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Milburn RK, Hopkinson AC, Bohme DK: Extraordinary cluster formation and intramolecular ligand-ligand interactions in cyanoacetylene [corrected] mediated by Mg+*: implications for the atmospheric chemistry of titan and for circumstellar chemistry. J Am Chem Soc. 2005 Sep 21;127(37):13070-8.
Experimental results are reported that track the kinetics of gas-phase reactions initiated by Mg+*, (c-C5H5) Mg+ and (c-C5H5) 2Mg+* in hydrogen cyanide and cyanoacetylene. The experiments were performed with a selected-ion flow tube (SIFT) tandem mass spectrometer at a helium buffer-gas pressure of 0.35 +/- 0.01 Torr and at 294 +/- 3 K. The observed chemistries of Mg+* and (c-C5H5) Mg+ are dominated by sequential ligation, while that of (c-C5H5) 2Mg+* is by ligand switching. The rate-coefficient measurements for sequential addition of cyanoacetylene to Mg+* indicate an extraordinary pattern in alternating chemical reactivity while multiple-collision induced dissociation experiments revealed an extraordinary stability for the Mg (HC3N) 4+* cluster ion. Molecular orbital calculations with density functional theory (DFT) at the B3LYP level, Hartree-Fock (HF) and second-order Mphiller-Plesset (MP2) levels, all performed with a 6-31+G (d) basis set, have been used to calculate structures and energies for the observed Mg (HC3N) 1-4 (+)* cations. These calculations indicate that the path of formation of Mg (HC3N) 4+* involves ligand-ligand interactions leading to two cyclic (HC3N) 2 ligands which then interact to form 2,4,6,8-tetracyanosemibullvalene-Mg+ or 1,2,5,6-tetracyano-1,3,5,7-cyclooctatetraene-Mg+ cations. A case is made for the formation of similar complex organomagnesium ions in the upper atmosphere of Titan where subsequent electron-ion recombination may produce cyano derivatives of large unsaturated hydrocarbons. In contrast, circumstellar environments with their much higher relative content of free electrons are less likely to give rise to such chemistry. |
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