Molecular Structure of Neodymium Diiodide

The molecular structure of neodymium diiodide was investigated in an electron diffraction study at T _exp = 795(10) K. The molecule has C_2v symmetry; the internuclear distance R_g(Nd–I) is 297.3(3) pm and _g = 132(3)°. The vibration frequencies were estimated from the experimental values of the root-mean-square vibration amplitudes.     

Molecular Structure of PrBr3 and HoBr3 According to Simultaneous Electron Diffraction and Mass Spectrometry Experiment

The saturated vapors of praseodymium and holmium tribromides were investigated for the first time by electron diffraction with mass spectral monitoring at 1100(10) and 991(10) K. It is established that the molecules have a pyramidal effective configuration with bond angles Br–Pr–Br = 114.7(10)° and Br–Ho–Br = 115.3(11)°. Given the low deformation vibration frequencies of lanthanide tribromide molecules, the insignificant pyramidality of the r_g configuration may correspond to the planar equilibrium geometry of D_3h symmetry for the molecules. The internuclear distances r_g(Pr–Br) = 2.696(6) and r_g(Ho–Br) = 2.594(5) point to the lanthanide compression effect. The vibration frequencies of PrBr₃ and HoBr₃ molecules were estimated from electron diffraction data.     

Equilibrium Structure of Beryllium Dibromide from Combined Gas Electron Diffraction and Vibrational Spectroscopy Analysis

The molecular structure of BeBr₂ has been investigated by gas-phase electron diffraction at the temperature 800(10) K. The conventional analysis yielded the following values: r _g(Be–Br) = 1.944(6)Å, l(Be–Br) = 0.068(4)Å, r _g(Br–Br) = 3.848(8)Å, l(Br–Br) = 0.109(3)Å, k(Be–Br) = 1.1(1.1) × 10^–5 ų, (Br–Br) = 2.1(1.0) × 10^–5 ų. Three models of nuclear dynamics were used to simulate the conventional analysis values—infinitesimal vibrations and two models, which take into account the kinematic and dynamic anharmonicity of the bending vibration. All models give similar values of bond angle, amplitudes, and shrinkage, excluding the harmonic model, which yields too low value l(Br–Br). The equilibrium bond distance r _e(Be–Br) = 1.932(11) Å was estimated, taking into account the anharmonicity corrections for stretching and bending vibrations and centrifugal distortion.     

Gas Electron Diffraction and Quantum-Mechanical Study of Dimethyloxalate

The geometrical structure and conformation of dimethyloxalate, CH₃OC(O)–C(O)OCH₃, have been studied by gas electron diffraction (GED) and quantum-chemical calculations (MP2 and B3LYP methods with 6-31G* and cc-pVTZ basis sets). The GED analysis with a dynamic model (T = 323 K) results in a mixture of two planar conformers, anti (C_2h symmetry) and syn (C_2v symmetry) orientation of the two C=O bonds. The energy difference between these conformers is 0.02(0.18) kcal/mol and barrier to internal rotation around the C–C bond is 0.44(0.41) kcal/mol. The CH₃ groups occupy synperiplanar positions with respect to the C=O bonds. The following main geometrical parameters for the anti conformer (Å and degrees) have been derived: r_g(C–C) = 1.532(3), r_g(C=O) = 1.203(2), r_g(C_sp3–O) = 1.436(3), r_g(C_sp2–O) = 1.333(3), (C_sp2–C_sp2–O) = 111.9(1.9), (C_sp2–O–C_sp3) = 116.3(1.6), (O–C= O) = 127.0(1.8).This paper is devoted to the 75th anniversary of gas electron diffraction method.     

Synthesis,crystal structure and spectroscopic characterization of a hydrogen-bonded 1D copper(II) complex

Novel 1:2 and 1:1 (M:L) copper(II) complexes have been prepared from the tridentate ligand 2-(1-methyl-2-aza-5-oxapentyl)phenol (H₂L¹). The crystal structure of [Cu(HL¹)₂] (1) exhibits a noncentrosymmetric square-planar geometry with a slightly tetrahedral distortion. The Cu^II atom is coordinated by two amino N and two phenoxo O atoms of two (HL¹)^– ligands. The phenoxo and the alkoxy groups are involved in two strong intramolecular hydrogen bonds. The coordination moieties are further connected to a 1D linear structure by the action of intermolecular hydrogen bonds between the alkoxyl and the amino groups. The importance of steric hindrance introduced by the methyl group in the molecular structure and the packing of the complex molecules has been demonstrated. The e.p.r. parameters of (1) have been obtained: g = 2.231, g = 2.005, g _iso = 2.080, A = 185.0 G, A _iso = 86.5 G, A = (3A _iso – A )/2 = 37.3 G. These results confirm a distorted square planar stereochemistry with a ( )¹ ground state.     

Sorption of Cu2+ ions by chitin and chitosan from aqueous solutions. Molecular structure of complexes formed

The sorption of Cu²⁺ ions by chitin and chitosan from aqueous solutions has been investigated, as well as the molecular structure of the complexes formed. The static exchange capacities have been determined, equal to 3.5 and 0.25 mmole/g for chitosan and chitin, respectively, and the partition coefficients (5000 and 70 g/ml). It has been shown that in complex formation a bond with the amino group is formed as the result of the substitution of a proton in the latter. The EPR spectra of these complexes have been obtained and their radiospectroscopic parameters determined (g = 2.334,g = 2.054,A = 0.0156 cm^–1, andB = 0.0028 cm^–1 for chitin, andg = 2.231,g = 2.048,A = 0.0192 cm^–1, andB = 0.0025 cm^–1 for chitosan). For chitosan the ligands are two nitrogen atoms of the amino groups and two oxygen atoms of the hydroxyl groups in the position C³ of adjacent glucosamine rings; for chitin, the oxygen atoms of the acetyl groups take part in addition in the complex formation. The analysis of the radiospectroscopic parameters and their comparison with published data lead to the conclusion that the Cu²⁺ complex with chitosan has a tetragonal symmetry, while the complex with chitin most probably has an octahedral structure.Institute of Physical Chemistry, Russian Academy of Sciences, 117915 Moscow. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 10, pp. 2305–2311, October, 1992.     

The Photochemical Behavior of the Uranyl Ion (UO$\rm{{_{2}^{2+}}}$): a Dimer Perspective and Outlook

The complex formation of uranyl (UO ) with oxalic acid (HOOC? COOH) in acetone is studied by UV/VIS, absorption, luminescence, and excitation spectroscopy. Based on solid‐state crystallographic data, we propose a dimer structure with D_2h symmetry for the complex in solution. This symmetry is vibrationally distorted to D₂ by the out‐of‐plane equatorial‐ligand vibration. From the spectroscopic point of view, this vibration induces intensity in the transitions Π_g←Σ and one component of Δ_g←Σ . From the photochemical point of view, this vibration induces a twisting mechanism that destroys the complex. From the theoretical point of view, it is worthwhile to notice that the symmetry of the odd out‐of‐plane vibration is the same as the symmetry of the odd LUMO (f_xyz). By vibrating accordingly to the LUMO symmetry, the complex is self‐destroying by absorption of light, and the uranyl is regenerated. A small comment is devoted to a possible δ–δ interaction and the quintuple U₂ bond distance proposed by Gagliardi and Ross [29].     

An e.p.r. study of tetradentate Schiff base copper complexes with an N–(CH2)n–N,n = 3–6 backbone

Cu^II complexes of Schiff-base ligands with a general formulation o-HOC₆H₄CH=N–(CH₂) _n –N=CHC₆H₄OH-o, where n = 3–6 have been prepared and their e.p.r. spectra investigated in order to determine the effect of the flexible methylene backbone length on the structure. The room temperature and 77 K e.p.r. spectra of the compounds, n = 3 and 4, are typical of the axially symmetric ground state with g > g . When n = 5, on the other hand, the complex gives an isotropic spectrum at room temperature. For n = 6, g appears to be greater than g . The g _iso value increases gradually from n = 3 to n = 6 indicating deviation from planarity. Simulation of the 77 K spectrum for n = 6 shows the presence of two distinct Cu²⁺ sites of equal probability. The Q-band spectrum of this compound exhibits a narrowed g signal indicative of exchange coupling. The spectrum is a consequence of intermolecular electron exchange giving a pseudo d² _z state.     

Intensity distribution in the vibronic Γ7(2 T 2g )→Γ8(4 A 2g ) transition of ReBr 6 2− in Hexahalogenostannate host crystals

The phosphorescence spectra of ReBr ₆ ^2– doped A₂SnX₆ (A = K, Rb, Cs; X = Cl, Br) have been measured at 10 K. The spectra consist of a weighted sum of progressions associated with the local modes of the ReBr ₆ ^2– center. By a fit to a generalized Lorentzian line shape function the totally symmetric distortion of the ₇(² T _2g ) excited state relative to the ₈(⁴ A _2g ) ground state has been determined.Dedicated to Professor Dr. H.-H. Schmidtke on the occasion of his 50th birthday.     

Distorted pentacoordinate copper(II) complexes containing a tridentate ligand

Summary Copper(II) complexes with a tridentate chelating ligand within the general 2N, X (X = O or S) donor class, containing abis(benzimidazolyl) donor set, were prepared and characterized. X-band e.p.r. spectra of the complexes indicateg >g and the largeg and lowA have been interpreted in terms of a distorted basal plane. Superimposed on theg component are five SHF lines withA _N = 16±2G, supporting the interaction of two nitrogen atoms with the copper nucleus. Thus, the basal plane of the complex comprises 2N atoms, with the ligand hetero atom being axially coordinated.     

Molecule-Based Magnets with 1D Chain Structure of Bifurcated H-Bonding: Syntheses,Structure, and Magnetic Properties

Two complexes [PyH][Ni(Mnt)₂] (I) and [QlH][Ni(Mnt)₂] (II) (Mnt^2– = maleonitriledithiolate, [PyH]⁺ = pyridinium, [QlH]⁺ = quinolinium) were synthesized and characterized by elemental analysis and IR spectra. ESR spectra of polycrystalline samples collected at room temperature are isotropic with g = 2.039 for Iand anisotropic with g _z = 1.997, g _y g _x = 2.102 (g _av = 2.068) for II. The magnetic susceptibility data of I and II have been measured in the temperature range of 2–300 K. The results obtained suggest the antiferromagnetic coupling interactions between neighboring Ni³⁺ ions for I and ferromagnetic coupling interactions for II, respectively. Based on the crystal structure analysis of I, the magnetic properties of two complexes were discussed.     

Synthesis and magnetic properties of new nickel(II)-copper(II)-nickel(II) trinuclear complexes with irregular spin-state structure

Summary Four novel heterotrinuclear complexes were prepared, namely {[Ni(L)₂]₂[Cu(pba)]}(ClO₄)₂, where pba = pro-pylene-1,3-bis(oxamato) and L = 1,10-phenanthroline (phen), 5-nitro-1,10-phenanthroline (NO₂-phen), 2,2-bipyridyl (bpy), 4,4-dimethyl-2,2-bipyridyl (Me₂bpy). Based on i.r., elemental analyses, conductivity measurements and electronic spectra oxamato-bridged structures are proposed for these complexes, consisting of two nickel(II) ions, each in a distorted octahedral environment, and a copper(II) ion in a square planar environment, respectively. The temperature dependence of the magnetic susceptibility of {[Ni(phen)₂]₂[Cu(pba)]}(ClO₄)₂·H₂O was studied in the 4–300 K range, giving the exchange integral J = - 106 cm^–1. TheX _scm T versusT plot exhibited a minimum at ca. 98 K, characteristic of this kind of coupled polymetallic complex with an irregular spin-state structure. E.s.r. spectra of these complexes clearly indicated a strong rhombicity withg ₁ = 5.20,g ₂ = 2.29 andg ₃ = 2.02 (approximately), which agrees with an anisotropicS = ±1/2 Kramer doublet in the ground state.     

Effects of an avoided crossing on linewidths of resonances in the H2 i3Π g state

We have investigated the influence of nonadiabatic effects on the linewidths of quasi-bound rovibrational levels in the i³ _g state of molecular hydrogen. This state has a potential barrier due to the interaction of then=3³ _g Rydberg series and the Q1³ _g valence state. The radial coupling with the higher lying³ _g states has been modelled into a diabatic problem. The widths of thev=4 and 5 levels, a measure for their lifetimes, have been determined both in the adiabatic i³ _g potential, and in the two-state system. The results show small shifts of the i³ _g (v=4,5;N=1) levels of a few wavenumbers, and a significant increase of 49% in the lifetimes of these levels. Both effects are large enough to be important in spectroscopic experiments. The magnitude of the effect is in accordance with observed linewidths obtained in recent experiments. Model calculations for differently shaped potential barriers have been carried out to study the generality of the above conclusions.     

Mechanism of the cooperative vibronic emission of1δg singlet oxygen in solutions

The mechanism for inducing the probability of the ¹_g-X³_g ^– transition in an oxygen molecule with the simultaneous excitation of a vibration state of the solvent has been considered. The intensification of the 0-0 band of this transition under the influence of the solvent is attributed to the induction of the electric dipole moment of the b¹_g ⁺- ¹_g (m_ba) transition during a collision between O₂ and solvent molecules. It has been shown that m_ba is strongly dependent on the intermolecular distance and, consequently, on the normal coordinate of the vibrations of the solvent (Q) at a given collision parameter. Calculation of the derivative m_ba/Q (with consideration of the strong spin-orbit coupling of the b¹_g ⁺ and X³_g ^– states) makes it possible to account for the intensity of the new luminescence band of¹_g singlet oxygen, which is shifted toward longer wavelengths relative to the 0-0 band of the ¹_g-X³_g ^– by the value of the vibrational quantum of the solvent.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 5, pp. 594–596, September–October, 1985.     

ESR study of reversible complex formation between bis(η5-cyclopentadienyl)vanadium and fullerene C60 in solution

Using the ESR method, equilibrium complex formation of fullerene C₆₀ with bis-(⁵-cyclopentadienyl)vanadium in solutions in aromatic solvents was found to occur. Based on an analysis of isotropic (g _i = 2.0008;A _i ,(V⁵¹)=–45.7 mT) and anisotropic (g _i =1.9808;g ₂ = 2.0135; <g ₃> = 2.0060;A ₁(⁵¹V)=–7.85 mT;A ₂(⁵¹V) = –6.50 mT; <A ₃(⁵¹V)> = 0.61 mT) ESR spectra parameters, it was established that the complex formed, Cp₂V(²-C₆₀), corresponds to vanadocene d¹-complexes with ²-bonded ligands.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1945–1947, November, 1994.     

Biogels of cytochrome c : Cytochrome c peroxidase complex studied by electron paramagnetic resonance spectroscopy

Cytochrome c: cytochrome c peroxidase (Cc: CcP in 1: 1 ratio) complex was successfully encapsulated in sol-gel derived glass. The electron paramagnetic resonance (e.p.r.) and optical absorption techniques were used to characterize the coordination number, spin state, charge-transfer activity and structural orientation of Cc: CcP complex and its constituents. The sol-gel encapsulation of metalloproteins allows, for the first time, the detection of e.p.r. signals of biological systems at room temperature. CcP exhibits an e.p.r. spectrum representing the high spin and purely axial symmetry with parameters at g 6 and g 2 and an electronic absorption spectrum with a descent in spectral intensity of shoulder band at 380 nm and a blue-shifted charge-transfer band at 620 nm. Cc shows an e.p.r. spectrum characterizing a mixture of high spin (g 6 and g 2) and low spin (g _x=2.7, g _y=2.2 and g _z=1.8) components. Upon complexation, Cc:CcP pair displays a single and broad e.p.r. spectrum at g 2 and a light absorption spectrum with a red-shifted Soret band at 423 nm, a blue-shifted charge-transfer band at 620 nm and an intensified charge-transfer band at 507 nm. These results suggest that the sol-gel encapsulated Cc:CcP complex has the following chemical and physical characteristics: (a) a hexa-coordination, (b) a high-spin state, (c) an active charge-transfer (or redox) pair, and (d) the direction of the g paramagnetic center of Cc : CcP complex lies nearly parallel to that of the heme normal. The structural coordinations of the sol-gel encapsulated Cc, CcP and Cc : CcP are examined. Moreover, the possible use of biogels at the sol, gelation, and xerogel stages during gel processing to control the structural rigidity and spatial separation/orientation of the encapsulated heme proteins and to study their possible routes of long-range electron transfer reactions are also discussed.     

The hyperfine structure of electron spin resonance spectrum of tricobalt cluster radical anion CH3CCo3(CO)

CH₃CCo₃(CO)₉ was synthesized from the reaction between chloralose and Co₂(CO)₈. The radical anion was generated by electrochemical reduction, and electron spin resonance spectra in THF were recorded by in situ electrolysis in the sample tube in the ESR cavity at 298 and 110K with the spectral data <g> =2.015, g =2.013, g =2.016; <a> = —35.2G, a₁ = —78.6G and a =—13.5G. The effect on shift of the magnetic resonance field by using the second order perturbation theory was discussed, and the electron spin density on Co atom orbitals 4s, 4p and 3d was calculated, respectively to be 4s: —0.08, 3d: 0.855 and 4p: 0.225.     

Crystal structure of the 1 : 4 complex between 18-crown-6 and a polyfunctional guest: 2-hydroxymethyl-4-(1,1,3,3-tetramethylbutyl)phenol

Single crystal X-ray analysis of the 1 : 4 complex between 18-crown-6 and 2-hydroxymethyl-4-(1,1,3,3-tetramethylbutyl)phenol is reported. Crystals of the complex are triclinic witha = 11.929 (2),b = 18.655(2),c = 8.313(1)Å, = 93.14(1), = 94.02(1), = 100.89(1)^o, andD _c = 1.111 g cm^–3 forZ = 1. TheR index is 0.057 for 5935 reflections measured at 293 K. The complex lies on a center of symmetry, the macroring has the Ci symmetry (g ⁺ g ⁺ aag ^– aag ^– ag ^– g ^– aag ⁺ aag ⁺ a). The CH₂-OH group at theortho position to the phenolic OH lies near the benzene ring plane; this conformation differs from the one found for uncomplexed phenol-alcohol molecules. The packing is characterized by layers of hydrogen bonded entities parallel toac; alongb the layers are stabilized by van der Waals interactions.     

Structure and Energetics of β–Diketonates. XI. Molecular Structure of Sc(thd)3 from Gas–Phase Electron Diffraction Data

Electron–diffraction and mass–spectrometric studies of saturated vapor of scandium tris–dipivaloyl–methanate showed that at 135(5)°, the vapor contains only monomeric Sc(thd)₃, whose structural parameters r _a , r _g , and r were determined. The internuclear distances in the chelate ring were found to be rSc=O) = 2.066(5), r(O=C) = 1.272(3), and r(C=C_r) = 1.385(3). The ScO₆ coordination polyhedron has a D ₃ symmetry configuration close to a regular antiprism. The angle of rotation of the O=O=O trigonal faces relative to their position in a regular prism is 25.7(1.5)°.     

What are the relative steric demands of carboxyl and methyl groups?

The relative steric demands of carboxyl and methyl groups are compared by contrasting the difference quantity H _f ^o (g, ArCOOH) — H _f ^o (g, ArCH₃) for a collection of alkylated benzoic acids and toluenes with the value for Ar=C₆H₅, the archetypical (i.e., unsubstituted) benzoic acid and toluene. We conclude that carboxyl and methyl groups are nearly the same size.