Room temperature response of SnO2-electrode modified solid state electrochemical CO2 sensors

A thin film solid state electrochemical gas sensor has been investigated for CO₂ detection based on the cell reaction: Na⁺+OH+CO₂=NaHCO₃. The galvanic cell arrangement is Au | Na_xCoO_2−δ (ref.) | NASICON | Au, SnO₂ where the right hand electrode is in contact with CO₂ and O₂ in a humid atmosphere. The response has been compared to results obtained with a conventional pellet type sensor. Furthermore, both devices have been exposed to CO and humidity. Strong cross-sensitivities were observed leading to large changes in the emf in both cases. The response to moisture is reversible and fast with a response time of about 1 min according to a fast surface reaction of H₂O with SnO₂. The presence of CO leads to a signal change with a high response time and a very slow reverse reaction. However, the response to CO₂ is not influenced by the presence of CO or H₂O with regard to the signal height and response time. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Isotopieverschiebung der natürlichen geraden und ungeraden Sm- und Nd-Isotope

The isotope shift of the natural isotopes of Sm and Nd was investigated by a digital recording Fabry-Perot spectrometer using highly enriched samples. By the accuracy achieved in the measurements it was possible to verify earlier results indicating different relative isotope shifts for lines with positive and negative displacements, and to show that also lines with the same sign of the displacement may have different relative isotope shifts. It is shown that these non-constant relative isotope shifts can be explained for the even isotopes by the different contributions of the mass and volume effect to the isotope shifts in the different lines, but this explanation seems only to be partly valid for some distances between even and odd isotopes. The size of the mass effect is estimated and its influence especially on the results of the odd-even staggering is considered. The odd-even staggering is compared to earlier results for Hg. The estimation of the mass effect shows that the results forδ〈r ²〉 determined from the measured isotope shifts without correcting for the mass effect may be considerably wrong. Nevertheless it is possible by a suitable combination of the measured distances to calculate a quantity which only depends on nuclear properties and which therefore enables to check the known theories of the volume dependent isotope shift also without calculating the probability density of thes-electrons at the nucleus.     

Isomers of cyclo-heptasulfur and their coordination to Li(+): an ab initio molecular orbital study

The potential energy hypersurfaces (PESs) of heptasulfur (S7) and of [LiS7]+ have been investigated by ab initio molecular orbital calculations at the G3X(MP2) level of theory. Besides the chair-like seven-membered ring (1a) as the global minimum structure, eight local minimum structures and one transition state have been located on the PES of S7. The barrier for pseudorotation of 1a is only 5.6 kJ mol(-1). The boat-like S7 ring (1b) is 12.1 kJ mol(-1) less stable than 1a, followed by three isomers of connectivity S6=S and four open-chain isomers. On the basis of multireference calculations at the MRCI(4,4)+Q/6-311G(d) level, the most stable open-chain form of S7 is a triplet of relative energy 133.1 kJ mol(-1). Thus, the reaction energy (deltaE0) for the ring opening of 1a is 133.1 kJ mol(-1), halfway between those of the highly symmetrical rings S6 and S8. Because of their strong multireference characters, the stabilities of the biradicalic singlet chains are significantly overestimated by the single-reference-based G3X(MP2) method. The calculated vibrational spectrum of 1a is in good agreement with experimental data. The various isomers of S7 form stable complexes with Li+ with coordination numbers of 1-4 for the metal atom and binding energies in the range of -93.8 to -165.7 kJ mol(-1). A total of 15 isomeric complexes have been located, with 13 of them containing cyclic ligands. The global minimum structure (2a) is composed of 1a, with the Li+ cation linked to the four negatively charged sulfur atoms (symmetry C(s)). The corresponding complex 2c containing the ligand 1b is by 23.4 kJ mol(-1) less stable than 2a, and a bicyclic crown-shaped LiS7 cation (2e) is by 34.9 kJ mol(-1) less stable than 2a. Even less stable are four complexes with the branched S6=S ligand. SS bond activation by polarization of the valence electrons takes place on coordination of Li+ to cyclo-S7 (1a).     

Vibrational spectra,force constants and thermodynamic functions of cycloheptasulfur,S71

Raman and IR spectra of four allotropes of cycloheptasulfur are reported. All 15 fundamental vibrations of the S₇ molecule have been observed and were assigned in accordance with the molecular point group C_s. A normal-coordinate analysis using an extended Urey—Bradley force field with 10 independent force constants resulted in very good agreement between observed and calculated wavenumbers. Entropy, heat capacity and other functions of gaseous S₇ have been calculated by statistical methods and evidence for pseudorotation of S₇ in the vapor state is presented. The molecular structure as well as the valence force constants of S₇ are rationalized by a qualitative molecular orbital treatment.     

Radiative lifetimes of levels of the configuration 4f 7 (8 S) 6s 6p in Eu I

Radiative lifetimes of the 4f ⁷ (⁸ S) 6s 6p z ¹⁰ P _{7/2, 9/2},z ⁸ P _{5/2, 7/2, 9/2} andz ⁶ P _{7/2, 5/2} levels of Eu I were measured by the time resolved recording of the exponential decay of the fluorescence. Three of them were measured for the first time, the others agree with previous determinations. From the values of the lifetimes absolute oscillator strengths are deduced, taking into account the branching ratios for the transitions to the 4f ⁷ 6s ² and 4f ⁷ 5d 6s levels. From these absolutef values the radial integralI(6s ², 6s 6p) was derived by a parametric analysis. Theoretical values of oscillator strengths which were calculated using this radial integral show good agreement with the experimental data.     

Does the interconversion of polysulfur compounds proceed via hypervalent intermediates? An ab initio MO study

Ab initio MO calculations at the CCSD(T)/6-311++G(2df,p)//MP2/6-311++G** level have been carried out to determine the reaction energies and Gibbs energies of the homolytic dissociation of the S-S bonds in the chainlike sulfanes H2Sn (n = 2-4). Good agreement with the experimental data is observed. At the same level of theory, the formation of the hypothetical sulfuranes H2S(SH)2, H2S(SSH)2, and S(SH)4 from H2S and the mentioned sulfanes has been studied. Species of this type had been proposed as intermediates in the interconversion reactions of polysulfur compounds (e.g., formation of S7 from S8 and vice versa). The three sulfuranes serve here as model compounds. On the basis of the Gibbs energies and activation energies at 298 K, it is shown that the formation of the three sulfuranes from sulfanes requires too much energy and activation energy to successfully compete with homolytic dissociation reactions. In addition, the formation of the methylsubstituted sulfurane S(SMe)4 from the sulfanes Me2S2 and Me2S3 was studied to elucidate the mechanism of the formal exchange of sulfur atoms between polysulfane molecules. However, both the reaction energy of 199 kJ mol(-1) and the activation energy of 287 kJ mol(-1), calculated at the MP2/6-31G* level, are much higher than the homolytic dissociation energy of the S-S bonds in chain- and ringlike polysulfur compounds, such as Me2S4 (140 kJ mol(-1)) and sulfur homocycles (150 kJ mol(-1)). Therefore, it is concluded that the observed interconversion reactions of sulfur rings and of chainlike polysulfanes do not proceed via sulfurane-type intermediates. Instead, these reactions will take place by a radical chain mechanism at high temperatures, while at temperatures below 100 degrees C they are most probably initiated either by traces of nucleophiles that are present as impurities or by the polar surface groups usually present on the walls of the vessels used.