Closo clusters with unusual electron numbers: Localized bond schemes for n-atom clusters with n, (n + 1) or (n + 2) skeletal electron pairs

This paper considers whether satisfactory localized 2- and 3-centre bond schemes may be devized for real or hypothetical clusters B_nH_n^c? (c = 0, 2 or 4; n = 4 → 12) or isoelectronic analogues thereof, with closo deltahedral shapes. As expected, localized bond schemes can be devized for all the species that would be predicted, using MO approaches, to have closed shell electronic configurations: these include species B_nH_2? (n = 5 → 12) and B_nH_n^c? (c = 0 or 4; n = 4, 8, 9 or 11). There is also generally good agreement with MO treatments as to which systems are not expected to be stable: these include B₄H₄^2? and B_nH_n^c? (c = 0 or 4; n = 5, 7 or 10). Localized bond treatments have value for estimating the bond orders of cluster bonds in species B_nH_n and B_nH_n^2?, though they overestimate the bond orders in species B_nH_n^4? (except when n = 4). They are misleading, when applied to octahedral and icosahedral systems, in indicating the former shape to be feasible for hypothetical species B₆H₆ and B₆H₆^4?, and the latter shape to be feasible for hypothetical species B₁₂H₁₂ and B₁₂H₁₂^4?.     

Rules for macropolyhedral boranes

Based on extensive computational studies, rules to derive the thermodynamically most stable macropolyhedral borane for any formula between B_nH_n−4 to B_nH_n+8 were identified. Formally, the macropolyhedral boranes may be obtained by condensing regular convex borane clusters where as many BH₃ moieties are eliminated as vertexes are shared in the macropolyhedral framework. Macropolyhedral boranes consisting of two cluster fragments may be classified according to their general formulae ranging from B_nH_n−4 to B_nH_n+8. For each of these formulae, various structure types are conceivable differing in the number of shared vertexes and in the types of combined cluster fragments. However, for each general formula, only one structure type is known experimentally and this one is also computationally found to be thermodynamically preferred! For each class of macropolyhedral B_nH_m boranes, a preferred number of shared vertexes is identified, and this determines the number of skeletal electron pairs. With this knowledge, the type of fused clusters, i.e. the most favourable framework, may be predicted. The concept of preferred fragments may be applied to even predict the distribution of vertexes among the fused fragments in the thermodynamically most stable isomers. When there is at least one closo fragment it has 12-vertexes. Without any closo fragment the most stable macropolyhedral borane has a nido 10-vertex cluster fragment.     

The infrared spectrum of chd2cn and the ground state geometry of methyl cyanide

Analysis of the rovibration spectrum of CHD₂CN enables the rotation parameter (A_o—B?_o) to be determined, from which the value of A_o may be found using the microwave B_o and C_o constants. The use of A_o (CHD₂CN) in conjunction with all other available B_o and C_o constants for isotopes of methyl cyanide enables the ground state geometry to be defined within fairly narrow limits. The CH bond length is in agreement with the value predicted from the CH vibration frequency of CHD₂CN, and with the value calculated from the results of a recent analysis of the infrared spectrum of CD₃CN.     

Ermittlung kinetischer Parameter aus der Kurvenform für Reaktionen unter Aufheizung

Form indexes for DTA or TG curves (S _T orS _x) must be treated separately. Only forS _x can clear relations to be developed for the order of reactionn. In the rational range ofn between 0.5 and 3.0 we found for linear, exponential and hyperbolic programmes these functions were found to be of the type $$S_x = an^{0,5} + b$$ Since a low dependence on frequency factork ₀ has been established, for a linear programme the simultaneous determination ofn andk ₀ may be performed.     

Space group determination of the BaY(Cu0.5Fe0.5)2O5+δ phase using a convergent-beam electron-diffraction technique

The space group of the BaY(Cu_0.5Fe_0.5)₂O_5+δ (δ=0.03-0.17) phase was studied by selected-area electron diffraction and convergent-beam electron diffraction (CBED). The CBED patterns for BaY(Cu_0.5Fe_0.5)₂O_5.03 grains taken from the zone axes of [111], [001] and [010] had the symmetries of m, 4mm and 2mm, respectively. Forbidden reflections were observed neither in selected-area electron diffraction nor in the CBED patterns. From these results, the space group of the BaY(Cu_0.5Fe_0.5)₂O_5.03 was determined to be P4/mmm. Since the presence of a mirror plane parallel to the (Cu,Fe)O₂ planes was confirmed, Cu and Fe were found to be randomly distributed in the (Cu,Fe)O₂ planes. The same analyses were performed for BaY(Cu_0.5Fe_0.5)₂O_5.17 grains and the space group was also found to be P4/mmm. The change in the magnetic properties of BaY(Cu_0.5Fe_0.5)₂O_5+δ samples due to the high-pressure heat-treatment was concluded to be caused by excess of oxygen.     

The 99Ru Mössbauer spectrum of β-RuCl3

The ⁹⁹Ru nuclear quadrupole interaction in β-RuCl₃ can be related to the large trigonal distortion at the ruthenium site. The ratio of the quadrupole moments has been determined to be Q_e/Q_g = +3.06 ± 0.10, and it is confirmed that Q_c and Q_g are both positive.     

Anti-bacterial effect of four extracts from leaves of Dracontomelon dao on Escherichia coli growth using microcalorimetry coupled with principal component analysis

The anti-Escherichia coli activities of four extracts in leaves of Dracontomelon dao, a traditional folk herb in China were investigated and compared by microcalorimetry. The four extracts are PE fraction, CHCl₃ fraction, EtOAc fraction, and n-BuOH fraction. The heat flow power–time (HFP–time) curves of E. coli growth in the presence of the four extracts were measured using an ampoule method. Then the nine thermal kinetic parameters were obtained from the curves. From the result of principal component analysis, it can be seen that parameters k ₁, k ₂, P ₁, and Q _p2 might be the main parameters in evaluating the anti-E. coli effects. In the presence of CHCl₃ fraction, EtOAc fraction, and n-BuOH fraction, k ₂, Q _p2 of E. coli decreased with increasing concentrations of the extracts. The EtOAc fraction was observed to have the strongest anti-bacterial activity with half-inhibitory concentration IC50 of 98.5 μg mL^?1. So, it can be concluded that EtOAc fraction can be further developed as anti-bacterial bioactive fraction of leaves of Dracontomelon dao.     

Pulsed NMR of cis-polyisoprene solutions T1 and T2 relaxations,free volume,viscosity relationships

Previous pulsed NMR studies of polyisoprene have largely been concerned with entangled or crosslinked networks. This paper deals with (i) the relaxation of high molecular weight entangled; (ii) cross-linked; (iii) monodisperse low molecular weight; and (iv) high molecular weight polymer in the presence of tetrachloroethylene which, by increasing molecular mobility, can be expected to influence the NMR relaxation. For all four types of polyisoprene, the spin-lattice T₁, relaxation shows a minimum with position depending only on the free volume, as influenced by changes in temperature T and polymer concentration v₁,. For monodisperse polyisoprene of molecular weight 7200, insufficient to form an entangled network, the spin-spin relaxation decay constant T_2L is quantitatively related to the free volume ₁ by two parameters A′ and B″ when the free volume is altered by a change in temperature, or in polymer concentration (10–100/). This can also be expressed in the form where the parameter T_∞ at 100% concentration agrees with the value used to describe rheological properties. At other concentrations of polymer, T_∞ and B′ can be derived quantitatively from the coefficients of volume expansion of polymer and solvent. The variation of T_2L with molecular weight (T_2L ∝ M^?0.5) occurs via the A′ parameter. It is concluded that T_2L can be quantitatively related to the free volume available for molecular motion (as influenced by temperature and solvent concentration) as well as to molecular weight. Furthermore T_2L is simply related to viscosity n, over a wide range of temperatures and concentrations. T₂ can be used to analyse the molecular motions involved in theology.     

The infrared spectrum of CHD2NC and the ground state structure of methyl isocyanide

Analysis of the rovibration spectrum of CHD₂NC has enabled the rotation parameter (A₀B₀) to be determined. Knowledge of the microwave B₀ and C₀ constants enables A₀(CHD₂NC) to be determined, which, when used in conjunction with all other B₀ and C₀ constants for isotopic methyl isocyanides, allows the ground state structure to be defined within fairly narrow limits. The CH bond length is much shorter than that determined by the substitution technique (r_s structure). It is now in almost perfect agreement with the value predicted by the CH vibration frequency in CHD₂NC, and falls in the range of CH bond lengths exhibited by all other symmetric top methyl group molecules.     

First example of monodispersed (Mo3Se3)1∞ clusters

The close relationship of the structure of ternary molybdenum chalcogenides to molecular cluster compounds suggests that these chalcogenides might be synthesized from such clusters or be solubilized as clusters. However, in contrast to molybdenum halide clusters which can be obtained in solution from the solid phase, many attempts to solubilize these chalcogenide clusters were unsuccessful. This is the first report that some of the pseudo-one-dimensional compounds (M₂Mo₆X₆, X = Se, Te; M = Li, Na) can be dissolved when exposed to highly polar solvents such as dimethylsulfoxide or N-methylformamide. We show using optical microscopy, TEM, and light scattering that some of these solutions contain individual (Mo₃X₃)¹_∞ chains. This is also the first example of a purely inorganic transition-metal polymer solution. The behavior with respect to flocculation is consistent with the double layer theory. We show that the (Mo₃X₃)¹_∞ chains can be oriented in solution.     

The trans-influence in polyenyl—metal complexes; The crystal and molecular structure of hydrido-(η-pentamethylcyclopentadienyl)bis(triphenylphosphine)-rhodium(III) hexafluorophosphate

The structure of [Rh(η-C₅Me₅)H(PPh₃)₂]PF₆ has been determined by ¹H NMR studies and a single-crystal X-ray analysis. The compound crystallises in the orthrhombic space group P2₁2₁2₁ with lattice constants a 12. 926(3), b 15.216-(3) and c = 20.957(4) Å, with Z = 4. The structure was determined using diffractometer data and by least-squares techniques to R = 0.0887 based on 2805 independent reflections with F₀?4o(F₀). The geometry about the metal atom may be inferred to be a “piano-stool” arrangement with the C₅Me₅ ring representing the seat and the PPh₃ and H ligands the legs, although the hydrogen atom was not directly located in the crystallographic analysis. The observed distortions in the C₅Me₅ ring may be attributed to the large trans-influence of the hydrido ligand.     

Broadening and shift of thalliumnP 1/2,3/2 states (n=6–10) perturbed by noble gases

Impact broadening and shift of the transitions Tl 6P _1/2?nP _1/2,3/2 (n=7, 9, 10) and 6P _3/2?9P _{1/2, 3/2} are measured by high resolution Doppler-free two-photon spectroscopy. For excited states with small principal quantum numbers the results are in accord with values obtained fromC ₆-C ₈-C ₁₂ potentials calculated semiempirically. For intermediate principal quantum numbers the experiments show that the elastic scattering of the valence electron at the noble gas atom must be considered additionally. The experimental shift rates ofP _1/2 states are found to be larger than ofP _3/2 states. Furthermore, the line shifts of the one- and two-photon transitions concerning the 6P _1/2, 6P _3/2, 9P _1/2, 9P _3/2, 7S _1/2 states show that the contribution of the lower level of the transition must be considered too.     

Structural, spectroscopic and photoluminescence studies of LiEu(WO4)2−x(MoO4)x as a near-UV convertible phosphor

A series of lithium europium double tungsto-molybdate phosphors LiEu(WO₄)_2−x(MoO₄)_x (x=0, 0.4, 0.8, 1.2, 1.6, 2.0) have been synthesized by solid-state reactions and their crystal structure, optical and luminescent properties were studied. As the molybdate content increases, the intensity of the ⁵D₀→⁷F₂ emission of Eu³⁺ activated at wavelength of 396 nm was found to increase and reach a maximum when the relative ratio of Mo/W is 2:0. These changes were found to be accompanied with the changes in the spectral feature, which can be attributed to the crystal field splitting of the ⁵D₀→⁷F₂ transition. As the molybdate content increases the emission intensity of the 615 nm peak also increases. The intense red-emission of the tungstomolybdate phosphors under near-UV excitation suggests them to be potential candidate for white light generation by using near-UV LEDs. In this study the effect of chemical compositions and crystal structure on the photoluminescent properties of LiEu(WO₄)_2−x(MoO₄)_x is investigated and discussed.     

Normalized potential sweep voltammetry: Part III. Elimination of the effect of uncompensated Resistance and the application to heterogeneous kinetics of rapid charge-transfer processes

Uncompensated resistance (R_u) has a distoring effect on normalized potential sweep voltammetry (NPSV) slopes. This provides a simple and effective method to determine R_f^o, the value of the potentiostat feedback resistance necessary for full compensation. If the NPSV range is divided into overlapping segment, 1 and 2, corresponding to I_N of 0.20–0.50 and 0.50–0.80 respectively, the slopes m₁ and m₂ differ significantly when R_f differs form R_f^o. The difference, m₁-m₂, is negative for R_f<R_f^o and positive for R_f>R_f^o. Fine tuning of the potentiostat R_f setting so that the average value of the difference is the theoretical value can be accomplished in a minimum of time. Under these conditions, m₁ and m₂, as well as m_T, the slope of the entire correlation have very nearly the same values. Linear equations were derived from theoretical data which allow heterogeneous rate constants to be obtained directly from NPSV slopes. The precision in the NPSV slopes was observed to be of the order of ±0.002 which implies that the method should give reliable rate constants as great as 10cm s^? at a voltage sweep rate of 100V s^?1. The method is demonstrated using the reduction of benzonitrile and perylene in N,N-dimethylformamide and acetonitrile as examples.     

Binary metal alloy clusters

Binary alloy clustersA _n B _m (A,B: Bi, Sb, Pb, Ag, in different compositions) have been generated by simultaneous inert gas condensation of the two vapours. Under suitable condensation conditions the adsorption of single atomsA to clustersB _m can be studied. The adsorption probabilities are found to vary with cluster size. The cluster reactivities depend on the number of absorbed foreign atoms. Lead and bismuth clusters are found to be highly reactive in cracking the strongly bound Sb₄-tetrahedron. One single interaction channel is found for Pb _n /Sb₄ (symmetrical dissociation) and three interaction channels are found for Bi _n /Sb₄ (symmetrical and nonsymmetrical dissociation). Pb- and Bi-clusters have different onset thresholds for dissociative reactivity.     

Studies of cluster anions C n X− (X=N,P, As,Sb, Bi) produced by laser ablation

A series of cluster anions C _n X^? are produced from laser ablation of appropriate samples, where X is selected as a group-VB element. The recorded mass spectra of these cluster anions display a drastic even/odd alternation on ion intensities: For C _n N^?, only anions with oddn can be observed; For C _n P^? and C _n As^?, cluster anions with evenn are produced but with lower signal intensities; For C _n Sb^?, the signal intensity of clusters does not show even/odd alternation; Finally, for C _n Bi^?, the intensities of cluster anions with evenn are higher than those with oddn. This parity effect can be attributed to the linear structure of the cluster anions, and the parity reversal of C _n X^? from C _n N^? to C _n Bi^? can be explained from the electronegativity decreasing of the heteroatom X as it descends in the group. The Hückel model was applied to account the structural feature of these clusters.     

Magnetic properties of ultrafine ferrihydrite clusters studied by Mossbauer spectroscopy and by thermodynamical analysis

Magnetic properties of ultrafine clusters of Fe₅HO₈·₄H₂O (ferrihydrite, FH), isolated in pores of polysorb, were studied by Mossbauer spectroscopy and by thermodynamical analysis. Thermodynamical analysis allowed the conclusion that magnetic properties of ultrafine clusters cannot be interpreted in terms of a secondorder magnetic phase transition or of superparamagnetic behavior alone but require the consideration of a jumplike first order magnetic phase transition (JMT). The critical radius R _cr below which the JMT is to be expected in clusters was derived from thermodynamic criteria. It was determined as R _cr = 2 α β η/(1 - T _cc/T ₀), where α, β and η are constants derived from surface energy, magnetostriction, compressibility and T _cc = 3/2 Nκ _B T _o ² ηβ ² (N is the number of iron atoms, κ _B is the Boltzmann constant, T _o is the Curie temperature of the clusters). For the smallest FH clusters isolated in pores of polysorb, the critical radius and the JMT temperature were estimated by Mossbauer spectroscopy to be R _cr ～ 1.5–2.0 nm and T _JMT ～ 4.2–6 K, respectively. Satisfactory agreement between the value R _cr, estimated from the experimental data and the one derived by thermodynamical analysis was achieved. Interfacial (cluster-surface) and intercluster interactions were found to destroy the JMT effect and to give rise to a second-order magnetic phase transition.     

On the assignment of SiH and SiD stretching frequencies: A reanalysis of the ν7 bands of Si2H6 and Si2D6 and a harmonic local mode force field for disilane

Assignments of ν(SiH) and ν(SiD) modes in SiHX₃, SiH₂X₂ and SiH₃X compounds are surveyed. With a few exceptions, ν(SiH)/ν(SiD) ratios are found to be those expected on the basis of harmonic local mode calculations, if ν(SiD) values are first divided by a factor R which normally lies in the range 1.010–1.011. Major deviations from this range indicate misassignments or Fermi resonances. Stretch-stretch interaction constants f′_a are tabulated, and agree excellently with ab initio values, where these are available. Where ν^is (SiH) values are known, the frequency sum rule is found to be obeyed closely. The ν₇ bands of Si₂H₆ and Si₂D₆ are reanalysed to yield an R value of 1.010. The ν(SiH) data for disilanes then yield, in a “6 × 6” refinement, values of the three interaction force constants f′_a, f′_g and f′_g, together with predictions of hitherto unlocated stretching fundamentals. Constancy of the R factor for different types of XH bonds is shown to be a consequence of Dennison's rules, and similar anharmonicity factors.     

Thermodynamic properties and molecular dynamics of (NH4)2Zn(SO4)2·6H2O studied by single-crystal NMR and MAS NMR

The thermodynamic properties and molecular dynamics of (NH₄)₂Zn(SO₄)₂·6H₂O were investigated using thermogravimetric analysis, differential scanning calorimetry, and nuclear magnetic resonance observations. The first mass loss occurs near 350 K (= T _d) and is interpreted as the onset of partial thermal decomposition. The temperature dependences of the spin–lattice relaxation time in laboratory frame T ₁ and in rotating frame T _1ρ for H nuclei were studied near T _d and T _C1. The increase in T ₁ near T _d seems to be related to the ammonium protons, and the abrupt decrease in T _1ρ near T _d can be explained due to the loss of H₂O.     

Spin coherence in crystalline complexes. Naphthalene: 2(1,4-diiodotetrafluorobenzene)

The weak crystalline complex of naphthalene : 2(1,4-diiodotetrafluorobenzene), which exhibits naphthalene phosphorescence with a lifetime in milliseconds, is conveniently investigated by a combination of conventional and pulse techniques of optically detected magnetic resonance. The D + E and D ? E resonances, investigated here, reveal no transferred hyperfine interaction, but increased D and E values because of the external spin-orbit coupling which is found to be anisotropic. Measurements are made of T₂, T₂*, and T₂? for this complex. This coherence study finds T₂ to be similar in magnitude to what has been observed for mixed organic crystals. T₂*, when compared with T₂, confirms the earlier suggestion that the crystalline complex is highly ordered. T_2? measurement suggests interaction with the nuclear spins of C₆I₂F₄.