl hydride half-sandwich phosphane complexes of the Group 9 metals has been investigated by DFT calculations on the model system [CpM(PH(3))(CH(3))(H)] (M = Co, Rh, Ir). For each metal, the unsaturated product has a triplet ground state; thus, spin crossover occurs during the reaction. All relevant stationary points on the two potential energy surfaces (PES) and the minimum energy crossing point (MECP) were optimized. Spin crossover occurs very near the sigma-CH(4) complex local minimum for the Co system, whereas the heavier Rh and Ir systems remain in the singlet state until the CH(4) molecule is almost completely expelled from the metal coordination sphere. No local sigma-CH(4) minimum was found for the Ir system. The energetic profiles agree with the nonexistence of the Co(III) methyl hydride complex and with the greater thermal stability of the Ir complex relative to the Rh complex. Reductive elimination of methane from the related oxidized complexes [CpM(PH(3))(CH(3))(H)](+) (M = Rh, Ir) proceeds entirely on the spin doublet PES, because the 15-electron [CpM(PH(3))](+) products have a doublet ground state. This process is thermodynamically favored by about 25 kcal mol(-1) relative to the corresponding neutral system. It is essentially barrierless for the Rh system and has a relatively small barrier (ca. 7.5 kcal mol(-1)) for the Ir system. In both cases, the reaction involves a sigma-CH(4) intermediate. Reductive elimination of ethane from [CpM(PH(3))(CH(3))(2)](+) (M = Rh, Ir) shows a similar thermodynamic profile, but is kinetically quite different from methane elimination from [CpM(PH(3))(CH(3))(H)](+): the reductive elimination barrier is much greater and does not involve a sigma-complex intermediate. The large difference in the calculated activation barriers (ca. 12.0 and ca. 30.5 kcal mol(-1) for the Rh and Ir systems, respectively) agrees with the experimental observation, for related systems, of oxidatively induced ethane elimination when M = Rh, whereas the related Ir systems prefer to decompose by alternative pathways.

Computational Exploration of the Catalytic Mechanism of Dopamine β-Monooxygenase: Modeling of Its Mononuclear Copper Active Sites

Dopamine hydroxylation by the copper-superoxo, -hydroperoxo, and -oxo species of dopamine beta-monooxygenase (DBM) is investigated using theoretical calculations to identify the active species in its reaction and to reveal the key functions of the surrounding amino acid residues in substrate binding. A 3D model of rat DBM is constructed by homology modeling using the crystal structure of peptidylglycine alpha-hydroxylating monooxygenase (PHM) with a high sequence identity of 30% as a template. In the constructed 3D model, the CuA site in domain 1 is coordinated by three histidine residues, His265, His266, and His336, while the CuB site in domain 2 is coordinated by two histidine residues, His415 and His417, and by a methionine residue, Met490. The three Glu268, Glu369, and Tyr494 residues are suggested to play an important role in the substrate binding at the active site of DBM to enable the stereospecific hydrogen-atom abstraction. Quantum mechanical/molecular mechanical (QM/MM) calculations are performed to determine the structure of the copper-superoxo, -hydroperoxo, and -oxo species in the whole-enzyme model with about 4700 atoms. The reactivity of the three oxidants is evaluated in terms of density-functional-theory calculations with small models extracted from the QM region of the whole-enzyme model.

Gas-Phase Catalysis by Atomic and Cluster Metal Ions: The Ultimate Single-Site Catalysts

Gas-phase experiments with state-of-the-art techniques of mass spectrometry provide detailed insights into numerous elementary processes. The focus of this Review is on elementary reactions of ions that achieve complete catalytic cycles under thermal conditions. The examples chosen cover aspects of catalysis pertinent to areas as diverse as atmospheric chemistry and surface chemistry. We describe how transfer of oxygen atoms, bond activation, and coupling of fragments can be mediated by atomic or cluster metal ions. In some cases truly unexpected analogies of the idealized gas-phase ion catalysis can be drawn with related chemical transformations in solution or the solid state, and so improve our understanding of the intrinsic operation of a practical catalyst at a strictly molecular level.

Comprehensive DFT Study of Nitrous Oxide Decomposition over Fe-ZSM-5†

The reaction mechanism for nitrous oxide decomposition has been studied on hydrated and dehydrated mononuclear iron sites in Fe-ZSM-5 using density functional theory. In total, 46 different surface species with different spin states (spin multiplicity M(S) = 4 or 6) and 63 elementary reactions were considered. Heats of adsorption, activation barriers, reaction rates, and minimum energy pathways were determined. The approximate minimum energy pathways and transition states were calculated using the "growing string method" and a modified "dimer method". Spin surface crossing (e.g., O(2) desorption) was considered. The minimum potential energy structure on the seam of two potential energy surfaces was determined with a multiplier penalty function algorithm by Powell and approximate rates of spin surface crossings were calculated. It was found that nitrous oxide decomposition is first order with respect to nitrous oxide concentration and zero order with respect to oxygen concentration. Water impurities in the gas stream have a strong inhibiting effect. In the concentration range of 1-100 ppb, the presence of water vapor influences the surface composition and the apparent rate coefficient. This is especially relevant in the temperature range of 600-700 K where most experimental kinetic studies are performed. Apparent activation barriers determined over this temperature range vary from 28.4 (1 ppb H(2)O) to 54.8 kcal/mol (100 ppb H(2)O). These results give an explanation why different research groups and different catalyst pretreatments often result in very different activation barriers and preexponential factors. Altogether perfect agreement with experimental results could be achieved.

Selectivity control in electron spin inversion processes: regio- and stereochemistry of Paternò-Büchi photocycloadditions as a powerful tool for mapping intersystem crossing processes

Regio- and stereoselectivity in ene-carbonyl photocycloadditions depend on the spin multiplicity of the excited carbonyl state, although both singlet and triplet states produce the cycloadducts with comparable chemoselectivity. The correlation between selectivity and spin state was evaluated by concentration, temperature, and solvent viscosity studies. The higher selectivity observed for triplet reactions is rationalized by the optimal conformations of the intermediate 2-oxabutane-1,4-diyls for intersystem crossing (ISC) to the singlet manifold, controlled preferentially by spin-orbit coupling. This weak interaction connected with ISC can lead to substantial control of regio- and stereoselectivity. The role of hyperfine coupling is demonstrated by magnetic isotope effects.