44,676 research outputs found
Testing for the presence of a terrane boundary within Neoproterozoic (Dalradian) to Cambrian siliceous turbidites at Callander, Perthshire, Scotland
No full textThe Southern Highland Group (Dalradian) and Keltie Water Grit Formation, which includes the Lower Cambrian Leny Limestone, form an inverted, 1.4 km thick, largely arenaceous, sequence at Callander. The grits have the same detrital mineralogy throughout, mainly quartz, plagioclase (An(1-3)), muscovite, and biotite. Chlorite formed from detrital biotite during low-grade regional metamorphism (T less than 270 °C). There are some vertical changes in major element (but not trace element) chemistry of the grits, and detrital muscovites have a wide, but comparable, range in composition throughout, apart from an influx of Na-rich micas in the Keltie Water Grits. 40Ar/39Ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600-1800 Ma in the Dalradian rocks; similar old ages occur in the Keltie Water Grits but are diluted by ages of 507 - 886 Ma. We interpret these new data as showing that the rocks were most likely deposited as a single sequence, possibly with a disconformity, in Neoproterozoic to Early Cambrian times, before the onset of Grampian orogenesis in the Early Palaeozoic. No major structural or straitigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes
A 2 h periodic variation in the low-mass X-ray binary Ser X-1
No full textSpectroscopy of the low-mass X-ray binary Ser X-1 using the Gran Telescopio Canarias have revealed a ?2 h periodic variability that is present in the three strongest emission lines. We tentatively interpret this variability as due to orbital motion, making it the first indication of the orbital period of Ser X-1. Together with the fact that the emission lines are remarkably narrow, but still resolved, we show that a main-sequence K dwarf together with a canonical 1.4 M? neutron star gives a good description of the system. In this scenario, the most likely place for the emission lines to arise is the accretion disc, instead of a localized region in the binary (such as the irradiated surface or the stream-impact point), and their narrowness is due instead to the low inclination (?10°) of Ser X-1
Houghton, J. T., Meira Filho, L.G., Callander, B. A., Harris, N., Kattenberg, A. & Maskell, K. (eds). — Climate change 1995. The Science of Climate change. Cambridge University Press. Cambridge, New York & Melbourne. 1996.Watson, R. T., Zinyowera, M.C., Moss, R. H. & Dokken, D.J. (eds). — Climate change 1995. Impacts, adaptations and mitigation of climate change : Scientific-technical analyses. Cambridge University Press. Cambridge, New York & Melbourne. 1996.Bruce, J. R, Lee, H. & Haïtes, E.F. (eds). — Climate change 1995. Economie and social dimensions of climate change. Cambridge University Press. Cambridge, New York & Melbourne. 1996
No full textTranier Michel. Houghton, J. T., Meira Filho, L.G., Callander, B. A., Harris, N., Kattenberg, A. & Maskell, K. (eds). — Climate change 1995. The Science of Climate change. Cambridge University Press. Cambridge, New York & Melbourne. 1996.Watson, R. T., Zinyowera, M.C., Moss, R. H. & Dokken, D.J. (eds). — Climate change 1995. Impacts, adaptations and mitigation of climate change : Scientific-technical analyses. Cambridge University Press. Cambridge, New York & Melbourne. 1996.Bruce, J. R, Lee, H. & Haïtes, E.F. (eds). — Climate change 1995. Economie and social dimensions of climate change. Cambridge University Press. Cambridge, New York & Melbourne. 1996. In: Revue d'Écologie (La Terre et La Vie), tome 51, n°4, 1996. pp. 406-407
Rearrangement of alkylchlorocarbenes: 1,2-H shift in free carbene, carbene-olefin complex, and excited states of carbene precursors
No full textPhotolysis of alkylchlorodiazirines (1) in the presence of olefins gives a cyclopropane (3) by addition of the generated carbene to the olefin and a vinyl chloride derivative (2) resulting from a 1,2-H shift rearrangement. This rearrangement may occur either in the carbene or in some excited state, precursor of the carbene (RIES mechanism), or in a ''carbene + olefin complex'' on the way to the formation of 3 (COC mechanism). Results obtained by time-resolved photoacoustic calorimetry as well as by thermolysis and photolysis of ClCH2C(N-2)Cl and CH3(CH2)(2)C(N-2)Cl in the presence of tetramethylethylene clearly indicate that both the RIES and COC mechanisms play a role but with efficiencies which greatly depend on the nature of the diazirine. Reexamination of the results previously obtained with benzylchlorodiazirines indicates that, for this class of diazirines, the RIES mechanism is temperature dependent and has a very low efficiency at room temperature and below, whereas the nonlinearity of the plots [3]/[2] vs [olefin] is mainly due to the COC mechanism.PT: J; CR: BIGOT B, 1978, J AM CHEM SOC, V100, P8575 CHANG KT, 1979, J AM CHEM SOC, V101, P5082 FREY HM, 1966, ADV PHOTOCHEM, V4, P225 GANZER GA, 1986, J AM CHEM SOC, V108, P1517 GRAHAM WH, 1965, J AM CHEM SOC, V87, P4396 HEIHOFF K, 1987, BIOCHEMISTRY-US, V22, P1422 HOUK KN, 1984, J AM CHEM SOC, V106, P4291 HOUK KN, 1984, J AM CHEM SOC, V106, P4293 JACKSON JE, 1994, ADV CARBENE CHEM, V1 KANABUSKAMINSKA JM, 1987, J AM CHEM SOC, V109, P5267 LAVILLA JA, 1989, J AM CHEM SOC, V111, P6877 LAVILLA JA, 1989, J AM CHEM SOC, V111, P712 LAVILLA JA, 1990, TETRAHEDRON LETT, V31, P5109 LIU MTH, 1987, J ORG CHEM, V52, P4223 LIU MTH, 1989, J CHEM SOC CHEM COMM, P12 LIU MTH, 1990, J AM CHEM SOC, V112, P3915 MODARELLI DA, 1991, J AM CHEM SOC, V113, P8985 MODARELLI DA, 1992, J AM CHEM SOC, V114, P7034 MOSS RA, 1993, J CHEM SOC CHEM COMM, P1597 MOSS RA, 1994, ADV CARBENE CHEM, V1 MULDER P, 1988, J AM CHEM SOC, V110, P4090 MULLERREMMERS PL, 1985, J AM CHEM SOC, V107, P7275 NI T, 1989, J AM CHEM SOC, V111, P457 NICKON A, 1993, ACCOUNTS CHEM RES, V26, P84 RUDZKI JE, 1985, J AM CHEM SOC, V107, P7849 SKELL PS, 1969, J AM CHEM SOC, V91, P7131 TOMIOKA H, 1984, J AM CHEM SOC, V106, P454 TURRO NJ, 1982, J AM CHEM SOC, V104, P1754 WARNER PM, 1984, TETRAHEDRON LETT, V25, P4211 WESTRICK JA, 1987, BIOCHEMISTRY-US, V26, P8313 WHITE WR, 1992, J ORG CHEM, V57, P2841 WIERLACHER S, 1993, J AM CHEM SOC, V115, P8943 YAMAMOTO N, 1994, J AM CHEM SOC, V116, P2064; NR: 33; TC: 30; J9: J AMER CHEM SOC; PG: 9; GA: UG695Source type: Electronic(1
Catalytic P-H activation by Ti and Zr catalysts
No full textCatalytic dehydrocoupling of phosphines was investigated using the anionic zirconocene trihydride salts [Cp*Zr-2(mu-H)(3)Li](3) (1a) or [Cp*Zr-2(mu-H)(3)K(thf)(4)] (1b), and the metallocycles [CpTi(NPtBu3)(CH2)(4)] (6) and [Cp*M(NPtBu3)(CH2)(4)] (M = Ti 20, Zr 21) as catalyst precursors. Dehydrocoupling of primary phosphines RPH2 (R = Ph, C6H2Me3, Cy, C10H7) gave both dehydrocoupled dimers RP(H)P(H)R or cyclic oligophosphines (RP)(n) (n = 4, 5) while reaction of tBu(3)C(6)H(2)PH(2) gave the phosphaindoline tBu(2)(Me2CCH2)C6H2PH (9). Stoichiometric reactions of these catalyst precursors with primary phosphines afforded [Cp*Zr-2((PR)(2))H][K(thf)(4)] (R = Ph 2, Cy 3, C6H2Me3 4), [Cp*Zr-2((PPh)(3))H] [K(thf)(4)] (5), [CpTi(NPtBu3)(PPh)(3)] (7) and [CpTi(NPtBu3)(mu-PHPh)](2) (8), while reaction of 6 with (C(6)H(2)tBu3)PH2 in the presence of PMe3 afforded [CpTi(NPtBu3)(PMe3)(p(C(6)H(2)tBu(3))] (10). The secondary phosphines Ph2PH and (PhHPCH2)(2)CH2 also undergo dehydrocoupling affording (Ph2P)(2) and (PhPCH2)(2)CH2. The bisphosphines (CH2PH2)(2) and C6H4(PH2)(2) are dehydrocoupled to give (PCH2CH2PH)(2) (12) and (C6H4P(PH))(2) (13) while prolonged reaction of 13 gave (C6H4P2)(8) (14). The analogous bisphosphine Me2C6H4(PH)(2) (17) was prepared and dehydrocoupling catalysis afforded (Me2C6H2P(PH))(2) (18) and subsequently [(Me2C6H2P2)(2)(mu-Me2C6H2P2)](2) (19). Stoichiometric reactions with these bisphosphines gave [Cp*Zr-2(H)(PH)(2)C6H4] [Li(thf)(4)] (22), [Cp*Ti(NPtBu3)(PH)(2)C6H4](2) (23) and [Cp*Ti(NPtBu3)(PH)(2)C6H4] (24). 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