Thermodynamics, kinetics, and dynamics of the two alternative aniomesolytic fragmentations of C-O bonds: an electrochemical and theoretical study

Fragmentation reactions of radical anions (mesolytic cleavages) of cyanobenzyl alkyl ethers (intramolecular dissociative electron transfer, heterolytic cleavages) have been studied electrochemically. The intrinsic barriers for the processes have been established from the experimental thermodynamic and kinetic parameters. These values are more than 3 kcal/mol lower as an average than the related homolytic mesolytic fragmentations of radical anions of 4-cyanophenyl ethers. In the particular case of isomers 4-cyanobenzyl phenyl ether and 4-cyanophenyl benzyl ether, the difference in intrinsic barriers amounts to 5.5 kcal/mol, and this produces an energetic crossing where the thermodynamically more favorable process (homolytic) is the kinetically slower one. The fundamental reasons for this behavior have been established by means of theoretical calculations within the density functional theory framework, showing that, in this case, the factors that determine the kinetics are clearly different (mainly present in the transition state) from those that determine the thermodynamics and they are not related to the regioconservation of the spin density ("spin regioconservation principle"). Our theoretical results reproduce quite well the experimental energetic difference of barriers and demonstrate the main structural origin of the difference.     

Gas-phase properties and fragmentation behavior of cationic, dinuclear iron chloride clusters Fe(2)Cl(n)()(+) (n = 1-6)

Sector-field mass spectrometry is used to probe the fragmentation patterns of cationic dinuclear iron chloride clusters Fe(2)Cl(n)()(+) (n = 1-6). For the chlorine-rich, high-valent Fe(2)Cl(n)()(+) ions (n = 4-6), losses of atomic and molecular chlorine prevail in the unimolecular and collision-induced dissociation patterns. Instead, the chlorine deficient, formally low-valent Fe(2)Cl(n)()(+) clusters (n = 1-3) preferentially undergo unimolecular degradation to mononuclear FeCl(m)()(+) ions. In addition, photoionization is used to determine IE(Fe(2)Cl(6)) = 10.85 +/- 0.05 eV along with appearance energy measurements for the production of Fe(2)Cl(5)(+) and Fe(2)Cl(4)(+) cations from iron(III) chloride vapor. The combination of the experimental results allows an evaluation of some of the thermochemical properties of the dinuclear Fe(2)Cl(n)()(+) cations: e.g., Delta(f)H(Fe(2)Cl(+)) = 232 +/- 15 kcal/mol, Delta(f)H(Fe(2)Cl(2)(+)) = 167 +/- 4 kcal/mol, Delta(f)H(Fe(2)Cl(3)(+)) = 139 +/- 4 kcal/mol, Delta(f)H(Fe(2)Cl(4)(+)) = 113 +/- 4 kcal/mol, Delta(f)H(Fe(2)Cl(5)(+)) = 79 +/- 5 kcal/mol, and Delta(f)H(Fe(2)Cl(6)(+)) = 93 +/- 2 kcal/mol. The analysis of the data suggests that structural effects are more important than the formal valency of iron as far as the Fe-Cl bond strengths in the Fe(2)Cl(n)()(+) ions are concerned.     

Free energies for degradation reactions of 1,2,3-trichloropropane from ab initio electronic structure theory

Electronic structure methods were used to calculate the gas and aqueous phase reaction energies for reductive dechlorination (i.e., hydrogenolysis), reductive β-elimination, dehydrochlorination, and nucleophilic substitution by OH? of 1,2,3-trichloropropane. The thermochemical properties ΔH(f)°(298.15 K), S°(298.15 K, 1 bar), and ΔG(S)(298.15 K, 1 bar) were calculated by using ab initio electronic structure calculations, isodesmic reactions schemes, gas-phase entropy estimates, and continuum solvation models for 1,2,3-trichloropropane and several likely degradation products: CH3?CHCl?CH2Cl, CH2Cl?CH2?CH2Cl, C?H2?CHCl?CH2Cl, CH2Cl?C?H?CH2Cl, CH2═CCl?CH2Cl, cis-CHCl═CH?CH2Cl, trans-CHCl═CH?CH2Cl, CH2═CH?CH2Cl, CH2Cl?CHCl?CH2OH, CH2Cl?CHOH?CH2Cl, CH2═CCl?CH2OH, CH2═COH?CH2Cl, cis-CHOH═CH?CH2Cl, trans-CHOH═CH?CH2Cl, CH(═O)?CH2?CH2Cl, and CH3?C(═O)?CH2Cl. On the basis of these thermochemical estimates, together with a Fe(II)/Fe(III) chemical equilibrium model for natural reducing environments, all of the reactions studied were predicted to be very favorable in the standard state and under a wide range of pH conditions. The most favorable reaction was reductive β-elimination (ΔG(rxn)° ≈ ?32 kcal/mol), followed closely by reductive dechlorination (ΔG(rxn)° ≈ ?27 kcal/mol), dehydrochlorination (ΔG(rxn)° ≈ ?27 kcal/mol), and nucleophilic substitution by OH? (ΔG(rxn)° ≈ ?25 kcal/mol). For both reduction reactions studied, it was found that the first electron-transfer step, yielding the intermediate C?H2?CHCl?CH2Cl and the CH2Cl?C?H?CH2Cl species, was not favorable in the standard state (ΔG(rxn)° ≈ +15 kcal/mol) and was predicted to occur only at relatively high pH values. This result suggests that reduction by natural attenuation is unlikely.     

Hybrid quantum mechanical and molecular mechanics study of the S(N)2 Reaction of CCl4 + OH- in aqueous solution: the potential of mean force, reaction energetics, and rate constants

The bimolecular nucleophilic substitution reaction of CCl(4) and OH(-) in aqueous solution was investigated on the basis of a combined quantum mechanical and molecular mechanics method. A multilayered representation approach is employed to achieve high accuracy results at the CCSD(T) level of theory. The potential of mean force calculations at the DFT level and CCSD(T) level of theory yield reaction barrier heights of 22.7 and 27.9 kcal/mol, respectively. Both the solvation effects and the solvent-induced polarization effect have significant contributions to the reaction energetics, for example, the solvation effect raises the saddle point by 10.6 kcal/mol. The calculated rate constant coefficient is 8.6 × 10(-28) cm(3) molecule(-1) s(-1) at the standard state condition, which is about 17 orders magnitude smaller than that in the gas phase. Among the four chloromethanes (CH(3)Cl, CH(2)Cl(2), CHCl(3), and CCl(4)), CCl(4) has the lowest free energy activation barrier for the reaction with OH(-) in aqueous solution, confirming the trend that substitution of Cl by H in chloromethanes diminishes the reactivity.     

Ab initio chemical kinetic study on Cl + ClO and related reverse processes

The reaction of ClO with Cl and its related reverse processes have been studied theoretically by ab initio quantum chemical and statistical mechanical calculations. The geometric parameters of the reactants, products, and transition states are optimized by both UMPW1PW91 and unrestricted coupled-cluster single and double excitation (UCCSD) methods with the 6-311+G(3df) basis set. The potential energy surface has been further refined (with triple excitations, T) at the UCCSD(T)/6-311+G(3df) level of theory. The results show that Cl(2) and O ((3)P) can be produced by chlorine atom abstraction via a tight transition state, while ClOCl ((1)A(1)) and ClClO ((1)A') can be formed by barrierless association processes with exothermicities of 31.8 and 16.0 kcal/mol, respectively. In principle the O ((1)D) atom can be generated with a large endothermicity of 56.9 kcal/mol; on the other hand, its barrierless reaction with Cl(2) can readily form ClClO ((1)A'), which fragments rapidly to give ClO + Cl. The rate constants of both forward and reverse processes have been predicted at 150-2000 K by the microcanonical variational transition state theory (VTST)/Rice-Ramsperger-Kassel-Marcus (RRKM) theory. The predicted rate constants are in good agreement with available experimental data within reported errors.     

Density functional theory investigation of the reactions of isodihalomethanes (CH(2)X-X where X = Cl, Br, or I) with ethylene: substituent effects on the carbenoid behavior of the CH(2)X-X species

We investigated the chemical reactions of isodihalomethane (CH(2)X-X) and CH(2)X radical species (where X = Cl, Br, or I) with ethylene and the isomerization reactions of CH(2)X-X using density functional theory calculations. The CH(2)X-X species readily reacts with ethylene to give the cyclopropane product and an X(2) product via a one-step reaction with barrier heights of approximately 2.9 kcal/mol for CH(2)I-I, 6.8 kcal/mol for CH(2)Br-Br, and 8.9 kcal/mol for CH(2)Cl-Cl. The CH(2)X reactions with ethylene proceed via a two-step reaction mechanism to give a cyclopropane product and X atom product with much larger barriers to reaction. This suggests that photocyclopropanation reactions using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH(2)X species. The isomerization reactions of CH(2)X-X had barrier heights of approximately 14.4 kcal/mol for CH(2)I-I, 11.8 kcal/mol for CH(2)Br-Br, and 9.1 kcal/mol for CH(2)Cl-Cl. We compare our results for the CH(2)X-X carbenoids to results from previous calculations of the Simmons-Smith-type carbenoids (XCH(2)ZnX) and Li-type carbenoids (LiCH(2)X) and discuss their differences and similarities as methylene transfer agents.     

Theoretical study of the mechanism of valence tautomerism in cobalt complexes

Valence tautomerism is studied in the [Co(II-HS)(sq)(2)(bpy)]/[Co(III-LS)(sq)(cat)(bpy)] mononuclear cobalt complex by using DFT methods (HS, high spin; LS, low spin; cat, catecholate; sq, semiquinone; bpy, 2,2'-bipyridine). Calculations at the B3LYP* level of theory reproduce well the energy gap between the Co(II-HS) and Co(III-LS) forms giving an energy gap of 4.4 kcal/mol, which is comparable to the experimental value of 8.9 kcal/mol. Potential energy surfaces and crossing seams of the electronic states of the doublet, quartet, and sextet spin states are calculated along minimum energy paths connecting the energy minima corresponding to the different spin states. The calculated minimum energy crossing points (MECPs) are located at 8.8 kcal/mol in the doublet/sextet surfaces, at 10.2 kcal/mol in the doublet/quartet surfaces, and at 8.4 kcal/mol in the quartet/sextet surfaces relative to the doublet ground state. Considering the energy of the three spin states and the crossing points, the one-step relaxation mechanism between the Co(II-HS) and Co(III-LS) forms is the most probable. This research shows that mapping MECPs can be a useful strategy to analyze the potential energy surfaces of systems with complex deformation modes.     

Transition states of the retro-ene reactions of allylic diazenes

[reaction: see text] Density functional theory studies of intramolecular retro-ene reactions of allyldiazenes show that the reaction is a concerted process involving a six-center cyclic transition state. The activation barriers for deazetization for X = H, Me, F, Cl, and Br (3a-e) are 2.4, 40.2, 22.3, 9.3, and 8.8 kcal/mol, respectively.     

Theoretical Study on Gas Phase Reactions of OH Hydrogen-Abstraction from Formyl Fluoride with Di erent Catalysts

The mechanisms and kinetics of the gas phase reactions that the hydrogen atom in formyl uoride (FCHO) abstracted by OH in the presence of water, formic acid (FA), or sulfuric acid (SA) are theoretically investigated at the CCSD(T)/6-311++G(3df, 3pd)//M06-2X/6-311++G(3df, 3pd) level of theory. The calculated results show that the barriers of the transition states involving catalysts are lowered to -2.89, -6.25, and -7.76 kcal/mol from 3.64 kcal/mol with respect to the separate reactants, respectively, which re ects that those catalysts play an important role in reducing the barrier of the hydrogen abstraction reaction of FCHO with OH. Additionally, using conventional transition state theory with Eckart tun-neling correction, the kinetic data demonstrate that the entrance channel X FCHO+OH (X=H₂O, FA, or SA) is signi cantly more favorable than the pathway X OH+FCHO. More-over, the rate constants of the reactions of FCHO with OH radical with H₂O, FA, or SA introduced are computed to be smaller than that of the naked OH+FCHO reaction because the concentration of the formed X FCHO or X OH complex is quite low in the atmosphere.     

What is the active species of cytochrome P450 during camphor hydroxylation? QM/MM studies of different electronic states of compound I and of reduced and oxidized iron-oxo intermediates

We have investigated C-H hydroxylation of camphor by Compound I (Cpd I) of cytochrome P450cam in different electronic states and by its one-electron reduced and oxidized forms, using QM/MM calculations in the native protein/solvent environment. Cpd I species with five unpaired electrons (pentaradicaloids) are ca. 12 kcal/mol higher in energy than the ground state Cpd I species with three unpaired electrons (triradicaloids). The H-abstraction transition states of pentaradicaloids lie ca. 21 (9) kcal/mol above the triradicaloid (pentaradicaloid) reactants. Hydroxylation via pentaradicaloids is thus facile provided that they can react before relaxing to the ground-state triradicaloids. Excited states of Cpd I with an Fe(V)-oxo moiety lie more than 20 kcal/mol above the triradicaloid ground state in single-point gas-phase calculations, but these electronic configurations are not stable upon including the point-charge protein environment which causes SCF convergence to the triradicaloid ground state. One-electron reduced species (Cpd II) show sluggish reactivity compared with Cpd I in agreement with experimental model studies. One-electron oxidized species are more reactive than Cpd I but seem too high in energy to be accessible. The barriers to hydrogen abstraction for the various forms of Cpd I are generally not affected much by the chosen protonation states of the Asp297 and His355 residues near the propionate side chains of the heme or by the appearance of radical character at Asp297, His355, or the propionates.     

Performance of the correlation consistent composite approach for transition states: a comparison to G3B theory

The correlation consistent composite approach (ccCA) was applied to the prediction of reaction barrier heights (i.e., transition state energy relative to reactants and products) for a standard benchmark set of reactions comprised of both hydrogen transfer reactions and nonhydrogen transfer reactions (i.e., heavy-atom transfer, SN2, and unimolecular reactions). The ccCA method was compared against G3B for the same set of reactions. Error metrics indicate that ccCA achieves "chemical accuracy" with a mean unsigned error (MUE) of 0.89 kcal/mol with respect to the benchmark data for barrier heights; G3B has a mean unsigned error of 1.94 kcal/mol. Further, the greater accuracy of ccCA for predicted reaction barriers is compared to other benchmarked literature methods, including density functional (BB1K, MUE=1.16 kcal/mol) and wavefunction-based [QCISD(T), MUE=1.10 kcal/mol] methods.     

New reaction model for O-O bond formation and O2 evolution catalyzed by dinuclear manganese complex

A new mechanism of the oxygen evolving reaction catalyzed by [H(2)O(terpy)Mn(μ-O)(2)Mn(terpy)OH(2)](3+) is proposed by using density functional theory. This proton coupled electron transfer (PCET) model shows reasonable barriers. Because in experiments excess oxidants (OCl(-) or HSO(5)(-)) are required to evolve oxygen from water, we considered the Mn(2) complex neutralized by three counterions. Structure optimization made the coordinated OCl(-) withdraw a H(+) from the water ligand and produces the reaction space for H(2)O(2) formation with the deprotonated OH(-) ligand. The reaction barrier for the H(2)O(2) formation from OH(-) and protonated OCl(-) depends significantly on the system charge and is 14.0 kcal/mol when the system is neutralized. The H(2)O(2) decomposes to O(2) during two PCET processes to the Mn(2) complex, both with barriers lower than 12.0 kcal/mol. In both PCET processes the spin moment of transferred electrons prefers to be parallel to that of Mn 3d electrons because of the exchange interaction. This model thus explains how the triplet O(2) molecule is produced.     

过渡金属离子液体[C2mim][FeCl4]的溶解热

在干燥氩气氛下, 用等摩尔的高纯无水FeCl3和氯化1-甲基-3-乙基咪唑([C2mim][Cl])直接搅拌混合, 制备棕色透明的含过渡金属铁的离子液体[C2mim][FeCl4]. 在298.15 K下, 利用具有恒温环境的溶解反应热量计测定了这种离子液体的摩尔溶解焓(ΔsHm). 针对[C2mim][FeCl4]溶解于水后即分解的特点, 在Pitzer电解质溶液理论基础上, 提出了确定这种离子液体标准摩尔溶解焓的新方法, 得到了[C2mim][FeCl4]的标准摩尔溶解焓(ΔsH 0—m=-76.6 kJ/mol), 以及Pitzer焓参数组合: β(0)LFe,Cl+β(0)L[C2mim], Cl+ΦLFe,[C2mim]=0.072209和β(1)LFe,Cl+β(1)L[C2mim], Cl=0.15527. 借助热力学循环和Glasser离子液体晶格能理论, 用Fe3+, Cl-和[C2mim]+的离子水化焓数据以及[C2mim][FeCl4]标准摩尔溶解焓, 估算得到了配离子[FeCl4]-(g)解离成Fe3+(g)和4Cl-(g)的解离焓为5659 kJ/mol. 这个结果揭示了离子液体[C2mim][FeCl4]的标准摩尔溶解焓绝对值并不很大的原因, 即很大的离子水化焓被很大的[FeCl4]-(g)的解离焓相互抵消.     

Reactivity of isobutane on zeolites: a first principles study

In this work, ab initio and density functional theory methods are used to study isobutane protolytic cracking, primary hydrogen exchange, tertiary hydrogen exchange, and dehydrogenation reactions catalyzed by zeolites. The reactants, products, and transition-state structures are optimized at the B3LYP/6-31G* level, and the final energies are calculated using the CBS-QB3 composite energy method. The computed activation barriers are 52.3 kcal/mol for cracking, 29.4 kcal/mol for primary hydrogen exchange, 29.9 kcal/mol for tertiary hydrogen exchange, and 59.4 kcal/mol for dehydrogenation. The zeolite acidity effects on the reaction barriers are also investigated by changing the cluster terminal Si-H bond lengths. The analytical expressions between activation barriers and zeolite deprotonation energies for each reaction are proposed so that accurate activation barriers can be obtained when using different zeolites as catalysts.     

Unimolecular elimination of HF and HCl from chemically activated CF3CFClCH2Cl

The unimolecular reactions of CF3CFClCH2Cl molecules formed with 87 kcal mol(-1) of vibrational energy by recombination of CF3CFCl and CH2Cl radicals at room temperature have been characterized by the chemical activation technique. The 2,3-ClH and 2,3-FH elimination reactions, which have rate constants of (2.5 +/- 0.8) x 10(4) and (0.38 +/- 0.11) x 10(4) s(-1), respectively, are the major reactions. The 2,3-FCl interchange reaction was not observed. The trans (or E)-isomers of CF3CFCHCl and CF3CClCHCl are favored over the cis (or Z)-isomers. Density functional theory at the B3PW91/6-31G(d',p') level was used to evaluate thermochemistry and structures of the molecule and transition states. This information was used to calculate statistical rate constants. Matching the calculated to the experimental rate constants for the trans-isomers gave threshold energies of 62 and 63 kcal mol(-1) for HCl and HF elimination, respectively. The threshold energy for FCl interchange must be 3-4 kcal mol(-1) higher than for HF elimination. The results for CF3CFClCH2Cl are compared to those from CF3CFClCH3; the remarkable reduction in rate constants for HCl and HF elimination upon substitution of one Cl atom for one H atom is a consequence of both a lower E and higher threshold energies for CF3CFClCH2Cl.     

Geminate charge recombination in liquid alkane with concentrated CCl4: effects of CCl4 radical anion and narrowing of initial distribution of Cl-

Dynamics of radical cations and electrons in an admixture of a linear saturated hydrocarbon (n-dodecane) and halocarbon (carbon tetrachloride, CCl(4)) were investigated by picosecond electron beam pulse radiolysis. The decay of thermalized electrons (e(th)(-)) observed in infrared transient photoabsorption were simply accelerated by the addition of CCl(4), giving a high rate constant of 2.3 × 10(11) mol(-1) dm(3) s(-1). The decrease of the initial yield of e(th)(-) was quantified by C(37) (50 mmol), which is linked to the reaction of epithermal electrons (e(-)) with CCl(4). In contrast, the n-dodecane radical cation (RH(2)(?+)) monitored in the near-infrared indicated a convex-type dependence of the decay rate on CCl(4) concentration, although the initial yield of RH(2)(?+) remained almost constant up to a much higher CCl(4) concentration. The decay of RH(2)(?+) was analyzed by Monte Carlo simulations of geminate ion recombination with e(th)(-), chlorine anion (Cl(-)) formed via dissociative electron attachment, and CCl(4) radical anion. The results showed a good agreement with the experiments by considering two assumptions: (1) CCl(4) radical anion formed via e(th)(-) attachment and (2) narrowing of the initial distribution of Cl(-). The decrease in the initial yield of RH(2)(?+) at high CCl(4) concentration was well explained by immediate decomposition of CCl(4)(?+) to CCl(3)(+) and hole transfer from CCl(4)(?+) to adjacent RH(2) without diffusive motion of the reactants. Time-dependent density functional theory supported the spectroscopic assignment of intermediate species in the n-dodecane/CCl(4) system. The present results would be of help in understanding the electron capture reaction in multicomponent systems such as a chemically amplified resist in lithography.     

Fe(IV)═O(TBC)(CH3CN)]2+: comparative reactivity of iron(IV)-oxo species with constrained equatorial cyclam ligation

[Fe(IV)═O(TBC)(CH(3)CN)](2+) (TBC = 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclotetradecane) is characterized, and its reactivity differences relative to [Fe(IV)═O(TMC)(CH(3)CN)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) are evaluated in hydrogen atom (H-atom) abstraction and oxo-transfer reactions. Structural differences are defined using X-ray absorption spectroscopy and correlated to reactivities using density functional theory. The S = 1 ground states are highly similar and result in large activation barriers (~25 kcal/mol) due to steric interactions between the cyclam chelate and the substrate (e.g., ethylbenzene) associated with the equatorial π-attack required by this spin state. Conversely, H-atom abstraction reactivity on an S = 2 surface allows for a σ-attack with an axial substrate approach. This results in decreased steric interactions with the cyclam and a lower barrier (~9 kcal/mol). For [Fe(IV)═O(TBC)(CH(3)CN)](2+), the S = 2 excited state in the reactant is lower in energy and therefore more accessible at the transition state due to a weaker ligand field associated with the steric interactions of the benzyl substituents with the trans-axial ligand. This study is further extended to the oxo-transfer reaction, which is a two-electron process requiring both σ- and π-electron transfer and thus a nonlinear transition state. In oxo-transfer, the S = 2 has a lower barrier due to sequential vs concerted (S = 1) two electron transfer which gives a high-spin ferric intermediate at the transition state. The [Fe(IV)═O(TBC)(CH(3)CN)](2+) complex is more distorted at the transition state, with the iron farther out of the equatorial plane due to the steric interaction of the benzyl groups with the trans-axial ligand. This allows for better orbital overlap with the substrate, a lower barrier, and an increased rate of oxo-transfer.     

Quantum chemical study of adsorption and dissociation of H2S on the gallium-rich GaAs (001)-4 x 2 surface

H(2)S adsorption and dissociation on the gallium-rich GaAs(001)-4 x 2 surface is investigated using hybrid density functional theory. Starting from chemisorbed H(2)S on the GaAs(001)-4 x 2 surface, two possible reaction routes have been proposed. We find that H(2)S adsorbs molecularly onto GaAs(001)-4 x 2 via the formation of a dative bond, and this process is exothermic with adsorption energy of 6.6 kcal/mol. For the first reaction route, one of the H atoms from the chemisorbed H(2)S is transferred to a second-layer As atom and the dissociated SH is inserted into the Ga-As bond with an activation barrier of 8.2 kcal/mol, which is found to be 29.3 kcal/mol more stable than the reactants. For the second case, the dissociated species may insert themselves into the Ga-Ga dimer resulting in the Ga-H-Ga and Ga-HS-Ga bridge-bonded states, which are found to be 29.8 and 22.2 kcal/mol more stable than the reactants, respectively. However, the calculations also show that the activation barrier (16.1 kcal/mol) for chemisorbed H(2)S dissociation through the second route is higher than the transfer of one H atom into a second-layer As atom. As a result, we conclude that sulfur insertion into the Ga-As bond is more kinetically favorable.     

Optimized spin crossings and transition states for short-range electron transfer in transition metal dimers

Electron-transfer reactions in eight mixed-valence manganese dimers are studied using B3LYP. One of the dimers is a model of the active site of manganese catalase, while another represents a basic building block of the oxygen-evolving complex in photosystem II. The adiabatic reactions are characterized by fully optimized transition states where the single imaginary frequency represents the electron-transfer coordinate. When there is antiferromagnetic coupling between different high-spin centers, electron transfer must be accompanied by a spin transition. Spin transitions are characterized by minimum-energy crossing points between spin surfaces. Three reaction mechanisms have been investigated. First, a single-step reaction where spin flip is concerted with electron transfer. Second, an initial transition to a center with intermediate spin that can be followed by electron transfer. Third, an initial transition to a ferromagnetic state from which the electron can be transferred adiabatically. The complexes prefer the third route with rate-determining barriers ranging from 5.7 kcal/mol to 17.2 kcal/mol for different complexes. The origins of these differences are discussed in terms of oxidation states and ligand environments. Many DFT functionals overestimate charge-transfer interactions, but for the present complexes, the error should be limited because of short Mn-Mn distances.     

A centred, elongated "ferric tetrahedron" with an S= 15/2 spin ground state

The reaction of anhydrous FeCl(3) with 1H-benzotriazole-1-methanol (Bta-CH(2)OH) in MeOH produces the pentanuclear complex [Fe(5)O(2)(OMe)(2)(Bta)(4)(BtaH)(MeOH)(5)Cl(5)], containing a distorted tetrahedron of four Fe ions centred on a fifth. The central Fe is antiferromagnetically coupled to the peripheral Fe ions resulting in an S= 15/2 spin ground state.