Static Jahn-Teller Distortion Research Articles

Manifestation of the Jahn-Teller active vibrations in the vibronic spectra of tetracoordinate Cu 2+, Ni 2+ and Zn 2+ chlorides

An attempt is made to track the changes in the vibrational modes that are Jahn-Teller active of some pseudotetrahedral MCl 4 2− ions. The infrared and Raman spectra of powder (Et 4N) 2MCl 4, M=Ni, Cu, Zn and of Cu- and Ni-doped (Et 4N) 2ZnCl 4 were recorded both at room temperature and at 77, 100 and 170 K. The room temperature spectra show that for NiCl 4 2− and CuCl 4 2− if present the Jahn-Teller distortions are dynamically averaged and the transitions can be accounted for as intrasheet vibronic transitions. The low-temperature spectra however show splittings that are indicative of a low symmetry field thus suggestingthat at 77 K the NiCl 4 2− and CuCl 4 2− are vibronically stabilized in a static C 2v field. The doped compound spectra reveal that the Zn matrix effect overrides the static Jahn-Teller distortions present in the free ion spectra. In the CuCl 4 2− spectrum a band at 2700 cm −1 with side bands + v(a 1), − v(t 2 s) was detected and it was assigned to the 2B 2g→ 2B 1g transition arising from the split 2T 2 term in D 2h site symmetry.

Reduktion von (Cyclobutadien)(cyclopentadienyl)‐ und (1,5‐Cyclooctadien)‐ (cyclopentadienyl)nickel‐Kationen

Reduction of (Cyclobutadiene)(cyclopentadienyl)- and (1,5-Cyclooctadiene)(cyclopentadienyl)nickel Cations Cyclic voltammetry of the four cationic Ni sandwich complexes [Ni(C5R5)(C4R4')]+ (2: R = H, R' = Ph; 3: R = H, R' = Me; 4; R = Me, R' = Ph; 5: R = R' = Me) shows reversible one-electron reductions for the phenyl derivatives 2 and 4 and a peak pattern characteristic of reductive dimerization/oxidative monomerization for the methyl derivatives 3 and 5. The product of the reduction of 3 was isolated and characterized analytically and by an X-ray structure determination as the dimer 6 of 3, linked through cyclobutenyl rings, the first example for a dimerization of an electron-rich sandwich complex at a substituted C atom. EPR spectra of the neutral complexes 2 and 4 are compared to those of (1,5-cyclooctadiene)(cyclopentadienyl)nickel (7) and are interpreted in terms of a substantial static Jahn-Teller distortion.

Optical properties of group-V atom-vacancy pairs in silicon

Abstract Two optical absorption bands are identified as arising from the negative charge state of each of the P-, As-, and Sb-vacancy pairs in silicon. The more prominent band for each is at 6150, 6000, and 5500 cm−1, respectively. A second broader partially overlapping band at ∼8500 cm−1 is also present for each. Dichroism produced in the bands by uniaxial stress reveals a static Jahn-Teller distortion of opposite sign to that for the neutral state previously studied by EPR. The absorption bands can be identified as electron transitions from filled to empty orbitals of a simple one-electron molecular orbital model for the defect.

ChemInform Abstract: Crystal Structure and Spectroscopic Studies of Bis(N‐2‐pyridinylcarbonyl‐2‐pyridinecarboximidato)copper(II) Monohydrate. Local Bonding Effects

Abstract The crystal and molecular structure of bis(N-2- pyridinylcarbonyl-2-pyridinecarboximidato)copper(II) monohydrate, Cu(BPCA)2·H2O, has been determined from single crystal X-ray data. It crystallizes in the monoclinic space group P21/c with four formula units in a cell of dimensions: a = 8.917(1), b = 8.932(1), c = 28.794(17) A, β= 95.49(2)°.Least- squares refinement of 2754 reflections with I > 2.5σ(I) and 379 parameters gave a final R = 0.037 (Rw = 0.036). The structure consists of discrete neutral Cu(BPCA)2 entities linked two by two through water molecules hydrogen bonded to ligand carbonyl groups. The coordination geometry around copper ions can approximately be described as orthorhombically distorted octahedral. EPR and ligand field spectroscopic results have been analyzed in terms of a static Jahn-Teller distortion, and bonding parameters have been derived from EPR hyperfine and superhyperfine structures in the spectra of copper(II)-doped Zn(BPCA)2·H2O powder samples and frozen solution.

Crystal structure and spectroscopic studies of bis( N-2-pyridinylcarbonyl-2-pyridinecarboximidato)copper(II) monohydrate. Local bonding effects

The crystal and molecular structure of bis( N-2- pyridinylcarbonyl-2-pyridinecarboximidato)copper( II) monohydrate, Cu(BPCA) 2·H 2O, has been determined from single crystal X-ray data. It crystallizes in the monoclinic space group P2 1/ c with four formula units in a cell of dimensions: a = 8.917(1), b = 8.932(1), c = 28.794(17) Å, β= 95.49(2)°.Least- squares refinement of 2754 reflections with I > 2.5σ( I) and 379 parameters gave a final R = 0.037 ( R w = 0.036). The structure consists of discrete neutral Cu(BPCA) 2 entities linked two by two through water molecules hydrogen bonded to ligand carbonyl groups. The coordination geometry around copper ions can approximately be described as orthorhombically distorted octahedral. EPR and ligand field spectroscopic results have been analyzed in terms of a static Jahn-Teller distortion, and bonding parameters have been derived from EPR hyperfine and superhyperfine structures in the spectra of copper(II)-doped Zn(BPCA) 2·H 2O powder samples and frozen solution.

Calculated electronic structure of Au13 clusters.

The electronic structure of cubo-octahedral and icosahedral ${\mathrm{Au}}_{13}$ clusters has been investigated by the self-consistent-field molecular-orbital Dirac scattered-wave method. Double-point-group--symmetry considerations indicate that the icosahedral cluster will undergo static Jahn-Teller distortion, while the cubo-octahedral cluster cannot distort because of a Kramers degeneracy in the ground state. Molecular Zeeman and hyperfine interactions are calculated for the cubo-octahedral ${\mathrm{Au}}_{13}$ cluster through a first-order perturbation to the Dirac Hamiltonian. The predicted g tensors and $^{197}\mathrm{Au}$ hyperfine tensors are consistent with a nearly isotropic paramagnetic-resonance spectrum. The calculated density-of-states (DOS) curves for the ${\mathrm{Au}}_{13}$ clusters show similar features to those obtained in photoemission experiments of small clusters of gold. Relativistic effects increase the d-band width by more than 1 eV, and spin-orbit interaction splits the occupied d band by about 2 eV in both clusters. These calculated values are approximately 75% of the observed values for gold in the bulk metal. It is clearly shown that the overlap of the d band by the s-p band is mainly due to relativistic effects.

Identification of a paramagnetic Bi 0 center in X-irradiated KCl.Bi crystals

Crystals of KCl doped with bismuth exhibit after X-irradiation at room temperature the presence of a paramagnetic defect with tetragonal 〈100〉 symmetry. Based on the analysis of the ESR spectra a structural model based on a Bi 06 p 3 atom center is proposed. The strong axial crystal field is due to the presence of a vacancy in the first neighbourhood and/or a static Jahn-Teller distortion of E g character.

Dynamic Jahn-Teller Effect in the ODMR Spectrum of the Self-Trapped Exciton in AgCl Crystals

An isotropic ODMR line other than the ODMR spectra of the self-trapped exciton (STE) with the tetragonal symmetry in AgCl at 4.2 K was observed using X-band and K-band spectrometers. The g value satisfied the relation of g =( g // +2 g ⊥ )/3 where g // and g ⊥ are the g values of the parallel and perpendicular spectra of the STE. The isotropic line is ascribed to motional narrowing of the tetragonal spectra of the STE. This observation of thermal dynamic Jahn-Teller effect of the STE at 4.2 K indicates that the bound electron in the STE lowers the barrier height between three configurations of the static Jahn-Teller distortion of the self-trapped hole (STH).

Dynamic to static Jahn-Teller distortion in the E.P.R. of Cu(II) doped hexaimidazolecadmium(II) nitrate

E.P.R. of single crystals of hexaimidazolecadmium(II) nitrate containing substitutionally incorporated Cu(II) has been investigated in the temperature range 77 to 300 K. At 300 K the spectrum is isotropic with only broad Cu hyperfine features showing m I-dependent linewidth anisotropy indicative of incomplete averaging. At 77 K the spectrum becomes anisotropic showing well resolved ligand nitrogen superhyperfine features and there are three spatially distinct sites corresponding to the three equally probable tetragonal elongations of the CuN6 octahedron brought about by static Jahn-Teller distortion. At 77 K the parameters are: g 11 = g 22 = 2·0693 ± 0·0005 and g 33 = 2·3083 ± 0·0005 63Cu A 11 = A 22 = 13·5 ± 0·5 and = 159·8 ± 0·5 and 14N A 11 = A 22 = 13·5 ± 0·5 and A 33 = 12·0 ± 0·5 all in units of 10-4 cm-1. The isotropic parameters of the dynamically averaged spectrum are: g iso = 2·158 ± 0·0005 and 63Cu A iso = 53 ± 2 × 10-4 cm-1.

EPR studies on pairs of Jahn-Teller distorted hexakis pyridine-N-oxide ions in Zn(C 5H 5NO) 6(NO 3) 2

Semi-dilute 10% 63Cu:Zn(pyO) 6(NO 3) 2 (pyO = C 5H 5NO, pyridine-N-oxide) has been studied by X- and Q-band EPR. Below the transition from dynamic to static Jahn-Teller distortion, various pairs of Cu(pyO) 2+ 6 ions are observed, including the antiferrodistortive one with the largest possible exchange interaction, which pair was not present in the corresponding ClO − 4 crystal. From the AB type of spectrum which is observed at Q-band, the weak exchange interaction in antiferro-distortive pairs was determined: J′ = 329 10 −4 cm −1, larger than the corresponding values in the ClO − 4 and BF − 4 complexes.

Struktur und Bindung in Übergangsmetall-Fluoriden MIIMe IVF 6: Neutronenbeugungs-Strukturuntersuchungen an CaSnF 6, FeZrF 6, und CrZrF 6

Compounds M IIMe IVF 6 requently undergo phase transitions from the cubic ordered ReO 3 to the trigonal LiSbF 6 structure when lowering the temperature. In case of a strongly Jahn-Teller unstable cation in the M II position additional phases may occur. Results of powder neutron-diffraction studies on CaSnF 6, FeZrF 6, and CrZrF 6 at different temperatures are reported. The high-temperature phases have the space group Fm3 m; the F − ligands are either statistically displaced from the M IIMe IV directions or undergo a strong thermal motion perpendicular to these directions ( ∢M II  FMe IV : 165–180°). The thermal ellipsoids of the CrF bonds are strongly indicative of a dynamical Jahn-Teller effect in addition. In the low-temperature phases of CaSnF 6 and FeZrF 6 (space group R 3 ) the ∢M II  FMe IV is more distinctly bent (⋍155–160°). CrZrF 6 undergoes two reversible phase transitions, which are determined to occur at 415 ± 5 K (cubic → tetragonal, dynamic to static Jahn-Teller distortion of CrF 6 octahedra and 150 ± 10 K (tetragonal → (pseudo)monoclinic).

Zeeman spectroscopy of crystal-field transitions of Co-doped InP

The 474.0 meV and 785.8 meV crystal-field transitions of CoI2n+ in InP observed in photoluminescence (PL) and optical absorption respectively are studied in magnetic fields of up to 10 T. The 474.0 meV PL line is shown to arise from a Gamma 8- Gamma 8 transition in Td symmetry between the lowest spin-orbit state ( Gamma 8) of the 4T2(F) level of CoI2n+ and the spin-only Gamma 8(4A2(F)) ground state. The anisotropic g-values of the excited state, g1 and g2, are 5.48+or-0.2 and -2.98+or-0.15 respectively. An isotropic g-value of 2.14+or-0.05 is obtained for the ground state, in agreement with the results of electron spin resonance on CoI2n+. The Zeeman pattern of the 785.8 meV absorption line shows a marked trigonal (C3v) symmetry due to Jahn-Teller (JT) coupling to T2 distortion modes in the excited state. The Zeeman data are shown to be consistent with the behaviour expected from the 4A2(F) to 4T1(F) transition of CoI2n+, the 4T1(F) state undergoing an almost static JT distortion. Anticrossings in the anisotropy plot of the Zeeman pattern are observed at the main symmetry directions. This cannot be explained on a model of static C3v trigonal complexes, and is attributed to tunnelling between the magnetically equivalent distortion directions. Analysis of the Zeeman data gives gmod mod =0.96+or-0.05 and gperpendicular to =3.76+or-0.05 for the 4T1(F) excited state and g=2.10+or-0.10 for the 4A2(F) ground state.

Phase transitions in CuZrF 6 and CrZrF 6: A Mössbauer and EPR study of local and cooperative Jahn-Teller distortions

CuZrF 6 and CrZrF 6 undergo phase transitions between 100 and 450K, which are induced by crystal packing effects and changes from dynamical to static Jahn-Teller distortions of the Cu(Cr)F 6 octahedra. We analyzed in particular the transitions of the Jahn-Teller type using 57Fe 2+, doped into the Cu 2+(Cr 2+) sites, as a Mössbauer probe. The quadrupole splitting is large in the region of static distortions and essentially reflects the distortion symmetry of the host Cu(Cr)F 6 polyhedra, while it vanishes in the case of a dynamical Jahn-Teller effect. Ligand field, EPR, and magnetic data are given in addition and are discussed with respect to the structures of the host compounds and the cooperative Jahn-Teller order of the tetragonally elongated Cu(Cr)F 6 polyhedra in the low-temperature phases.

E.P.R. studies on pairs of Jahn-Teller distorted hexakis pyridine-N-oxide copper(II), Cu(C5H5NO)62+, ions

Semi-dilute 10 per cent 63Cu : Zn(pyO)6 X 2 (pyO = C5H5NO, pyridine-N-oxide], with X - = ClO- - or BF4 -, has been studied by X- and Q-band E.P.R.. Below the transition from dynamic to static Jahn-Teller distortion (at ∼ 60 K), pairs of Cu(pyO)6 2+ ions are observed in both systems. In the ClO4 - salt which is antiferrodistortively ordered in the pure copper crystal, antiferrodistortive pairs and a small amount of ferrodistortive pairs are detected. For both types of pairs all types of configurations are found to exist except the ones with the largest possible exchange interaction. However, in the BF4 - salt all configurations do appear with similar intensities, whereas the pure copper system is ferrodistortive. From the AB type of spectra which are observed at Q-band, for both the ClO4 - and BF4 - complexes the weak exchange interaction in antiferrodistortive pairs could be determined : |J'|/k = 0·041 K and 0·032 K, respectively. For the ClO4 - salt this exchange coupling constant corresponds to the interchain constant existing in the pure copper system which is known to be a pseudo-one-dimensional antiferromagnet at low temperatures. The above conclusions are drawn from a comparison between experimental and calculated spectra.

EPR studies on exchange interactions in pairs of Jahn-Teller ions in Cu : Zn(pyO) 6(ClO 4) 2

Semi-dilute 10% 63Cu : Zn(pyO) 6(ClO 4) 2 [ pyO = C 5 H 5 NO, pyridine-N-oxide] has been studied by EPR. Spectra of pairs of Cu(pyO) 2+ 6 ions are observed below the transition from dynamic to static Jahn-Teller distortion. Most of these pairs are antiferrodistortively oriented. They exhibit only the type of exchange which corresponds to the interchain interaction J′ in the pure Cu compound. From the Q-band AB type of spectrum it was found that | J′|/ k = 0.041 K. An estimate of the zero- point spin reduction could be derived (∼36%).

Electron-spin-resonance study ofPb−6p3in KC1: A possible Jahn-Teller system

Four $S=\frac{1}{2}$ electron-spin-resonance spectra exhibiting orthorhombic symmetry ($[1\overline{1}0]$, [001], [110]) and observed in KC1: ${\mathrm{Pb}}^{2+}$ after x irradiation at temperatures above 220 K are identified as originating from ${\mathrm{Pb}}^{\ensuremath{-}}$ centers. The mobility of the anion and cation vacancies above this temperature is shown to be essential in the production of these defects. The ${\mathrm{Pb}}^{\ensuremath{-}}$ spectra are characterized by strongly anisotropic $g$ values ranging from \ensuremath{\sim}0.7 to \ensuremath{\sim}4 and their explanation requires a coupling scheme for ${\mathrm{Pb}}^{\ensuremath{-}}$ intermediate between $\mathrm{LS}$ and $j\ensuremath{-}j$ and a full diagonalization of the crystal field, the spin-orbit interaction, and the Coulomb repulsion between the 20 states of the ${p}^{3}$ configuration. A comparison is made between the crystal fields experienced by ${\mathrm{Pb}}^{\ensuremath{-}}6{p}^{3}$ and by the previously studied $\mathrm{LS}$-coupled system ${\mathrm{Sn}}^{\ensuremath{-}}5{p}^{3}$. Because of the sizable presence of orbital angular momentum in its ${\ensuremath{\Gamma}}_{8}$ ground state, the ${\mathrm{Pb}}^{\ensuremath{-}}$ is shown to induce a static Jahn-Teller distortion when occupying an unperturbed octahedral anion site. The Jahn-Teller energies are estimated to be \ensuremath{\sim}${10}^{3}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ for ${\mathrm{Pb}}^{\ensuremath{-}}$, and \ensuremath{\sim}${10\phantom{\rule{0ex}{0ex}}\mathrm{c}\mathrm{m}}^{\ensuremath{-}1}$ for ${\mathrm{Sn}}^{\ensuremath{-}}$. It is concluded that the orthorhombic symmetry in all four ${\mathrm{Pb}}^{\ensuremath{-}}$ defects is caused by a static Jahn-Teller distortion of mixed ${E}_{g}$ and ${T}_{2g}$ character and that vacancies, if present, merely exert an additional perturbation. This viewpoint is shown to be a fruitful one: It allows us (i) to explain the near constancy throughout the four defects of the dominant crystal-field parameter (\ensuremath{\sim} -2 \ifmmode\times\else\texttimes\fi{} ${10}^{4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$) and (ii) to propose specific models for two of the ${\mathrm{Pb}}^{\ensuremath{-}}$ centers.

A comparative study of group IA and IIA homonuclear clusters

Pseudopotential calculations for clusters K x , Be y , Mg y , Ca z ( x ⩽ 8, y ⩽ 5, z ⩽ 4) are presented and c previous results for Li and Na clusters. In the series Li 3, Na 3, K 3 the static Jahn-Teller distortion of the D 3h geometry increases, the energy difference between the obtuse C 2v and the linear structure becoming essentially zero for K 3. In the series Li 4, Na 4, K 4 the stability of the “T-form” (C 2v) increases with respect to the rhombic geometry, the latter becoming unstable for K 4 For the alkaline-earth clusters binding energies per atom are mostly very small-comparable to the diatomic case-with the exception of Be 4 and Ca 4 in tetrahedral, and Be 5 in trigonal-bipyramidal geometry.

Preparation and spectroscopic properties of pure tetrabenzoporphyrins

Cadmium tetrabenzoporphyrin, tetrabenzoporphyrin, and the bispyridine complex of magnesium tetrabenzoporphyrin were synthesized. As judged from the visible absorption spectra these compounds are not contaminated with porphyrin-like impurities. 100-MHz 1H Fourier-transform n.m.r. spectra of the above mentioned compounds are presented. From the low-temperature e.s.r. spectra it can be concluded that the lowest triplet state of biligated magnesium tetrabenzoporphyrin exhibits a static Jahn–Teller distortion resulting in a loss of four-fold molecular symmetry.

Analysis of the vibrational spectra of the hexakis(pyridine N-oxide)metal(II)perchlorates

The far infrared (far i.r.) and laser Raman spectra of M(C 5H 5NO) 6(ClO 4) 2 (M = Co 2+, Ni 2+, CU 2+ and Zn 2+) are reported in the 500-100 cm −1 region. The compounds are all isomorphous and crystallize in the space group R 3 . The metal coordination sphere has S 6 symmetry. A normal coordinate analysis of the vibrational spectra in the metal—ligand region yields excellent agreement between observed and calculated bands, however, anomalous vibrational absorption in the copper(II) analog requires reduction of the copper site symmetry from S 6 to C i . This indicates the presence of a static Jahn—Teller distortion on the vibrational time scale.

Effect of oxygen in photoluminescence from chromium-doped semi-insulating GaAs

Photoluminescence measurements have been performed on chromiumdoped semi-insulating GaAs as a function of temperature. The effect of oxygen is explained in terms of the relative strength of the oxygen-emission at ∼ 0.62 eV and the chromium-emission at ∼ 0.81 eV. An explanation of chromium emission at ∼ 0.81 and 0.54 eV in terms of a static Jahn-Teller distortion in the ground state of Cr 2+ is proposed.