Oxidation of methanol by FeO2+ in water: DFT calculations in the gas phase and ab initio MD simulations in water solution

We investigate the mechanism of methanol oxidation to formaldehyde by ironoxido ([Fe(IV)O]2+), the alleged active intermediate in the Fenton reaction. The most likely reaction mechanisms are explored with density functional theory (DFT) calculations on microsolvated clusters in the gas phase and, for a selected set of mechanisms, with constrained Car-Parrinello molecular dynamics (CPMD) simulations in water solution. Helmholtz free energy differences are calculated using thermodynamic integration in a simulation box with 31 water molecules at 300 K. The mechanism of the reaction is investigated with an emphasis on whether FeO2+ attacks methanol at a C-H bond or at the O-H bond. We conclude that the most likely mechanism is attack by the oxido oxygen at the C-H bond ("direct CH mechanism"). We calculate an upper bound for the reaction Helmholtz free energy barrier in solution of 50 kJ/mol for the C-H hydrogen transfer, after which transfer of the O-H hydrogen proceeds spontaneously. An alternative mechanism, starting with coordination of methanol directly to Fe ("coordination OH mechanism"), cannot be ruled out, as it involves a reaction Helmholtz free energy barrier in solution of 44 +/- 10 kJ/mol. However, this coordination mechanism has the disadvantage of requiring a prior ligand substitution reaction, to replace a water ligand by methanol. Because of the strong acidity of [FeO(H2O)5]2+, we also investigate the effect of deprotonation of a first-shell water molecule. However, this is found to increase the barriers for all mechanisms.     

气相中HRnCCH和X(X=H_2O,NH_3)反应机理的理论研究

在CCSD(T)//MP2/aug-cc-pVTZ-pp理论水平上,研究了HRnCCH与大气中H2O及NH3分子反应的机理,反应主要包括HRnCCH与HRnOH及HRnNH2之间的转化、H2O和NH3在HRnCCH中的碳碳三键上的加成反应以及HRnCCH与双分子水反应等.结果表明,HRnCCH与H2O反应生成HCCH和HRnOH及HRnCCH与NH3反应生成HCCH和HRnNH2的能垒分别为54.1和75.2 kJ/mol,而生成HRnCHC(OH)H,HRnC(OH)CH2,HRnCHC(NH2)H和HRnC(NH2)CH2的活化能分别为219.6,220.5,174.4和182.4kJ/mol,此结果表明HRnCCH反应性较弱且是稳态存在的.此外,在HRnCCH与H2O反应中加入单个水分子,仍然生成HRnCHC(OH)H,但反应活化能却降低了96.4 kJ/mol,说明水分子对该反应有明显的催化作用.     

(H_2O)_n(n=0～2)催化HS和HOCl反应机理的理论研究

在CCSD(T)/6-311+G(3df,2p)//M06-2X/6-311+G(3df,2p)水平上研究了(H_2O)n(n=0~2)催化HS和HOCl的反应机理.结果表明,HS与HOCl反应中HS夺取HOCl上的H原子形成产物H_2S和ClO.在无水催化时,该反应存在2种不同的路径(分别经过过渡态TS1和TS2,二者互为顺反结构),对应的能垒分别为100.28和100.91kJ/mol,到达产物(H_2S+ClO)需吸收18.99kJ/mol能量,反应不易发生;在单个水分子参与时,水分子可通过形成弱相互作用或者作为H原子转移桥梁影响反应机理,获得了4种水催化路径,能垒(间于53.97~92.39kJ/mol之间)均低于无水催化过程.同时发现,在反应到达产物前,水分子可以与产物形成中间体IM,IM相对能仅为0.46kJ/mol,有利于产物形成;有2个水分子参与反应时,找到了3条催化路径,最优反应路径过渡态TS7的能垒为45.05kJ/mol,低于无水催化过程,相比单个水分子最优路径能垒(53.97kJ/mol)并无显著降低.     

The "somersault" mechanism for the p-450 hydroxylation of hydrocarbons. The intervention of transient inverted metastable hydroperoxides

A series of model theoretical calculations are described that suggest a new mechanism for the oxidation step in enzymatic cytochrome P450 hydroxylation of saturated hydrocarbons. A new class of metastable metal hydroperoxides is described that involves the rearrangement of the ground-state metal hydroperoxide to its inverted isomeric form with a hydroxyl radical hydrogen bonded to the metal oxide (MO-OH --> MO....HO). The activation energy for this somersault motion of the FeO-OH group is 20.3 kcal/mol for the P450 model porphyrin iron(III) hydroperoxide [Por(SH)Fe(III)-OOH(-)] to produce the isomeric ferryl oxygen hydrogen bonded to an *OH radical [Por(SH)Fe(III)-O....HO(-)]. This isomeric metastable hydroperoxide, the proposed primary oxidant in the P450 hydroxylation reaction, is calculated to be 17.8 kcal/mol higher in energy than the ground-state iron(III) hydroperoxide Cpd 0. The first step of the proposed mechanism for isobutane oxidation is abstraction of a hydrogen atom from the C-H bond of isobutane by the hydrogen-bonded hydroxyl radical to produce a water molecule strongly hydrogen bonded to anionic Cpd II. The hydroxylation step involves a concerted but nonsynchronous transfer of a hydrogen atom from this newly formed, bound, water molecule to the ferryl oxygen with a concomitant rebound of the incipient *OH radical to the carbon radical of isobutane to produce the C-O bond of the final product, tert-butyl alcohol. The TS for the oxygen rebound step is 2 kcal/mol lower in energy than the hydrogen abstraction TS (DeltaE() = 19.5 kcal/mol). The overall proposed new mechanism is consistent with a lot of the ancillary experimental data for this enzymatic hydroxylation reaction.     

Dynamic study of metal-ion incorporation into porphyrins based on the dynamic characterization of metal ions and on sitting-atop complex formation.

We succeeded in the detection of the sitting-atop (SAT) copper(II) complex of TPP (5,10,15,20-tetraphenylporphyrin) in acetonitrile (AN) as a solvent with a very low Br?nsted basicity, where two pyrrolenine nitrogens in the Cu(II)-SAT complex coordinate to the metal ion and two protons still remain on the pyrrole nitrogens. The structure parameters around the copper(II) ion in the Cu(II)-SAT complex, as determined by a fluorescent EXAFS method, suggest an axially elongated and equatorially distorted six-coordinate geometry. We measured the rates of the formation reaction of the SAT complexes for a series of transition metal(II) ions in AN using the stopped-flow technique. We propose the mechanism where there is a rapid deformation equilibrium of the porphyrin ring prior to the rate-determining step of the bond rupture of a coordinated solvent molecule on the metal(II) ion. Furthermore, we measured the rates of the deprotonation reaction of the Cu(II)-SAT complex by some Br?nsted bases and indicated that the rate-determining step is the attack of the base on the proton of the pyrrole nitrogen in the SAT complex. Finally, a unified mechanism relevant to the porphyrin metalation mechanism has been proposed.     

CO_(2)加氢制甲酸理论研究及高效铁基催化剂设计北大核心CSCD

CO_(2)加氢制甲酸由于需同时活化惰性氢气及CO_(2)而富有挑战性,同时此过程原子经济性100%,具有很好的理论和现实研究价值,但文献中报道的活性较好的催化剂均为贵金属催化剂.为了开发活性更高的用于CO_(2)加氢制甲酸的铁基催化剂,我们采用理论计算方法研究了12种不同种类的PNP-Fe(PNP=2,6-(二-叔丁基-磷甲基)吡啶)化合物催化CO_(2)加氢制甲酸的过程.理论研究结果表明,CO_(2)加氢制甲酸反应过程包括H2活化及CO_(2)插入金属氢键两个步骤,H_(2)活化过程是整个反应的速控步骤.催化剂吡啶环上进行P原子取代可以显著降低H_(2)活化能垒.基于以上发现,我们设计了一种新颖的高效铁基催化剂,使用此催化剂催化CO_(2)加氢制甲酸反应,速控步骤能垒只有85.6 kJ/mol,催化活性与贵金属的比较接近.我们研究的12种铁基催化剂速控步骤能垒范围为85.6~126.4 kJ/mol,显示了配体良好的调控催化活性能力.     

单分子水对CH_2SH+NO_2反应机理影响的理论研究

在B3LYP/6-311++G(2df,p)水平下对单分子水参与下的CH_2SH+NO_2反应的微观机理进行了研究.为了获得更准确的能量信息,采用HL复合方法和CCSD(T)/aug-ccpvtz方法进行单点能校正.结果表明,加入单分子水后的CH_2SH+NO_2反应体系,共经过10条不同的反应路径,得到6种反应产物.与裸反应(CH_2SH+NO_2)相比,水分子在反应中起到了明显的正催化作用.不仅使生成产物trans-HONO的能垒(-52.84kJ·mol~(-1))降低了176.94kJ·mol~(-1),而且不需经过复杂的重排和异构化过程便可得到产物cis-HONO.在生成产物cis-HONO通道(Path3和Path4)中,活化能垒分别为143.65和126.70kJ·mol~(-1),而其裸反应的活化能垒却高达238.34kJ·mol~(-1).生成HNO_2的通道中(Path5和Path6)活化能垒分别为295.23和-42.19kJ·mol~(-1).其中Path6的无势垒过程使HNO_2也成为该反应的主要产物.另外,单分子水还可通过氢迁移的方式直接参与CH_2SH+NO_2的反应,活化能垒(TS7-TS10)分别为-10.62,151.03,186.22和155.10kJ·mol~(-1).除直接抽氢通道中的(Path8-Path10)外,其余反应通道均为放热反应,在热力学上是可行的.     

Role of the direct reaction H2S + SO2 in the homogeneous Claus reaction

Quantum chemical methods at the Gaussian-2 and -3 levels of theory have been used to investigate the reactions between H(2)S, SO(2), and S(2)O such as might occur in the front-end furnace of the Claus process. The direct reaction between H(2)S and SO(2) occurs via a 5-centered transition state with an initial barrier of approximately 135 kJ mol(-1) and an overall barrier of approximately 153 kJ mol(-1) to produce S(2)O and H(2)O. We indicate approximate values here because there are a number of isomers in the reaction pathway that have barriers slightly different from those quoted. The presence of a water molecule lowers this by approximately 60 kJ mol(-1), but the van der Waals complex required for catalysis by water is thermodynamically unfavorable under the conditions in the Claus reactor. The direct reaction between H(2)S and S(2)O can occur via two possible pathways; the analogous reaction to H(2)S + SO(2) has an initial barrier of approximately 117 kJ mol(-1) and an overall barrier of approximately 126 kJ mol(-1) producing S(3) and H(2)O, and a pathway with a 6-centred transition state has a barrier of approximately 111 kJ mol(-1), producing HSSSOH. Rate constants, including a QRRK analysis of intermediate stabilization, are reported for the kinetic scheme proposed here.     

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).     

Enthalpies of formation, bond dissociation energies and reaction paths for the decomposition of model biofuels: ethyl propanoate and methyl butanoate

The complete basis set method CBS-QB3 has been used to study the thermochemistry and kinetics of the esters ethyl propanoate (EP) and methyl butanoate (MB) to evaluate initiation reactions and intermediate products from unimolecular decomposition reactions. Using isodesmic and isogeitonic equations and atomization energies, we have estimated chemically accurate enthalpies of formation and bond dissociation energies for the esters and species derived from them. In addition it is shown that controversial literature values may be resolved by adopting, for the acetate radical, CH3C(O)O(.-), DeltaH(o)(f)298.15K) = -197.8 kJ mol(-1) and for the trans-hydrocarboxyl radical, C(.-)(O)OH, -181.6 +/- 2.9 kJ mol(-1). For EP, the lowest energy decomposition path encounters an energy barrier of approximately 210 kJ mol(-1) (approximately 50 kcal mol(-1)), which proceeds through a six-membered ring transition state (retro-ene reaction) via transfer of the primary methyl H atom from the ethyl group to the carbonyl oxygen, while cleaving the carbon-ether oxygen to form ethene and propanoic acid. On the other hand, the lowest energy path for MB has a barrier of approximately 285 kJ mol(-1), producing ethene. Other routes leading to the formation of aldehydes, alcohols, ketene, and propene are also discussed. Most of these intramolecular hydrogen transfers have energy barriers lower than that needed for homolytic bond fission (the lowest of which is 353 kJ mol(-1) for the C(alpha)-C(beta) bond in MB). Propene formation is a much higher energy demanding process, 402 kJ mol(-1), and it should be competitive with some C-C, C-O, and C-H bond cleavage processes.     

Bonding rearrangements of hydrogen-bonded complexes involving alkynes

Molecules containing a C-C triple bond, such as HC[triple bond]CH, FC[triple bond]CF, and the C[triple bond]CH radical, are allowed to interact with a partner molecule of H2O, NH3, or HF. Quantum chemical calculations show that these C[triple bond]CH...X H-bonded complexes are bound by up to 4 kcal x mol(-1). More importantly, they can rearrange in such a way that the partner molecule adds to the triple bond so as to form a double C=C bond. Whereas this process is strongly exoergic, there is a high-energy barrier to this rearrangement process. On the other hand, when a second water molecule is added to the complex, it can shuttle protons from the donor part of the complex to the acceptor, and thereby greatly reduce the rearrangement energy barrier. In the case of CCH + 2H2O, this barrier is computed to be less than 4 kcal x mol(-1).     

A "push-pull" mechanism for heterolytic o-o bond cleavage in hydroperoxo manganese porphyrins

A water-soluble manganese porphyrin, 5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrinatomanganese(III) (Mn(III)TDMImP) is shown to react with H(2)O(2) to generate a relatively stable dioxomanganese(V) porphyrin complex (a compound I analog). Stopped-flow kinetic studies revealed Michaelis Menton-type saturation kinetics for H(2)O(2). The visible spectrum of a compound 0 type intermediate, assigned as Mn(III)(OH)(OOH)TDMImP, can be directly observed under saturating H(2)O(2) conditions (Soret band at 428 nm and Q bands at 545 and 578 nm). The rate-determining O-O heterolysis step was found to have a very small activation enthalpy (ΔH(≠) = 4.2 ± 0.2 kcal mol(-1)) and a large, negative activation entropy (ΔS(≠) = -36 ± 1 cal mol(-1) K(-1)). The O-O bond cleavage reaction was pH independent at 8.8 < pH < 10.4 with a first-order rate constant of 66 ± 12 s(-1). These observations indicate that the O-O bond in Mn(III)(OH)(OOH)TDMImP is cleaved via a concerted "push-pull" mechanism. In the transition state, the axial (proximal) (-)OH is partially deprotonated ("push"), while the terminal oxygen in (-)OOH is partially protonated ("pull") as a water molecule is released to the medium. This mechanism is reminiscent of O-O bond cleavage in heme enzymes, such as peroxidases and cytochrome P450, and similar to the fast, reversible O-Br bond breaking and forming reaction mediated by similar manganese porphyrins. The small enthalpy of activation suggests that this O-O bond cleavage could also be made reversible.     

Is the peptide bond formation activated by Cu(2+) interactions? Insights from density functional calculations

The catalytic role that Cu(2+) cations play in the peptide bond formation has been addressed by means of density functional calculations. First, the Cu(2+)-(glycine)2 --> Cu(2+)-(glycylglycine) + H2O reaction was investigated since mass spectrometry low collision activated dissociation (CAD) spectra of Cu(2+)-(glycine)2 led to the elimination of a water molecule, which suggested that an intracomplex peptide bond formation might have occurred. Results show that this intracomplex condensation is associated to a very high free energy barrier (97 kcal mol(-1)) and reaction free energy (66 kcal mol(-1)) because of the loss of metal coordination during the reaction. Second, on the basis of the salt-induced peptide formation theory, the condensation reaction between two glycines was studied in aqueous solution using discrete water molecules and the conductor polarized continuum model (CPCM) continuous method. It is found that the synergy between the interaction of glycines with Cu(2+) and the presence of water molecules acting as proton-transfer helpers significantly lower the activation barrier (from 55 kcal/mol for the uncatalyzed system to 20 kcal/mol for the Cu(2+) solvated system) which largely favors the formation of the peptide bond.     

铂催化2-烯炔苯甲醛水合环化反应的机理及化学选择性的理论研究

采用密度泛函理论(DFT)的计算方法, 研究了铂催化2-烯炔基苯甲醛水合环化反应的微观机理及化学选择性的根源. 计算结果表明, 首先炔基被催化活化而发生亲核环化生成吡喃铂中间体; 接着吡喃铂中间体与烯烃双键发生[3+2]环加成生成铂-碳卡宾复合物; 之后, 反应将沿2条路径进行, 得到产物3a或4a, 其中4a的生成需经两步水分子辅助的质子转移过程. 生成产物3a需要克服的活化能垒为146.5 kJ/mol; 对4a的生成, 烯醇式和酮式互变异构是决速步聚, 当一个水分子参与反应时, 对应的能垒为185.8 kJ/mol, 当2个和3个水分子参与反应时, 能垒分别降低到128.1和64.9 kJ/mol. 因此, 水分子参与催化得到产物4a的路径是有利的. 另外, 反应的选择性与在异构化过程中水的共催化作用有关. 以上结果很好地解释了实验现象, 并为铂催化水环化反应提供新的见解.     

Water-assisted oxo mechanism for heme metabolism

A mechanism of heme metabolism by heme oxygenase (HO) is discussed from B3LYP density functional theory calculations. The concerted OH group attack to the alpha-carbon by the iron-hydroperoxo species is investigated using a model with full protoporphyrin IX to confirm our previous conclusion that this species does not have sufficient oxidizing power for heme oxidation (J. Am. Chem. Soc. 2004, 126, 3672). Calculated activation energies and structures of the intermediates and transition state for this process remain unchanged from those for a small model with porphine in the previous study, which shows that the inclusion of the side chain of the porphyrin ring is not essential in describing the OH group transfer. The activation barrier for a direct oxo attack to the alpha-carbon by an iron-oxo model is calculated to be 49.8 kcal/mol, the barrier height of which looks very high for the enzymatic reaction under physiological conditions. This large activation energy is due to a highly bent porphyrin structure in the transition state. However, a bridging water molecule plays an important role in reducing the porphyrin distortion in the transition state, resulting in a remarkable decrease of the activation barrier to 13.9 kcal/mol. A whole-enzyme model with about 4000 atoms is constructed to elucidate functions of the protein environment in this enzymatic reaction using QM/MM calculations. The key water molecule is fixed in the protein environment to ensure the low-barrier and regioselective heme oxidation. A water-assisted oxo mechanism of heme oxidation by heme oxygenase is proposed from these calculational results.     

二氟亚烷基卡宾与甲醛环加成反应机理的理论研究

用二阶微扰理论研究单重态二氟亚烷基卡宾与甲醛发生的环加成反应机理,采用MP2/6-31G^*方法计算了势能面上各驻点的构型参数、振动频率和能量.结果表明,单重态二氟亚烷基卡宾与甲醛的环加成反应主要有两种反应通道,通道1中,两个反应物经a,b和c三条反应途径生成三元环构型的产物P1,其中途径c是主反应途径,该途径有两步组成:(Ⅰ)二氟亚烷基卡宾与甲醛生成了1个富能中间体(INT1c),是无势垒放热反应,放出能量为219.18kJ/mol;(Ⅱ)中间体(INT1c)异构化为产物二氟亚烷基环氧乙烷,其势垒为134.71kJ/mol.通道2的反应途径由三步组成:(Ⅰ)反应物首先生成了1个富能中间体(INT1b),为无势垒的放热反应,放出的能量142.77kJ/mol;(Ⅱ)中间体(INT1b)异构化成另一中间体(INT2),其势垒为22.31kJ/mol;(Ⅲ)中间体(INT2)异构化成四元环构型产物P2,其势垒为11.98kJ/mol.     

Accurate measurement of the relative bond energies of CO and H2O ligands in Fe+ mono- and bis-ligated complexes

The systems Fe(H(2)O)(n) (+)/CO[bond]H(2)O and Fe(CO)(n) (+)/CO[bond]H(2)O (n = 1 and 2) were investigated in a triple cell Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Using mixtures of CO with a very small amount of water, the ligand exchange equilibrium was reached, allowing experimental determination of the relevant equilibrium constants and free energies of reaction. Quantum chemical calculations at the B3LYP level of theory on the reactant and product species allowed us to determine the entropic terms and to derive the relative bond energies of CO and H(2)O in the mono- and bis-ligated complexes. For n = 1, H(2)O is more strongly bound to Fe(+) than CO by 4.1 +/- 1.6 kJ x mol(-1) at 298 K. For n = 2, at the same temperature, H(2)O is more strongly bound than CO to (H(2)O)Fe(+) by 7.6 +/- 1.6 kJ x mol(-1), and to (CO)Fe(+) by more than 20.1 kJ x mol(-1).     

On the hydrolysis mechanism of the second-generation anticancer drug carboplatin

The hydrolysis reaction mechanisms of carboplatin, a second-generation anticancer drug, have been explored by combining density functional theory (DFT) with the conductor-like dielectric continuum model (CPCM) approach. The decomposition of carboplatin in water is expected to take place through a biphasic mechanism with a ring-opening process followed by the loss of the malonato ligand. We have investigated this reaction in water and acid conditions and established that the number of protons present in the malonato ligand has a direct effect on the energetics of this system. Close observation of the optimised structures revealed a necessary systematic water molecule in the vicinity of the amino groups of carboplatin. For this reason we have also investigated this reaction with an explicit water molecule. From the computed potential-energy surfaces it is established that the water hydrolysis takes place with an activation barrier of 30 kcal mol(-1), confirming the very slow reaction observed experimentally. The decomposition of carboplatin upon acidification was also investigated and we have computed a 21 kcal mol(-1) barrier to be overcome (experimental value 23 kcal mol(-1)). We have also established that the rate-limiting process is the first hydration, and ascertained the importance of a water molecule close to the two amine groups in lowering the activation barriers for the ring-opening reaction.     

一种非血红素四氮杂轮烯配合物[Fe(Ⅲ)TMTAA]催化过氧化氢歧化反应机理的理论研究

采用密度泛函理论B3LYP方法计算了一种非血红素四氮杂轮烯配合物[Fe(Ⅲ)TMTAA]催化H2O2歧化的反应机理.对二重态、四重态和六重态势能面上各驻点进行了全优化,发现反应易于沿四重态势能面发生.整个反应分两阶段进行,第一阶段通过氧氧均裂形成中间体IM6和第一个水,第二阶段经两次氢转移形成第二个水.反应决速步骤为O—O均裂步骤,能垒为63.9kJ·mol-1,相对于自由H2O2均裂所需能垒226.7kJ·mol-1有较大的降低.这表明标题配合物可有效地降低标题反应的能垒,有可能作为一种潜在的过氧化氢仿酶.     

A comparative study of O2, CO,and NO binding to iron–porphyrin

Minimum-energy structures of O₂, CO, and NO iron–porphyrin (FeP) complexes, computed with the Car–Parrinello molecular dynamics, agree well with the available experimental data for synthetic heme models. The diatomic molecule induces a 0.3–0.4 Å displacement of the Fe atom out of the porphyrin nitrogen (N_p) plane and a doming of the overall porphyrin ring. The energy of the iron–diatomic bond increases in the order Fe(SINGLE BOND)O₂ (9 kcal/mol) < Fe(SINGLE BOND)CO (26 kcal/mol) < Fe(SINGLE BOND)NO (35 kcal/mol). The presence of an imidazole axial ligand increases the strength of the Fe(SINGLE BOND)O₂ and Fe(SINGLE BOND)CO bonds (15 and 35 kcal/mol, respectively), with few structural changes with respect to the FeP(CO) and FeP(O₂) complexes. In contrast, the imidazole ligand does not affect the energy of the Fe(SINGLE BOND)NO bond, but induces significant structural changes with respect to the FeP(NO) complex. Similar variations in the iron–imidazole bond with respect to the addition of CO, O₂, and NO are also discussed. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 69: 31–35, 1998