Doublet Spin States Research Articles

Reductions of oxygen, carbon dioxide, and acetonitrile by the magnesium(II)/magnesium(I) couple in aqueous media: theoretical insights from a nano-sized water droplet.

Reductions of O2, CO2, and CH3CN by the half-reaction of the Mg(II)/Mg(I) couple (Mg(2+) + e(-) → Mg(+•)) confined in a nanosized water droplet ([Mg(H2O)16](•+)) have been examined theoretically by means of density functional theory based molecular dynamics methods. The present works have revealed many intriguing aspects of the reaction dynamics of the water clusters within several picoseconds or even in subpicoseconds. The reduction of O2 requires an overall doublet spin state of the system. The reductions of CO2 and CH3CN are facilitated by their bending vibrations and the electron-transfer processes complete within 0.5 ps. For all reactions studied, the radical anions, i.e., O2(•-), CO2(•-), and CH3CN(•-), are initially formed on the cluster surface. O2(•-) and CO2(•-) can integrate into the clusters due to their high hydrophilicity. They are either solvated in the second solvation shell of Mg(2+) as a solvent-separated ion pair (ssip) or directly coordinated to Mg(2+) as a contact-ion pair (cip) having the (1)η-[MgO2](•+) and (1)η-[MgOCO](•+) coordination modes. The (1)η-[MgO2](•+) core is more crowded than the (1)η-[MgOCO](•+) core. The reaction enthalpies of the formation of ssip and cip of [Mg(CO2)(H2O)16](•+) are -36 ± 4 kJ mol(-1) and -30 ± 9 kJ mol(-1), respectively, which were estimated based on the average temperature changes during the ion-molecule reaction between CO2 and [Mg(H2O)16](•+). The values for the formation of ssip and cip of [Mg(O2)(H2O)16](•+) are estimated to be -112 ± 18 kJ mol(-1) and -128 ± 28 kJ mol(-1), respectively. CH3CN(•-) undergoes protonation spontaneously to form the hydrophobic [CH3CN, H](•). Both CH3CN and [CH3CN, H](•) cannot efficiently penetrate into the clusters with activation barriers of 22 kJ mol(-1) and ∼40 kJ mol(-1), respectively. These results provide fundamental insights into the solvation dynamics of the Mg(2+)/Mg(•+) couple on the molecular level.

Computational biotransformation profile of paracetamol catalyzed by cytochrome P450.

The P450-catalyzed biotransformation of the analgesic drug paracetamol (PAR) is a long-debated topic, involving different mechanistic hypotheses as well as experimental evidence for the metabolites N-acetyl-p-benzoquinone imine (NAPQI), p-benzoquinone, acetamide, and 3-hydroxy-PAR. During the catalytic cycle of P450, a high-valent iron(IV)-oxo species known as Compound I (Cpd I) is formed as the ultimate oxidant, featuring two energetically close-lying ground states in the doublet (low-spin) and quartet (high-spin) spin states, respectively. In order to clarify the catalytic mechanism, a computational chemistry analysis has been undertaken for both the high- and low-spin routes, employing density functional theory (DFT) including PCM (polarized continuum-solvation model) that yields an approximate simulation of the bulk polarization exerted through the protein. The results demonstrate that hydrogen abstraction transfer (HAT) by the P450 oxidant Cpd I (FeO) is kinetically strongly preferred over the alternative pathways of an oxygen addition reaction (OAR) or two consecutive single-electron transfers (SET). Moreover, only the respective high-spin route yields N-acetyl-p-semiquinone imine (NAPSQI) as an intermediate that is converted to the electrophile N-acetyl-p-benzoquinone imine (NAPQI). By contrast, 3-hydroxy-PAR, acetamide, and p-benzoquinone as electrophilic and redox-active agent are formed predominantly in the low-spin state through reactions that do not involve NAPSQI. Thus, all experimentally observed PAR metabolites are in accord with an initial HAT from the phenolic oxygen, and NAPSQI should indeed be the precursor of NAPQI, both of which are generated only via the high-spin pathway.

Structure and bonding analysis of intermediate model heme-imidazole and heme-thiolate enzymes complexed with formate, acetate and nitrate: A theoretical study

Structure and bonding analysis of the FeO bonds in the model heme-imidazole and heme-thiolate complexes [(por)Fe(Im)(R)] (R=HC(O)O−, 1FeIm, R=H3CC(O)O−, 2FeIm, R=NO3−, 3FeIm) and [(por)Fe(SMe))(R)]− (R=HC(O)O−, 4FeSMe, R=H3CC(O)O−, 5FeSMe, R=NO3−, 6FeSMe) were investigated at the DFT/TZP level of theory using density functionals BP86, PW91, PBE, TPSS and B3LYP. The FeO bond distances at BP86/TZP in the complexes [(por)Fe(Im)(R)] (1.858Å in 1FeIm, 1.868Å in 2FeIm and 1.919Å in 3FeIm) and [(por)Fe(SMe)(R)]− (1.955Å in 4FeSMe, 1.981Å in 5FeSMe and 1.991Å in 6FeSMe) are longer than those expected for a FeO single bond estimated on the basis of covalent radii predictions (FeO=1.79Å). The FeO bond distances in [(por)Fe(Im)(R)] 1FeIm–3FeIm are shorter than those in [(por)Fe(SMe)(R)]−4FeSMe–6FeSMe. The thiolate is more trans-directing ligand than the imidazole ligand. The results of optimized geometries, NPA charges, Wiberg bond indices, NHO analysis and BDEs clearly indicate that the formate, acetate and nitrate ligands bind more weakly in heme-thiolate complexes than in heme-imidazole complexes. The reaction intermediate of chloroperoxidase enzyme would be more reactive with organic substrates than the myeloperoxidase (MPO) enzyme. The quartet and the sextet spin states are less stable than the doublet spin state. The optimized bond distances are shortest for doublet spin state.

Complete active space second order perturbation theory (CASPT2) study of N(²D) + H₂O reaction paths on D₁ and D₀ potential energy surfaces: direct and roaming pathways.

We report reaction paths starting from N((2)D) + H2O for doublet spin states, D0 and D1. The potential energy surfaces are explored in an automated fashion using the global reaction route mapping strategy. The critical points and reaction paths have been fully optimized at the complete active space second order perturbation theory level taking all valence electrons in the active space. In addition to direct dissociation pathways that would be dominant, three roaming processes, two roaming dissociation, and one roaming isomerization: (1) H2ON → H-O(H)N → H-HON → NO((2)Π) + H2, (2) cis-HNOH → HNO-H → H-HNO → NO + H2, (3) H2NO → H-HNO → HNO-H → trans-HNOH, are confirmed on the D0 surface.

Density functional theory calculations on oxidative C-C bond cleavage and N-O bond formation of [Ru(II)(bpy)2(diamine)](2+) via reactive ruthenium imide intermediates.

DFT calculations are performed on [Ru(II)(bpy)2(tmen)](2+) (M1, tmen = 2,3-dimethyl-2,3-butanediamine) and [Ru(II)(bpy)2(heda)](2+) (M2, head = 2,5-dimethyl-2,5-hexanediamine), and on the oxidation reactions of M1 to give the C-C bond cleavage product [Ru(II)(bpy)2(NH=CMe2)2](2+) (M3) and the N-O bond formation product [Ru(II)(bpy)2(ONCMe2CMe2NO)](2+) (M4). The calculated geometrical parameters and oxidation potentials are in good agreement with the experimental data. As revealed by the DFT calculations, [Ru(II)(bpy)2(tmen)](2+) (M1) can undergo oxidative deprotonation to generate Ru-bis(imide) [Ru(bpy)2(tmen-4 H)](+) (A) or Ru-imide/amide [Ru(bpy)2(tmen-3 H)](2+) (A') intermediates. Both A and A' are prone to C-C bond cleavage, with low reaction barriers (ΔG(≠)) of 6.8 and 2.9 kcal mol(-1) for their doublet spin states (2)A and (2)A', respectively. The calculated reaction barrier for the nucleophilic attack of water molecules on (2)A' is relatively high (14.2 kcal mol(-1)). These calculation results are in agreement with the formation of the Ru(II)-bis(imine) complex M3 from the electrochemical oxidation of M1 in aqueous solution. The oxidation of M1 with Ce(IV) in aqueous solution to afford the Ru(II)-dinitrosoalkane complex M4 is proposed to proceed by attack of the cerium oxidant on the ruthenium imide intermediate. The findings of ESI-MS experiments are consistent with the generation of a ruthenium imide intermediate in the course of the oxidation.

Correlating net charges and the activity of bis(imino)pyridylcobalt complexes in ethylene polymerization

A series of unsymmetrical bis(imino)pyridylcobalt complexes was investigated by DFT–QEq method to correlate the net charges of cobalt center and their catalytic activities. The effective net charge values on cobalt atom, calculated by QEq method at doublet spin state, reflected the influences of substituents within their ligands. The catalytic activities of these cobalt complexes in ethylene polymerization continuously increase along with higher values of the effective net charges on cobalt atom. The energy differences at doublet and quartet states kept the constant tendency for the cobalt complexes without the steric influences of substituents within ligands.

Infrared multiple photon dissociation spectroscopy of a gas-phase oxo-molybdenum complex with 1,2-dithiolene ligands.

Electrospray ionization (ESI) in the negative ion mode was used to create anionic, gas-phase oxo-molybdenum complexes with dithiolene ligands. By varying ESI and ion transfer conditions, both doubly and singly charged forms of the complex, with identical formulas, could be observed. Collision-induced dissociation (CID) of the dianion generated exclusively the monoanion, while fragmentation of the monoanion involved decomposition of the dithiolene ligands. The intrinsic structure of the monoanion and the dianion were determined by using wavelength-selective infrared multiple-photon dissociation (IRMPD) spectroscopy and density functional theory calculations. The IRMPD spectrum for the dianion exhibits absorptions that can be assigned to (ligand) C=C, C–S, C—C≡N, and Mo=O stretches. Comparison of the IRMPD spectrum to spectra predicted for various possible conformations allows assignment of a pseudo square pyramidal structure with C2v symmetry, equatorial coordination of MoO2+ by the S atoms of the dithiolene ligands, and a singlet spin state. A single absorption was observed for the oxidized complex. When the same scaling factor employed for the dianion is used for the oxidized version, theoretical spectra suggest that the absorption is the Mo=O stretch for a distorted square pyramidal structure and doublet spin state. A predicted change in conformation upon oxidation of the dianion is consistent with a proposed bonding scheme for the bent-metallocene dithiolene compounds [Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc.1976, 98, 1729−1742], where a large folding of the dithiolene moiety along the S···S vector is dependent on the occupancy of the in-plane metal d-orbital.

Synthesis of 2D polymeric dicyanamide bridged hexa-coordinated Cu(II) complex: Structural characterization, spectral studies and TDDFT calculation

A rare 2D polymeric dicyanamide bridged hexa-coordinated copper(II) complex [Cu(L1′)(μ1,5-dca)2]n (1) (L1′=2-carboxypyrazine) has been synthesized from the reaction of Cu(NO3)2⋅6H2O, 2-pyrazinecarbonitrile (L1) and sodium dicyanamide (Nadca) in methanolic medium. Single crystal X-ray analysis reveals that the complex has a 2D infinite zigzag chain structure in which copper(II) ions are bridged by single dicyanamide ligand in an end-to-end fashion. Such 2-carboxypyrazine can be obtained on the way of metal-assisted nitrile hydrolysis which well connected with Cu(NO3)2⋅6H2O and dicyanamide (dca) to give rare 2D Cu(II) polymeric complex due to the flexibility in the coordination ability of the copper(II) ions within the polymeric chain. The geometry of the asymmetric unit of the complex was optimized in singlet state by DFT method with multilayer ONIOM model at doublet spin state accordance with repeating asymmetric unit only. The electronic spectrum of the complex is explained using TDDFT calculation.

Redox chemistry of nickel(II) complexes supported by a series of noninnocent β-diketiminate ligands.

Nickel complexes of a series of β-diketiminate ligands ((R)L(-), deprotonated form of 2-substituted N-[3-(phenylamino)allylidene]aniline derivatives (R)LH, R = Me, H, Br, CN, and NO2) have been synthesized and structurally characterized. One-electron oxidation of the neutral complexes [Ni(II)((R)L(-))2] by AgSbF6 or [Ru(III)(bpy)3](PF6)3 (bpy = 2,2'-bipyridine) gave the corresponding metastable cationic complexes, which exhibit an EPR spectrum due to a doublet species (S = 1/2) and a characteristic absorption band in near IR region ascribable to a ligand-to-ligand intervalence charge-transfer (LLIVCT) transition. DFT calculations have indicated that the divalent oxidation state of nickel ion (Ni(II)) is retained, whereas one of the β-diketiminate ligands is oxidized to give formally a mixed-valence complex, [Ni(II)((R)L(-))((R)L(•))](+). Thus, the doublet spin state of the oxidized cationic complex can be explained by taking account of the antiferromagnetic interaction between the high-spin nickel(II) ion (S = 1) and the organic radical (S = 1/2) of supporting ligand. A single-crystal structure of one of the cationic complexes (R = H) has been successfully determined to show that both ligands in the cationic complex are structurally equivalent. On the basis of theoretical analysis of the LLIVCT band and DFT calculations as well as the crystal structure, the mixed-valence complexes have been assigned to Robin-Day class III species, where the radical spin is equally delocalized between the two ligands to give the cationic complex, which is best described as [Ni(II)((R)L(0.5•-))2](+). One-electron reduction of the neutral complexes with decamethylcobaltocene gave the anionic complexes when the ligand has the electron-withdrawing substituent (R = CN, NO2, Br). The generated anionic complexes exhibited EPR spectra due to a doublet species (S = 1/2) but showed no LLIVCT band in the near-IR region. Thus, the reduced complexes are best described as the d(9) nickel(I) complexes supported by two anionic β-diketiminate ligands, [Ni(I)((R)L(-))2](-). This conclusion was also supported by DFT calculations. Substituent effects on the electronic structures of the three oxidation states (neutral, cationic, and anionic) of the complexes are systematically evaluated on the basis of DFT calculations.

ChemInform Abstract: Developments in the Biomimetic Chemistry of Cubane‐Type and Higher Nuclearity Iron‐Sulfur Clusters

AbstractReview: 169 refs.

Bis(methylborabenzene) sandwich compounds of the first row transition metals

The bis(methylborabenzene)metal sandwich compounds (η6-C5H5BCH3)2M of the first row transition metals from Ti to Ni have been examined by density functional theory. The four of these seven methylborabenzene sandwich compounds that have been synthesized, namely the V, Cr, Fe, and Co derivatives, are predicted to have quartet, triplet, singlet, and doublet spin states, respectively, in accord with experiment. For (η6-C5H5BCH3)2Ti the singlet spin state structure lies only ∼2 kcal/mol below the triplet spin state structure. This energetic relationship is reversed for (η6-C5H5BCH3)2Ni for which the triplet spin state structure lies ∼2 kcal/mol in energy below the singlet spin state structure. Similarly the sextet (η6-C5H5BCH3)2Mn spin state structure lies only ∼2 kcal/mol below the corresponding doublet spin state structures. These observations suggest possible spin-crossover behavior for these three still unknown (η6-C5H5BCH3)2M (M = Ti, Mn, Ni) systems. The M–C and M–B distances from the metal to the methylborabenzene ligand increase for a given (η6-C5H5BCH3)2M system as the spin multiplicity is increased. This suggests a weakening of the metal–ligand bond with an increase in the spin multiplicity. The energy surfaces of all of these (η6-C5H5BCH3)2M sandwich compounds are complicated by multiple local minima differing only by the rotation of one methylborabenzene ring relative to the other such ring. The energy differences between these rotamers for the same spin state are generally less than the energy differences between different spin states for a given (η6-C5H5BCH3)2M sandwich compound.

C603– versus C604– /C602– – Synthesis and Characterization of Five Salts Containing Discrete Fullerene Anions

AbstractFive new compounds, [Rb(18crown‐6)]3[C60] (1), [Rb(18crown‐6)]6[C60]2(C3H7NO)2(C4H8O)2 (2), [Rb(benzo18crown‐6)]6[C60]2(C2H8N2)5 (3), [Cs(benzo18crown‐6)]3C60(C2H8N2)2 (4), and [Cs3(benzo18crown‐6)5]C60(C2H8N2)(4.5+x) (5) were synthesized and characterized by single‐crystal X‐ray structure determination. All compounds contain discrete C60 anions, which are ordered in 1, 2, and 4, where direct cation‐anion contacts occur. The unit cells of 1 and 2 contain two independent fullerides, which coordinate to the rubidium atoms either of two or of four [Rb(18crown‐6)] units. Owing to the presence of differently coordinated fullerene units in compounds 1 and 2, a possible disproportionation of C603– into C602– and C604– anions is discussed. In 3 and 4 the C60 anions are coordinated by three Rb and Cs atoms, respectively. In all compounds the average charge of the anion is –3. Magnetic data reveal a doublet spin state for 3. The EPR spectra are discussed for compounds 3 and 5. The role of a dynamic Jahn‐Teller distortion is discussed, and we report the first IR spectroscopic data of fullerene trianions, which have been obtained in solution.

Ruthenium, Rhodium, Osmium, and Iridium Complexes of Osazones (Osazones = Bis-Arylhydrazones of Glyoxal): Radical versus Nonradical States

Phenyl osazone (L(NHPh)H2), phenyl osazone anion radical (L(NHPh)H2(•-)), benzoyl osazone (L(NHCOPh)H2), benzoyl osazone anion radical (L(NHCOPh)H2(•-)), benzoyl osazone monoanion (L(NCOPh)HMe(-)), and anilido osazone (L(NHCONHPh)HMe) complexes of ruthenium, osmium, rhodium, and iridium of the types trans-[Os(L(NHPh)H2)(PPh3)2Br2] (3), trans-[Ir(L(NHPh)H2(•-))(PPh3)2Cl2] (4), trans-[Ru(L(NHCOPh)H2)(PPh3)2Cl2] (5), trans-[Os(L(NHCOPh)H2)(PPh3)2Br2] (6), trans- [Rh(L(NHCOPh)H2(•-))(PPh3)2Cl2] (7), trans-[Rh(L(NHCOPh)HMe(-))(PPh3)2Cl]PF6 ([8]PF6), and trans-[Ru(L(NHCONHPh)HMe)(PPh3)2Cl]Cl ([9]Cl) have been isolated and compared (osazones = bis-arylhydrazones of glyoxal). The complexes have been characterized by elemental analyses and IR, mass, and (1)H NMR spectra; in addition, single-crystal X-ray structure determinations of 5, 6, [8]PF6, and [9]Cl have been carried out. EPR spectra of 4 and 7 reveal that in the solid state they are osazone anion radical complexes (4, gav = 1.989; 7, 2.028 (Δg = 0.103)), while in solution the contribution of the M(II) ions is greater (4, gav = 2.052 (Δg = 0.189); 7, gav = 2.102 (Δg = 0.238)). Mulliken spin densities on L(NHPh)H2 and L(NHCOPh)H2 obtained from unrestricted density functional theory (DFT) calculations on trans-[Ir(L(NHPh)H2)(PMe3)2Cl2] (4(Me)) and trans-[Rh(L(NHCOPh)H2)(PMe3)2Cl2] (7(Me)) in the gas phase with doublet spin states authenticated the existence of L(NHPh)H2(•-) and L(NHCOPh)H2(•-) anion radicals in 4 and 7 coordinated to iridium(III) and rhodium(III) ions. DFT calculations on trans-[Os(L(NHPh)H2)(PMe3)2Br2] (3(Me)), trans-[Os(L(NHCOPh)H2)(PMe3)2Br2] (6(Me)), and trans-[Ru(L(NHCONHPh)HMe(-))(PMe3)2Cl] [9(Me)](+) with singlet spin states established that the closed-shell singlet state (CSS) solutions of 3, 5, 6, and [9]Cl are stable. The lower value of M(III)/M(II) reduction potentials and lower energy absorption bands corroborate the higher extent of mixing of d orbitals with the π* orbital in the case of 3 and 6. Time-dependent (TD) DFT calculations elucidated the MLCT as the origin of the lower energy absorption bands of 3, 5, and 6 and π → π* as the origin of transitions in 4 and 7.

Developments in the biomimetic chemistry of cubane-type and higher nuclearity iron-sulfur clusters.

A prior thematic issue of Chemical Reviews in 20041 provides broad coverage of the field of biomimetic inorganic chemistry. One principal objective of this field, which is a component of the continually burgeoning multidisciplinary enterprise that is bioinorganic chemistry, is the synthesis of analogues of mononuclear and polynuclear sites in proteins which convey information of significance in interpreting the physical and chemical properties of such sites. This article focuses on polynuclear analogues, specifically higher nuclearity, biomimetic metal-sulfur clusters. Many synthetic and biological clusters can be designated as either strong-field or weak-field. Strong-field clusters are formed by first transition series elements with π-acceptor ligands and by second and third series elements regardless of ligation, and manifest properties arising from large splittings of the d-orbital manifold at individual metal sites. Such species are subsumed under a restrictive definition of clusters as containing two or more metal atoms where direct and substantial metal-metal bonding is present.2 A broader definition now normally employed leads to recognition of weak-field clusters. These species contain σ/π-donor ligands that induce smaller d-orbital splittings favoring individual metal sites with high-spin configurations, magnetic interactions among these individual sites, paramagnetic molecular ground states, and labile ligand binding. These clusters contain first transition series elements. Prominent examples of metals in native weak-field cluster sites include (but are not limited to) Mn3–6 (catalases, photosystem II), non-heme Fe7–10 (O2 carriers, oxygenases, reductases, hydrogenase), Fe-S11–14 (electron transfer, nitrogenase, numerous nonredox functions), Ni15,16 (urease), and Ni-Fe17–19 (hydrogenase). The only exceptions to the weak-field designation are Fe sites in [FeFe]- and [NiFe]-hydrogenases, which occur as the fragments Fe(CO)(CN)(μ2-CO) and Fe(CO)(CN)2(μ2-H), respectively. We also note that the weak-field/strong field distinction does not strictly apply to zinc and copper complexes because ZnII and CuI are necessarily diamagnetic, CuII has a spin-doublet ground state, and CuIII is uniformly diamagnetic. However, all but CuIII manifest the substitutional lability associated with the weak-field case. As examples of weak-field clusters, structures of selected protein-bound sites 1–8 containing Mn, non-heme Fe, Fe-S, Ni, and Ni-Fe (Ni site) are provided in Figure 1. Because of the restricted scope of this article, a selected bibliography of summary accounts of the biomimetic chemistry of mono- and polynuclear sites is presented in Table 1. Citations are in the period 2004–2013 and do not include the contents of the previous thematic issue.1 We note in particular a book devoted to bioinorganic synthesis,20 a treatment of biosynthetic inorganic chemistry involving manipulation of protein-bound sites,21 and biomimetic research on sulfur-ligated sites.22 Other examples of weak-field metal sites in proteins are found in this issue. Figure 1 Schematic structures of illustrative weak-field protein-bound clusters: O2-evolving center in photosystem II (1), [FeFe]-hydrogenase (2), [NiFe]-hydrogenase (3); dinuclear (4), trinuclear cuboidal (5), and tetranuclear cubane-type (6) iron-sulfur clusters; ... Table 1 Selected Bibliography of Biomimetic Inorganic Chemisty: 2004–2013 The purview of this article is the biomimetic chemistry of metal-sulfur clusters as confined to post-2004 advances of homo- and heterometallic iron-sulfur clusters of nuclearity four and higher. The emphasis is on the synthetic approaches to such clusters, some of which in their native condition are implicated directly in numerous enzymological processes.

Electronic properties of the low-lying spin states of dimethylnitrosamine coordinated to Fe(III) heme models: An ab initio study

The unusual O-coordination mode of nitrosamines to Fe(III) heme models has been observed in the bis(dimethylnitrosamine)(meso-tetraphenylporphyrinate)iron(III) cation. For the first time, this latter as well as the simpler bis(dimethylnitrosamine)(porphinate)iron(III) heme model cations have been studied through ab initio methods. The sextet, quartet, and doublet spin states of both cations have been studied through single-point calculations based on the experimental (X-ray) geometry. Their energies, charges, and spin densities have been analyzed. The obtained results (at the UHF/cc-pVDZ and ROHF/cc-pVDZ levels) indicate that the peripheral benzene rings are of secondary importance for the coordination of dimethylnitrosamine to the Fe(III) porphyrin core. The obtained energy ordering is sextet < quartet < doublet, at all computational levels. The UHF, ROHF, and UMP2 results indicate an excess of alpha spin density around the Fe atom, a low covalency for the FeO bond and a substantial charge transfer to the Fe atom. Our best estimates [obtained at ROMP2 level with the mixed cc-pVDZ/cc-pVTZ-DK(Fe) basis set] for the energy differences (in eV) between the three spin states considered are 0.929 for the sextet-quartet gap and 0.812 for the quartet-doublet gap, which indicate that the spin crossover (at room temperature) is very unlikely. These results represent the substantial decrease in the uncorrelated values. The implications of spin contaminations at the UHF and UMP2 levels for subsequent geometry optimizations to be performed in the smaller cation have also been discussed. © 2013 Wiley Periodicals, Inc.

Success in Making Zn+ from Atomic Zn0 Encapsulated in an MFI-Type Zeolite with UV Light Irradiation

For the first time, the paramagnetic Zn(+) species was prepared successfully by the excitation with ultraviolet light in the region ascribed to the absorption band resulting from the 4s-4p transition of an atomic Zn(0) species encapsulated in an MFI-type zeolite. The formed species gives a specific electron spin resonance band at g = 1.998 and also peculiar absorption bands around 38,000 and 32,500 cm(-1) which originate from 4s-4p transitions due to the Zn(+) species with paramagnetic nature that is formed in MFI. The transformation process (Zn(0) → Zn(+)) was explained by considering the mechanism via the excited triplet state ((3)P) caused by the intersystem crossing from the excited singlet state ((1)P) produced through the excitation of the 4s-4p transition of an atomic Zn(0) species grafted in MFI by UV light. The transformation process was well reproduced with the aid of a density functional theory calculation. The thus-formed Zn(+) species which has the doublet spin state exhibits characteristic reaction nature at room temperature for an O2 molecule having a triplet spin state in the ground state, forming an η(1) type of Zn(2+)-O2(-) species. These features clearly indicate the peculiar reactivity of Zn(+) in MFI, whereas Zn(0)-(H(+))2MFI hardly reacts with O2 at room temperature. The bonding nature of [Zn(2+)-O2(-)] species was also evidenced by ESR measurements and was also discussed on the basis of the results obtained through DFT calculations.

Pulsed electron spin nutation spectroscopy for weakly exchange-coupled multi-spin molecular systems with nuclear hyperfine couplings: a general approach to bi- and triradicals and determination of their spin dipolar and exchange interactions

Weakly exchange-coupled biradicals have attracted much attention in terms of their dynamic nuclear polarisation application in NMR spectroscopy for biological systems or the use of synthetic electron-spin qubits in quantum information processing/quantum-computing technology. Analogues multi-partite molecular systems are important in entering a new phase of the relevant fields. Many stable organic biradicals known so far have nitrogen nuclei at their electron spin sites, where singly occupied molecular orbitals are dominating and large hyperfine couplings occur. A salient feature of such weakly exchange-coupled molecular systems in terms of electronic spin structures is underlain by small zero-field splitting (ZFS) parameters comparable with nuclear hyperfine and/or exchange interactions. Pulse-based electron spin nutation (ESN) spectroscopy of weakly exchange-coupled biradicals, applicable to oriented or non-oriented media, has proven to be a useful and facile approach to the determination of ZFS parameters, which reflect relatively short distances between unpaired electron spins. In the present study, we first treat two-dimensional single-crystal ESN spectroscopy (Q-band) of a 15N-labelled weakly exchange-coupled biradical, showing the nuclear hyperfine effects on the ESN phenomena from both the experimental and theoretical side. ESN spectroscopy is transition moment spectroscopy, in which the nutation frequency as a function of the microwave irradiation strength ω1 (angular frequency) for any cases of weakly exchange-coupled systems can be treated. The results provide a testing ground for the simplified but general approach to the ESN analysis. In this study, we have invoked single-crystal electron-electron double resonance measurements on a typical biradical well incorporated in a diamagnetic host lattice and checked the accuracy of our ESN analysis for the spin dipolar tensor and exchange interaction. Next, we extend the general approach to analogues multi-partite molecular systems such as stable organic triradical, in which the exchange interaction can be governed by a significant amount of the delocalisation of three unpaired spins over the molecular frame of a triangular structure. The triangular structure maintains π-conjugation in which each spin-bearing nitroxide at the vertex participates and the exchange interaction is greatly controlled by the dihedral angle between the π-conjugation plane and nitroxide moiety at the vertex. In this context, the ZFS parameters do not correspond to spin distances (1.0 nm) in a straightforward manner, but reflect a salient electronic structure associated with both the π-electron network and the symmetry property of the triradical under study. Thus, both the D-value and exchange interaction J have been controlled in this study. In order to interpret the experimental ZFS parameters and exchange interaction, which is three-order of magnitude reduced in the present poly(methyl methacrylate) polymer matrix compared with that in the crystal, sophisticated quantum chemical calculations of the ZFS tensor and exchange interaction were carried out and reproduced the experimental values, concluding that the present triradical of the triangular structure undergoes significant twisting at the nitroxide sites in the polymer matrix. In this study, we observed electron spin resonance forbidden transitions between the Ms-manifolds belonging to the spin-doublet ground state and spin-quartet excited state. The observation enables us to derive the magnitude of the exchange coupling.

Carbene Generation by Cytochromes and Electronic Structure of Heme-Iron-Porphyrin-Carbene Complex: A Quantum Chemical Study

Carbene-heme-iron-porphyrin complexes generated from cytochrome P450 (CYP450)-mediated metabolism of compounds containing methylenedioxyphenyl (MDP) moiety lead to the mechanism-based inhibition (MBI) of CYPs. This coordination complex is termed as the metabolic-intermediate complex (MIC). The bioinorganic chemistry of MDP carbenes has been studied using quantum chemical methods employing density functional theory (B3LYP functional with implicit solvent corrections) to (i) analyze the characteristics of MDP-carbene in terms of singlet-triplet energy difference, protonation, and dimerization energies, etc.; (ii) determine the electronic structure and analyze the Fe-carbene interactions; and (iii) elucidate the potential reaction pathways for the generation of carbene, using Cpd I (iron(IV)-oxo-porphine with SH(-) as the axial ligand) as the model oxidant to mimic the activity of CYP450. The results show that MDP-carbenes are sufficiently stable and nucleophilic, leading to the formation of stable MIC (-40.35 kcal/mol) on the doublet spin state, formed via interaction between σ(LP) of carbene and empty dz(2) orbital of heme-iron. This was aided by the back-bonding between filled d(xz) orbital of heme-iron and the empty p orbital of carbene. The mechanistic pathway proposed in the literature for the generation of MDP-carbene (CH hydroxylation followed by water elimination) was studied, and observed to be unfavorable, owing to the formation of highly stable hydroxylated product (-57.12 kcal/mol). An intriguing pathway involving hydride ion abstraction and proton transfer followed by water elimination step was observed to be the most probable pathway.

A new type of sandwich compound: homoleptic bis(trimethylenemethane) complexes of the first row transition metals

The metal carbonyl complexes [η4-(CH2)3C]Fe(CO)3 and [η4-(CH2)3C]Cr(CO)4 have been synthesized containing the umbrella-shaped trimethylenemethane ligand. The prospect of using this ligand in the metal sandwich complexes [(CH2)3C]2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) has now been investigated by density functional theory. The lowest energy structures of such complexes have the metal atom sandwiched between two tetrahapto trimethylenemethane ligands. Singlet spin state structures are strongly preferred for the titanium and nickel derivatives and doublet spin state structures for the vanadium and cobalt derivatives. Similarly, the triplet spin state is preferred for the iron derivative by more than 11 kcal mol−1. The preferred spin states for the chromium and manganese derivatives depend on the functional used for the structure optimization. Thus the B3LYP and B3LYP* methods predict the higher spin states, namely triplet for chromium and quartet for manganese. However, the BP86 method predicts the lower spin states of singlet for chromium and doublet for manganese. The higher spin state structures for the late transition metal derivatives, namely quintet [(CH2)3C]2Fe, quartet [(CH2)3C]2Co, and triplet [(CH2)3C]2Ni, have trihapto rather than tetrahapto trimethylenemethane ligands. The frontier molecular orbitals in the singlet [(CH2)3C]2M derivatives (M = Ti, Ni) suggest strong metal–ligand σ and π bonding but insignificant metal–ligand δ bonding.

Cover Picture: Shape-Defined and Shape-Shifting Paramagnetic Organic Nano/Microstructures Derived From a Doublet State Nitronyl Nitroxide Radical (ChemPlusChem 12/2012)

The cover picture shows generation of nano/microscale paramagnetic solid structures from a doublet spin state nitronyl nitroxide radical derivative by using a bottom-up self-assembly technique and their electron paramagnetic resonance spectroscopy studies as described by R. Chandrasekar and P. Hui in their Communication on page 1062 ff. In the background, the University of Hyderabad Lake represents the involvement of environment friendly water in the self-assembly process. At room temperature the evaporation of an acetonitrile solution of radical forms a porous film at a SiO2 surface, and in acetonitrile/water it forms solution stable nanovesicles, which shape-shift into metastable crystalline nanoparticles upon ultrasonication and after a month they revert back into vesicles.