Spectral hole-burning and magnetic circular dichroism of matrix-isolated copper phthalocyanine

Persistent spectral hole-burning in the Q-band region is reported for a concentrated (≈ 2 × 10⁻² mol l⁻¹) matrix of copper phthalocyanine in solid Ar at 1.6 K. Hole-burning occurs with a quantum efficiency of ≈ 10⁻⁶ and can be reversed by annealing at ≈ 28 K. Vibrational side holes allow determination of excited-state frequencies. A hole-burning mechanism involving triplet-state charge separation followed by rapid electron back-transfer is proposed. Temperature dependence of the MCD and the difference between MCD spectra taken before and after irradiation indicate that the excited state is split by crystal-field and spin-orbit interactions.     

π-d-Interaction and the temperature dependent magnetic circular dichroism spectra of low spin Fe(III)-heme compounds

The magnetic circular dichroism (MCD) of metmyoglobin cyanide, ferricytochromec and horseradish peroxidase cyanide were measured in the region 340–800 nm over a range of temperatures from 293 to 15 K. All three species show the temperature dependent MCD (TheC-type effects ∼1/T) in both visibleQ and near UVB bands. While the MCD and absorption inB- region as well as the absorption inQ region are quite similar for all three species the MCD inQ- bands reveal the marked differences, especially at low temperatures. To explain these observations, the theoretical treatment based on our previous model (A. P. Mineyev and Yu. A. Sharonov, 1978, Theoret. Chim. Acta (Berl.)49, 295–307) is proposed. The key point of this consideration is the configuration π-d- interaction which in addition to our previous analysis involves the first excited Fe(III)-ion Kramers doublet and theB-Q-mixing effects. The simultaneous least square fit of MCD and absorption data allows to evaluate the π —d- parameters which appear to be of the order of 10²−10³ cm⁻¹. The role of the π -d- interaction in the forming of hemoprotein spectra are discussed.     

Investigation of the photoinduced magnetization of copper octacyanomolybdates nanoparticles by X-ray magnetic circular dichroism

Through an extensive set of SQUID magnetic measurements, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism, we have determined the nature of the metastable photomagnetic phase in the cyano-bridged 3D network Cs(2)Cu(7)[Mo(CN)(8)](4). The photomagnetic effect is induced by the photoconversion of Mo(IV) ions in low spin (LS) configuration (S = 0) into Mo(IV) ions in high spin (HS) configuration (S = 1). The magnetic and spectroscopic measurements fully support the LS to HS conversion, whereas the previously invoked charge transfer mechanism Mo(IV) + Cu(II) ? Mo(V) + Cu(I) can be completely ruled out.     

Magnetic circular dichroism spectrum of the triphenylene-iodine complex

The MCD CT bands of the triphenylene (9,10-benzphenanthrene) -I₂, triphenylene-TCNE and I₂-I₂ complexes are indicative of a structure of C_3v symmetry where the I₂ is located centrally and axially over the triphenylene ring.     

Jahn-Teller instability and the magnetic circular dichroism of a tetragonal molecular system

The consequences for the absorption and magnetic circular dichroism (m.c.d.) spectra of Jahn-Teller coupling in an E orbital state of a molecule with C _4v symmetry are investigated. The models used cover both static and dynamic Jahn-Teller interactions, the former allowing simple illustration of the interactions and their effects on the m.c.d. spectra. The moments of the spectra are discussed, as is the orbital reduction factor which is found to be oscillatory as a function of excitation energy in the presence of Jahn-Teller coupling.     

Mutation in the flavin mononucleotide domain modulates magnetic circular dichroism spectra of the iNOS ferric cyano complex in a substrate-specific manner

We have obtained low-temperature magnetic circular dichroism (MCD) spectra for ferric cyano complexes of the wild type and E546N mutant of a human inducible nitric oxide synthase (iNOS) oxygenase/flavin mononucleotide (oxyFMN) construct. The mutation at the FMN domain has previously been shown to modulate the MCD spectra of the l-arginine-bound ferric iNOS heme (Sempombe, J.; et al. J. Am. Chem. Soc. 2009, 131, 6940-6941). The addition of l-arginine to the wild-type protein causes notable changes in the CN(-)-adduct MCD spectrum, while the E546N mutant spectrum is not perturbed. Moreover, the MCD spectral perturbation observed with l-arginine is absent in the CN(-) complexes incubated with N-hydroxy-L-arginine, which is the substrate for the second step of NOS catalysis. These results indicate that interdomain FMN-heme interactions exert a long-range effect on key heme axial ligand-substrate interactions that determine substrate oxidation pathways of NOS.     

Spin relaxation of a short-lived radical in zero magnetic field

A short-lived radical containing only one I = 1/2 nucleus, the muoniated 1,2-dicarboxyvinyl radical dianion, was produced in an aqueous solution by the reaction of muonium with the dicarboxyacetylene dianion. The identity of the radical was confirmed by measuring the muon hyperfine coupling constant (hfcc) by transverse field muon spin rotation spectroscopy and comparing this value with the hfcc obtained from DFT calculations. The muon spin relaxation rate of this radical was measured as a function of temperature in zero magnetic field by the zero field muon spin relaxation technique. The results have been interpreted using the theoretical model of Fedin et al. (J. Chem. Phys., 2003, 118, 192). The muon spin polarization decreases exponentially with time after muon implantation and the temperature dependence of the spin relaxation rate indicates that the dominant relaxation mechanism is the modulation of the anisotropic hyperfine interaction due to molecular rotation. The effective radius of the radical in solution was determined to be 1.12 ± 0.04 nm from the dependence of the muon spin relaxation rate on the temperature and viscosity of the solution, and is approximately 3.6 times larger than the value obtained from DFT calculations.     

Probing a complex of cytochrome c and cardiolipin by magnetic circular dichroism spectroscopy: implications for the initial events in apoptosis

Oxidation of cardiolipin (CL) by its complex with cytochrome c (cyt c) plays a crucial role in triggering apoptosis. Through a combination of magnetic circular dichroism spectroscopy and potentiometric titrations, we show that both the ferric and ferrous forms of the heme group of a CL:cyt c complex exist as multiple conformers at a physiologically relevant pH of 7.4. For the ferric state, these conformers are His/Lys- and His/OH(-)-ligated. The ferrous state is predominantly high-spin and, most likely, His/-. Interconversion of the ferric and ferrous conformers is described by a single midpoint potential of -80 ± 9 mV vs SHE. These results suggest that CL oxidation in mitochondria could occur by the reaction of molecular oxygen with the ferrous CL:cyt c complex in addition to the well-described reaction of peroxides with the ferric form.     

Recognition of guanines at a double helix-coil junction in DNA by a trinuclear copper complex

Coordination between guanine N7 and a trinuclear copper complex appears critical for selective and efficient strand scission of DNA at a helix-coil junction as indicated by the lack of reactivity of comparable DNA containing 7-deazaguanine in place of guanine; both the base pair at the junction and coil flexibility also modulate the specificity of DNA oxidation.     

Inhomogeneous electrodeposition of copper in a magnetic field

Normally, magnetoconvection driven by the Lorentz force increases the limiting current in the mass-transport limited regime, roughly as the one-third power of the applied magnetic field. Here we show that an applied field can actually diminish the rate of copper electrodeposition at low overpotentials. The effect is related to the formation of a vortex at the leading edge of the flow. Similar, but weaker effects are due to gravity.     

Rotationally resolved magnetic vibrational circular dichroism of the paramagnetic molecule NO

Magnetic vibrational circular dichroism (MVCD) enables the measurement of molecular magnetic moments with modest spectral resolution. Due to its paramagnetism, NO gives a much stronger spectral response, about 3 orders of magnitude more intense, than do typical diamagnetic molecules. The molecule thus provides a convenient test for the experiment and theory of paramagnetic rotors. We have measured and analyzed the MVCD, equivalent to the molecular Zeeman spectra, of NO in co-linear magnetic fields of 0.1, 0.2, 2, 4 and 8 Tesla. Similar MVCD intensities were observed for both the (2)Π(1/2) and (2)Π(3/2) components of NO, particularly for high J values, which demonstrates a considerable deviation from pure Hund's case (a) for NO. The g(J)-values for the (2)Π(1/2) components of NO, which can be determined from our experimental spectra by moment analysis, agree well with the predicted values from Radford's theory. For the (2)Π(3/2) components, we tested this theory by simulating the MVCD and absorption spectra, and comparing them with our experimental spectra by use of moment analysis to show that they match well in terms of magnetic properties. While 0.2 T experiments easily develop sufficient MVCD for analysis of NO spectra and these low field intensities have a linear field dependence, spectra in the strong fields accessible in our study showed non-linear response due to onset of saturation effects. We also observed a strong field dependence for the absorption intensities for some (2)Π(3/2) components that was not encompassed in the theoretical model. Finally, a full coupling scheme provided analytical MVCD and absorption intensities that were in good agreement with the experimental values.     

Electronic transitions and magnetic circular dichroism of the naphthalene monogenegative ion

The optical absorption and magnetic circular dichroism (MCD) spectra of the mononegative ion of naphthalene in a solution of 2-methyltetrahydrofuran have been measured at room temperature over the 25000–41000 cm^?1 spectral region. Experimental values of the MCD parameter B/D are compared with theoretical data obtained by means of an LCAO SCF CI calculation according to Pariser, Parr and Pople. The agreement between theory and experiment is rather good.     

Fourteen-electron ring model and the anomalous magnetic circular dichroism of meso-triarylsubporphyrins

The MCD spectra of meso-triarylsubporphyrins show a sign anomaly which is correlated with the acceptor properties of the aryl substituent. From the spectra, magnetic moments of the excited states are determined. In the context of a simplified orbital model, the sign change is attributed to the quenching of the magnetic moment of the LUMO by acceptor orbitals of the substituent. The actual calculation of this moment presents a major challenge to computational methods. It is shown that wave function techniques based on CASSCF underestimate the covalency effects that are responsible for the quenching. In contrast, a CI method based on DFT orbitals yields excellent results, which fully support the orbital model.     

The circular dichroism spectra of the cyclodextrin complex with 1,4-benzocyclooctenedione

The assignment of the absorption spectra of 1,4-benzocyclooctenedione (1) is reported by measuring the circular dichroism spectra of the β-cyclodextrin complex with 1. It is concluded from the signs of the circular dichroism bands that the first (16.6 × 10³−27.4 × 10³ cm⁻¹) absorption band is composed of two electronic transitions having perpendicular polarizations with respect to the long axis of 1, the second (27.4 × 10³−35.8 × 10³ cm⁻¹) absorption band has the transition dipole moment parallel to the long axis of 1 and the third (35.8 × 10³−44.3 × 10³ cm⁻¹) absorption band is composed of two electronic transitions having perpendicular and parallel polarizations with respect to the long axis of 1.     

The circular dichroism spectra of the β-cyclodextrin complex with azulene

When azulene is included in the cavity of β-cyclodextrin (β-CDx), induced circular dichroism (CD) bands are observed in the corresponding absorption bands of azulene. On the basis of the theoretical conclusions for β-CDx complexes with naphthalene derivatives of Harata and Uedaira, it is concluded from the signs of the CD bands that the first (about 455 to 715 nm), third (about 290 to 305 nm) and fifth (at about 238 nm) absorption bands have the transition dipole moments perpendicular to the long axis and the second 305 to 360 nm), fourth (about 240 to 290 nm) and sixth (shorter than 220 nm) absorption bands have the transition dipole moments parallel to the long axis of azulene. Our assignments are in complete agreement with earlier assignments. Our experimental results do not provide any information regarding two new electronic transitions suggested theoretically by Thulstrup et al.     

Inversion of the sign of the solid-state circular dichroism at low temperature

-Ni(H₂O)₆ · SO₄ and its selenate derivative exhibit chirality only in the solid-state. We have observed, for the first time, a sign inversion of CD (circular dichroism) in the ³A_2g → ³T_1g(P) Ni(II) d–d transition at near liquid nitrogen temperatures. The novel finding was achieved by building a new cooling unit to a solid-state specialized Universal Chiroptical Spectrophotometer (UCS-1) [R. Kuroda, T. Harada, Y. Shindo, Rev. Sci. Instrum. 72 (2001) 3802.] and by formulating an analytical procedure to obtain artifact-free CD signals based on the Mueller matrix method. The sign inversion is remarkable as the crystal structure hardly changes from 300 to 100 K. The origin of the sign inversion is discussed.     

Synthesis, crystal structure and DFT analysis of a new trinuclear complex of copper

The new trinuclear complex [Cu₂(μ-L)₂CuCl₂] has been synthesized and characterized by elemental analysis, IR, UV-Vis and X-ray spectroscopy, where L is a dianionic tetradentate Schiff base ligand with N₂O₂ donor atoms. The molecular structure of [Cu₂(μ-L)₂CuCl₂] was determined by X-ray crystallography. In the complex, the most remarkable aspect of the trinuclear complex is that it adopts a bent structure for the three copper atoms, with a Cu1Cu3Cu2 intramolecular angle of approximately 90.62(2)°. All three copper atoms are five coordinate, with a slightly distorted square pyramidal geometry. In the two terminals moieties, the basal plane of the square pyramidal is formed by two oxygen atoms and two nitrogen atoms of the Schiff base ligand, and the apical position at the Cu atom is occupied by the bridging Cl1 anion. The Cu1Cl1Cu2 angle is 110.51(5)°. The central copper atom also has a five-coordinate, slightly distorted square pyramidal geometry, with four phenolato oxygens belonging to the Schiff base ligands from Cu(salpn) units describing the square planar base and the Cl anions being apical. The optimized structure of the complex has been studied using the B3LYP/6-31G(d)/LanL2TZf level of theory. The calculation shows that all the copper atoms are five coordinate with distorted square pyramidal structures, which is consistent with experimental data.     

Numerical simulation of the onset of mass transfer and convection in copper electrolysis subjected to a magnetic field

Numerical investigations have been performed in order to simulate the transient behaviour of copper electrolysis in a rectangular cell with vertical electrodes in a galvanostatic regime where the kinetics of the electrodes is controlled by charge-transfer. The transient behaviour observed for a binary electrolyte reproduces the temporal evolution of the concentration distribution measured in recent experimental work. Horizontal magnetic fields that vary linearly between the electrodes create Lorentz forces that either enhance or attenuate natural convection. The different time scales of natural convection and convection driven by the Lorentz force lead to interesting transient effects. The simulations performed for the case of an attenuating Lorentz force explain the dynamics of vertical inhomogeneities of the concentration boundary layer during the initial stages of electrolysis that were previously observed experimentally.