The structure, properties, and nature of HArF-HOX (X = F, Cl, Br) complex: an ab initio study and an unusual short hydrogen bond

The structure and properties (geometric, energetic, electronic, spectroscopic, and thermodynamic properties) of HArF‐HOX (X = F, Cl, Br) complex have been investigated at the MP2/aug‐cc‐pVTZ level. Three types of complexes are formed through a hydrogen bond or a halogen bond. The HArF‐HOX complex is the most stable, followed by the FArH‐OHX complex, and the HArF‐XOH complex is the most unstable. The binding distance in FArH‐OHX complex is very short (1.1–1.7 Å) and is smaller than that in HArF‐HOX complex. However, the interaction strength in the former is weaker than that in the latter. Thus, an unusual short hydrogen bond is present in FArH‐OHX complex. The associated H‐Ar bond exhibits a red shift, whereas the distant one gives a blue shift. A similar result is also found for the O? H and O? X bonds. The isotropic chemical shift is negative for the associated hydrogen atom but is positive for the associated halogen atom. However, a reverse result is found for the anisotropic chemical shift. The analyses of natural bond orbital and atoms in molecules have been performed for these complexes to understand the nature and properties of hydrogen and halogen bonds. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011     

C59H3N异构体的结构与稳定性的理论研究

分别用MNDO,AM1和PM3三种半经验方法对C59HN所有1-2,1-4和1-6氢加成物C59H3N的异构体进行几何构型全优化,结合频率分析及HF/6-31G单点能计算,确定了各异构体的基态结构及其相对稳定性,计算结果表明,C59HN氢加成物的立体选择性规律与C60和C60H2的不同,最稳定异构体不是1-2加成物,而是1-4加成的6,18-或12,15-异构体,次稳定异构体为1-2加成物,三种半经验方法计算得到的两者能量差为13～15kJ/mol,N原子取代碳笼骨架C原子后,改变了碳笼氢加成物的立体选择性规律.     

(MN)nHm(M=Ga,In; n=1-4; m=1, 2)团簇的结构与稳定性

采用密度泛函理论(DFT)的B3LYP方法, 在6-31G**和Lanl2dz水平上分别对(MN)nHm(M=Ga, In; n=1-4; m=1, 2)进行了优化和振动频率计算. 得到了上述团簇的最稳定构型、H原子的结合能以及它们的能隙. 结果表明, (MN)nH(M=Ga, In; n=1-4)的基态构型均为双重态, (MN)nH2(M=Ga, In; n=1-4)的基态构型均为单重态; 当氢的个数为1时, 加在N原子上比加在M(M=Ga, In)原子上稳定, 如有N3单元, 那么加在N3单元两侧的构型是相同的, 且它是最稳定的; 当氢的个数为2时, 除n=1外, 分别加在两个N原子上的构型是最稳定的, 如有N3单元, 那么分别加在N3单元分离最远的两个N原子的构型是最稳定的. GaNH、(GaN)3H 和InNH的结合能和能隙都很大, 说明这些团簇都有很高的稳定性.     

Rotational spectroscopy and molecular structure of the 1,1,2-trifluoroethylene-acetylene complex

Guided by ab initio calculations, Fourier transform microwave rotational spectra in the 6-22 GHz region are obtained for the complex formed between 1,1,2-trifluoroethylene and acetylene, including the normal isotopomer, three of four singly substituted (13)C species obtained in natural abundance, and using commercially available isotopic varieties of acetylene, species containing HCCD and H(13)C(13)CH. Although the ab initio calculations suggest two possible low energy planar arrangements for the molecules in the complex, only a single, unique structure is obtained from a combined analysis of the rotational constants derived from the spectra and atomic positions determined using Kraitchman [Am. J. Phys. 21, 17 (1953)] substitution coordinates. This structure is similar to that obtained for the CF(2)CHF[Single Bond]HF complex [H. O. Leung and M. D. Marshall, J. Chem. Phys. 126, 114310 (2007)] in which both the primary and secondary interactions occur between the HCCH molecule and a F atom and a H atom bonded to the same carbon of CF(2)CHF. The 2.748(15) A hydrogen bond has acetylene as the donor and 1,1,2-trifluoroethylene as the acceptor and forms a 104.49(15) degrees C[Single Bond]Fcdots, three dots, centeredH angle. The 2.8694(9) A secondary interaction between the pi bond of acetylene and the H atom geminal to the acceptor F atom causes the hydrogen bond to deviate 69.24(67) degrees from linearity. This large deviation from linearity and the similarity of the two intermolecular bond lengths suggest that the two interactions are becoming comparable in importance.     

Isomerization and oxygen atom transfer reactivity in oxo-Mo complexes of relevance to molybdoenzymes

Both dioxo Mo(VI) and mono-oxo Mo(V) complexes of a sterically restrictive N2O heteroscorpionate ligand are found to exist as cis and trans isomers. The thermodynamically stable isomer differs for the two oxidation states, but in each case, we have isolated the kinetically labile isomer and followed its isomerization to the thermodynamically stable form. The Mo(VI) complex is more stable in the cis geometry and isomerizes more than 6 times faster than the Mo(V) complex, which prefers the trans geometry. In OAT reactions with PPh3, the trans isomer of the dioxo-Mo(VI) reacts approximately 20 times faster than the cis isomer. Thus, there are both oxidation state and donor atom dependent differences in isomeric stability and reactivity that could have significant functional implications for molybdoenzymes such as DMSO reductase.     

Isomerization and fragmentation reactions of gaseous phenylarsane radical cations and phenylarsanyl cations. A study by tandem mass spectrometry and theoretical calculations

The unimolecular reactions of radical cations and cations derived from phenylarsane, C6H5AsH2 (1) and dideutero phenylarsane, C6H5AsD2 (1-d2), were investigated by methods of tandem mass spectrometry and theoretical calculations. The mass spectrometric experiments reveal that the molecular ion of phenylarsane, 1*+, exhibits different reactivity at low and high internal excess energy. Only at low internal energy the observed fragmentations are as expected, that is the molecular ion 1*+ decomposes almost exclusively by loss of an H atom. The deuterated derivative 1-d2 with an AsD2 group eliminates selectively a D atom under these conditions. The resulting phenylarsenium ion [C6H5AsH]+, 2+, decomposes rather easily by loss of the As atom to give the benzene radical cation [C6H6]*+ and is therefore of low abundance in the 70 eV EI mass spectrum. At high internal excess energy, the ion 1*+ decomposes very differently either by elimination of an H2 molecule, or by release of the As atom, or by loss of an AsH fragment. Final products of these reactions are either the benzoarsenium ion 4*+, or the benzonium ion [C6H7]+, or the benzene radical cation, [C6H6]*+. As key-steps, these fragmentations contain reductive eliminations from the central As atom under H-H or C-H bond formation. Labeling experiments show that H/D exchange reactions precede these fragmentations and, specifically, that complete positional exchange of the H atoms in 1*+ occurs. Computations at the UMP2/6-311+G(d)//UHF/6-311+G(d) level agree best with the experimental results and suggest: (i) 1*+ rearranges (activation enthalpy of 93 kJ mol(-1)) to a distinctly more stable (DeltaH(r)(298) = -64 kJ mol(-1)) isomer 1 sigma*+ with a structure best represented as a distonic radical cation sigma complex between AsH and benzene. (ii) The six H atoms of the benzene moiety of 1 sigma*+ become equivalent by a fast ring walk of the AsH group. (iii) A reversible isomerization 1+<==>1 sigma*+ scrambles eventually all H atoms over all positions in 1*+. The distonic radical cation 1*+ is predisposed for the elimination of an As atom or an AsH fragment. The calculations are in accordance with the experimentally preferred reactions when the As atom and the AsH fragment are generated in the quartet and triplet state, respectively. Alternatively, 1*(+) undergoes a reductive elimination of H2 from the AsH2 group via a remarkably stable complex of the phenylarsandiyl radical cation, [C6H5As]*+ and an H2 molecule.     

Synthesis of the silaisocyanoacetylene molecule

The hitherto elusive silaisocyanoacetylene molecule (HCCNSi)-a member of the silaisocyanide family-has been synthesized for the first time through the reaction of the silicon nitride radical (SiN) with acetylene (C(2)H(2)) in the gas phase under single collision conditions. Compared to the isoelectronic reaction of the cyano radical (CN) with acetylene, the replacement of the carbon atom in the cyano group by an isovalent silicon atom has a pronounced effect on the reactivity. Whereas the silicon nitride radical was found to pass an entrance barrier and adds with the nitrogen atom to the acetylene molecule, the cyano radical adds barrierlessly with the carbon atom forming the HCCH(NSi) and HCCH(CN) intermediates, respectively. These structures undergo hydrogen loss to form the linear silaisocyanoacetylene (HCCNSi) and cyanoacetylene molecules (HCCCN), respectively. Therefore, the isovalency of the silicon atom was found to bear little resemblance with the carbon atom having a dramatic effect not only on the reactivity, but also on the reaction mechanism, thermochemistry, and chemical bonding of the isoelectronic silaisocyanoacetylene and cyanoacetylene products, effectively reversing the thermodynamical stability of the nitrile versus isonitrile and silanitrile versus isosilanitrile isomer pairs.     

Rotational spectroscopy and molecular structure of the 1-chloro-1-fluoroethylene-acetylene complex

Guided by ab initio calculations, Fourier transform microwave spectra in the 6-21 GHz region are obtained for seven isotopomers of the complex formed between 1-chloro-1-fluoroethylene and acetylene. These include the four possible combinations of (35)Cl- and (37)Cl-containing CH(2)CClF with the most abundant acetylene isotopic modification, HCCH, and its H(13)C(13)CH analogue, as well as three singly substituted deuterated isotopomers. Analysis of the spectra determines the rotational constants and additionally, the complete chlorine quadrupole hyperfine coupling tensors in both the inertial and principal electric field gradient axis systems, and where appropriate, the diagonal components of the deuterium quadrupole coupling tensors. The inertial information contained in the rotational constants provides the structure for CH(2)CClF-HCCH: a primary, hydrogen bonding interaction existing between the HCCH donor and the F atom acceptor on the 1-chloro-1-fluoroethylene moiety, while a secondary interaction occurs between the acetylenic bond on the HCCH molecule and the H atom cis to the hydrogen-bonded F atom on the substituted ethylene, which causes the hydrogen bond to deviate from linearity. This is similar to the structure obtained for 1,1-difluoroethylene-HCCH [H. O. Leung and M. D. Marshall, J. Chem. Phys. 126, 154301 (2006)], and indeed, to within experimental uncertainty, the intermolecular interactions in CH(2)CClF-HCCH and its 1,1-difluoroethylene counterpart are practically indistinguishable, even though ab initio calculations at the MP2∕6-311G++(2d, 2p) level suggest that the former complex is more strongly bound.     

C60NH5/6, 6/6异构体的理论研究

用半经验的AM1, PM3及ab initio方法对C60NH两种异构体的结构及光谱进行了理论计算。结果表明, 具有开环结构的C60NH的5/6异构体稳定性要高于具有闭环结构的6/6异构体。计算了两种异构体开环与闭环过程的反应坐标, 发现6/6开环异构体是势能面上的一局部最小点, 而5/6闭环异构体不存在6/6异构体的H可以在两种镜面异构体之间快速翻转, 使其核磁共振谱呈现C2v对称性。通过振动分析确认了所优化的构型确实为势能面的能量最低点, 并得到了C60NH各异构体的红外光谱。     

Complexes of HArF and AuX (X = F,Cl, Br,I). Comparison of H-bonds,halogen bonds,F-shared bonds and covalent bonds

The various sorts of complexes in which HArF and AuX (X = F, Cl, Br, I) can engage are probed by MP2/aug-cc-pVTZ calculations. The most weakly bound are those containing a halogen bond (XB) of the AuX⋯FArH sort, with binding energies less than 8 kcal/mol. H-bonded dimers FArH⋯XAu are a little stronger, held together by some 12 kcal/mol. Being the most strongly bound places the F atom of HArF roughly midway between Ar and Au in an F-shaped structure, bound by some 43–54 kcal/mol. The last sort of product involves atomic rearrangements wherein the H atom migrates from Ar to Au, followed by formation of a covalent Ar–Au bond. The resulting molecular unit is stabilized by 30–40 kcal/mol relative to the original HArF and AuX reactants. The H-bonded dimers are held together by an unusually large polarization component, surpassing electrostatic attraction, while dispersion predominates for the halogen bonds. Perturbations of the geometries and stretching frequencies offer a ready means of distinguishing the different types of complexes by spectroscopic techniques.     

Infrared photodissociation spectroscopy of protonated acetylene and its clusters

The protonated acetylene cation, C2H3+, (also known as the vinyl cation) and the proton-bound acetylene dimer cation (C4H5+) are produced by a pulsed supersonic nozzle/pulsed electrical discharge cluster source. The parent ions are also generated with weakly attached argon "tag" atoms, e.g., C2H3+Ar and C4H5+Ar. These ions are mass selected in a specially designed reflectron time-of-flight mass spectrometer and studied with infrared laser photodissociation spectroscopy in the 800-3600 cm-1 region. Vibrational resonances are detected for both ions in the C-H stretching region. C2H3+ has a strong vibrational resonance near 2200 cm-1 assigned to the bridged proton stretch of the nonclassical ion, while C4H5+ has no such free-proton vibration. Instead, C4H5+ has resonances near 1300 cm-1, consistent with a symmetrically shared proton in a di-bridged structure. Although the shared proton structure is not the lowest energy isomer of C4H5+, this species is apparently stabilized under the supersonic beam conditions. Larger clusters containing additional acetylene units are also investigated via the elimination of acetylene. These species have new IR bands indicating that rearrangement reactions have taken place to produce core C4H5+ ions with the methyl cyclopropane cation structure and/or the protonated cyclobutadiene isomer. Ab initio (MP2) calculations provide structures and predicted spectra consistent with all of these experiments.     

Gold as hydrogen. An experimental and theoretical study of the structures and bonding in disilicon gold clusters Si2Au(n)- and Si2Au(n) (n = 2 and 4) and comparisons to Si2H2 and Si2H4

In a previous communication, we showed that a single Au atom behaves like H in its bonding to Si in a series of Si-Au clusters, SiAu(n) (n = 2-4) (Kiran et al. Angew. Chem., Int. Ed. 2004, 43, 2125). In this article, we show that the H analogy of Au is more general. We find that the chemical bonding and potential energy surfaces of two disilicon Au clusters, Si(2)Au(2) and Si(2)Au(4), are analogous to Si(2)H(2) and Si(2)H(4), respectively. Photoelectron spectroscopy and ab initio calculations are used to investigate the geometrical and electronic structures of Si(2)Au(2)(-), Si(2)Au(4)(-), and their neutral species. The most stable structures for both Si(2)Au(2) and Si(2)Au(2)(-) are found to be C(2)(v), in which each Au bridges the two Si atoms. For Si(2)Au(4)(-), two nearly degenerate dibridged structures in a cis (C(2)(h)) and a trans (C(2)(v)) configuration are found to be the most stable isomers. However, in the neural potential energy surface of Si(2)Au(4), a monobridged isomer is the global minimum. The ground-state structures of Si(2)Au(2)(-) and Si(2)Au(4)(-) are confirmed by comparing the computed vertical detachment energies with the experimental data. The various stable isomers found for Si(2)Au(2) and Si(2)Au(4) are similar to those known for Si(2)H(2) and Si(2)H(4), respectively. Geometrical and electronic structure comparisons with the corresponding silicon hydrides are made to further establish the isolobal analogy between a gold atom and a hydrogen atom.     

Fluxional processes in diamagnetic and paramagnetic allyl dicarbonyl and 2-methylallyl dicarbonyl molybdenum histidinato complexes as revealed by spectroscopic data and density functional calculations

This work describes a detailed study on the structure and dynamics of pseudooctahedral low-valent complexes of the type [Mo(His-N(epsilon)-R)(eta-2-R'-allyl)(CO)(2)] (His=N(delta),N,O-L-histidinate; R=H, R'=H (1); R=C(2)H(4)CO(2)Me, R'=H (2); R=H, R'=Me (3); R=C(2)H(4)CO(2)Me, R'=Me (4)). These diamagnetic 18-electron complexes were comprehensively characterized spectroscopically and by X-ray crystallography. In the solid state, the (substituted) allyl ligand is in an endo position in all compounds, but it is trans to the His-N(delta) atom in 1 and 2, whereas it is trans to the carboxylate O atom for the 2-Me-allyl compounds 3 and 4. In solution, both isomers are present in a solvent-dependent equilibrium. The third isomer (allyl trans to His-NH(2)) is not spectroscopically observed in solution. This is in agreement with the results from density functional (DFT) computations (BPW 91 functional) for 1 and 3, which predict a considerably higher energy (+6.3 and +5.9 kJ mol(-1), respectively) for this isomer. A likely path for isomerization is calculated, which is consistent with the activation energy determined by variable temperature NMR measurements. At least for 3, the preferred path involves several intermediates and a rotation of the 2-Me-allyl ligand. For the paramagnetic 17-electron congeners, DFT predicts the exo isomer of 3(+) with the 2-Me-allyl ligand trans to the carboxylate O atom to be by far the most stable isomer. For 1(+), an endo-exo equilibrium between the isomers with the allyl ligand trans to the carboxylate O atom is suggested. These suggestions are confirmed by EPR spectroscopy on the electrochemically generated species, which show signals for one- (4) and two- (2) metal-containing compounds. The appearance of the EPR spectra may be rationalized by inspection of the SOMOs from DFT calculations of the species in question. The notion of a metal-centered oxidation is also substantiated by IR spectroelectrochemistry and by UV/Vis spectra of the 17-electron complexes. Upon depleting the metal of electron density, the stretching vibrations of the carbonyl ligands shift more than 100 cm(-1) to higher wavenumbers, and the carbonyl vibration of the metal-coordinated carboxylate shifts by about 50 cm(-1). A color change from yellow to green upon oxidation is observed visually and quantified by the appearance of a new band at 622 nm (2(+)) and 546 nm (4(+)), respectively.     

Hydrogen-atom tunneling in isomerization around the C-O bond of 2-chloro-6-fluorophenol in low-temperature argon matrixes

Infrared spectra of 2-chloro-6-fluorophenol in argon matrixes at 20 K revealed the presence of a "Cl-type" isomer, which has the OH···Cl hydrogen bond, but no "F-type" isomer with OH···F bonding, in striking contrast to the existence of both isomers in the gas and liquid phases at room temperature. This finding suggests that the F-type isomer changes to the more stable Cl-type one by hydrogen-atom tunneling in the matrixes. Similar experiments on the OD···X analog species were performed to confirm the tunneling isomerization, resulting in an O-D stretching band of the F-type isomer appearing as well as that of the Cl type, like the spectra reported in the gas and liquid phases. This implies that tunneling migration of the D atom is inhibited in the argon matrix. In addition, UV-induced photoreactions of 2-chloro-6-fluorophenol were studied by a joint use of matrix-isolation IR spectroscopy assisted by density functional theory calculations. It was found that 2-fluorocyclopentadienylidenemethanone and 4-chloro-2-fluorocyclohexadienone were produced from the Cl type; the former was by the Wolff rearrangement after dissociation of the H atom in the OH group and the Cl atom, and the latter was by intramolecular migration of the H and Cl atoms. As for the deuterated F-type isomer, however, 2-chlorocyclopentadienylidenemethanone was produced by the Wolff rearrangement after dissociation of the D atom in the OD group and the F atom, besides other photoproducts of the deuterated Cl-type isomer. It is thus concluded that the tunneling isomerization around the C-O bond occurs in the OH···X species but not in the OD···X species.     

Intramolekulare Beweglichkeit eines Dibenzo-[b,f]-1,4-thiazepin-5-oxids. 13. Mitteilung über siebengliedrige Heterocyclen

The kinetics of the equilibrium in aqueous solution between the diasteromers of 2-Chloro-11-(4-methyl-1-piperazinyl-4-oxide)-dibenzo-[b, f]-1,4-thiazepine-5-oxide (V) have been investigated. Both isomers differ in the position of the oxygen atom of the sulphoxide group. The sulphoxide group. The isomer with a pseudo-equatorial oxygen atom is 0.75 kcal/mol more stable than the isomer with the pseudo-axial oxygen atom. The calculated free energies of activation δG≠ are 24.6 kcal/mol and 23.9 kcal/mol.     

Influence of single impurity atoms on the structure, electronic, and magnetic properties of Ni5 clusters

With a gradient-corrected density functional method, we have studied computationally the influence of single impurity atoms on the structure, electronic, and magnetic properties of Ni5 clusters. The square-pyramidal isomer of bare Ni5 with six unpaired electrons was calculated 23 kJ/mol more stable than the trigonal bipyramid in its lowest-energy electronic configuration with four unpaired electrons. In a previous study on the cluster Ni4, we had obtained only one stable isomer with an O or an H impurity, but we located six minima for ONi5 and five minima for HNi5. In the most stable structures of HNi5, the H atom bridges a Ni-Ni edge at the base or the side of the square pyramid, similarly to the coordination of an H atom at the tetrahedral cluster Ni4. The most stable ONi5 isomers exhibit a trigonal bipyramidal structure of the Ni5 moiety, with the impurity coordinated at a facet, (micro3-O)Ni5, or at an apex edge, (micro-O)Ni5. We located four stable structures for a C impurity at a Ni5 cluster. As for CNi4, the most stable structure of the corresponding Ni5 complex comprises a four-coordinated C atom, (micro4-C)Ni5, and can be considered as insertion of the impurity into a Ni-Ni bond of the bare cluster. All structures with C and five with O impurity have four unpaired electrons, while the number of unpaired electrons in the clusters HNi5 varies between 3 and 7. As a rough trend, the ionization potentials and electron affinities of the clusters with impurity atoms decrease with the coordination number of the impurity. However, the position of the impurity and the shape of the metal moiety also affect the results. Coordination of an impurity atom leads to a partial oxidation of the metal atoms.     

C-H···π interactions in the CHBrF2···HCCH weakly bound dimer

The microwave spectra of four isotopologues of the CHBrF(2)···HCCH weakly bound dimer have been measured in the 6-18 GHz region using chirped-pulse and Balle-Flygare Fourier-transform microwave spectroscopy. Spectra of (13)CH(79)BrF(2) and (13)CH(81)BrF(2) monomers have also been measured, and spectroscopic constants are reported. Measurement of spectra for the (79)Br and (81)Br isotopologues of CHBrF(2) complexed with both (12)C(2)H(2) and (13)C(2)H(2) have allowed the determination of a structure with C(s) symmetry for this complex. CHBrF(2) interacts with the triple bond of acetylene via a C-H···π contact (R(H···π) = 2.670(8) ?) with the Br atom lying in the ab plane, located 3.293(40) ? from a hydrogen atom of the HCCH molecule. The structure of CHBrF(2)···HCCH has been compared with recently studied related acetylene complexes, including a comparison with (and further structural analysis of) the CHClF(2)···HCCH complex.     

Observation of Au2H- impurity in pure gold clusters and implications for the anomalous Au-Au distances in gold nanowires

Au(2)H(-) was recognized and confirmed as a minor contamination to typical photoelectron spectra of Au(2) (-), produced by laser vaporization of a pure Au target using an ultrahigh purity helium carrier gas. The hydrogen source was shown to be from trace H impurities present in the bulk gold target. Carefully designed experiments using H(2)- and D(2)-seeded helium carrier gas were used to study the electronic structure of Au(2)H(-) and Au(2)D(-) using photoelectron spectroscopy and density functional calculations. Well-resolved photoelectron spectra with vibrational resolution were obtained for Au(2)H(-) and Au(2)D(-). Two isomers were observed both experimentally and theoretically. The ground state of Au(2)H(-) turned out to be linear with a terminal H atom [Au-Au-H](-) ((1)A(1),C(infinity v)), whereas a linear [Au-H-Au](-) ((1)A(1),D(infinity h)) structure with a bridging H atom was found to be a minor isomer 0.6 eV higher in energy. Calculated electron detachment energies for both isomers agree well with the experimental spectra, confirming their existence in the cluster beam. The observation and confirmation of H impurity in pure gold clusters and the 3.44 A Au-Au distance in the [Au-H-Au](-) isomer presented in the current work provide indirect experimental evidence that the anomalous 3.6 A Au-Au distances observed in gold nanowires is due to an "invisible" hydrogen impurity atom.     

Insights into the mechanism of electrocatalytic hydrogen evolution mediated by Fe2(S2C3H6)(CO)6: the simplest functional model of the Fe-hydrogenase active site

The di-iron complex Fe2(S2C3H6)(CO)6 (a), one of the simplest functional models of the Fe-hydrogenases active site, is able to electrocatalyze proton reduction. In the present study, the H2 evolving path catalyzed by a has been characterized using density functional theory. It is showed that, in the early stages of the catalytic cycle, a neutral mu-H adduct is formed; monoelectron reduction and subsequent protonation can give rise to a diprotonated neutral species (a-muH-SH), which is characterized by a mu-H group, a protonated sulfur atom, and a CO group bridging the two iron centers, in agreement with experimental IR data indicating the formation of a long-lived mu7-CO species. H2 release from a-muH-SH, and its less stable isomer a-H2 is kinetically unfavorable, while the corresponding monoanionic compounds (a-muH-SH- and a-H2-) are more reactive in terms of dihydrogen evolution, in agreement with experimental data. The key species involved in electrocatalysis have structural features different from the hypothetical intermediates recently proposed to be involved in the enzymatic process, an observation that is possibly correlated with the reduced catalytic efficiency of the biomimetic di-iron assembly.