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In the present investigation, a QM/MM approach was used to better understand the effect of the second environmental shell of the active site on the catalytic conversion of acetylene to acetaldehyde by acetylene hydratase (AH). In addition, the effect of substituting W-coordinating sulfur atoms with selenium atoms was done to provide insight into the influence of the W-coordinating atoms on the catalytic reaction. From the results, it found that the presence of the second shell environment had a significant effect on the reaction. Specifically, in the absence of the MM second shell environment (i.e., QM-cluster model), the rate-determining step is defined by the first proton transfer step. In contrast, for the QM/MM model, the rate-determining step is defined by the water attacking step. Moreover, with the presence of the MM second shell environment, a key intermediate found in the DFT-cluster investigation does not exist in the QM/MM investigation. Rather, what was a two-step process in the DFT-cluster study was calculated to occur in a single step for the QM/MM study. Regarding the sulfur to selenium substitutions, it was found that Gibbs energy for the acetylene binding phase was significantly affected. Notably, the trans-position selenium made the binding of acetylene 65.6 kJ [mol.sup.-1] less endergonic. Moreover, the overall reaction became 38.2 kJ [mol.sup.-1] less endergonic compared with the wild type (WT) AH model. Thus, the substitution of key W-coordinating sulfur atoms with selenium atoms may offer a means to enhance the catalytic mechanism of AH considerably. Key words: acetylene hydratase, dithiolene, diselenolene, density functional theory, QM/MM. Dans le cadre de la presente etude, nous avons utilise une methode de mecanique quantique et de mecanique moleculaire (QM/MM) afin de mieux comprendre l'effet de la seconde couche dans l'environnement du site actif de l'acetylène hydratase (AH) sur la transformation catalytique de l'acetylène en acetaldehyde. De plus, nous avons substitue les atomes de soufre coordonnant l'atome de W par des atomes de selenium afin d'expliquer l'influence des atomes coordonnant le W sur la reaction catalytique. D'après les resultats, la presence de la deuxième couche de l'environnement a un effet considerable sur la reaction. En effet, en absence de la deuxième couche MM de l'environnement (c.-a-d. le modèle QM de l'agregat), l'etape limitante de la reaction est celle du transfert du premier proton. Par contre, avec le modèle QM/MM, l'etape limitante de la reaction est celle de l'attaque par la molecule d'eau. De plus, en presence de la deuxième couche MM de l'environnement, nous avons trouve dans la modelisation de l'agregat par DFT un intermediaire cle qui n'existe pas dans la modelisation par QM/MM. Ce qui etait un processus en deux etapes dans la modelisation de l'agregat par DFT a plutot ete calcule comme se produisant en une seule etape dans la modelisation par QM/MM. En ce qui concerne la substitution des atomes de soufre par des atomes de selenium, nous avons constate qu'elle avait un effet considerable sur l'energie de Gibbs de la phase de liaison de l'acetylène. Notons par ailleurs que la configuration trans des atomes de selenium rendait la liaison de l'acetylène moins endergonique de 65,6 kJ*[mol.sup.1]. De plus, la reaction globale s'est revelee 38,2 kJ*[mol.sup.1] moins endergonique comparativement au modèle de l'AH de type sauvage. Par consequent, la substitution d'atomes de soufre determinants qui coordonnent l'atome de W par des atomes de selenium pourrait se reveler un moyen d'ameliorer considerablement le mecanisme catalytique de l'AH. [Traduit par la Redaction] Mots-cles : acetylène hydratase, dithiolène, diselenolène, theorie de la fonctionnelle de la densite, QM/MM., Introduction Due to the dithiolene ligand's ability to form stable complexes with various highly oxidized metals and the unique optical, electrical, and magnetic properties of the metal complexes formed, the [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.03.033 Byline: Sandrine Guerlet (a), Thierry Fouchet (a)(b), Bruno Bezard (a), Julianne I. Moses (c), Leigh N. Fletcher (d), Amy A. Simon-Miller (e), F. Michael Flasar (e) Keywords: Saturn; Infrared observations; Atmospheres, Composition Abstract: Limb and nadir spectra acquired by Cassini/CIRS (Composite InfraRed Spectrometer) are analyzed in order to derive, for the first time, the meridional variations of diacetylene (C.sub.4H.sub.2) and methylacetylene (CH.sub.3C.sub.2H) mixing ratios in Saturn's stratosphere, from 5hPa up to 0.05hPa and 80[degrees]S to 45[degrees]N. We find that the C.sub.4H.sub.2 and CH.sub.3C.sub.2H meridional distributions mimic that of acetylene (C.sub.2H.sub.2), exhibiting small-scale variations that are not present in photochemical model predictions. The most striking feature of the meridional distribution of both molecules is an asymmetry between mid-southern and mid-northern latitudes. The mid-southern latitudes are found depleted in hydrocarbons relative to their northern counterparts. In contrast, photochemical models predict similar abundances at north and south mid-latitudes. We favor a dynamical explanation for this asymmetry, with upwelling in the south and downwelling in the north, the latter coinciding with the region undergoing ring shadowing. The depletion in hydrocarbons at mid-southern latitudes could also result from chemical reactions with oxygen-bearing molecules. Poleward of 60[degrees]S, at 0.1 and 0.05hPa, we find that the CH.sub.3C.sub.2H and C.sub.4H.sub.2 abundances increase dramatically. This behavior is in sharp contradiction with photochemical model predictions, which exhibit a strong decrease towards the south pole. Several processes could explain our observations, such as subsidence, a large vertical eddy diffusion coefficient at high altitudes, auroral chemistry that enhances CH.sub.3C.sub.2H and C.sub.4H.sub.2 production, or shielding from photolysis by aerosols or molecules produced from auroral chemistry. However, problems remain with all these hypotheses, including the lack of similar behavior at lower altitudes. Our derived mean mixing ratios at 0.5hPa of (2.4[+ or -]0.3)x10.sup.-10 for C.sub.4H.sub.2 and of (1.1[+ or -]0.3)x10.sup.-9 for CH.sub.3C.sub.2H are compatible with the analysis of global-average ISO observations performed by Moses et al. (Moses, J.I., Bezard, B., Lellouch, E., Gladstone, G.R., Feuchtgruber, H., Allen, M. [2000a]. Icarus 143, 244-298). Finally, we provide values for the ratios [CH.sub.3C.sub.2H]/[C.sub.2H.sub.2] and [C.sub.4H.sub.2]/[C.sub.2H.sub.2] that can constrain the coupled chemistry of these hydrocarbons. Author Affiliation: (a) LESIA - Observatoire de Paris, 5 place Janssen, 92195 Meudon, France (b) Universite Pierre et Marie Curie, UMR 8109, 4 place Jussieu, 75006 Paris, France (c) Space Science Institute,1602 Old Orchard Ln, Seabrook, TX 77586, USA (d) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA (e) NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA Article History: Received 30 November 2009; Revised 24 March 2010; Accepted 29 March 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.02.002 Byline: Thomas K. Greathouse (a), G.R. Gladstone (a), J.I. Moses (b), S.A. Stern (c), K.D. Retherford (a), R.J. Vervack (d), D.C. Slater (a), M.H. Versteeg (a), M.W. Davis (a), L.A. Young (d), A.J. Steffl (d), H. Throop (d), J.Wm. Parker (d) Keywords: Jupiter; Abundances, Atmospheres; Occultations; Ultraviolet observations Abstract: The Alice ultraviolet spectrograph onboard the New Horizons spacecraft observed two occultations of the bright star I Ophiucus by Jupiter's atmosphere on February 22 and 23, 2007 during the approach phase of the Jupiter flyby. The ingress occultation probed the atmosphere at 32[degrees]N latitude near the dawn terminator, while egress probed 18[degrees]N latitude near the dusk terminator. A detailed analysis of both the ingress and egress occultations, including the effects of molecular hydrogen, methane, acetylene, ethylene, and ethane absorptions in the far ultraviolet (FUV), constrains the eddy diffusion coefficient at the homopause level to be 3.4.sub.-2.8.sup.+9.0x10.sup.6 cm.sup.2 s.sup.-1, consistent with Voyager measurements and other analyses (Festou, M.C., Atreya, S.K., Donahue, T.M., Sandel, B.R., Shemansky, D.E., Broadfoot, A.L. [1981]. J. Geophys. Res. 86, 5717-5725; Vervack Jr., R.J., Sandel, B.R., Gladstone, G.R., McConnell, J.C., Parkinson, C.D. [1995]. Icarus 114, 163-173; Yelle, R.V., Young, L.A., Vervack Jr., R.J., Young, R., Pfister, L., Sandel, B.R. [1996]. J. Geophys. Res. 101 (E1), 2149-2162). However, the actual derived pressure level of the methane homopause for both occultations differs from that derived by from the Voyager ultraviolet occultations, suggesting possible changes in the strength of atmospheric mixing with time. We find that at 32[degrees]N latitude, the methane concentration is 3.1.sub.-0.5.sup.+0.5x10.sup.8 cm.sup.-3 at 70,397km, the methane concentration is 1.2.sub.-0.3.sup.+0.3x10.sup.9 cm.sup.-3 at 70,383km, the acetylene concentration is 1.4.sub.-0.2.sup.+0.4x10.sup.8 cm.sup.-3 at 70,364km, and the ethane concentration is 6.8.sub.-0.8.sup.+1.1x10.sup.8 cm.sup.-3 at 70,360km. At 18[degrees]N latitude, the methane concentration is 3.2.sub.-0.7.sup.+0.7x10.sup.8 cm.sup.-3 at 71,345km, the methane concentration is 1.2.sub.-0.2.sup.+0.6x10.sup.9 cm.sup.-3 at 71,332km, the acetylene concentration is 1.6.sub.-0.6.sup.+0.3x10.sup.8 cm.sup.-3 at 71,318km, and the ethane concentration is 7.0.sub.-2.5.sup.+2.4x10.sup.8 cm.sup.-3 at 71,315km. We also find that the H.sub.2 occultation light curve is best reproduced if the atmosphere remains cold in the microbar region such that the base of the thermosphere is located at a lower pressure level than that determined by in situ instruments aboard the Galileo probe (Seiff, A., Kirk, D.B., Knight, T.C.D., Young, R.E., Mihalov, J.D., Young, L.A., Milos, F.S., Schubert, G., Blanchard, R.C., Atkinson, D. [1998]. J. Geophys. Res. 103 (E10), 22857-22889) - the Sieff et al. temperature profile leads to too much absorption from H.sub.2 at high altitudes. However, this result is highly model dependent and non-unique. The observations and analysis help constrain photochemical models of Jupiter's atmosphere. Author Affiliation: (a) Southwest Research Institute, Division 15, 6220 Culebra Road, San Antonio, TX 78228, USA (b) Lunar and Planetary Institute, 3600 Bay Area Blvd., Houston, TX 77058, USA (c) Southwest Research Institute, Boulder, 1050 Walnut Street, CO 80302, USA (d) JHU/Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA Article History: Received 29 April 2008; Revised 4 January 2010; Accepted 23 January 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.11.027 Byline: A. Coustenis (a), D.E. Jennings (b), C.A. Nixon (b)(c), R.K. Achterberg (c), P. Lavvas (d), S. Vinatier (a)(e), N.A. Teanby (f), G.L. Bjoraker (b), R.C. Carlson (g), L. Piani (a)(h), G. Bampasidis (i), F.M. Flasar (b), P.N. Romani (b) Keywords: Titan; Abundances, atmospheres; Organic chemistry; Satellites, atmospheres; Spectroscopy; Atmospheres, chemistry; Atmospheres, composition; Satellites, composition Abstract: This paper reports on the results from an extensive study of all nadir-looking spectra acquired by Cassini/CIRS during the 44 flybys performed in the course of the nominal mission (2004-2008). With respect to the previous study (Coustenis, A., and 24 colleagues [2007]. Icarus 189, 35-62, on flybys TB-T10) we present here a significantly richer dataset with, in particular, more data at high northern and southern latitudes so that the abundances inferred here at these regions are more reliable. Our enhanced high-resolution dataset allows us to infer more precisely the chemical composition of Titan all over the disk. We also include improved spectroscopic data for some molecules and updated temperature profiles. The latitudinal distributions of all of the gaseous species are inferred. We furthermore test vertical distributions essentially for acetylene (C.sub.2H.sub.2) from CIRS limb-inferred data and from current General Circulation Models for Titan and compare our results on all the gaseous abundances with predictions from 1-D photochemical-radiative models to check the reliability of the chemical reactions and pathways. Author Affiliation: (a) Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA), Observatoire de Paris-Meudon, 5, Place Jules Janssen, 92195 Meudon Cedex, France (b) NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA (c) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (d) Department of Planet. Science, University of Arizona, Tucson, AZ 85721, USA (e) Laboratoire de Meteorologie Dynamique, IPSL, CNRS/UPMC, F-75252 Paris Cedex 05, France (f) AOPP, Department of Physics, Univ. of Oxford, Clarendon Lab., Parks Rd., Oxford OX1 3PU, UK (g) IACS, Catholic University of America, and NASA/Goddard Space Flight Center, Planet. Systems Lab. - Code 693.0, Greenbelt, MD 20771, USA (h) Museum de l'Histoire Naturelle, Case Postale 52-61, rue Buffon, F-75005 Paris, France (i) Faculty of Physics, National and Kapodistrian Univ. of Athens, Panepistimiopolis, GR-15783 Zographos, Athens, Greece Article History: Received 14 July 2009; Revised 20 November 2009; Accepted 20 November 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.04.002 Byline: Sandrine Guerlet (a), Thierry Fouchet (a), Bruno Bezard (a), Amy A. Simon-Miller (b), F. Michael Flasar (b) Keywords: Saturn; Infrared observations; Atmospheres; Composition Abstract: Measuring the spatial distribution of chemical compounds in Saturn's stratosphere is critical to better understand the planet's photochemistry and dynamics. Here we present an analysis of infrared spectra in the range 600-1400cm.sup.-1 acquired in limb geometry by the Cassini spacecraft between March 2005 and January 2008. We first determine the vertical temperature profiles from 3 to 0.01hPa, at latitudes ranging from 70[degrees]N to 80[degrees]S. We infer a similar meridional temperature gradient at 1-2hPa as in recent previous studies [Fletcher, L.N., Irwin, P.G.J., Teanby, N.A., Orton, G.S., Parrish, P.D., de Kok, R., Howett, C., Calcutt, S.B., Bowles, N., Taylor, F.W., 2007. Icarus 189, 457-478; Howett, C.J.A., Irwin, P.G.J., Teanby, N.A., Simon-Miller, A., Calcutt, S.B., Fletcher, L.N., de Kok, R., 2007. Icarus 190, 556-572]. We then retrieve the vertical profiles of C.sub.2H.sub.6 and C.sub.2H.sub.2 from 3 to 0.01hPa and of C.sub.3H.sub.8 around 1hPa. At 1hPa, the meridional variation of C.sub.2H.sub.2 is found to follow the yearly averaged solar insolation, except for a strong equatorial mole fraction of 8x10.sup.-7, nearly two times higher than expected. This enhancement in abundance can be explained by the descent of hydrocarbon-rich air, with a vertical wind speed at the equator of 0.25[+ or -]0.1 mm/s at 1hPa and 0.4[+ or -]0.15 mm/s at 0.1hPa. The ethane distribution is relatively uniform at 1hPa, with only a moderate 25% increase from 35[degrees]S to 80[degrees]S. Propane is found to increase from north to south by a factor of 1.9, suggesting that its lifetime may be shorter than Saturn's year at 1hPa. At high altitudes (1Pa), C.sub.2H.sub.2 and C.sub.2H.sub.6 abundances depart significantly from the photochemical model predictions of Moses and Greathouse [Moses, J.I., Greathouse, T.K., 2005. J. Geophys. Res. 110, 9007], except at high southern latitudes (62, 70 and 80[degrees]S) and near the equator. The observed abundances are found strongly depleted in the 20-40[degrees]S region and enhanced in the 20-30[degrees]N region, the latter coinciding with the ring's shadow. We favor a dynamical explanation for these anomalies. Author Affiliation: (a) LESIA - Observatoire de Paris, 92195 Meudon, France (b) NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA Article History: Received 18 December 2008; Revised 3 April 2009; Accepted 13 April 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.solener.2009.05.010 Byline: Sylvain Rodat, Stephane Abanades, Jean-Louis Sans, Gilles Flamant Keywords: Hydrogen production; Solar thermal energy; Methane dissociation; Solar reactor; Thermal simulation Abstract: This study addresses the solar thermal decomposition of natural gas for the co-production of hydrogen, as well as Carbon Black as a high-value nano-material, with the bonus of zero CO.sub.2 emissions. The work focused on the development of a medium-scale solar reactor (10kW) based on the concept of indirect heating. The solar reactor is composed of a cubic cavity receiver (20cm side), which absorbs concentrated solar irradiation through a quartz window via a 9cm-diameter aperture. The reacting gas flows inside four graphite tubular reaction zones that are settled vertically inside the cavity. Experimental results were as follows: methane conversion and hydrogen yield of up to 98% and 90%, respectively, were achieved at 1770K, and acetylene was the most important by-product, with a mole fraction up to about 5%. The effect of the methane mole fraction in the feed gas, the residence time and the temperature on the reaction extent was analyzed. In addition to the experimental section, thermal simulations were carried out. They showed a homogeneous temperature distribution inside the cavity receiver of the reactor and permit to draw up a thermal balance. Author Affiliation: Processes, Materials, and Solar Energy Laboratory, CNRS (PROMES-CNRS, UPR 8521), 7 Rue du Four Solaire, 66120 Odeillo Font-Romeu, France Article History: Received 14 April 2008; Revised 6 April 2009; Accepted 24 May 2009 Article Note: (miscellaneous) Communicated by: Associate editor S.A. Sherif

We report on an experimental approach that reveals crucial details of the composition of singlet-triplet mixed eigenstates in acetylene. Intersystem crossing in this prototypical polyatomic molecule embodies the mixing of the lowest excited singlet state ([S.sub.1]) with 3 triplet states ([T.sub.1], [T.sub.2], and [T.sub.3]). Using high-energy (157-nm) photons from an [F.sub.2] laser to record excited-state photoelectron spectra, we have decomposed the mixed eigenstates into their [S.sub.1], [T.sub.3], [T.sub.2], and [T.sub.1] constituent parts. One example of the interpretive power that ensues from the selective sensitivity of the experiment to the individual electronic state characters is the discovery and examination of destructive interference between two doorway-mediated intersystem crossing pathways. This observation of an interference effect in nonradiative decay opens up possibilities for rational coherent control over molecular excited state dynamics. Decomposition mixed eigenstates | Nonradiative decay interference | Photoelectron spectroscopy

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.06.003 Byline: Athena Coustenis (a), Donald E. Jennings (b), Antoine Jolly (c), Yves Benilan (c), Conor A. Nixon (b)(d), Sandrine Vinatier (a), Daniel Gautier (a), Gordon L. Bjoraker (b), Paul N. Romani (b), Ronald C. Carlson (e), F. Michael Flasar (b) Keywords: Titan; Atmospheres; composition; Infrared observations; Satellites; atmospheres; Satellites; composition Abstract: We report here the first detection of mono-deuterated acetylene (acetylene-d1, C.sub.2HD) in Titan's atmosphere from the presence of two of its emission bands at 678 and 519 cm.sup.-1 as observed in CIRS spectral averages of nadir and limb observations taken between July 2004 and mid-2007. By using new laboratory spectra for this molecule, we were able to derive its abundance at different locations over Titan's disk. We find the C.sub.2HD value (1.27.sub.-0.22.sup.-0.13x10.sup.-9) to be roughly constant with latitude from the South to about 45[degrees] N and then to increase slightly in the North, as is the case for C.sub.2H.sub.2. Fitting the 678 cm.sup.-1 [nu].sub.5 band simultaneously with the nearby C.sub.2H.sub.2 729 cm.sup.-1 [nu].sub.5 band, allows us to infer a D/H ratio in acetylene on Titan with an average of the modal values of 2.09[+ or -]0.45x10.sup.-4 from the nadir observations, the uncertainties being mainly due to the vertical profile used for the fit of the acetylene band. Although still subject to significant uncertainty, this D/H ratio appears to be significantly larger than the one derived in methane from the CH.sub.3D band (upper limit of 1.5x10.sup.-4; Bezard, B., Nixon, C.A., Kleiner, I., Jennings, D.E., 2007. Icarus, 191, 397-400; Coustenis, A., Achterberg, R., Conrath, B., Jennings, D., Marten, A., Gautier, D., Bjoraker, G., Nixon, C., Romani, P., Carlson, R., Flasar, M., Samuelson, R.E., Teanby, N., Irwin, P., Bezard, B., Orton, G., Kunde, V., Abbas, M., Courtin, R., Fouchet, Th., Hubert, A., Lellouch, E., Mondellini, J., Taylor, F.W., Vinatier, S., 2007. Icarus 189, 35-62). From the analysis of limb data we infer D/H values of 9.6.sub.-3.1.sup.-4.5x10.sup.-5 (at 54[degrees] S), 2.4.sub.-0.7.sup.-0.9x10.sup.-4 (at 15[degrees] S), 2.7.sub.-0.6.sup.-0.7x10.sup.-4 (at 54[degrees] N) and 1.9.sub.-0.5.sup.-0.7x10.sup.-4 (at 80[degrees] N), which average to a mean value of 1.63[+ or -]0.27x10.sup.-4. Author Affiliation: (a) Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA), Observatoire de Paris-Meudon, 5, Place Jules Janssen, 92195 Meudon Cedex, France (b) NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA (c) LISA, CNRS, Univ. Paris 12 and Paris 7, 94010 Creteil Cedex, France (d) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (e) IACS, Catholic University of America, and NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA Article History: Received 1 December 2007; Revised 31 May 2008

The studies were done to obtain the first principles calculations for the atomic and electronic structure of the diamond-(111)-(2 X 1) face adsorbed with C2 or C2H2 based on the density functional theory (DFT). The results show that depending on the initial bonding configuration of the C2 biradical on the Pandey chain, different superstructures are formed.

Photofragment translational spectroscopy was employed to obtain correlated final state distributions for the acetylene-HCl complex. Vibrational predissociation was induced by exciting the asymmetric acetylene C-H stretching vibration. In contrast to previous investigations proposing that the primary dissociation channel involves intermolecular V-V energy transfer, it was observed that the dominant channel involves intramolecular V-V energy transfer, with most of the excess energy remaining with the acetylene subunit.

The organometallic compound vinylene-Co,N-linked multi(porphyrin)s, (Por)CoIII-CH=CH-(N-Por)H2 was a product of the nucleophilic addition reaction of a pyrrole nitrogen of free-base porphyrins to a phi-complexed acetylene ligand. The formation of such compound was also influenced by the CoIII porphyrin intermediate. Such cobalt complexes underwent oxidative migration for the Co-bound vinyl group to a porphyrin pyrrole nitrogen.

The Pauson-Khand reaction of cyclobutenes 3-5 with a variety of acetylenes yielded cis-anti cis-tricyclo[[5.3.0.0.sup.2,6]]decanes 9-17. The unwanted regioisomers 9a and 10a were the sole products using acetylene, but there was a remarkable reversal in orientation of the cyclobutene component yielding the desired regioisomer 13b upon using (trimethylsilyl)acetylene. The importance of the allylic methyl group in the cyclobutenes in directing the regiochemical outcome was substantiated by the lack of selectivity in Pauson-Khand reactions of desmethylcyclobutene 5 with acetylene and (trimethylsilyl)acetylene. The relative unimportance of electronic control of regiochemistry was concluded from the consistent ratio of Pauson-Khand reaction products from norbornenone 22 with various acetylenes. A hypothesis rationalizing the regiochemical outcome was based on steric interactions of the allylic methyl group from the cyclobutene component with either the smaller acetylene substituent or the CO ligands on the cobalt. This steric interaction was further hypothesized to be influenced by the larger acetylene substituent sterically crowding the CO ligands on the cobalt.

This paper considers the failure of an acetylene gas cylinder which occurred under extreme loading conditions resulting in severe fragmentation of the cylinder. The characteristics of the failure morphology, which is entirely ductile, are discussed. The loading experienced and the fragmentation are put in the context of literature information and events such as failure using explosives and other high-velocity loadings. A model of failure is developed which demonstrates the energies involved in the incident and the explanation for the development of the failure characteristics are discussed.

The fixed potential amperometric sensing of acetylene in the gas phase on a gold-Nation electrode was found to suffer from catalytic poisoning effects with resultant loss of sensitivity. However, by applying a potential pulse program, the sensitivity of the electrode could be restored. with suitable choice of applied potentials, electrochemical adsorption of acetylene took place, allowing an increase in sensitivity. For an adsorption time of 60 s, a 20-fold increase in sensitivity and a detection limit of 20 ppb were obtained. Compounds which, in a fixed potential amperometric mode, would be considerable interferants were found not to preadsorb along with acetylene. An increase in selectivity was, therefore, also achieved.

The solid-phase synthesis of phenylacetylene oligomers which uses a nolvel 3-propyl-3-(benzyl-supported) triazene linkage is described. Phenylacetylene oligomers comprised of functionalized monomers are synthesized in gram quantities by solid-phase methods. The 1-aryl-3-propyl-3-(benzyl-supported) triazene moiety is used to attach growing oligomers to a divinylbenzene cross-linked polystyrene support, a linkage achieved by direct reaction of arenediazonium tetrafluoroborate salts with a n-propylamino-modified Merrifield resin. The yields and purities of oligomers obtained using this solid-phase method are compared with those of oligomers produced by solution methods.

The assembly of a new class of disk-shaped molecules based on amphiphilic phenylacetylene macrocycles (PAMs) as monolayers, at the air-water interface and on solid surfaces, is described. Results of characterization experiments affirm the hypothesis that the dicarboxylate PAMs form a well-ordered and stable two-dimensional array. PAMs are also found to adopt the edge-on configuration with near-vertical orientation of the macrocycle plane.

Recent experimental data on the relative rates of metastable diacetylene [Mathematical Expression Omitted] reactions are used to estimate the potential effects of their inclusion into the photochemical model of the atmosphere of Titan. It is demonstrated that the reactions of [Mathematical Expression Omitted] have the potential for successfully competing with free radical routes to formation of larger hydrocarbon species in Titan's atmosphere, especially at altitudes below about 200 km. At an altitude of 105 km, the net production rate of larger hydrocarbons from [Mathematical Expression Omitted], [C.sub.4]H, and [C.sub.2]H is estimated to be in the ratio 90:30:1.

Acetylene inhibition of [N.sub.2]O reduction to [N.sub.2] is traditionally used to estimate denitrification. Although NO is also produced in the denitrification process, the effect of [C.sub.2][H.sub.2] on NO production in denitrification has not previously been examined, mainly because [C.sub.2][H.sub.2] interferes with chemiluminescent analysis of NO. We used a gas-flow system to study the effect of [C.sub.2][H.sub.2] on NO and [N.sub.2]O production in soil columns under denitrifying conditions and developed a method to avoid [C.sub.2][H.sub.2] interference of NO analysis. Nitric oxide, [N.sub.2]O, and [C.sub.2][H.sub.2] were measured in [Mathematical Expression Omitted]-amended Brookston clay loam (fine-loamy, mixed, mesic Typic Argiaquoll) and Fox sandy loam (fine-loamy over sandy or sandy-skeletal, mixed, mesic Typic Hapludalf) with various moisture contents with and without added glucose during 50-h anaerobic incubations. In glucose-amended Brookston soil with [approximately equal to]8% [C.sub.2][H.sub.2] added to the [N.sub.2] carrier gas, net NO production rates were increasingly inhibited after the first 24 h, and by 50 h were only 2 to 7% of the rates obtained in the absence of [C.sub.2][H.sub.2]. Without glucose amendment and with [C.sub.2][H.sub.2], net NO production rates decreased by 24 to 62% in the Brookston soil. In the Fox soil inhibition occurred almost immediately on [C.sub.2][H.sub.2] addition and net NO production during 50 h was reduced by 36 to 62% with glucose amendment, and by 65 to 76% without glucose amendment. In most cases, net [N.sub.2]O production rates were decreased by [C.sub.2][H.sub.2] due to the decrease in production of its precursor, NO, and in some cases, increased due to inhibition of [N.sub.2]O reduction to [N.sub.2]. Our data suggest that the [C.sub.2][H.sub.2] inhibition method may underestimate total denitrification rates under low water contents or high levels of C substrate.

The reaction of acetylene with B4H8 is studied by evaluating 18 intermediates and 12 transition states involved in carborane formation. Two general mechanisms are considered, namely, the addition of C2H2 to the boron hydride and the insertion of C2H2 into a B-H bond of the boron hydride. Results suggest that methylated products are formed during the reaction due to the migration of a boron hydrogen to a methylene carbon, particularly in the insertion mechanism.

A carborane analogue of benzocyclobutadiene, C11H18B10, having no colour and needle-like crystals is synthesized by a reaction of 1,2-dehydro-o-carborane with acetylenes. Strong orbital connections between the cage and exo-cage pi system are absent in the C11H18B10. A photochemical reaction of C11H18B10 with 1,3-butadiene yielded 2 + 2 adduct C15H24B10. Three dimensional crystal structure of C11H18B10, obtained with single crystal x-ray diffraction, is described.

The reaction between terminal alkynes with nitrones using CuI catalyst results in the formation of 1-aza-1-buten-3-yne and 2-azetidinone as major products. The composition of the resulting product is affected by the ligand use and the substitutents on the employed acetylene. The experimental set up of the synthesis of 1-aza-1-buten-3-yne and 2-azetidinone is discussed.

A new synthetic technique enables the preparation of a series of polypyridyl ruthenium(II) complexes (1) with monodentate pyridyl- and cyano-substituted acetylenes. Electronic and structural characteristics of the complexes are not influenced by the acetylenic unit. Excited-state characteristics of Rh(II) complexes having both terpyridine (tpy) and bipyridine (bpy) ligands are intermediate between those of the parent Rh(tpy)(sub 2)(super 2+) and Rh(bpy)(sub 3)(super 2+) complexes. X-ray crystallographic data indicate the cations of 1 to have a planar structure with the acetylenic ligand being sterically unhindered and this structure facilitates the polymerization reactions by the phenylpyridylacetelyene.

Spectroscopic and ab initio theoretical studies of the reaction between RuX2(PPh3)3 (X = Cl, Br) and tert-butylacetylene that yields vinylidene complexes and help examine the characteristics of acetylene molecule in transition metal complexes. X-ray studies of the final trans-bis(phosphine) complex reveal that it is a thermodynamically-stable five-coordinate complex. The mechanism involves formation of an eta(2)-CH coordinated complex by slippage of the eta(2)-CC coordinated alkyne. 1, 2-hydrogen migration in the eta(2)-CH complex occurs within the acetylene to yield a metastable vinylidene complex having a cis-bis(phosphine) that isomerizes to the final trans-bis(phosphine) complex.

Acetylene undergoes Diels-Alder reaction with various 2-alkyl-furans to produce cyclohex-1-ene-1,6-dicarbaldehydes. Addition of Na2CO3 and antioxidant 2,6-di-tert-butyl-4-methylphenol (BHT) prevents polymerization and side products. An increase in alkyl group chain length increases the rate of the reactions, while solvents inhibit the rate. Experimental NMR coupling constants reveal the stereochemistry of the product as endo isomer.

Phenylacetylene dendritic macromolecules are synthesized employing divergent, convergent and double-stage convergent methods and the efficiencies of these methods are compared. Convergent approach is efficient in solving the insolubility problem. Various chromatographic techniques and spectroscopic analyses facilitate the characterization of these compounds. Combustion analysis is used to determine the elemental composition.

A synthesis procedure yields rosefuran from propargyl bromide, acetone and 1-buten-3-ol in two steps, yielding water as the reaction by-product. The scheme is atom-economical and is based on acid-catalyzed cyclization of diols to furans. The osmium tetroxide-catalyzed dihydroxylation of acetylene- and allyl alcohol-derived beta,gamma-unsaturated ketones yields the diols.

A one dimensional photochemical model analyzes Voyager observations of hydrocarbons and hazes in Neptunes's stratosphere. The total average C2H2 and C2H6 mixing ratios are determined from the Voyager IRIS spectra. The UVS spectra yields the limitation on the addition of CH4 and C2H4 to the C2H2 and C2H6 at solar occultation latitudes at low pressures. Height profile influences the eddy diffusion coefficient of the hydrocarbons.

The lithium-iodine exchange-initiated intramolecular conjugate addition reactions of certain model lambda-iodo-alpha,beta-acetylenic esters were explored. Reactions leading to the formation of 5-membered rings exhibited product mixtures emerging from proton abstraction reactions. Reactions conducted in the presence of trimethylsilyl chloride, to capture intermediate allenolate ions, ensured better cyclizations. The extremely efficient formation of bicyclic ester ensued from allenolate ion trapping by an internal electrophilic center in another ester.

Mono- and di-carbon interpolations into polyhedral boron clusters facilitate the synthesis of distinct carborane clusters such as arachno-tricarbaboranes and arachno-12-vertex, di- and tetracarbaboranes. The pathway underlying mono- and di-carbon interpolations involves the induced reactivity of terminal acetylenes bearing electron-retrieving groups with numerous polyhedral borane anions. Neighboring carbon atoms in the carborane clusters are located on the puckered open faces. These clusters conform to 10-vertex arachno cage structures. This is ascribed to an icosahedron lacking two vertices.

The genus Petrosia reveals the presence of an Okinawan marine sponge which exhibits the occurrence of two acetylenic enol ether glycerides. Spectroscopic studies are used to determine the plane geometries of the glycerides. All possible stereoisomers are completely prepared enantioselectively to analyze the total geometries of the glycerides. D-mannitol and L-ascorbic acid are used to synthesize (R)-1-O-benzylglycerol and its (S)-enantiomer, respectively, which are used as chiral building blocks in the process. An important part in the preparation is the interaction of bromo enol ether with enediyne, which undergoes pairing calalyzed by palladium.

An improvised pulsed-nozzle Fourier-transform microwave spectrometer was used to explore the ketene-acetylene CH2CO-C2H2 complex. The hydrogen nucleus of ketene and acetylene are exchanged by tunneling motions, and this is revealed by relative intensity estimations. The complex preserved a planar structure that is unlike the crossed configuration normally associated with a (2-pi(s) + 2-pi(a)) cycloaddition reaction.

Thermal intramolecular interactions of acetylenic compounds reveal the participation of the heteroatom. The participation is seen as being a nonsynchronous but concerted process. Heteroatoms are surrounded by polar bonds in the nonsynchronous process which help the primary polarizations inducing the interaction method. Heteroatom analogues are studied to reveal the high-energy reaction of all-carbon retro-ene processes. The bond energies vary due to the higher involvement of nitrogen compounds over oxygen analogues.

The enhanced acetylene (C2H2) biodegradation in field soils with a history of exposure to the gas is analyzed. Five strains of gram-positive aerobic bacteria, that are capable of metabolizing C2H2 as sole carbon source, are isolated and identified as Rhodococcus spp. Results show that enhanced C2H2 degradation in field soils persists for a long period, and spreads when the capabilities are transferred to indigenous soil microorganisms.

An infrared spectrum analysis of the volatile material content of Comet Hyakutake shows an abundance of acetylene. The estimated abundance of acetylene is about 0.2-0.9% relative to water and is comparable to the abundance estimated in the solid-phase of cold interstellar clouds. The findings suggest that the volatile materials of the comet may have originated from the ice-covered interstellar grains.

High-performance liquid chromotography (HPLC) studies reveal that Acinetobacter sp. strain 4-CB1 degrades 4-chlorobenzoate by initially producing 4-chlorobenzoyl coenzyme A from 4-chlorobenzoate, ATP and CoA. This 4-chlorobenzoyl CoA is then dehalogenated and hydrolyzed to the finalproducts 4-hydroxybenzoate and CoA. An amended pathway for 4-chlorobenzoate degradation based on the results of this HPLC study is presented. The results show the importance of Coenzyme A in this process but its exact role is still unknown.

Glassy, disubstituted acetylene-based polymers, which exhibit extremely high gas permeabilities and high vapor/gas selectivities, which is quite unusual for conventional glassy polymers such as polysulfone is presented. It is observed that cavity size distributions of both polymers are obtained by using an energetic-based algorithm.

It is reported that the irradiation of cyanoacetylene and ozone on a water ice surface at 255 nm yields both hydrogen peroxide and cyanoacetylene (NCCHCO). It is proposed that the overall process may involve a crucial step where NCCCHO rearranges to NCCHCO.

The pulse Laval nozzle apparatus is used to study the reactions of C2H with NH3 and ND3 at low temperatures. The strong negative temperature dependence of the rate coefficients for the C2H plus NH3 clearly shows that the C2H plus NH3 reaction does not have a barrier.

Histone acetyltransferases CREB-binding protein (CBP), p300/CBP-associated factor (PCAF), and Tat-interactive protein, 60 kDa (Tip60) is experimented for their ability to in vitro acetylate high-mobility group box-containing proteins 1 and 2(HMGB-1 and -2) and the role of C-terminal domain on the pattern of acetylation as well as some properties of the modified truncated protein. It is found that the modifications in the acetylation pattern of HMGB-1 and -2 upon removal of the C-terminal tail are regarded as a means by which the acidic domain modulates some properties of these proteins.