Sulfur, Selenium and Tellurium Ring Clusters: Experimentaland Density-Functional Theoretical Study of Polarized OpticalAbsorption and Raman Spectra, Light-Induced Effects and Conical Intersections

The Raman and IR absorption spectra of single crystals of germanium isotopes 72Ge, 73Ge, 74Ge, and 76Ge in the region of phonon absorption and interband electronic transitions are studied at room temperature. The dependence of the Raman peak position on the atomic mass has the form ν ~ M –1/2. The shifts of the phonon absorption peaks of individual isotopes with respect to germanium of natural isotopic composition natGe are determined. With increasing average atomic mass of germanium, these peaks shift to longer wavelengths. In the region of interband electronic transitions, the intrinsic absorption edge of 76Ge is observed to shift by 1 meV to higher energies with respect to Ge of natural isotopic composition. For isotopes with atomic masses close to that of natural germanium (72Ge,73Ge, 74Ge), we found no significant difference in the band gap width at room temperature.

Polarized Raman and resonance Raman spectra of single crystals of one‐dimensional mixed valence platinum complexes have been recorded and studied. Polarized Raman spectra of single crystals of [M(II) (en)2M(IV)(en)2X2](ClO4)4, where X = Cl or Br, M = Pt or Pd and en = ethylenediamine, show strong resonance enhancement of the…XM(IV)X…symmetric stretching vibration in the chain. Analysis of the spectra shows that the metals are coupled in the chain through the halide atoms. The coupling occurs by electron charge transfer via the dz2‐orbitals of the metal from the lower to the higher oxidation state.

Raman and optical absorption spectra of oriented Se8 and Se12 rings formed in zeolites: Dependence on the Se loading density

Polarized Raman and optical absorption spectra of the mordenite single crystals containing sulfur, selenium, or tellurium in the one-dimensional nanochannels

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C70 at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Gruneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the dω/dP value for certain phonon modes abruptly changes at pressures of P1≈2 GPa and P2≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C70 crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C70, which is about 2.1±0.1 eV.

Infrared and Raman spectra of potassium hydrogen malonate single crystals

Raman spectra of sulfur, selenium or tellurium clusters confined in nano-cavities of zeolite A

The infrared spectra of polycrystalline hexamethylenetetramine-h12 and -d12 (HMT-h12 and HMT-d12) at 300 and 100°K, and the Raman spectra of polycrystals, single crystals, and saturated aqueous solutions of HMT-h12 and HMT-d12 at 300°K, have been recorded. The infrared spectra and the Raman spectra of the single crystals yielded an unambiguous assignment of most of the zero wave vector fundamental vibrations in the solid. Product rule and normal coordinate calculations verified the mutual compatibility of the assignments for the two isotopic molecules, and gave a detailed description of the vibrational modes. The spectra of HMT-h12 and their assignment differ slightly from those of previous authors, while the spectra of HMT-d12 have not been reported previously. The only F1 mode whose frequency is well established is ν16, at 378 cm−1 in HMT-h12 and 315 cm−1 in HMT-d12. The longitudinal and transverse components of several of the infrared-active F2 modes have been unambiguously assigned in the Raman spectra.

The Te8 ring molecule (cluster) is poorly investigated due to the lack of experimental data. Here, we report an experimental and theoretical study of a regular array of oriented Te8 rings formed in the ∼1.14 nm diameter cavities of zeolite LTA, which are arranged in a cubic lattice with a spacing of ∼1.2 nm. Single crystals of LTA with encapsulated tellurium (LTA-Te) were studied using Raman spectroscopy (RS) and optical absorption spectroscopy (OAS). The experimental LTA-Te spectra were found to be in agreement with those calculated using density functional theory (PBE0 hybrid functional and def2-TZVP basis sets) for the crown-shaped Te8 ring molecule with D4d symmetry. Using polarization-orientation RS, we show that the Te8 rings are oriented by their major axes along the 4-fold axes of cubic LTA. We also show that the site symmetry of Te8 in LTA-Te is lower than D4d. Te8 bond-bending modes are well described in the harmonic approximation, while bond-stretching modes are mixed due to the reduced ring symmetry and, probably, anharmonicity. Importantly, OAS data of LTA-Te display dependence on the Te8 concentration, implying the interaction of the rings from neighbouring LTA cavities with the generation of the valence and conduction electron bands of such a cluster crystal.

Single-crystal polarized Raman spectra (80 to 4000 cm-1 at 4 ≤ T ≤ 700 K) and powder IR spectra (1500 to 4000 cm-1 at 50 < T < 300 K) were measured for two microporous zeolites natrolite, Na16[Al16Si24O80]·16H2O, and scolecite, Ca8[Al16Si24O80]·24H2O to determine the behavior of H2O molecules in the channels. Both IR and Raman spectra show intense O-H stretching and H2O bending modes derived from the hydrogen-bonded H2O molecule(s) in the channels. Using published crystal structural data for natrolite and scolecite, and a consideration of Raman mode intensities that are sensitive to the H2O orientation in the framework channels, the internal stretching and bending modes could be assigned. The Raman spectra also show lower energy lattice modes and, in addition, second-order scattering in the wavenumber range where O-H stretching vibrations occur. The stretching vibrations of H2O molecules of natrolite and scolecite are located between 3200 and 3700 cm-1 and bending vibrations occur around 1650 cm-1. In the case of natrolite, two intense O-H stretching modes can be observed and also several weaker combination modes. The latter was used to derive a low energy external H2O translational vibration, T(H2O), which is also observed directly in single-crystal Raman spectra. In addition, two H2O librational modes are located at about 440 and 500 cm-1. For scolecite, six O-H stretching modes are observed in the Raman spectra recorded at 4 K, but only five are found at room temperature in the IR or Raman. The single-crystal Raman spectra also show several second-order combination modes consisting of external and internal H2O vibrations. They permit the wavenumber of several T(H2O) modes at low wavenumbers to be determined. These combination bands are analyzed based on their temperature behavior between 0 and 300 K. It is shown that the wavenumber of the H2O bending modes decreases with an increase of the H-O-H angle of the H2O molecule in natrolite and scolecite. The dehydration behavior of H2O in natrolite and scolecite was investigated by Raman measurements of the intensities of the O-H stretching modes at temperatures from 300 K to 570 K and 720 K, respectively. IR and Raman spectra, obtained over a large temperature range, permit one to obtain a better understanding of inner surface H2O-molecule behavior in microporous silicates and energetics and the behavior of hydrogen bonding.

Raman spectra of single crystal and powdered samples of dianthracene have been recorded at temperatures from 77 to 410 K and at pressures to 32 kbar. The Raman and infrared spectra of dianthracene dispersed in KBr demonstrate the apparent stability of dianthracene under these conditions, contrary to other reports. At pressures above 10 kbar, However, a fluorescence background appeared with the Raman spectra excited with Ar and He–Ne lasers. This fluorescence is attributed to direct excitation of close-coupled sandwich-like dimer defects created and trapped in the crystal under pressure.

The self-complementary oligonucleotides [r(CGC)d(CGC)]2 and [d(CCCCGGGG)]2 in single-crystal and solution forms have been investigated by Raman spectroscopy. Comparison of the Raman spectra with results of single-crystal X-ray diffraction and with data from polynucleotides permits the identification of a number of Raman frequencies diagnostic of the A-helix structure for GC sequences. The guanine ring frequency characteristic of C3'-endo pucker and anti base orientation is assigned at 668 +/- 2 cm-1 for both dG and rG residues of the DNA/RNA hybrid [r(GCG)d(CGC)]2. The A-helix backbone of crystalline [r(GCG)d(CGC)]2 is altered slightly in the aqueous structure, consistent with the conversion of at least two residues to the C2'-endo/anti conformation. For crystalline [d(CCCCGGGG)]2, the Raman and X-ray data indicate nucleosides of alternating 2'-endo-3'-endo pucker sandwiched between terminal and penultimate pairs of C3'-endo pucker. The A-A-B-A-B-A-A-A backbone of the crystalline octamer is converted completely to a B-DNA fragment in aqueous solution with Raman markers characteristic of C2'-endo/anti-G (682 +/- 2) and the B backbone (826 +/- 2 cm-1). In the case of poly(dG).poly(dC), considerable structural variability is detected. A 4% solution of the duplex is largely A DNA, but a 2% solution is predominantly B DNA. On the other hand, an oriented fiber drawn at 75% relative humidity reveals Raman markers characteristic of both A DNA and a modified B DNA, not unlike the [d-(CCCCGGGG)]2 crystal. A comparison of Raman and CD spectra of the aqueous [d(CCCCGGGG)]2 and poly(dG).poly(dC) structures suggests the need for caution in the interpretation of CD data from G clusters in DNA.

Photostructural changes in amorphous selenium: an in situ EXAFS study at low temperature

Oriented Se6 ring clusters in zeolite single crystals: Polarized Raman microscopy, optical absorption spectra and photo-induced effects

A density functional study for the isomers of anionic sulfur clusters S n− ( n=3–20)