S-electron Densities Research Articles

Extending the knowledge on the quaternary rare earth nickel aluminum germanides of the RENiAl4Ge2 series (RE=Y, Sm, Gd–Tm, Lu) – structural, magnetic and NMR-spectroscopic investigations

Abstract The quaternary rare earth nickel aluminum germanide series RENiAl4Ge2 (RE = Y, Sm, Gd–Tm, Lu) has been extended by several members. The compounds were synthesized from the elements by arc-melting, and single crystals of YNiAl4Ge2, GdNiAl4Ge2, and LuNiAl4Ge2 were grown from an aluminum flux. All members crystallize isostructurally in the rhombohedral SmNiAl4Ge2-type structure (R3̅m, Z = 3). The compounds can be described as a stacking of RE δ+ and [NiAl4Ge2] δ− slabs with an ABC stacking sequence, or alternatively as stacking of CsCl and CdI2 building blocks. The results of the magnetic measurements indicate that all rare earth atoms are in a trivalent oxidation state. Of the RENiAl4Ge2 series, the members with RE = Sm, Gd–Dy exhibit antiferromagnetic ordering with a maximum Néel temperature of T N = 16.4(1) K observed for GdNiAl4Ge2. 27Al NMR spectroscopic investigations yielded spectra with two distinct signals, in line with the crystal structure, however, significantly different resonance frequencies of δ iso ms(YNiAl4Ge2) = 77(1) and 482(1) ppm as well as δ iso ms(LuNiAl4Ge2) = 90(1) and 467(1) ppm were observed. These indicate significantly different s-electron densities at the two crystallographically different Al atoms, in line with the results from DFT calculations. The Bader charge analysis confirms that the present compounds must be considered as germanides, as expected from the relative electronegativities of the constituent elements, while the low charges on Al and Y indicate significant covalent bonding.

Two-Bond Spin−Spin Coupling across a Hydrogen Bond: X−Y Coupling in the Presence and Absence of the Proton

This paper addresses the question of whether spin−spin coupling between X and Y across an X−H−Y hydrogen bond is proof that the hydrogen bond is covalent. The results of ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations are used to compare coupling constants (2hJX-Y) for hydrogen-bonded complexes and related complexes that have the same X−Y distance but no hydrogen bond. The results show that even in the absence of a hydrogen bond, X−Y coupling can occur. The magnitude of this coupling constant may be greater or less than it is in the hydrogen-bonded complex. Thus, these data suggest that X−Y spin−spin coupling is not a proof of covalency. The role of the hydrogen bond is to allow X and Y to approach each other close enough to form a stable complex, for which an X−Y coupling constant can be measured experimentally. The presence of the proton does influence the magnitude of the coupling constant by altering the s-electron densities on X and Y.

X-ray diffraction, Raman spectroscopic, and solid-state NMR studies of the group 14 metal-(tetracarbonyl)cobalt complexes Ph3MCo(CO)4(M = Si, Sn, Pb)

Single-crystal X-ray diffraction studies illustrate that the three title compounds are isomorphous, belonging to the triclinic space group P[Formula: see text], with slightly distorted trigonal bipyramidal geometry about cobalt. The solid-state29Si,119Sn, and207Pb cross-polarization magic angle spinning (CP MAS) NMR spectra are presented. The indirect spin–spin coupling constant (J), quadrupolar–dipolar shift (d), direct dipolar coupling constant (D' ), anisotropy in spin–spin coupling (ΔJ), and the chemical shift tensor were extracted. A plot of the reduced coupling constant vs. s-electron densities at the nucleus indicates that the Fermi contact term may be dominant for the tin and lead complexes; however, the large ΔJ for all complexes indicate that there are also significant anisotropic terms. Trends in the Raman scattering spectra are also discussed.Key words:29Si,119Sn, and207Pb CP MAS NMR, tetracarbonyl cobalt, spin–spin coupling, chemical shift tensor, quadrupole coupling, Fermi contact, cobalt–group 14.

Solid-state nuclear magnetic resonance studies of triphenylsilyl-, triphenyltin-, and triphenyllead(pentacarbonyl)manganese(I) complexes

The solid-state CP MAS (29Si, 119Sn, and 207Pb) NMR spectra of the triphenylsilyl-, triphenyltin-, and triphenyllead(pentacarbonyl)manganese(I) complexes, (Ph3E)Mn(CO)5 (E = Si, Sn, Pb), have been analyzed to give the chemical shifts, one-bond spin-spin coupling constants, 1JE-Mn, the "effective-dipolar" coupling constants (D - ΔJ/3), the chemical shift tensors, and the spin-spin anisotropy (ΔJ), where the analysis permits. For the tin and lead compounds, three and four sets of chemical shifts, respectively, were observed, and two different polymorphs occur for the lead complex, depending on the solvent used for recrystallization. The average values of the reduced coupling constants, 1KMn-Si (2.64 × 1020 T2 J-1), 1KSn-Mn (1.25 × 1020 T2 J-1), and 1KPb-Mn (4.18 × 1020 T2 J-1) showed a linear correlation with the s-electron densities at the respective metal nuclei. The principal components of the chemical shift tensors have been determined for the tin and lead compounds.Key words: manganese-group-14 compounds, solid-state 29Si, 119Sn, and 207Pb CP MAS NMR, spin-spin coupling, chemical shift anisotropy, quadrupole coupling.

Solid-state nuclear magnetic resonance studies of triphenylsilyl-, triphenyltin-, and triphenyllead(pentacarbonyl)manganese(I) complexes

The solid-state CP MAS (29Si, 119Sn, and 207Pb) NMR spectra of the triphenylsilyl-, triphenyltin-, and triphenyllead(pentacarbonyl)manganese(I) complexes, (Ph3E)Mn(CO)5 (E = Si, Sn, Pb), have been analyzed to give the chemical shifts, one-bond spin-spin coupling constants, 1JE-Mn, the "effective-dipolar" coupling constants (D - ΔJ/3), the chemical shift tensors, and the spin-spin anisotropy (ΔJ), where the analysis permits. For the tin and lead compounds, three and four sets of chemical shifts, respectively, were observed, and two different polymorphs occur for the lead complex, depending on the solvent used for recrystallization. The average values of the reduced coupling constants, 1KMn-Si (2.64 × 1020 T2 J-1), 1KSn-Mn (1.25 × 1020 T2 J-1), and 1KPb-Mn (4.18 × 1020 T2 J-1) showed a linear correlation with the s-electron densities at the respective metal nuclei. The principal components of the chemical shift tensors have been determined for the tin and lead compounds.Key words: manganese-group-14 compounds, solid-state 29Si, 119Sn, and 207Pb CP MAS NMR, spin-spin coupling, chemical shift anisotropy, quadrupole coupling.

High-resolution67Zn-Mössbauer study of oxide spinels

We report on first experiments to investigate the electronic structure in the normal spinels ZnAl2O4 and ZnFe2O4 and in the inverse spinels Zn2SnO4 and Zn2TiO4 using the high-resolution67Zn-Mossbauer spectroscopy. The electric field gradient for67Zn at the B (octahedral) site in ZnAl2O4 is negative, whereas the A (tetrahedral) site remains essentially cubic, however, with a more positive center shift. ZnFe2O4 orders antiferromagnetically at ≈10K. Due to superexchange a magnetic field is observed at67Zn. In the inverse spinels short range order leads to only few (Zn,Sn) and (Zn,Ti) configurations at the octahedral sites. The s-electron densities at the67Zn sites are distinct and cover a surprisingly broad range for Zn2TiO4. This strongly suggests that d-electrons of Ti play an essential role in the chemical bond of this compound.

Properties of 57Fe hot-implanted into diamond crystals studied by Mössbauer emission spectroscopy between 4 and 300 K

We investigated the hyperfine interactions of 57Fe ions implanted into diamond crystals that were heated to 830 K during implantation. The implantation of radioactive parent 57Co ions was performed to a fluence of 2 × 10 13 ions/cm 2, using the 50 kV isotope separator. The Mössbauer emission spectra were measured in the temperature range between 4 and 300 K. using a helium flow cryostat. About 20% of the implanted ions end up in high-symmetry (HS, presumably substitutional) sites, with the remaining 80% of ions appearing in low-symmetry (LS, presumably hexagonal interstitial) sites. The HS sites are characterized by very high Debye temperature θ D ~ 1300 K and by high s-electron density characteristic for Fe 4+ (3d 4sp 3) electron configuration. Impurities at LS sites exhibit a Debye temperature of 550 ± 50 K and a very large electric field gradient, V zz = 1.2 × 10 18 V cm 2 . An anomalous rise of the recoil-free factor for LS sites below 100 K was observed. This effect can be an evidence of localized jumps of interstitial impurity atoms between adjacent tetrahedral and hexagonal positions in the host lattice. The values of the s-electron densities and electric field gradients at Fe nuclei implanted in diamond are compared with the corresponding values in other IV-group matrices and are found to be correlated with the atomic volumes of the corresponding host elements.

Mössbauer investigation of the anomalous properties of the optically bistable [Fe(II)(CN) 5NO] 2- complex

The dependence of the Mössbauer parameters of the light induced metastable state upon temperature and concentration in crystals and solid solutions is identical with that of the ground state of the nitroprusside complex. This concerns s-electron densities, electric field gradients, phonon spectra and intermolecular interactions despite different magnitudes of the parameters.

Physisorption interaction of H 2 with simple metals

Calculations of the physisorption interaction between a H 2-molecule and simple metal surfaces are reported. The physisorption wells of simple and noble metals having the same s-electron densities are markedly different. The former are shallower because there is no contribution of d-electrons to the Van der Waals force. The physisorption energies are found to depend strongly on the bulk density, varying from 25 meV (Al) to 2.7 meV (K). Results for three Al surfaces show a face dependence due to the different work functions that may be measurable.

Physisorption interaction of H 2 with simple metals

Calculations of the physisorption interaction between a H 2-molecule and simple metal surfaces are reported. The physisorption wells of simple and noble metals having the same s-electron densities are markedly different. The former are shallower because there is no contribution of d-electrons to the Van der Waals force. The physisorption energies are found to depend strongly on the bulk density, varying from 25 meV (Al) to 2.7 meV (K). Results for three Al surfaces show a face dependence due to the different work functions that may be measurable.

The syntheses and characterisation of, and the determination of1J(103Rh–103Rh) in [(η5-C5Me5)2Rh2(µ-CH2)2{µ-CH2CR(CH2CRCH2)CH2}](R = H or Me) and [(η5-C5Me5)2Rh2(µ-CH2)2{µ-Ph2P(CH2)nPPh2}]2+(n= 1 or 2)

Reaction of [(η5-C5Me5)2Rh2(µ-CH2)2Cl2] with CH2CRCH2MgCl gave [(η5-C5Me5)2Rh2(µ-CH2)2{µ-CH2CR(CH2CRCH2)CH2}](3, R = H; 4, R = Me). Reaction of [(η5-C5Me5)2Rh2(µ-CH2)2(MeCN)2]2+ with Ph2P(CH2)nPPh2gave [(η5-C5Me5)2Rh2(µ-CH2)2{µ- Ph2P(CH2)nPPh2}]2+(5, n= 1; 6, n= 2). The complexes (3)–(6) were characterised largely by their n.m.r. spectra which also allowed the determination of 1J(103Rh–103Rh) as 13.5 for (3), 13.2 for (4), 12.4 for (5), and 11.9 Hz for (6). After allowance for gyromagnetic ratios and s-electron densities at the nuclei, these coupling constants are comparable with a 1J(l95Pt–195Pt) coupling of ca. 8 000 Hz.

Chemical and hyperfine field analysis of sputtered iron

The previously reported amorphous structure of sputtered iron is shown to be stabilized by carbon impurities as deduced from high energy ion beam scattering and Mössbauer spectroscopy. The minimum amount of carbon needed has not been determined but amorphous samples were obtained for carbon contents of 6% or even less. Spin wave excitation behaviour up to room temperature is revealed. A possible relation between s-electron densities and magnetic hyperfine fields at the iron nuclei, as indicated by the Mössbauer spectra, is discussed.

Molecular orbital (MSX?) calculations of s-electron densities of tetrahedrally coordinated ferric iron: Comparison with experimental isomer shifts

The MSXα method has been used to calculate the s-electron densities at the nucleus for tetrahedrally coordinated ferric iron, (FeO4)5−, comparing the observed increase in isomer shift values with increasing Fe-O separation. The results give an isomer shift calibration constant of −0.3 (a.u. mm×s−1) assuming a constant ratio for the iron and oxygen sphere radii for the different polyhedra sizes. It is suggested that increasing bonding distances in tetrahedral coordination polyhedra are the dominant factors determining the value of the isomer shifts in Fe-Mg-silicates.

Variation of core-electron contributions to Mössbauer isomer shift with ionic size

Using nonrelativistic Hartree-Fock-Slater wave functions, the core s-electron densities at the nucleus have been calculated for several nd 5 (n=3,4,5) ions. The results show a significant variation of core densities with the ion size.

Correlation of isotope shifts with |ψ(0)|^2 for actinide configurations*

We have used both Hartree-Fock functions and the pseudorelativistic HX functions of Cowan to examine the relationship between observed isotope shifts and s-electron densities at the nucleus for Cm, Am, Pu, and U. There are significant changes in relative electron densities when the relativistic correction is included which greatly improve the correlation between observed isotope shifts and electron density. The improvement is especially dramatic for those configurations which fit most poorly with HF values of |ψ(0)|2. The slope [S=ΔIS/Δ(4π|ψ(0)|tot2)] per unit effective mass difference is found to be nearly constant for the actinides considered. This allows prediction of isotope shifts for 252Cf-250Cf which have not been measured. Isotope shifts are also predicted for Cm, Am, Pu, and U configurations which have not yet been identified.

Calculation of chemical shifts in the Mössbauer spectra of some tin(IV) compounds

The Pople-Segal-Santry SCMO method is used to calculate the electronic structures of a selected series of compounds of Sn(IV) with basis sets including 5s, 5p, and 5d orbitals on tin. The occupation number of the 5s orbital of tin in each compound is correlated with their known experimental chemical shifts and an approximately linear relationship is found. The 5s electron density at the tin nucleus, calculated from the Fermi-Segré equation using a modified form of Burns' screening rules, also exhibits a linear relation with the observed shifts. The inclusion of 5d orbitals does not appear to exert a marked effect on the calculated s-electron densities. From the results the value of ΔR/R for the 119Sn nucleus on excitation to the 23.8 keV level is 3.5 × 10–4. The differential contribution, due to 5s screening, of 4s electron density to the total at the nucleus is calculated for the series. Finally, the occupation numbers of the three individual 5p orbitals lead to a qualitative understanding of experimental quadrupole splitting data.