Semi-occupied Molecular Orbital Research Articles

The soft X-ray absorption spectrum of the allyl free radical

The first experimental study of the X-ray absorption spectrum (XAS) of the allyl free radical, CH(2)CHCH(2), is reported. A supersonic He seeded beam of hyperthermal allyl radicals was crossed by a high resolution synchrotron radiation (SR) in the focus of a 3D ion momentum imaging time-of-flight (TOF) spectrometer to investigate the soft X-ray absorption and fragmentation processes. The XAS, recorded as Total-Ion-Yield (TIY), is dominated by C1s electron excitations from either the central carbon atom, C(C), or the two terminal carbon atoms, C(T), to the frontier orbitals, the semi-occupied-molecular-orbital (SOMO) and the lowest-unoccupied-molecular-orbital (LUMO). All of the intense features in the XAS could only be assigned with the aid of ab initio spectral simulation at the Multi-Configuration Self-Consistent-Field (MCSCF) level of theory, this level being required because of the multi-reference nature of the core-excited state wavefunctions of the open shell molecule. The ionization energies (IEs) of the singlet and triplet states of the C1s ionized allyl radical (XPS) were also calculated at the MCSCF level.

EPR Spectroscopy of 4, 4′‐Bis(tert‐butyl)‐2, 2′‐bipyridine‐1, 2‐dithiolatocuprates(II) in Host Lattices with Different Coordination Geometries

AbstractA series of new heteroleptic MN2S2 transition metal complexes with M = Cu2+ for EPR measurements and as diamagnetic hosts Ni2+, Zn2+, and Pd2+ were synthesized and characterized. The ligands are N2 = 4, 4′‐bis(tert‐butyl)‐2, 2′‐bipyridine (tBu2bpy) and S2 =1, 2‐dithiooxalate, (dto), 1, 2‐dithiosquarate, (dtsq), maleonitrile‐1, 2‐dithiolate, or 1, 2‐dicyanoethene‐1, 2‐dithiolate, (mnt). The CuII complexes were studied by EPR in solution and as powders, diamagnetically diluted in the isostructural planar [NiII(tBu2bpy)(S2)] or[PdII(tBu2bpy)(S2)] as well as in tetrahedrally coordinated[ZnII(tBu2bpy)(S2)] host structures to put steric stress on the coordination geometry of the central CuN2S2 unit. The spin density contributions for different geometries calculated from experimental parameters are compared with the electronic situation in the frontier orbital, namely in the semi‐occupied molecular orbital (SOMO) of the copper complex, derived from quantum chemical calculations on different levels (EHT and DFT). One of the hosts, [NiII(tBu2bpy)(mnt)], is characterized by X‐ray structure analysis to prove the coordination geometry. The complex crystallizes in a square‐planar coordination mode in the monoclinic space group P21/a with Z = 4 and the unit cell parameters a = 10.4508(10) Å, b = 18.266(2) Å, c = 12.6566(12) Å, β = 112.095(7)°. Oxidation and reductions potentials of one of the host complexes, [Ni(tBu2bpy)(mnt)], were obtained by cyclovoltammetric measurements.

Heterogeneous Binding of Dioxygen: EPR and DFT Evidence for Side-On Nickel(II)–Superoxo Adduct with Unprecedented Magnetic Structure Hosted in MFI Zeolite

This article reports on the activation of dioxygen on nickel(I) dispersed inside the nanopores of the ZSM-5 zeolite, which can be regarded as a heterogeneous mimetic system (zeozyme) for Ni-bearing enzymes. The side-on η(2)-coordination of the resulting nickel-bound superoxo adduct was ascertained by detailed analysis of the EPR spectra of both (16)O(2) and (17)O(2) species supported by computer simulations of the spectra and relativistic DFT calculations of the EPR signatures. Molecular analysis of the g and A((17)O) tensors (g(xx) = 2.0635, g(yy) = 2.0884, g(zz) = 2.1675; |A(xx)| ≈ 1.0 mT, |A(yy)| = 5.67 mT, |A(zz)| ≈ 1.3 mT) and quantum chemical modeling revealed an unusual electronic and magnetic structure of the observed adduct (with g(zz)(g(max)) > g(yy)(g(mid)) > g(xx)(g(min)) and the largest O-17 hyperfine splitting along the g(mid) direction) in comparison to the known homogeneous and enzymatic nickel-superoxo systems. It is best described as a mixed metalloradical with two supporting oxygen donor ligands and even triangular spin-density redistribution within the η(2)-{NiO(2)}(11) magnetophore. The semioccupied molecular orbital (SOMO) is constituted by highly covalent δ overlap between the out-of-plane 2p(π(g)*) MO of dioxygen and the 3d(x(2)(-y(2))) MO of nickel. By means of the extended transition state-natural orbitals for the chemical valence approach (ETS-NOCV), three distinct orbital channels (associated with σ, π, and δ overlap) of congruent and incongruent charge and spin density flows within the η(2)-{NiO(2)}(11) unit, contributing jointly to activation of the attached dioxygen, were identified. Their individual energetic relevance was quantified, which allowed for explaining the oxygen binding mechanism with unprecedented accuracy. The nature and structure sensitivity of the g tensor was rationalized in terms of the contributions due to the magnetic field-induced couplings of the relevant molecular orbitals that control the g-tensor anisotropy. The calculated O-17 hyperfine coupling constants correspond well with the experimental parameters, supporting assignment of the adduct. To the best of our knowledge, the η(2)-{NiO(2)}(11) superoxo adducts have not been observed yet for digonal mononuclear nickel(I) centers supported by oxygen donor ligands.

The first unpaired electron placed inside a C3-symmetry P-chirogenic cluster

The Pd(3)(dppm*)(3)(CO)(n+) enantiomers (n = 2 (2), 1 (3)) were prepared either from (R,R)- or (S,S)-P-chirogenic bis(phenyl-m-xylylphosphino)methane (dppm*; 1) and Pd(OAc)(2) in the presence of CF(3)CO(2)H, CO and water (n = 2), and then by reductive electrolysis (n = 1). The stable enantiomeric [Pd(3)((S,S)-dppm*)(3)(CO)](+)˙ (3), is the first C(3)-symmetry radical-cation M-M bonded cluster, therefore the odd electron is delocalized onto the Pd(3) frame within this symmetry. The novel chiral species have been characterized by circular dichroism (CD) of both enantiomers of the Pd(3)(dppm*)(3)(CO)(2+) clusters (2) and by EPR spectroscopy for the Pd(3)((S,S)-dppm*)(3)(CO)(+)˙ paramagnetic compounds (3, g = 2.041). Evidence for reduced symmetry with respect to the achiral cluster was also obvious from the hyperfine splittings of the EPR signal which display three different hyperfine coupling values: 3 ×A((31)P) = 83.9 × 10(-4) cm(-1), 3 ×A((31)P) = 69.7 × 10(-4) cm(-1), 3 ×A((105)Pd) = 12.5 × 10(-4) cm(-1). In the absence of an X-ray structure for the paramagnetic clusters, DFT computations were performed to address the geometry. The optimized geometry of the Pd(3)((S,S)-dppm*)(3)(CO)(+)˙ radicals (3) exhibits three phosphorus atoms placed well above the Pd(3) plane, while the three others are located below the trimetallic frame within C(3)-symmetry due to intramolecular steric hindrance. This makes them chemically different with respect to the carbonyl group and explains the experimental EPR spectrum well. Consequently this C(3)-symmetry deformation also induces a change in the shape of the SOMO (semi-occupied molecular orbital) towards this same symmetry compared to the corresponding achiral C(3v) species.

Synthesis and Properties of [NiCp*(2,5‐tBu2PC4H2)], a 20‐Valence‐Electron Phosphanickelocene

The reaction of the bulky phospholide salt Li(2,5-tBu2PC4H2) x 2THF (1; THF = tetrahydrofuran) with [NiCp*(acac)] (HCp* = pentamethylcyclopentadiene, Hacac = acetylacetone) gives the green air-sensitive phosphanickelocene [NiCp*(2,5-tBu2PC4H2)] (2) in yields of about 85%. An X-ray structural determination of 2 shows long Ni-ring bonds and "delocalised" ring P-C and C-C bonds characteristic of a classical 20-valence-electron (ve) nickelocene. The electronic structure of 2 has been clarified through a combined Amsterdam density functional (ADF) and photoelectron spectroscopic study, which indicates that the higher lying semi-occupied molecular orbital (SOMO) (-5.82 eV) has a' symmetry and that the phosphorus "lone pair" is energetically low-lying (-8.15 eV). Oxidation of phosphanickelocene 2 by AgBF4 occurs quantitatively to give the corresponding air-sensitive orange phosphanickelocenium salt [NiCp*(2,5-tBu2PC4H2)][BF4] (3). This complex has also been characterised by an X-ray crystallographic study, which reveals long Ni-C(alpha) and short C(alpha)-C(beta) bonds in the phospholyl ligand indicative of a SOMO having a'' symmetry. PMe3 reacts with 2 at room temperature to provoke a ring-slip reaction that gives the 18ve complex [NiCp*eta1-(2,5-tBu2PC4H2)(PMe3)] (4), but shows no reaction with the phosphanickelocenium salt 3 under the same conditions.

Deactivation of Triplet-Excited Riboflavin by Purine Derivatives: Important Role of Uric Acid in Light-Induced Oxidation of Milk Sensitized by Riboflavin

The reactivity of purine derivatives (uric acid, xanthine, hypoxanthine, and purine) toward triplet-excited riboflavin in aqueous solution at pH 6.4 is described on the basis of kinetic (laser flash photolysis), electrochemical (square-wave voltammetry), and theoretical data (density functional theory, DFT). Direct deactivation of triplet-excited riboflavin in aqueous solution, pH 6.4 at 24 degrees C, in the presence of uric acid, xanthine, and hypoxanthine strongly suggests a direct electron transfer from the purine to the triplet-excited riboflavin with k = 2.9 x 10(9) M(-1) s(-1) (DeltaH(++) = 14.7 kJ mol(-1), DeltaS(++) = -15.6 J mol(-1) K(-1)), 1.2 x 10(9) M(-1) s(-1) (DeltaH(++) = 34.3 kJ mol(-1), DeltaS(++) = +45.3 J mol(-1) K(-1)), and 1.7 x10(8) M(-1) s(-1) (DeltaH(++) = 122 kJ mol(-1), DeltaS(++) = +319 J mol(-1) K(-1)), respectively. From the respective one-electron oxidation potentials collected in aqueous solution at pH 6.4 for uric acid (E = +0.686 vs normal hydrogen electrode, NHE), xanthine (E = +1.106 vs NHE), and hypoxanthine (E = +1.654 vs NHE), the overall free energy changes for electron transfer from the quencher to the triplet-excited riboflavin are as follows: uric acid (DeltaG(o) = -114 kJ mol(-1)), xanthine (DeltaG(o) = -73.5 kJ mol(-1)), hypoxanthine (DeltaG(o) = -20.6 kJ mol(-1)), and purine (DeltaG(o) > 0). The inertness observed for purine toward triplet-excited riboflavin corroborates with its electrochemical inactivity in the potential range from 0 up to 2 V vs NHE. These data are in agreement with the DFT results, which show that the energy of the purine highest occupied molecular orbital (HOMO) (-0.2685 arbitrary unit) is lower than the energy of the semioccupied molecular orbital (SOMO) (-0.2557 a.u.) of triplet-excited riboflavin, indicating an endergonic process for the electron-transfer process. The rate-determining step for deactivation by purine derivatives can be assigned to an electron transfer from the purine derivative to the SOMO orbital of the triplet-excited riboflavin. The results show that uric acid may compete with oxygen and other antioxidants to deactivate triplet-excited riboflavin in milk serum and other biological fluids leading to a free radical process.

Synthesis and properties of [CoCp*(2,5-PC4tBu2H2)]: the first monophosphacobaltocene.

The reduction of the phosphacobaltocenium salt [CoCp*(2,5-PC(4)tBu(2)H(2))](+)[BPh(4)](-) (3; Cp*=pentamethylcyclopentadienyl) by magnesium in tetrahydrofuran (THF) furnishes the stable air-sensitive phosphacobaltocene [CoCp*(2,5-PC(4)tBu(2)H(2))] (4) in yields of up to 80 %. The crystal structure of 4 shows long Co-C(alpha) and short C(alpha)-C(beta)bonds in the phospholyl ligand, consistent with a semi-occupied molecular orbital (SOMO) having a" symmetry. A combined Amsterdam density functional (ADF)/photoelectron spectroscopic study, which confirms this assignment, gives ionisation energies (IE) of 5.02 eV from the SOMO and 8.89 eV from the phosphorus "lone pair". A comparison of cyclovoltammograms for 3 and the corresponding cyclopentadienyl complex [CoCp*(1,3-C(5)tBu(2)H(3))](+) [BPh(4)](-)(5) shows that replacing a CH group by an sp(2) phosphorus atom results in an anodic first reduction potential shift of 0.29 V.

Possible structures for H-Cu-CH3 molecules

EPR spectra for H-Cu-CH3 molecules in glasses at 77 K show 63Cu hyperfine splitting that accords with a d1x2 – y2 orbital for the semi-occupied molecular orbital (SOMO) rather than a d1z2 SOMO. The g-values also strongly imply a d1x2 – y2 configuration. The estimated spin densities on the H- and -CH3 ligands are small. It is argued that the molecule must be strongly bent in order to favour this SOMO and that this bending may be induced by weak solvation.Key words : H-Cu-CH3, EPR spectra, molecular structure, hyperfine splitting.

Dramatic tensor-axis non-coincidence effects in the electron spin resonance spectra of some low-spin manganese(II) complexes

ESR spectra are reported for frozen CH2Cl2/ClCH2CH2Cl solutions of 14 low-spin manganese(II) cations: [(η5-C5H5)Mn(CO)L2]+, L = 1/2 Ph2PCH2CH2PPh2(dppe), 1/2 Me2PCH2CH2PMe2(dmpe), 1/2 Ph2PCH2PPh2(dppm), PMe3, PPh3; [(η5-MeC5H4)Mn(CO)dppe]+; [(η5-6-exo-PhC6H6)Mn(CO)L2]+, L = 1/2 dppe, 1/2 dmpe, PMe3; [(η5-6-exo-PhC6HMe5)Mn(CO)dppe]+; [(η5-6-exo-PhC7H8)Mn(CO)dppe]+; [(η5-C5H5-Mn(CO)2PPh3]+; and [(η5-6-exo-PhC6H6)Mn(CO)2L]+, L = PMe3, PnBu3. The spectra show anisotropic 55Mn hyperfine coupling and nearly isotropic 31P hyperfine coupling. There are dramatic departures from first-order line spacings which result from non-coincidence of the X and Z principal axes of the g and the 55Mn hyperfine tensors. In most cases, the ESR parameters can be interpreted in terms of a semi-occupied molecular orbital (SOMO) primarily dx2–y2 in character, but rotated about the y axis to avoid an antibonding interaction with the dienyl ring. The spectrum of [(η5-6-exo-PhC6H6)Mn(CO)(PMe3)2]+ shows non-equivalent 31P couplings, suggesting an unsymmetrical dienyl ring conformation. This spectrum, and those of the monophosphine cations, are best explained in terms of a SOMO primarily dyz in character.

Radical cations of trialkylphosphine oxides, trialkylphosphates, hexamethylphosphoramide, dimethyl sulphoxide and various sulphones, sulphites and sulphates. An electron spin resonance and radiation chemical study

Exposure of dilute solutions of the title compounds in trichlorofluoromethane to 60Co γ-rays at 77 K or to X-rays at ca. 4 K gave species identified as radical cations by e.s.r. spectroscopy. The parent cations were formed from dimethyl sulphoxide, dimethyl sulphite, tetramethylene sulphone and hexamethylphosphoramide. Trimethylphosphate and dimethylsulphate gave well defined solvent adducts with large hyperfine coupling to a single chlorine nucleus. This result is surprising for the latter cation since it implies localisation of the semi-occupied molecular orbital (SOMO) on one of the two oxygen ligands, which should share the unpaired electron equally in the uncomplexed cation. In the other cases hydrogen-atom migration dominated, giving carbon-centred radicals. This change was also found for the cation complex of trimethylphosphate and dimethylsulphate on photolysis or annealing. For the hexamethyl phosphoramide cation, the SOMO appears to be localised on one of the three Me2N moieties. This species was stable up to ca. 160 K. The phenyl derivatives, Ph3PO and (PhO)3PO gave aromatic radical cations in preference to oxygen-centred cations or their solvent complexes.

Structures and reactions of radical cations of some bicycloalkanes and their related alkenes as studied by 4 k matrix esr

The σ radical cations of most typical bicycloalkanes such as norbornane and bicyclo[2,2,2]octane are radiolytically produced at 4 K in halogenocarbon matrices and are studied by ESR spectroscopy. Their electronic and geometrical structures as well as their dynamical behaviors have been elucidated from the hyperfine structures and their temperature changes. The semi occupied molecular orbital (SOMO) of the former cation is 4a 2, in which the unpaired electron delocalizes over the four exo C-H bonds giving large hyperfine coupling. The latter is a Jahn-Teller active species and exhibits static distortion from D 3h to C 2v at 4 K in CFCl 3, and the SOMO is likely to be 6b 2, in which the unpaired electron delocalizes over the four endo C-H bonds giving large proton coupling, although a dynamically averaged structure with 12 equivalent methylene protons is observed in C-C 6F 12 as well as in CFCl 2CF 2Cl matrices at 77 K. The unpaired electron distribution in bicycloalkane radical cations is similar to that in cycloalkane radical cations previously studied. Upon warming both the cations undergo deprotonation to give 2-yl alkyl radicals from the exo or endo C-H bond, at which the higher unpaired electron density is populated. In addition to these radical cations, the structures and reactions of the radical cations of the related bicycloalkenes such as norbornadiene, quadricyclane, and bicyclo[2,2,2]octene have also been studied. The hydride ion transfer to an olefinic radical cation to form an alkyl radical is observed for the bicyclo[2,2,2]octene radical cation as the first example observed by ESR.

Structures of diazabenzene cations formed by radiolysis. An electron spin resonance study

Exposure of 1,2-, 1,3- and 1,4-diazabenzene in dilute solution in trichlorofluoromethane at 77 K to 60Co γ-rays gives the corresponding radical cations, characterised by their e.s.r. spectra. The 1,2 derivative has a SOMO (semi-occupied molecular orbital) largely confined to the two nitrogen n orbitals, with a 2s-orbital contribution of approximately twice that observed for pyridyl cations. This suggests that the extent of distortion for these radicals is far less than that for pyridyl cations. In contrast, the 1,4-diazabenzene cation has a considerably reduced 14N isotropic coupling to two equivalent nitrogen nuclei, and hence a reduced 2s-orbital contribution, suggesting more extensive flattening at both nitrogen atoms.In marked contrast, the pyrimidine cation has a SOMO largely localised on one of the nitrogen atoms in a non-bonding orbital closely resembling that for the pyridine cation. This is interpreted in terms of a preferred asymmetric distortion which lowers this SOMO below that of the expected symmetrical SOMO for this cation.