Molecular orbital calculation of hyperfine interactions,electric field gradient and quadrupole splitting in reduced rubredoxins

The active site of rubredoxins, the simplest class of iron-sulfur, electron-transfer proteins consists of a single Fe atom surrounded by a distorted tetrahedral array of four cysteine sulfur atoms. In a previous paper on the oxidized rubredoxins, we calculated the electric field gradient at the Fe⁵⁷ nucleus, the resultant quadrupole splitting and the hyperfine field due to interaction between the Fe nucleus spins and the net electron spin of the molecular complex. In this paper, we are going to present our calculated results of the same set of properties for the reduced state, using the molecular orbitals similarly obtained from an Iterated Extended Hückel calculation. Significant contribution of the anisotropy to the hyperfine field as well as the sign of the electric field gradient, and hence the quadrupole splitting allow us to pin down conformations which give the best agreement with the experimental results. In fact, the conformation (conformer A) which represents a small movement of one of the S atoms from its crystal structure position in such a way that pairs of S atoms lie in ⊥ planes yield remarkably good results for all the physical properties calculated in both oxidation states. This consistency also suggests that there is no appreciable conformational change through the oxidation process.     

Spin State in (Porphinato)iron(III) Complex. A Spin‐Crossover Complex with Two Magnetically Distinct Sites

The results of low temperature X‐ray determination, Mössbauer and magnetic measurement of spin‐crossover (isothiocyanato)(porphinato)iron(III) hemipyridine are reported. The features in 77 K Mössbauer spectrum include two doublets, one with a quadrupole splitting (ΔE_Q) of 1.961 mm s^?1 (low‐spin site) and the other with ΔE_Q = 0.792 mm s^?1 (high‐spin site). As the temperature of the sample is increased to 300 K, the signal intensity of the high‐spin site grows to 92% at the expense of the low‐spin signal. The variable‐temperature magnetic susceptibility data also support that the tetraphenyl complex is a spin‐crossover complex.     

A comment on the dependence of the C-H proton isotropic hyperfine constants on carbon hybridization in planar radicalS

Properties of the McConnell Q_CH parameter relating proton isotropic hyperfine coupling constants to spin densities in planar nng radicals are examined. Q_CH is a sensitive function of the hybridization of the C—H bond attached to radical carbocycles C_nH_n (n = 3,5,6,7,8). The hybridization effect is most pronounced in the highly strained cyclopropenyl radical. Q_CH spans a narrow range in most of the common π-radicals, including moderately strained fused systems like azulene, biphenylene, acenaphthylene and acepleiadylene anions.     

Electronic structure and energy decomposition of binuclear transition metal complexes containing β-diketiminate and imido ligands: spin state and metal’s nature effects

DFT calculations with full geometry optimization using BP86-D and OPBE functionals have been performed on series of [(BDI)M(NH)]₂(Bz) and [(BDI)M(NH)]₂(Tol) (M = Ti, V, Nb, Cr, Mn, Fe, Co, and Ni; BDI = β-diketiminate; NH = imido group; Bz = benzene; and Tol = toluene) of various spin states (singlet S = 0, triplet S = 1, quintet S = 2, and singlet S = 0 of broken symmetry method). Depending on the metal nature and its electron count and the spin state, the six-membered ring in [(BDI)M(NH)]₂(Bz) and [(BDI)M(NH)]₂(Tol) adopts various hapticities that involve full or partial coordination, giving rise to a flat or a distorted ring, respectively. The NH^2? imido group is linear or bent with respect to its sp or sp² hybridization acting as a six- or a four-electron donor, respectively. The (BDI)^? anion is a bidentate ligand as a six-electron donor. The optimized geometries do not show direct metal-metal bonding and correspond to long separations. The optimized structures for Nb metal are comparable to the available experimental ones. The Ziegler-Rauk energy decomposition analysis scheme was employed to characterize the geometry distortion, the steric interaction (electrostatic and Pauli), and the orbital interaction terms in the total bonding energy. The results showed that the interaction terms in all the studied complexes are governed by one third covalent and two thirds ionic characters, which are in agreement with the ΔE_elstat (electrostatic) and ΔE_orb (orbital) contributions, respectively, into the total attractive interaction (ΔE_elstat + ΔE_orb).     

Rotational and hyperfine parameters of NH2(X2B1) from LMR spectra

Seventeen rotational transitions up to N=7 of the NH₂ radical in its ground vibronic state have been measured by far infrared laser magnetic resonance spectroscopy. Analysis of the Zeeman patterns and hyperfine structure yields precise rotational, spin-rotational and distortion parameters, and isotropic and anisotropic hyperfine interaction parameters for both ¹⁴N and ¹H nuclei.     

On the INDO calculations of N32− and N4− free radicals

?-tensors as well as anisotropic and isotropic hyperfine coupling constants have been calculated for the N₃^2? and N₄^? free radicals. The adequacy of the INDO method is discussed in relation with the results of a previous ab initio calculation.     

Modified statistical theory of energy transfer in ion-molecule collisions

The modified statistical theory developed previously for potentials appropriate to interactions in neutral-neutral collisions, is now extended to more strongly attractive potentials involved in ion-neutral collisions. The model system is the collisional deactivation of C₅H₉⁺ by a variety of both polar and non-polar neutral molecules. A 12 - 6 - 4 potential is used for ion interaction with non-polar neutrals, and a 12 - 6 - 4 - 2 potential, as modified by Su and Bowers to take into account the rotational energy of the neutral, for interaction with polar neutrals. Calculated is (ΔE), the average energy lost by the ion in a collision, and compared with experiment. For C₅H₉⁺-CH₄ collisions, the calculated (ΔE) agrees with experiment within 5%. Predictions of the theory, namely that (ΔE) should increase with excitation energy and should decrease with the size of the excited reactant, are found to be in fair agreement with the somewhat ambiguous experimental evidence.     

Reactivity of haloalkanes in their reactions with the chlorine atom

Experimental kinetic data on reactions of the chlorine atom with halogenated derivatives of methane and ethane (37 reactions) have been analyzed by the intersecting-parabolas method. The following five factors have an effect on the activation energy of these reactions: the enthalpy of reaction, triplet repulsion, the electronegativities of the reaction center atoms, the dipole–dipole and multidipole interactions between the reaction center and polar groups, and the effect of π electrons in the vicinity of the reaction center. The increments characterizing the contribution from each factor to the activation energy of the reaction have been calculated. The contribution from the polar interaction, ΔE _μ, to the activation energy depends on the dipole moment of the polar group and obeys the following empirical equation: ln(Δ_{E μ}/Σμ) = ?0.74 + 0.87(ΔE _μ/Σμ) ? 0.084(ΔE _μ/Σμ)².     

Hyperfine Shifts of the 13C-NMR. in Protoporphyrin IX Iron (III) Dicyanide and Deuteroporphyrin IX Iron (III) Dicyanide

The ¹³C-NMR. in Zn(II) (Protoporphyrin IX), Fe(III) (Protoporphrin IX) (CN)₂, Zn(II) (Deuteroporphyrin IX dimethylester), and Fe(III) (Deuteroporphyrin IX) (CN)₂ have been identified, and the ¹³C hyperfine shifts in the iron complexes evaluated. In a partial analysis of these data the parameters Q^H, Q_CCH^H, and Q_C′CH^C, which characterize the isotropic coupling through hyperconjugation between the ¹H and ¹³C nuclei of the porphyrin side chains and the unpaired electron spin density on the aromatic ring carbon atoms, have been re-examined. This paper is part of an investigation of the electronic states in low spin ferric hemes and hemoproteins, and the relations between the electronic structures and the biological roles of these molecules.     

Factors determining the reactivity of the bromine atom toward haloalkanes

Experimental data concerning reactions of the bromine atoms with haloalkanes and carbonyl compounds (25 reactions) have been analyzed within the intersecting parabolas model. The following factors have an effect on the activation energy of these reactions: enthalpy of reaction, triplet repulsion, electronegativity of reaction center atoms, dipole–dipole and multidipole interactions of the reaction center with polar groups, and the interaction of π electrons with electrons of the reaction center. The increments characterizing the contribution from each factor to the activation energy of the reaction have been calculated. The increment ΔEμ, which characterizes the dipole–dipole interaction in the transition state, and the dipole moment of the polar group (μ) are correlated by the following empirical equation: ln(ΔE _μ/Σμ) = ?0.14 + 0.47(ΔE _μ/Σμ) ? 0.024(ΔE _μ/Σμ)².     

Micromechanism of Cu and Fe alloying process on the martensitic phase transformation of NiTi-based alloys: First-principles calculation

Using first-principles pseudo-potential plane wave method, the formation enthalpy ΔH, binding energy ΔE, elastic constants, and electronic structure were calculated and analyzed carefully for NiTiX (X = Cu, Fe) shape memory alloy. The results show that the Cu or Fe element prefers to occupy the Ni site in the NiTi matrix phase respectively. Compared with the NiTi matrix phase, the ΔH, ΔE, c ₄₄ and c′ of NiTi (Cu) are similar to each other. However, the structural stability of the NiTi phase is improved obviously by the Fe alloying process. Simultaneously, the shear modulus c ₄₄ and c′ of NiTi (Fe) are larger than those of the NiTi matrix phase. Furthermore, Milliken population results indicate that Q _Cu–Ti is smaller than Q _Ni–Ti after the Cu alloying process, but Q _Fe–Ti is larger than Q _Ni–Ti. The electron density difference shows that some covalent bonding exists between Fe and Ti elements. Based on the upward analysis, the difference in the phase stability and elastic constants of NiTiX (X = Cu, Fe) is the substantial mechanism for the different M _s of NiTiX (X = Cu, Fe) although Cu or Fe substitutes for the same atom Ni elements in the NiTi matrix phase.     

The ESR spectrum of the C5F5 radical

Experimental results of an investigation of the ESR spectrum of the pentafluorocyclopentadienyl radical C₅F₅ in various liquid and solid matrices are reported. At temperatures above 120 K the ESR spectra indicate a five-fold symmetry with a fluorine isotropic splitting of 16 G and an isotropic g tensor of 2.0041. From liquid solutions a value A_iso¹³C ≈ 2.1 G for the isotropic coupling of the ¹³C₁ species in natural abundance was found. The ESR spectra exhibit a pronounced temperature dependence, which is interpreted by lineshape analysis to originate from an anisotropic hyperfine interaction tensor of fluoroine, partially averaged by a uniaxial rotational reorientation. From ESR spectra of polycrystalline samples the principal value A6 = 57.5–58.3 G depending on the matrix was derived.     

ESR and structure of SO2Cl−√2

A single crystal ESR study is reported on the radial anion of sulphury chloride. In addition to the main spectrum which can be ananyzed in terms of anisotropic hyperfine coupling to two equivalent chlorines, the satellite spectrum of ³³SO₂Cl^?√₂ has been observed in natural abundance. The isotropic value of ca. 200 G for the ³³S hyperfine splitting corresponds to a spin population of 0.21–0.24 in the 3s orbital of sulphur. From the anisotropic chlorine coupling, a spin population of 0.31 in the 3p orbital on each chlorine atom has been derived. These results are consistent with a C_2v trigonal-bipyramid structure, the unpaired electron occupying a molecular orbital of a₁ representation which is localized to a considerable extent in the σ orbitals of the chlorine ligands in the apical positions. A similar structure is indicated by other results for certain isoelectronic phosphorus-centered radicals.     

Calculated hyperfine coupling constants for 5,5-dimethyl-1-pyrroline N-oxide radical products in water and benzene

The optimized structures of some radical adducts of 5,5-dimethyl-1-pyrroline N-oxide were computed by different methods on ESR spectra. As trapped radicals, H, N₃, NH₂, CH₃, CCl₃, OOH in water and F, OH, CF₃, CH₂OH, OC₂H₅ in benzene solutions were used. The calculated isotropic hyperfine coupling constants of all the trapped radicals were compared with the corresponding experimental data. The hyperfine coupling constant due to the β proton of the nitroxide radical was seen to be consist with the McConnel’s relation α_β = B ₀ + B ₁cos²θ and, to be effected with the opposite spin density of oxygen nucleus bonded to the nitrogen. It was concluded that in hyperfine calculations the DFT(B3PW91)/LanL2DZ level is superior computational quantum model relative to the used other level. Also, the study has been enriched by the computational of the optimized geometrical parameters, the hyper conjugative interaction energies, the atomic charges and spin densities for all the radical adducts.     

A theoretical study on the inhibition efficiencies of some quinoxalines as corrosion inhibitors of copper in nitric acid

Quantum chemical calculations based on DFT method were performed on three quinoxalines compounds namely ethyl 2-(4-(2-ethoxy-2-oxoethyl)-2-p-tolylquinoxalin-1(4H)-yl)acetate (Q1), 1-[4-acetyl-2-(4-chlorophenyl)quinoxalin-1(4H)-yl]acetone (Q2) and 2-(4-methylphenyl)-1,4-dihydroquinoxaline (Q3), used as corrosion inhibitors for copper in nitric acid media to determine the relationship between the molecular structure of quinoxalines and inhibition efficiency. Quantum chemical parameters such as the highest occupied molecular orbital energy (E_HOMO), the lowest unoccupied molecular orbital energy (E_LUMO), energy gap (ΔE), dipole moment (μ), electronegativity (χ), electron affinity (A), global hardness (η), softness (σ), ionization potential (I), the fraction of electrons transferred (ΔN), and the total energy (TE), were calculated. The theoretically obtained results were found to be consistent with the experimental data reported.     

Single crystal EPR study of Mn(II)-doped cis-catena-μ-sulphato-aquotris(imidazole)cadmium(II): Mn(II) in imidazole ligated low symmetry site

EPR of Mn(II)-doped single crystals of cis-catena-μ-sulphato-aquotris(imidazole)cadmium(II) has been studied in the temperature range 300-90 K. Mn(II) replaces Cd(II) substitutionally giving rise to two magnetically inequivalent sites. The large magnitude and high asymmetry component of the D-tensor is in accordance with the low symmetry of the substitutional site. Due to the non-first order nature of the spectrum all the work had to be carried out at Q-band frequency. Forbidden transitions corresponding to Δm_I = ± 1 and ±2 are observed for a number of orientations and their positions have been calculated by perturbation theory taking into account the second order admixtures due to cross terms in D and hyperfine coupling constant A. The spin—Hamiltonian parameters at 300 K are A = −83.5, D = −790.0, E = −80.4 (all in units of 10⁻⁴ cm⁻¹) and g = 2.0014.     

Angular dependent linewidths of ESR spin probes in oriented smectic systems. An application to oriented lecithin—cholesterol multibilayers

A theory, based on the diffusion model of molecular reorientation extended with an orienting potential, is developed for the angular dependence of the ESR linewidths of symmetrical top radicals in oriented smectic liquid crystals. The theoretical results have been compared with linewidth data obtained with the cholestane spin label in oriented multibilayers of equimolar quantities of dipalmitoyl lecithin and cholesterol. For the determiination of the linewidths, the ESR spectra were fitted to a line shape function, which includes unresolved proton hyperfine interactions.From the comparison of theory and experiment the rotational diffusion tensor and order parameter S can be found, giving D_⊥ = 3.4 × 10⁶ s^?1, D|/D_⊥ = 40 and S = 0.86 at 49°C. The high value of D|/D_⊥ indicates a strong anisotropic motion of the cholestane spin label. The order parameter found is in good agreement with the order parameter calculated from the angular dependence of the ESR line positions.     

The spin-coupling model of zero-field splitting for trimeric [3Fe–4S] and mixed-metal [3FeZn–4S] clusters of ferredoxins from Pyrococcus furiosus

The spin-coupling model of zero-field splitting (ZFS) is developed for trimeric [3Fe–4S] clusters. The correlations between the cluster ZFS parameters D_S and E_S of the states with total spin S and ZFS parameters D_i and E_i of individual ions were obtained for mixed-valent (MV) [3Fe–4S]⁰ clusters with high-spin ground state S_gr=2, for the MV iron core of the hetero-metal [3FeZn–4S]⁺ cluster with S_gr=5/2 and for the monovalent [3Fe–4S]⁺ cluster with S_gr=5/2 of Pyrococcus furiosus ferredoxin (Pf Fd). These correlations and the cluster ZFS parameters D_S and E_S depend on total spin S, intermediate spin S₁₂ and individual spins s_i. The spin-coupling model explains the experimentally observed negative cluster ZFS parameters of MV trimers in the [3Fe–4S]⁰ and [3FeZn–4S]⁺ clusters and the positive cluster ZFS parameter of the tetrameric MV [4Fe–4S]⁺ cluster and the monovalent [3Fe–4S]⁺ trimer of Pf Fd. Single-particle ZFS parameters D_i were obtained for the [3Fe–4S] trimers and [4Fe–4S]⁺ tetramer (S_gr=3/2) of Pf Fd. In distorted trimers, the cluster ZFS parameter D_S of anisotropy changes the value and sign under the variation of isotropic Heisenberg exchange or/and double exchange coupling due to the exchange admixture of the excited states. Experimentally observed peculiarities of effective hyperfine constants A_i for the MV trimer with S_gr=5/2 of the hetero-metal [3FeZn–4S]⁺ cluster were described in the spin-coupling exchange-resonance model with the exchange admixture of the excited states and non-equivalence of the states of different localization.     

Solid state NMR spectroscopy as a probe of structure and bonding in the carbides Sc3TC4 (T = Co,Ni, Ru,Rh, Os,Ir)

Scandium transition metal carbides having the formula Sc₃TC₄ (T = Co, Ni, Ru, Rh, Os, Ir) have been structurally characterized by solid state ¹³C and ⁴⁵Sc nuclear magnetic resonance spectroscopy. In all the compounds investigated, well-resolved signals are observed for crystallographically distinct carbon and scandium sites, confirming the formation of superstructures in the Rh and Ir compounds at ambient temperature. ⁴⁵Sc NMR spectra are dominated by anisotropic broadening due to second-order quadrupolar perturbations. The nuclear electric quadrupolar coupling parameters (the coupling constant C_Q and the asymmetry parameter η) are generally found in good agreement with values calculated theoretically from the crystal structure using the WIEN2k program. Furthermore, the spectra reveal large isotropic resonance shift differences between inequivalent Sc sites in a given compound and between sites of the same type for different compounds. Altogether the results illustrate that ⁴⁵Sc NMR is a sensitive method for detecting isotropic and anisotropic local electron density variations in the Sc₃TC₄ family.     

Performance of nonrelativistic and quasi-relativistic hybrid DFT for the prediction of electric and magnetic hyperfine parameters in 57Fe Mössbauer spectra

57Fe electric and magnetic hyperfine parameters were calculated for a series of 10 iron model complexes, covering a wide range of oxidation and spin states. Employing the B3LYP hybrid method, results from nonrelativistic density functional theory (DFT) and quasi-relativistic DFT within the zero-order regular approximation (ZORA) were compared. Electron densities at the iron nuclei were calculated and correlated with experimental isomer shifts. It was shown that the fit parameters do not depend on a specific training set of iron complexes and are, therefore, more universal than might be expected. The nonrelativistic and quasi-relativistic electron densities gave fit parameters of similar quality; the ZORA densities are only shifted by a factor of 1.32, upward in the direction of the four-component Dirac-Fock value. From a correlation of calculated electric field gradients and experimental quadrupole splittings, the value of the 57Fe nuclear quadrupole moment was redetermined to a value of 0.16 barn, in good agreement with other studies. The ZORA approach gave no additional improvement of the calculated quadrupole splittings in comparison to the nonrelativistic approach. The comparison of the calculated and measured 57Fe isotropic hyperfine coupling constants (hfcc's) revealed that both the ZORA approach and the inclusion of spin-orbit contributions lead to better agreement between theory and experiment in comparison to the nonrelativistic results. For all iron complexes with small spin-orbit contributions (high-spin ferric and ferryl systems), a distinct underestimation of the isotropic hfcc's was found. Scaling factors of 1.81 (nonrelativistic DFT) and 1.69 (ZORA) are suggested. The calculated 57Fe isotropic hfcc's of the remaining model systems (low-spin ferric and high-spin ferrous systems) contain 10-50% second-order contributions and were found to be in reasonable agreement with the experimental results. This is assumed to be the consequence of error cancellation because g-tensor calculations for these systems are of poor quality with the existing DFT approaches. Excellent agreement between theory and experiment was found for the 57Fe anisotropic hfcc's. Finally, all of the obtained fit parameters were used for an application study of the [Fe(H2O)6]3+ ion. The calculated spectroscopic data are in good agreement with the Mossbauer and electron paramagnetic resonance results discussed in detail in a forthcoming paper.