Dilute Mixed Crystals Research Articles

Valence Stabilization, Mixed Crystal Chemistry, and Electronic Transitions in Tetrahedral Oxo and Hydroxo Cr(IV), Mn(V), and Fe(VI) Clusters: A Theoretic Investigation

Ab initioHartree–Fock SCF (HF-SCF) and multiconfiguration complete active space SCF (CSCF) calculations have been carried out on tetrahedralM(OH)4zandMO4z′model clusters (M,z,z′: CrIV,0, −4; MnV, 1, −3; FeVI, 2, −2) in their3A2ground state and in selected triplet and singlet ligand field and charge transfer excited states. Ground state orbital energies and charge distributions supplemented with calculations using asemiempirical approach (Jørgensen) help characterize the stabilization of CrIV, MnV, and FeVIin terms of competing ionic and covalent forces. The crucial role of the Madelung energy in stabilizing these unusual oxidation states is emphasized. Frozen orbitals as obtained by state averaging overd2triplet and singlet states are used to compare results from Hartree–Fock and ligand field treatments. Calculations using these orbitals show that interelectronic repulsion parameters in tetra-oxo coordinated CrIV, MnV, and FeVIare considerably reduced compared to their free ionic values. Charge transfer states are found to further modify energy levels ofd2type, leading to an effective lowering of Coulomb repulsion parameters in the order of thee2,e1t21, andt22strong field configurations. Theories of isomorphic substitution for ionic solids are not applicable for the systems under consideration. Comparison between available structural and spectral data shows that CrIV, MnV, and FeVIions in dilute mixed crystals with isovalent tetrahedral host ions, such as SiIV, PV, and SVI, assume geometries close to those known for stoichiometric phases with CrO44−, MnO43−, and FeO42−tetrahedra. Covalent contributions to the lattice energy from guest (d2) and host tetrahedra are additive, and mixing enthalpies are small or negligible, allowing for continuous solid solutions even when guest and host ions, such as MnVand PV, FeVIand SVI, differ considerably in their size.

Optical Spectroscopy of Single Pentacene Molecules in a Naphthalene Crystal

Abstract Dilute mixed molecular crystals of pentacene in naphthalene were prepared by cosublimation of the two compounds. Single pentacene molecules could be detected in the thin platelets by fluorescence excitation spectroscopy. By analyzing the exponential decay of the fluorescence intensity autocorrelation function for a single molecule in the microsecond time regime we could determine the population (k23) and depopulation (k31) rate of the metastable triplet state. The increase of the population rate k23 by a factor of ∼ 4 when compared to pentacene in p-terphenyl is responsible for the weaker fluorescence signals of single pentacene molecules in naphthalene crystals.

Solid-State 13C CPMAS NMR and Molecular Mechanics Study of Conformational Recognition in Mixed Crystals of Two Phenylalkyl Ketones

A detailed structural analysis of a dilute mixed crystal of 13C-Me-labeled 1,2-diphenylpropanone (1) in 2-methyl-l,2-diphenylpropanone (2) has been carried out with the help solid state 13C cross polarization and magic angle spinning (CPMAS) NMR, molecular mechanics calculations of mixed crystal models, and X-ray diffraction techniques. After characterization of the crystal phases of the pure components, mixed crystals prepared with 1-5% 1 (99.9% 13C-Me labeled) were investigated by solid-state NMR with the goal of addressing structural questions at the molecular level. In the mixed crystals, the NMR signals assigned to the host remained unperturbed, while signals of 1 were different from those observed in its pure crystal phases (racemic and enantiomorphous). Two different types of spectra were observed from samples obtained in different mixed crystallization experiments. We postulate that two groups of signals in one type arise from disorder of the guest in the crystal host when guest molecules crystallize in their two lowest energy conformers. In contrast, ordered mixed crystallization in the second type results in one signal assigned to the guest in its lowest energy conformer. These conclusions are supported by molecular mechanics calculations carried out for gas phase and model crystal systems.

Interconfigurational absorption and two-photon excitation spectra of hexachloroplatinate(2-) containing crystals

Low-temperature absorption and two-photon excitation spectra of complexes containing PtCl{sub 6}{sup 2{minus}} are presented and discussed. One-photon absorption spectra with moderately well resolved vibronic structure were obtained for PtCl{sub 6}{sup 2{minus}} in dilute mixed crystals. The data show that a transition to a low-lying interconfigurational state is located at {approximately} 18,000 cm{sup {minus}1} in the spectral frequency region below the first absorption transition previously assigned by others. This transition cannot be unambiguously assigned. If it corresponds to the same excited state responsible for the PtCl{sub 6}{sup 2{minus}} emission spectrum, this would lead to a partial reassignment of the excited states from that of earlier work. Ligand field calculations consistent with such a reassignment are presented. The two-photon excitation (TPE) spectra of the mixed Cs{sub 2}ZrCl{sub 6}:PtCl{sub 6}{sup 2{minus}} and pure K{sub 2}PtCl{sub 6} (at 77 K), measured with an improved spectrometer, show a noticeable improvement in signal-to-noise ratio compared to the previously reported TPE spectra of K{sub 2}PtCl{sub 6} and are assigned to higher energy d-d transitions.

ESR evidence for the ferro- and antiferro-magnetic intermolecular interaction in pure and dilute mixed crystals of galvinoxyl

From the temperature dependence of ESR absorption intensity, it is shown that the intermolecular exchange interaction is ferromagnetic in the dilute mixed crystal and antiferromagnetic in the low-temperature phase of the pure crystals. The exchange energy is obtained as 2 J F = 1.5 ± 0.7 meV and 2 J AF = −45±2 meV on the basis of the singlet-triplet model.

ChemInform Abstract: NONLINEAR RAMAN STUDY OF LINE SHAPES AND RELAXATION OF VIBRATIONAL STATES OF ISOTOPICALLY PURE AND MIXED CRYSTALS OF BENZENE

New experimental results on vibrational dynamics in crystalline benzene are presented. High resolution CARS measurements on a number of Raman active modes are presented for crystals of natural isotopic composition and for a 12C6H6 isotopically pure crystal. This is the first measurement of vibrational linewidths of an isotopically pure crystal. This study indicates that the naturally occurring 13C substituted isotopic impurities make a large quantitative contribution to the Raman linewidths. The residual low temperature linewidths of the pure crystal are consistent with mode specific energy relaxation. The effect of the 13C impurities on these lines is caused by near resonant scattering or trapping of vibrational excitons. In addition, the site energy dispersion due to disorder has been measured directly for the 991 cm−1 mode in a dilute mixed crystal. Also the relative contributions of scattering and trapping to the linewidths of the Ag and B2g factor group components of the 991 cm−1 band have been assessed.

Nonlinear Raman study of line shapes and relaxation of vibrational states of isotopically pure and mixed crystals of benzene

New experimental results on vibrational dynamics in crystalline benzene are presented. High resolution CARS measurements on a number of Raman active modes are presented for crystals of natural isotopic composition and for a 12C6H6 isotopically pure crystal. This is the first measurement of vibrational linewidths of an isotopically pure crystal. This study indicates that the naturally occurring 13C substituted isotopic impurities make a large quantitative contribution to the Raman linewidths. The residual low temperature linewidths of the pure crystal are consistent with mode specific energy relaxation. The effect of the 13C impurities on these lines is caused by near resonant scattering or trapping of vibrational excitons. In addition, the site energy dispersion due to disorder has been measured directly for the 991 cm−1 mode in a dilute mixed crystal. Also the relative contributions of scattering and trapping to the linewidths of the Ag and B2g factor group components of the 991 cm−1 band have been assessed.

Transient vibrational grating scattering

A new method for measuring vibrational lifetimes in the solid state is demonstrated in a dilute mixed molecular crystal. Using a picosecond light pulse at ω 1 and simultaneous crossed pulses at ω 2, a vibrational population grating at ω 1 – ω 2 is prepared. The decay of this grating is followed by measuring at variable delay the Bragg-angle diffraction of a probe pulse, resonant with an optical transition.

Optical detection of chlorine NQR of p-dichlorobenzene in a mixed single crystal in a magnetic field

By a combined optical and microwave pumping cycle, chlorine NQR transitions could be observed for a dilute mixed single crystal of p-dichlorobenzene ( p-DCB) in p-dibromobenzene ( p-DBB). From the line splittings in the NQR spectrum of the singlet ground state of p-DCB for an orientation of the external magnetic field B o parallel to one of the in-plane fine structure axes, a considerable deviation (Δθ = 12°, Δφ = 20°) of this axis from a direction expected for an undistorted triplet-state molecule can be calculated. In addition, a lower limit of the electric field-gradient asymmetry parameter (η > 0.06) for singlet state p-DCB in the p-DBB matrix at 1.2 K is established.

Simultaneous optical detection of chlorine NQR of p-dichlorobenzene in its non-radiative S o and phosphorescent T 1 state in a mixed single crystal

By a combined optical and microwave pumping cycle electron spin alignment is transferred to the diluted chlorine nuclear spins. Because of the conservation of nuclear spin alignment during the electronic excitation and phosphorescence decay, NQR transitions within the excited triplet and electronic ground state of p-dichloro-benzene ( p-DCB) can be detected optically in a dilute mixed single crystal. A detailied analysis of NQR transition frequencies and intensities in the T 1 state has been performed. For the first time the change of the molecular field gradient under electronic excitation could be observed at the chlorine site. The NQR frequency change Δν q(S o, T 1) was found to be matrix dependent and varied from 2.52 MHz ( p-dibromobenzene) to 3.75 MHz (xylene).

On the Jahn-Teller effect in IrF6: the Γ8g(t2g)3state at 6800 Å

In this work we present a detailed discussion of the highest energy (t 2g )3 transitions of IrF6, Γ8g (2 T 2g )⇃Γ8g (4 A 2g ) at 14 900 cm-1 and Γ7g (2 T 2g ) ⇃ Γ8g (4 A 2g ) at 12 100 cm-1, as observed in dilute mixed crystals with UF6, MoF6, and WF6 at 4·2 and 1·8 K. The system is of particular interest because Γ8g states are Jahn-Teller active and the Γ7g state serves as an excellent example of what may be expected for a similar but non-Jahn-Teller active state. It is found that the usual linear Jahn-Teller interaction involving either v 2(e g ) or v 5(t 2g ) cannot account for the observations of both shifts and splittings of v 5(Γ8g ). In order to explain all the relevant observations, theories encompassing higher-order vibronic coupling terms and/or coupled v 2-v 5 linear effects must be considered. Based on calculations, IrF6 is assigned an approximate D 4 h symmetry in the mixed crystals. A new electronic charge-transfer state (Γ7g or Γ6g ) is located at ∼ 15 900 cm-1 and stretching vibrations n(v...

’’Effective’’ site symmetry in molecular crystals. Orientational energies of dilute guest probes in isotopically mixed, partially deuterated ethylene crystals

The midinfrared and Raman spectra of dilute mixed crystals of the partially deuterated ethylenes (C2H3D; cis-, trans- and 1,1-C2H2D2; C2HD3) in C2H4 and in one another have been obtained. The guest modes often exhibit multiplet structure arising from different relative orientations of the neighboring molecules. The number of energetically inequivalent orientations is diagnostic of the ’’effective’’ site symmetry of the host lattice (i.e., symmetry in the spectroscopic rather than the mathematical sense), which was found to be Ci for each of the partially deuterated ethylenes. Consistent with the predictions of the virtual crystal approximation, the number and energies of the orientational components of the guest molecules are independent of the isotopic nature of the host crystal, which indicates that the intermolecular force field is insensitive to the isotopic composition of the interacting ethylene molecules. The orientational effect can be used to distinguish among several possible effective site symmetries for any crystalline lattice, provided that the number of predicted orientational components differ.

Photon-echo relaxation measurements with two dye-lasers application to pentacene- h14 and - d14 in p-terphenyl- h crystals at 1.5 K

Electronically controlled photon-echo relaxation measurements, using two nitrogen pumped dye-lasers, are reported for mixed crystals of pentacene- h 14 and - d 14 in p-terphenyl- h 14 at 1.5 K. In dilute mixed crystals (ca. 10 −8 M) the photon-echo lifetime is found to be exclusively determined by the fluorescence lifetime. Homogeneous linewidths of 7.1 and 5.9 MHz at 1.5 K are then calculated for the electronic origin of the 1B 2u ← 1A 1g transition of pentacene- h 14 and - d 14 respectively. The decrease in photon-echo lifetime at higher guest concentrations (ca. 10 −7 M) is ascribed to energy transfer between excited and neutral guest molecules.

Zero-field paramagnetic resonance of trivalent gadolinium in several lanthanide tris(ethylsulfate) nonahydrates and nonadeuterates: Determination of crystal-field and hyperfine-structure parameters

Abstract : Employing isotopically pure trivalent 155Gd, 157Gd, and 160Gd as guests in dilute mixed crystals of lanthanum tris(ethylsulfate) nonahydrate (La3(C2H5SO4)3.9H2O), it has been possible to determine both accurate zero-field splitting parameters of crystal field origin and hyperfine structure constants due to magnetic dipole and electric quadrupole interaction terms. The zero-field paramagnetic resonance spectroscopic method is used. It is reasoned that the quadrupole hyperfine constant is due, almost entirely, to lattice contributions; moreover, it is shown to be of the expected order of magnitude and sign. The hyperfine anomaly is discussed. A set of crystal field parameters are reported for the first time for 160Gd in crystals of La(C2H5SC4)3.9D2O which differ significantly from those of the hydrate. Variations of crystal field parameters with various lanthanon host ions are also observed. Finally, a few comments concerning zero-field line widths and their measurement are made. (Modified author abstract)

Raman study of crystalline ethylenes and the structure determination through model calculations of the lattice spectra

Complete Raman spectra of the intra- and intermolecular regions of pure ethylene, ethylene-d4, and their dilute mixed crystals have been obtained. The observations agree with group theoretical predictions based on a C2h5 space group. Model calculations of the Raman lattice spectra for the two possible molecular orientations consistent with this space group were undertaken to evaluate which of these two structures more satisfactorily reproduces the experimental results. Various sets of carbon-carbon, carbon-hydrogen, and hydrogen-hydrogen interaction parameters were utilized in the calculation of crystal energies, librational frequencies, and relative Raman scattering intensities. It is noteworthy that the predicted relative intensities are only very slightly dependent on the nature of the intermolecular potential chosen. This investigation provides strong evidence that the crystalline ethylene structure with P 121/n 1 (C2h5) space symmetry, the so-called ``b structure,'' is correct.

Alkali-Halide Mixed Crystals

In earlier work we have used a model for dilute mixed crystals in which the sole perturbing effect of a foreign ion is a change in the nearest-neighbor Born-Mayer repulsion parameters. A simple experimental criterion for the validity of this model is deduced and compared with experiment for the cases NaF-Li and NaF-K. For these cases, the validity criterion is violated; hence, the model is inadequate.

Dipole Moments of the Excited States of Azulene

The splitting of spectral lines by an electric field, in the accepted sense, is dependent on the interaction of the field with degenerate states. However, molecular crystals having no degenerate molecular states can be subject to a linear Stark mixing due to the interaction of the field with the inequivalent molecules in the unit cell. In a dilute mixed crystal where the guest–guest interactions are vanishingly small, the resultant splitting of spectral lines, ΔS = EGΔμ, is directly proportional to the field strength E, a geometrical factor G, and the change in dipole moment Δμ between the ground and excited states. The substitutional solid of azulene in naphthalene was chosen to demonstrate this effect. The dipole moments of the first and second singlet states of azulene have geen determined and are −0.42 and −0.31 D, respectively.

Triplet States in Mixed Molecular Crystals

Triplet exciton density of states functions were calculated for crystalline naphthalene and benzene, and the off-diagonal matrix elements of the Green's function were obtained for crystalline naphthalene, using theoretical intermolecular electron-exchange terms. These results were then applied for the study of triplet states in doped molecular crystals, using the one-particle Green's function method. The localized impurity, the impurity pair, and the general impurity problem were considered. The experimental implications of these results concerning the absorption and emission spectra and energy-trapping effects in dilute mixed crystals were considered.

Electronic States of Mixed Molecular Crystals: Singlet Excited States of Isotopic Impurities in Crystalline Benzene and Naphthalene

In this paper the one-particle Green's-function method has been applied for the study of singlet excited states of isotopically mixed crystals of benzene and naphthalene. Calculations of energy levels, of excitation amplitudes, and of line-shape functions were performed, considering both bound and virtual impurity energy levels. The present treatment relates experimentally observable properties for the dilute mixed crystals with the exciton density-of-states function of the pure crystal. The available experimental data for mixed crystals can be adequately accounted for using empirical data for the exciton density-of-states function derived from hot-band spectroscopy. The theoretical density of states for naphthalene based on the transition-octupole coupling terms provide only qualitative agreement with the experimental mixed-crystal data. We consider the applicability of simple perturbation expressions by using a moments-expansion method of the Green's function. Finally, we discuss the information on the intermolecular interactions derived from the moments of the density-of-states functions.

Electronic States of Mixed Molecular Crystals

In this paper we consider the energy levels of dilute mixed crystals in the weak vibronic coupling limit using the one-particle Green's-function method. The perturbation Hamiltonian is treated explicitly with reference to impurity molecules differing from the host molecules by isotopic substitution. The general equations derived for the energy levels, for excitation amplitudes, and for the optical properties of the mixed crystal are expressed in terms of the weighted density-of-states function of the pure crystal. This treatment emphasizes the relation between the density of exciton states in the pure crystal and the optical properties of mixed crystals. The nature of bound and virtual impurity states are considered, and new experiments using the impurity-induced electronic-absorption method are suggested.