Magnetic-Field-Dependent Electronic Relaxation of Gd3+ in Aqueous Solutions of the Complexes [Gd(H2O)8]3+, [Gd(propane-1,3-diamine-N,N,N′,N′-tetraacetate)(H2O)2]−, and [Gd(N,N′-bis[(N-methylcarbamoyl)methyl]-3-azapentane-1,5-diamine-3,N,N′-triacetate)(H2O)] of interest in magnetic-resonance imaging

EPR Spectra have been measured for aqueous solutions of a series of Gd³⁺ complexes at variable temperature and a range of magnetic fields; S-band (0.14 T), X-band (0.34 T), Q-band (1.2 T), and 2-mm-band (5.0 T). The major contribution to the observed line widths is magnetic-field-dependent and is interpreted as being due to the modulation of the zero-field splitting produced by distortion of the complexes from perfect symmetry. The transverse and longitudinal relaxation matrices for an ⁸S ion with such an interaction have been calculated using Redfield theory with vector-coupling methods, and diagonalised numerically to obtain relaxation rates and intensities for the degenerate transitions which contribute to the multiplet. The observed line width, which is inversely proportional to the magnetic field at low temperatures, is best described by the intensity-weighted mean transverse relaxation time for the four transitions with non-zero intensity. A least-squares fit of the data yields the square of the zero-field splitting tensor, Δ², and a correlation time, τ_v, with activation energy, E_v. The physical significance of these parameters and the extent of validity of the theoretical approach are considered. The parameters are used to predict the magnetic-field dependence of the longitudinal and transverse electronic relaxation times, which are discussed in the context of their relevance to ¹H-NMR relaxivity.     

Laser magnetic resonance detection of rotational transitions in CH2

Laser magnetic resonance spectra observed at 163.0 μm have been identified as pure rotational transitions in the ground electronic state (³B₁) of the methylene radical. The identification was based on the observation of hyperfine spin triplets and by isotopic substitutions involving deuterium and ¹³C, as well as other chemical and spectroscopic evidence. A low-pressure flame produced by reacting discharged fluorine with methane was found to be an excellent new spectroscopic source of ground-state CH₂ radicals.     

The two-photon fluorescence excitation spectrum of dibenzothiophene

The two-photon fluorescence excitation spectra of dibenzothiophene crystal (≈77 K) and solution have been investigated in the spectral region 30000–38000 cm^?1 (≈3350-2600 Å). Two electronic transitions have been found. Both the oriented gas model results applied to the crystal and the circular/linear polarization ratio to the solution claim for an ¹A₁ ← ¹A₁ assignment of the transitions. Not completely resolved vibronic structures in the crystal spectrum were observed for both transitions for which a tentative assignment to A₁ total symmetry is discussed. It has been found that the intensity of the first electronic band system arises not only from a purely electronic (i.e., Franck-Condon) mechanism, but also from its interaction with a higher state through totally symmetric vibrations.     

Magnetic circular dichroism and molecular orbital studies on bicyclo[6,2,0] decapentaene

The magnetic circular dichroism (MCD) spectrum of bicyclo[6,2,0] decapentaene has revealed four skeletal π → π^* electronic transitions in the visible and ultraviolet region. The four MCD bands are assigned to the B₂ ← A₁, A₁ ← A₁. B₂ ← A₁ and A₁ ← A₁ electronic transitions in increasing order of energy.     

Conformation‐Determined Through‐Bond versus Through‐Space Electronic Communication in Mixed‐Valence Systems with a Cross‐Conjugated Urea Bridge

Bis‐triarylamine 2 and cyclometalated diruthenium 6 (PF₆)₂ with a linear trans,trans‐urea bridge have been prepared, together with the bis‐triarylamine 3 and cyclometalated diruthenium 8 (PF₆)₂ with a folded cis,cis‐N,N‐dimethylurea bridge. The linear or folded conformations of these molecules are supported by single‐crystal X‐ray structures of 2 , 3 , and other related compounds. These compounds display two consecutive anodic redox waves (N ^{. +/0} or Ru^III/II processes) with a potential separation of 110–170 mV. This suggests that an efficient electronic coupling is present between two redox termini through the cross‐conjugated urea bridge. The degree of electronic coupling has been investigated by using spectroelectrochemical measurements. Distinct intervalence charge‐transfer (IVCT) transitions have been observed for mixed‐valent (MV) compounds with a linear conformation. The IVCT transitions can also be identified for the folded MV compounds, albeit with a much weaker intensity. DFT results support that the electronic communication occurs by a through‐bond and through‐space pathway for the linear and folded compounds, respectively. The IVCT transitions of the MV compounds have been reproduced by TDDFT calculations. For the purpose of comparison, a bistriarylamine and a diruthenium complex with an imidazolidin‐2‐one bridge and a urea‐containing mono‐triarylamine and monoruthenium complex have been synthesized and studied.     

Intermolecular nuclear relaxation in paramagnetic solutions: from free radicals to rare earths

The principles of the intermolecular relaxation of a nuclear spin by its fluctuating magnetic dipolar interactions with the electronic spins of the paramagnetic surrounding species in solution are briefly recalled. It is shown that a very high dynamic nuclear polarization (DNP) of solvent protons is obtained by saturating allowed transitions of free radicals with a hyperfine structure, and that this effect can be used in efficient Earth field magnetometers. Recent work on trivalent lanthanide Ln³⁺ aqua complexes in heavy water solutions is discussed, including paramagnetic shift and relaxation rate measurements of the ¹H NMR lines of probe solutes. This allows a determination of the effective electronic magnetic moments of the various Ln³⁺ ions in these complexes, and an estimation of their longitudinal and transverse electronic relaxation times T_1e and T_2e. Particular attention is given to Gd(III) hydrated chelates which can serve as contrast agents in magnetic resonance imaging (MRI). The full experimental electronic paramagnetic resonance (EPR) spectra of these complexes can be interpreted within the Redfield relaxation theory. Monte-Carlo simulations are used to explore situations beyond the validity of the Redfield approximation. For each Gd(III) complex, the EPR study leads to an accurate prediction of T_1e, which can be also derived from an independent relaxation dispersion study of the protons of the probe solutes.     

First‐principles calculation of electronic structure of V‐doped anatase TiO2

The energetic and electronic structures of V‐doped anatase TiO₂ have been investigated systematically by the GGA+U approach, including replacement of Ti by V in the absence and presence of oxygen vacancies and the presence of an interstitial site. It was found that V should exist as a V⁴⁺ ion in the replacement of Ti in the anatase lattice, the electron transitions of which to the conduction band from V 3d states are responsible for the experimentally observed visible light absorption. The influence of V dopant concentration on the electronic and magnetic properties is also discussed, such as the influence of the U value in systems containing oxygen vacancies and spin flip phenomena for interstitial V‐doping.     

Time-resolved electronic Raman spectrum of terbium aluminum garnet excited with visible and UV laser sources

Excitation profiles for the intensities of electronic Raman transitions between crystal field components of the ⁷F₆ and ⁷F₅ manifolds of terbium aluminum garnet are recorded for excitation in the spectral region where absorption bands due to levels of the ⁵D₄ manifold occur. The intensities of the electronic transitions are not enhanced which is thought to be caused by the small values of electric dipole matrix elements of the resonating electronic states in comparison to the values of such elements to other intermediate states which occur in the expression for the scattering tensor. Fluorescence from the ⁵D₄ levels is induced and resonance fluorescence are time resolved with respect to the Raman transitions. We report electronic Raman transitions excited with the 308.0 nm line of an XeCl excimer laser. As opposed to excitation with visible laser sources, transitions are recorded which terminate on all the crystal field levels of the ⁷F_5…0 levels. In addition, fluorescence from ⁵D₃ to the ground state of terbium aluminum garnet is also observed.     

Density Functional Theory Investigation of Structures and Electronic Spectra of N-protonated Corroles

The geometries and electronic spectra of a series of N-protonated corroles, including unsub-stituted H₄Cor⁺ and meso-triaryl substituted H₄TPC⁺, H₄TpFPC⁺, and H₄TdCPC⁺, were theoretically studied with density functional theory (DFT). The results indicate that all these compounds have two conformers, one with C₂ symmetry (denoted as S1) is more stable than the other (denoted as S2, C₁ symmetry) by 15.8?18.5 kJ/mol. The corrole macrocycles of these compounds show significant out-of-plane deformation. The enantiomerizations of the chiral S1 conformers were found to be a multi-step process with the S2 conformers as the in-termediates. Electronic absorption spectra and electronic circular dichroism (ECD) of these compounds were calculated with time-dependent DFT. In comparison with H₄Cor⁺, the UV-Vis absorptions of meso-triaryl species are significantly red-shifted and their Q bands are enhanced due to the π-π conjugation between the aryl and corrole rings. Several neighboring electronic transitions were calculated with opposite signs in rotatory strengths, suggesting that ECD spectroscopy may be a useful tool in studying the electronic transitions of these compounds.     

ELECTRICAL PROPERTIES OF M(S2C2(CN)2)2 1-(M=Ni,Pt) COMPLEXES

Abstract

Single crystal conductivity studies over a temperature range of 200°K to 400°K have been carried out on various M(S₂C₂(CN)₂)₂ ^1-(M=Ni,Pt) salts. A room temperature conductivity of ～ 10^?10ohm^?1 cm^?1 and an E_a?0.5 eV was observed for each of these systems. These results are correlated with the magnetic, electronic, and crystallographic properties. The semiconducting behavior of these systems is related to the electronic structure of the molecular anion.     

Excited states of the polyacetylenes. Analysis of the near ultraviolet spectra of diacetylene and triacetylene

The 2000–2800 Å system of diacetylene and the 2300–3000 Å system of triacetylene have been investigated. Analysis of the vibrational structure indicates that we observe transitions to a ¹Δ_u upper electronic state for both molecules and transitions to a ¹Σ^?_u state for diacetylene.     

Metal Chelate Complexes of the Schiff Bases Derived from Salicylaldehyde and Methyl Ethers of Aminophenols (Anisidines)

Some transition metal chelates of the Schiff's bases derived from salicylaldehyde and o -anisidine, m -anisidine and p -anisidine have been isolated and characterized. The structures of these complexes have been discussed on the basis of their elemental analyses, conductivities, magnetic moment values, electronic and infrared spectral data. The Ni²⁺, Cu²⁺ and Zn²⁺ form ML₂ type compounds, UO₂²⁺ and Fe³⁺ form dimeric [UO₂ L₂]and [Fe L₂ Cl]₂ respectively.     

Potential energy curves and spectral properties for electronic states of F2 and F+2

Potential energy (PE) curves for the Rydberg states of F₂, and for the ground and lowest two electronic states each of symmetry ²Π_g,u, ²Δ_g,u and ²Σ^±_g,u of F⁺₂, have been obtained using modest-sized configuration-interaction calculations. These PE curves have been used to calculate spectroscopic constants for the electronic states and the results agree reasonably well with the limited experimental and theoretical results previously reported. The theoretical PE curves for the Rydberg states of F₂ are found to be strongly perturbed by valence-Rydberg-ionic interactions and these perturbations appear to be responsible for certain features in recently reported electron energy-loss spectra in F₂. The corresponding electronic wavefunctions have been used to calculate the electronic transition moment, as a function of the internuclear distance, for dipole-allowed transitions between the lowest excited electron state of each symmetry and the appropriate ground electronic state. The radiative emission probabilities, natural lifetimes, and absorption oscillator strengths, for each band system, are also reported here. The predicted lifetimes for vibrational levels of the A ²Π_u of electronic state in F⁺₂ vary from 1.3–1.5 μs and agree reasonably well with the single available set of measurements. The predicted radiative lifetimes for the higher electronic states of F⁺₂ are substantially longer and fall into the range 5–100 ms.     

UV and VUV spectrum of matrix-isolated In: an investigation by absorption,magnetic circular dichroism and emission yield spectroscopy

The electronic absorption spectra of In atoms isolated in neon, argon, krypton and xenon matrices have been measured in the energy range between 2.5 and 9.0 eV. This region includes the 5s²5p → 5s²6s and 5s²5p → 5s²5d resonance transitions, higher members of the corresponding s- and d-Rydberg series and the inner shell 5s²5p → 5s5p² transitions. A correlation of the absorption spectra with results obtained from magnetic circular dichroism and fluorescence measurements has made it possible to provide a detailed assignment of most of the features in the spectra in spite of the complexities associated with their behavior. For example, the transition to 5s²6s could not be detected in any of the matrices and the 5s²5d configuration was found to be strongly quenched in intensity as compared to the other transitions. In contrast, several Rydberg transitions could be observed for In in Ne. These were satisfactorily interpreted within the Frenkel formalism. Some of these observations have been rationalized by assuming that the average radius of the wavefunction for the excited state is the dominant parameter for the matrix interaction.     

Theoretically derived absorption and MCD spectral data for the cellular species produced in the hydrolysis of cis-diamminedichloroplatinum(II)

The cellular species formed in the hydrolysis of cis-Pt(NH₃)₂Cl₂ (DDP), namely, cis-[Pt(NH₃)₂XY]ⁿ⁺ (X, Y = Cl^?, H₂O, OH^?; n = 0, 1, 2) have been investigated theoretically using the relativistic and nonrelativistic extended Huckel molecular orbital method. Molecular orbital (MO) results for trans-DDP and its hydrolysis products are also reported for comparison. Transition energies, molar absorption coefficients (?), and B terms from magnetic circular dichroism (MCD) derived from theory are presented for each of the species studied. The electronic absorption and MCD spectra of all the complexes are predicted to exhibit ligand field transitions arising primarily from excitations between the occupied Pt 5d orbitals and the unoccupied Pt 5d and 6p_z orbitals, respectively. The 5d → 6p_z transitions are expected to yield intense absorptions in the UV spectral region. Some intensity is generated in the d → d transitions as a result of the low symmetry of these complexes. Correlation of available experimental data with theory allows spectral assignments to be made and predicted. Substituent effects in the cis- and trans-isomeric species are discussed. Finally, the applicability of the EHMO method to these systems is examined.     

Structural and electronic evolution of Cr2O3 on compression to 55 GPa

Synchrotron single-crystal x-ray diffraction experiments have been performed on corundum-type Cr₂O₃ up to a pressure of 55 GPa in Ne and He pressure transmitting media. Diffraction experiments were complemented by measurements of optical absorption spectra with single crystal samples up to 60 GPa. Results of the diffraction data analysis rule out the earlier reported monoclinic distortion at 15–30 GPa, but indicate evidence of two discontinuous transitions of electronic or magnetic nature, most likely associated with a change in magnetic ordering and charge transfer. The compression mechanism established from single crystal refinements indicates much smaller distortion of the Cr³⁺ coordination environment than was previously assumed.     

Elektronenspektren und Bindungsverhältnisse in den Ionen PS43−, AsS43−, und SbS43−

The electronic absorption spectra of the ions PS₄^3?, AsS₄^3? and SbS₄^3? have been measured in aqueous solution. The transitions are assigned and discussed, using a simple MO diagram.     

Near infrared electronic spectrum of TCNQ mono-valent anion

The high resolution near infrared electronic spectrum of TCNQ anion dissolved in 2-methyltetrahydrofuran glass at 77 K has been determined. The absorption bands are interpreted as simple progressions of two molecular vibrations in a single electronic excited state with ν₀₀ = 11661 cm^?1. The molecular vibrations (ω′₁ = 1264 ± 3 cm^?1, ω′₂ = 335 ± 3 cm^?1) of the vibrational progression agree well with observed Raman active transitions. The experimental data do not require the presence of two electronic transitions in the 1.3 to 2.1 eV region, contrary to what had been assumed previously on the basis of less well resolved room temperature spectra.     

Absorption spectrum of Yb3+(4f13) IN[(C6H5)3PH]3 YbCl6

Optical absorption spectra taken at 300 and 77 K are reported for six-fold octahedrally coordinated Yb³⁺(4f¹³) in [(C₆H₅)₃PH]₃ YbCl₆. In addition to vibronic spectra we observe electronic transitions which suggest that inversion symmetry is lifted by a small distortion similar to that reported for the Nd³⁺(4f³) spectrum of the corresponding salt. The vibronic as well as the electronic transition have been analyzed on the basis of six-fold octahedral symmetry. The analysis appears reasonable and consistent in comparison with other rare-earth ions that have been studied in similar cubic and octahedral environments.     

Synthesis,Crystal Structure and Density Functional Studies on 1N‐Phenyl‐3‐(2,4‐Dichlorophenyl)‐5‐(4‐Chlorophenyl)‐2‐Pyrazoline

1N‐Phenyl‐3‐(2,4‐dichlorophenyl)‐5‐(4‐chlorophenyl)‐2‐pyrazoline has been synthesized and characterized by elemental analysis, IR, UV‐Vis and X‐ray single crystal diffraction. Density functional calculations have been carried out for the title compound by using the B3LYP method with a 6‐311G** basis set. The calculated results show that the predicted geometry can reproduce well the structural parameters. The electronic absorption spectra calculated in the gas phase are better than those calculated in EtOH solvent to model the experimental electronic spectra. Natural Bond Orbital (NBO) analyses suggest that the above electronic transitions are mainly assigned to π → π* transitions. On the basis of vibrational analyses, the thermodynamic properties of the compound at different temperatures have been calculated, revealing the correlations between C⁰_{p, m}, S⁰_m, H⁰_m and temperature.