The structure of the monomeric oxygen adduct of vitamin B12r: A single crystal ESR study

From ESR single crystal spectra of B_12rO₂ substituted in B_12b the structure of the CoOO fragment was determined at ?190°C. The g and A^Co tensors were found to be non-coaxial with the following principal values: g₁ = 1.994,g₂ = 2.012, g₃ = 2.079; A^Co₁ = ?21 MHz, A^Co₂ = ?27 MHz, A^Co₃ = ?63 MHz. The CoOO bond angle is 111°.     

Exciton transitions in quasi-one-dimensional crystals. 9,10-dichloroanthracene

Polarized reflection spectra of the first singlet transition of the α-crystalline form of 9,10-dichloroanthracene are reported. Crystal faces (001), (011) and (010) were examined in spectral range 450 to 350 nm at two temperatures, 5 K and 300 K. Two systems of transitions were observed. The first system is assigned to neutral excitons. Spectral similarities with unsubstituted anthracene and arguments based on the one-dimensional stacking of molecules are used to construct a model of the exciten band structures. The M-polarized ππ* molecular transition gives rise to a four branch band with two allowed transitions. The 0-0_b (A_g → A_u) transition lies 50–100 cm^?1 above the bottom of the exciton band and the 0-0_c′ (A_g → B_u) transition lies at the top of the band. In the reflection spectrum the Davydov splitting c′b for transverse excitons is 210 cm^?1. The exciton band of the 00 molecular transition is not isolated but overlaps the two-particle manifold of the 0–1 vibronic transition. As a result of the 0–1_c′ transition is unexpectedly strong in the spectra of the (010) face. The second system is polarized along the stack-axis a and starts 2500 cm^?1 above the first system. It is tentatively assigned as |a(A_g → B_u) charge transfer exciton transition in agreement with earlier observations.     

Absorption,fluorescence and phosphorescence spectra of the singlet and triplet states of s-tetrazine in the crystal and in mixed crystals at low temperatures

The electronic absorption, fluorescence and phosphorescence spectra of s-tetrazine at low temperatures (4.2-1.5 K) are reported and analyzed in the neat crystal and in several mixed crystals. The ³B_3u-¹A_g (nπ^*) origin is at 18414 ± 5 cm^?1 for neat tetrazine. In the mixed crystal several sites identified. The lowest energy origin is at 17453 cm^?1 for tetrazine in pyrazine; 17 701 cm^?1 in pyrimidine; and 17 676 cm^?1 in pyridazine. The ^eB_3u-¹A_g (nπ^*) origin is at 14 096 ± 2 cm^?1 for the neat crystal. The phosphorescence lifetime of neat tetrazine is measured to be 96.8 ± 2.1 μs at 4.2 and 1.8 K. All the spectra are predominately composed of members of progressions in a single totally symmetric mode (ν_6a) built upon site origins and vibrational fundamentals. The ν_6a interval is: 743 (¹A_g), 715 (³B_3u), and 709 cm^?1 (¹B_3u) in the neat tetrazine crystal; 732 (¹A_g) and 705 cm^?1 (¹B_3u in pyrazine host, 737 (¹A_g) and 701 cm^?1 (¹B_3u) in pyrimidine host, and 732 (¹A_g) and 703 cm^?1 (¹B_3u) in pyridazine host mixed crystals. All emission spectra may be analyzed by Oi → (ν″_6a)^o_n (i), i indicating the observed s     

A Raman spectroscopic study of the antimonite mineral peretaite Ca(SbO)4(OH)2(SO4)2·2H2O

Raman spectra of mineral peretaite Ca(SbO)₄(OH)₂(SO₄)₂·2H₂O were studied, and related to the structure of the mineral. Raman bands observed at 978 and 980 cm^?1 and a series of overlapping bands observed at 1060, 1092, 1115, 1142 and 1152 cm^?1 are assigned to the SO₄^2? ν₁ symmetric and ν₃ antisymmetric stretching modes. Raman bands at 589 and 595 cm^?1 are attributed to the SbO symmetric stretching vibrations. The low intensity Raman bands at 650 and 710 cm^?1 may be attributed to SbO antisymmetric stretching modes. Raman bands at 610 cm^?1 and at 417, 434 and 482 cm^?1 are assigned to the SO₄^2? ν₄ and ν₂ bending modes, respectively. Raman bands at 337 and 373 cm^?1 are assigned to O–Sb–O bending modes. Multiple Raman bands for both SO₄^2? and SbO stretching vibrations support the concept of the non-equivalence of these units in the peretaite structure.     

The singlet nπ* states of para-benzoquinone

The vibronic nπ* singlet spectra of p-benzoquinone-h₄ and p-benzoquinone-d₄ have been observed in a supersonic jet and some as yet unknown excited state fundamentals in the vapor phase have been assigned. The electric dipole forbidden, magnetic dipole allowed origin of the ¹B_1g ← ¹A_g transition is observed at 20045 cm^?1. The origin of the¹A_u ← ¹A_g, transition has been indirectly determined at 19991 cm^?1 from the vibronic excitation spectra. Neither shows a deuterium shift.     

A Raman spectroscopic study of the mineral coquandite Sb6O8(SO4)·(H2O)

Raman spectra of coquandite Sb₆O₈(SO₄)·(H₂O) were studied, and related to the structure of the mineral. Raman bands observed at 970, 990 and 1007 cm^?1 and a series of overlapping bands are observed at 1072, 1100, 1151 and 1217 cm^?1 are assigned to the SO₄^2? ν₁ symmetric and ν₃ antisymmetric stretching modes respectively. Raman bands at 629, 638, 690, 751 and 787 cm^?1 are attributed to the SbO stretching vibrations. Raman bands at 600 and 610 cm^?1 and at 429 and 459 cm^?1 are assigned to the SO₄^2? ν₄ and ν₂ bending modes. Raman bands at 359 and 375 cm^?1 are assigned to O–Sb–O bending modes. Multiple Raman bands for both SO₄^2? and SbO stretching vibrations support the concept of the non-equivalence of these units in the coquandite structure.     

CN stretching and NO2 deformation vibrations and normal coordinate analysis of m-dinitrobenzene and m-dinitrobenzene-d4

Two bands appear for each CN stretching and nitro deformation vibration in the infrared and Raman spectra of m-dinitrobenzene and m-dinitrobenzene-d₄. The 907 cm^?1 bending mode in the vibrational spectra of m-dinitrobenzene undergo 30 cm^?1 upward shift upon d₄ substitution. A normal coordinate analysis pointed out that the 937 cm^?1 bending and 727 cm^?1 CN stretching vibrations as well as 18b CD in-plane deformation are mixed to a great extent. The other nitro bending mode undergo also an inverse isotopic effect (2 cm^?1 upward shift) due to coupling with the 18a CD in-plane deformation vibration.     

Comparison of the force field in various pyrochlore families. II. Phases presenting structural defects

A previous study of various A₂B₂O₆O′ pyrochlore families by vibrational spectroscopy allowed the analysis of the ir and Raman spectra and the force fields of pyrochlore phases presenting structural defects. The spectra of the following compounds were compared: (i) ideal pyrochlore Cd₂Ta₂O₇, (ii) lacunary Tl₂Ta₂O₆, (iii) nonstoichiometric Pb_1.5Ta₂O_6.5, and (iv) Pb_2.3Ta₂O_7.3 exhibiting regular shear planes. If the tridimensional network of TaO₆ octahedra is not modified and the perturbations concern only the A₄O′ tetrahedra network, a slight modification of the vibrational spectra and a weak decrease in the TaO stretching and OTaO bending force constants are observed. But, if the octahedra network is perturbed, large modifications appear on the spectra, revealing unambiguously the existence of new types of bonds created by the defects in the structure. The Raman intense lines observed in the low frequency range in lead compounds is also discussed.     

Electronic absorption spectrum of Fe3+ doped in ammonium chloride single crystal

The electronic absorption spectrum of the Fe²⁺ ion doped in ammonium chloride has been studied at room and liquid air temperatures. The observed bands have been assigned transitions from the ground ⁶A_1g(S) state to the excited ⁴A_1g(⁴E_g), ⁴T_1g(G) and ⁴T_2g(G) states. The cubic field approximation with D_q = 675 cm^?1, B = 645 cm^?1 and C = 4.4 B is found to give a good fit to the observed band positions.It is further concluded that the site symmetry of the Fe³⁺ ion in the crystal is lowered from O_h to C_4v symmetry at liquid air temperature.     

Raman spectra of aqueous lithium sulfate and potassium thiocyanate in strong electric fields

The effects of high-voltage pulsed discharge (HVPD) activation on the Raman spectra of saturated aqueous solutions of lithium sulfate (Li₂SO₄/H₂O) in the range of the ν₁(A) totally symmetric vibrations of the SO ₄ ^2? anion and on the spectra of potassium thiocyanate KSCN/H₂O in the region of the ν₁(C≡N) vibrations of the SCN^? anion have been studied. The temperature dependences of the width and frequency of the corresponding spectral lines have been investigated.     

ESR characterisation of SO−3 and SO−4 radicals in X-irradiated kainite (KMgClSO4.3H2O)

The ESR spectra of the kainite (KMgClSO₄.3H₂O) crystal revealed an intense isotropic (g = 2.004) peak C attributed to the SO⁻₃ radical and two pairs of lines (A₁, A₂) and (B₁, B₂) bearing intensity ratio 5:3. The intensity and linewidth variation of peak C suggested that the signal contains an unresolved shf structure. The power saturation studies on SO⁻₃ indicate that its ESR line is homogeneously broadened and its line shape is Lorentzian. The spin—lattice and spin—spin relaxation times (T₁ and T₂) of SO⁻₃ have been estimated to be 0.44 s and 656 μs, respectively. The analysis of the anisotropic pairs of lines show that they constitute two sets A and B and are due to two chemically inequivalent SO⁻₄ radical species in the lattice. The ESR spectra of the polycrystalline samples recorded at 300 and 77 K confirm the isotropic behaviour of SO⁻₃ and chemical inequivalence of two types of SO⁻₄ radicals. The principal g-values of the SO⁻₄ radical were evaluated to be: g₁ = 2.007, g₂ = 2.011, g₃ = 2.014 for species A and g₁ = 2.008, g₂ = 2.012, g₃ = 2.015 for species B. The low microsymmetry of the SO²⁻₄ ion in the lattice seems to promote the radiation damage.     

Single crystal EPR study of Mn(II)-doped magnesium potassium Tutton's salt

Single crystal EPR studies of Mn(II)-doped magnesium potassium Tutton's salt, MgK₂(SO₄)₂.6H₂O, was studied at room temperature. The spin-Hamiltonian parameters obtained are: g=2.0036(3), A = −96(3), D = 350(5), a = 14(2) and F = −5(1) (A, D, a and F are in units of 10⁻⁴ cm⁻¹). The tetragonal distortion axis corresponds to one of the MgO bond directions. The zero-field splitting parameter (D) shows a linear dependence in the temperature range 77–370 K. The percentage of covalency of the MnO bond has been estimated to be 8 per cent.     

An ab initio study of the structure and torsional modes of the H2SO4 molecule

Ab initio calculations at the STO—3G and 4—31G levels have been carried out for the H₂SO₄ molecule as a function of the pair of twist angles of the HO bonds about the respective OS bonds. Values for the remaining bond angles and lengths were taken from the recent microwave structural determination by Kuczkowski et al. The results indicate a minimum energy for a structure with a (sc, sc) conformation and C₂ symmetry, where sc denotes synclinal, or gauche. This structure corresponds to that observed. At a higher energy of 11.5 kJ mol^?1 (4—31G) there is a structure with a (+sc, ?sc) conformation and C_s symmetry. The torsional modes corresponding to the a and b irreducible representations of the C₂ point group are estimated to have frequencies of 280 and 265 cm^?1, respectively.     

Raman spectra of gases: XIV. Hexachlorodisiloxane

The Raman spectra (50–1200 cm^?1) of gaseous, liquid, and solid (Cl₃Si)₂O have been recorded. The infrared spectra of the gas and solid have been recorded from 55–2000 cm^?1 . The spectra of the gas have been interpreted in detail on the basis of C_2v symmetry with the A₁ skeletal Si-O-Si bend assigned at 63 cm^?1. The spectra gave evidence that there are structural changes upon condensation of the gas and the Si-O-Si angle approaches linearity in the solid state. The opening of this angle is probably due to crystal packing factors.     

Electronic structure and Raman spectroscopy study of dibarium magnesium orthoborate,Ba2Mg(BO3)2

The samples of dibarium magnesium orthoborate Ba₂Mg(BO₃)₂ were synthesized by solid-state reaction. The X-ray diffraction (XRD) patterns and Raman spectra of the samples were collected. Electronic structure and vibrational spectroscopy of Ba₂Mg(BO₃)₂ were systematically investigated by first principle calculation. A direct band gap of 4.4 eV was obtained from the calculated electronic structure results. The top valence band is constructed from O 2p states and the low conduction band mainly consists of Ba 5d states. Raman spectra for Ba₂Mg(BO₃)₂ polycrystalline were obtained at ambient temperature. The factor group analysis results show the total lattice modes are 5E_u + 4A_2u + 5E_g + 4A_1g + 1A_2g + 1A_1u, of which 5E_g + 4A_1g are Raman-active. Furthermore, we obtained the Raman active vibrational modes as well as their eigenfrequencies using first-principle calculation. With the assistance of the first-principle calculation and factor group analysis results, Raman bands of Ba₂Mg(BO₃)₂ were assigned as E_g (42 cm⁻¹), A_1g (85 cm⁻¹), E_g (156 cm⁻¹), E_g (237 cm⁻¹), A_1g (286 cm⁻¹), E_g (564 cm⁻¹), A_1g (761 cm⁻¹), A_1g (909 cm⁻¹), E_g (1165 cm⁻¹). The strongest band at 928 cm⁻¹ in the experimental spectrum is assigned to totally symmetric stretching mode of the BO₃ units.     

The 3400 Å band system of sulphur dioxide

The contour of the band of SO₂ at 29937 cm^?1 has been shown to be of type-c, and an approximate excited state structure derived as r_SO = 1.50 Å, valence angle = 112°. For a number of reasons it is proposed that the principal bands between 3150 and 3400 Å correspond to vibronically induced B₁-A₁ transitions of a ¹A₂-¹A₁ forbidden electronic transition rather than to an allowed ¹B₁-¹A₁ transition.     

Vibrational (i.r. and Raman) spectra and conformational studies of 1-formyl-3-thiosemicarbazide

The i.r. and Raman spectra (30–4000 cm⁻¹) of 1-formyl-3-thiosemicarbazide (FTSC) and deuterated ftsc-d₄, have been studied. Most of the vibration modes reveal pairs of bands and show strong temperature dependence. A band group {ν(NNH₂)} at ∼ 1100 cm⁻¹ exhibits well resolved doublet (1095 and 1112 cm⁻¹) structure below 100 k. The intensity in the 11 12 cm⁻¹ band decreases regularly (band disappears at 150 K) with the rise in temperature. Two new bands at 955 and 1070 cm⁻¹ appear while measured above 400 K. The system eventually exists in several conformers in simultaneous equilibria. Moreover, a few bands {e.g. ν(CO), ν(CS) and ν(CH)} that show strong intensifies in i.r. exhibit weak (or zero) intensifies in the Raman and vice-versa. The features (characteristic of u and g vibration species) could be explained by a C_2h pseudo symmetry space group proposed for the system. Both the FTSC and FTSC-d₄ represent strong molecular associations. This favours the maximum abundance in the dimer stabilized conformers.     

Two-photon absorption spectrum of (2,2)-paracyclophane

The two-photon absorption spectrum of crystalline (2,2)-paracyclophane was measured in the regions of 31200–33800 cm^?1, 34700–36500 cm^?1, and 40800–48600 cm^?1 by monitoring the fluorescence intensity using a tunable dye laser as a two-photon excitation source. Two absorption bands in the region 34700–36500 cm^?1 were assigned to the two even-parity allowed ¹B_1g ← ¹A_1g and ¹B_2g ← ¹A_1g transitions.     

Microwave spectrum of the s-trans form of 3-pyridinecarbaldehyde

The microwave spectra of the ground state and one excited state of the ON s-trans form of 3-pyridinecarbaldehyde have been measured and assigned. The ground state rotational constants and dipole moment components are: A = 5417.8, B = 1583.289, C = 1225.389 (in MHz) and ¦μ_a¦ = 1.35, ¦μ_b¦ = 0.5 (in debye). The excited state most probably belongs to the C₃C torsion, for which the vibrational frequency is estimated to be 135 ± 30 cm^?1.     

Raman scattering study of calcium hexaboride

We report significant difference in the Raman spectra of two different kinds of CaB₆ single crystals grown from boron purity 99.9% (3N) or 99.9999% (6N), respectively. Our Raman spectra of CaB₆ (3N), which are similar to those of previous measurement [N. Ogita, S. Nagai, N. Okamoto, M. Udagawa, F. Iga, M. Sera, J. Akimitsu, S. Kunii, Phys. Rev. B 68 (2003) 224305], show peaks at 781.3 cm⁻¹ (T_2g), 1140.1 cm⁻¹ (E_g), and 1283.5 cm⁻¹ (A_1g). The E_g mode shows a characteristic double-peak feature due to an additional weak broad peak centered at 1156.0 cm⁻¹. However, the Raman spectra of CaB₆ (6N) show sharp peaks at 772.5 cm⁻¹ (T_2g), 1137.9 cm⁻¹ (E_g), and 1266.6 cm⁻¹ (A_1g). The peak frequencies are down shifted as much as 17 cm⁻¹. In addition, no additional peak feature is observed for the E_g mode so that the mode is symmetric in the case of CaB₆ (6N). The X-ray powder diffraction patterns for both CaB₆ (3N) and CaB₆ (6N) show that the lattice parameters are essentially the same. The majority of the impurity in the 99.9% (3N) boron is assessed to be C. Thus we prepared Ca(B_0.995C_0.005)₆, CaB₆ (6N) doped with C, and looked for the difference in the Raman spectra. The Raman spectra of Ca(B_0.995C_0.005)₆ are nearly identical to those of CaB₆ (6N), indicating that the difference in the Raman spectra of CaB₆ (3N) and CaB₆ (6N) is not due to C impurity. However, presence of impurity, even if small amount, seems to be enough to trigger local-structure changes to lower symmetry inducing the difference in Raman spectra of CaB₆ (3N) and CaB₆ (6N).