Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with O(2). The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF(6) bath gas, at five temperatures in the range 297-600 K. The second order rate constants at 10 Torr were fitted to the Arrhenius equation: [see text] The decrease in rate constant values with increasing temperature, although systematic is very small. The rate constants showed slight increases in value with pressure at each temperature, but this was scarcely beyond experimental uncertainty. From estimates of Lennard-Jones collision rates, this reaction is occurring at ca. 1 in 20 collisions, almost independent of pressure and temperature. Ab initio calculations at the G3 level backed further by multi-configurational (MC) SCF calculations, augmented by second order perturbation theory (MRMP2), support a mechanism in which the initial adduct, H(2)SiOO, formed in the triplet state (T), undergoes intersystem crossing to the more stable singlet state (S) prior to further low energy isomerisation processes leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are H2O+SiO. The decomposition of the intermediate cyclo-siladioxirane, via O-O bond fission, plays an important role in the overall process. The bottleneck for the overall process appears to be the T-->S process in H2SiOO. This process has a small spin-orbit coupling matrix element, consistent with an estimate of its rate constant of 1x10(9) s-1 obtained with the aid of RRKM theory. This interpretation preserves the idea that, as in its reactions in general, SiH2 initially reacts at the encounter rate with O2. The low values for the secondary reaction barriers on the potential energy surface account for the lack of an observed pressure dependence. Some comparisons are drawn with the reactions of CH2+O2 and SiCl2+O2.

Time-resolved kinetic studies of the reaction of silylene, SiH(2), with H(2)O and with D(2)O have been carried out in the gas phase at 297 K and at 345 K, using laser flash photolysis to generate and monitor SiH(2). The reaction was studied independently as a function of H(2)O (or D(2)O) and SF(6) (bath gas) pressures. At a fixed pressure of SF(6) (5 Torr), [SiH(2)] decay constants, k(obs), showed a quadratic dependence on [H(2)O] or [D(2)O]. At a fixed pressure of H(2)O or D(2)O, k(obs) values were strongly dependent on [SF(6)]. The combined rate expression is consistent with a mechanism involving the reversible formation of a vibrationally excited zwitterionic donor-acceptor complex, H(2)Si...OH(2) (or H(2)Si...OD(2)). This complex can then either be stabilized by SF(6) or it reacts with a further molecule of H(2)O (or D(2)O) in the rate-determining step. Isotope effects are in the range 1.0-1.5 and are broadly consistent with this mechanism. The mechanism is further supported by RRKM theory, which shows the association reaction to be close to its third-order region of pressure (SF(6)) dependence. Ab initio quantum calculations, carried out at the G3 level, support the existence of a hydrated zwitterion H(2)Si...(OH(2))(2), which can rearrange to hydrated silanol, with an energy barrier below the reaction energy threshold. This is the first example of a gas-phase-catalyzed silylene reaction.

The reaction of Ga(CH,), with an excess of ammonia leads to the formation of (CH,),Ga-NH,, which, when heated at 1 2 0 "C under 450 Torr nitrogen, decomposes to yield CH, and the trimer [(CH,),GaNH,], 1 and not, as previously stated, a dimeric species. A single-crystal X-ray study of 1 revealed that it is a molecular species containing a (GaN), ring [Ga-N 1.93(2)-2.05(2) A] in which four consecutive ring atoms are * Tri-u-amido-1 : ~K ' N ; 1 : ~K'N; 2 : 3~~N-tris(dimethylgaIlium).

a b s t r a c tSuspected coprolites from midden, burial and room fill contexts at Çatalhöyük were analysed by GC/MS and thin section micromorphology. Assessment of sterol biomarkers enabled a distinction between faecal and non-faecal sources for the deposits to be made, with bile acid biomarkers indicating that many of the faecal deposits are human coprolites. The relative lack of ruminant faeces could be due to this material being used as a fuel source. Deposits in burials were observed to contain soil and plant derived sterols rather than their faecal counterparts. Further analysis in thin section enabled identification of associated materials and contents. Diagnostic inclusions such as bone and plant fragments were only present in some of the human coprolites, which were observed to have a very similar morphology to decayed plant remains. This study illustrates the difficulties in distinguishing coprolites in the field and under the microscope, and demonstrates the importance of integrating biogeochemical methods, particularly when such deposits are used as the basis for interpreting human health and diet, and use-of-space in settlements.

No abstract

No abstract

No abstract

No abstract