{"id":16619,"date":"2017-03-19T13:07:59","date_gmt":"2017-03-19T13:07:59","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=16619"},"modified":"2017-03-22T09:25:09","modified_gmt":"2017-03-22T09:25:09","slug":"pyrophoric-metals-the-mechanism-of-thermal-decomposition-of-magnesium-oxalate","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16619","title":{"rendered":"Pyrophoric metals + the mechanism of thermal decomposition of magnesium oxalate."},"content":{"rendered":"
\n

A pyrophoric metal is one that burns spontaneously in oxygen; I came across this phenomenon as a teenager doing experiments at home. Pyrophoric iron<\/a> for example is prepared by heating anhydrous iron (II) oxalate in a sealed test tube (i.e.<\/em> to 600\u00b0 or higher). When the tube is broken open and the contents released, a shower of sparks forms. Not all metals do this; early group metals such as calcium undergo a different reaction releasing carbon monoxide and forming calcium carbonate and not the metal itself. Here as a prelude to the pyrophoric reaction proper, I take a look at this alternative mechanism using calculations.<\/p>\n

\"\"<\/a>
\n <\/a>There are ~60\u00a0crystal structures of metal oxalates, of which several are naturally occurring minerals (Fe, humboldtine[1]<\/a><\/span>, Ca, Weddellite[2]<\/a><\/span>, Li[3]<\/a><\/span>, Na[4]<\/a><\/span>, K[5]<\/a><\/span>, Cs[6]<\/a><\/span>. The natural geometry of the oxalate di-anion is planar (torsion 0 or 180\u00b0) but a small number are twisted such as the caesium oxalate.<\/p>\n

\"\"<\/p>\n

The kinetics of pyrolysis of a number of metal\u00a0\u00a0oxalates were studied some years ago (Ca[7]<\/a><\/span>, Li[8]<\/a><\/span>) indicating barriers ranging from 53-68 kcal\/mol. One proposed mechanism is as shown in this article.[7]<\/a><\/span><\/p>\n

\"\"<\/p>\n

It was surmised from the kinetic analysis that the k<\/em>1<\/sub> activation step (rotation about the C-C bond from planar to twisted) was ~12\u00a0\u00b1 20 kcal\/mol, whilst steps k<\/em>2<\/sub>\u00a0or k<\/em>3<\/sub>\u00a0had the much higher activation energy noted above. A search (of Scifinder) for quantum mechanical “reality checks”\u00a0of this mechanism revealed a blank and so I apply such a check\u00a0here using Mg as the metal.<\/p>\n

The carbonyl extrusion step (\u03c9B97XD\/Def2-TZVPPD\/SCRF=water, DOI: 10.14469\/hpc\/2320<\/a>) was studied with a water solvent field applied in an effort to mimic the solid state crystal structure of the species as a better representation of the ionic lattice than a pure vacuum calculation.\"\"\"\"<\/a>An IRC (intrinsic reaction coordinate, DOI: 10.14469\/hpc\/2324<\/a>) reveals the start-point geometry\u00a0still has a very small negative force constant (-38 cm-1<\/sup>, DOI:\u00a010.14469\/hpc\/2321<\/a>)\u00a0which now corresponds to a small rotation about the C-C bond to give a C2<\/sub>-symmetric conformation:<\/p>\n

\"\"But the barrier for this process is tiny and nothing like the\u00a0~12\u00a0\u00b1 20 kcal\/mol inferred from the kinetic analysis. Perhaps most of the incentive to pack into a totally planar geometry comes from the interactions in the ionic lattice.\u00a0The calculated free energy barrier (\u0394G298<\/sub>\u2021<\/sup>\u00a054.7 kcal\/mol, \u0394G755<\/sub>\u2021<\/sup>\u00a055.1 kcal\/mol) is within\u00a0the reported measured range.<\/p>\n

\"\"<\/p>\n

The mechanism for production of pyrophoric metal itself is likely to be far more complex, involving (inter alia<\/em>) electron transfer from oxygen to metal. If I find anything I will report back here.<\/p>\n

References<\/h2>\n
  1. T. Echigo, and M. Kimata, \"Single-crystal X-ray diffraction and spectroscopic studies on humboldtine and lindbergite: weak Jahn\u2013Teller effect of Fe2+ ion\", Physics and Chemistry of Minerals<\/i>, vol. 35, pp. 467-475, 2008. https:\/\/doi.org\/10.1007\/s00269-008-0241-7<\/a>\n\n<\/li>\n
  2. C. Sterling, \"Crystal structure analysis of weddellite, CaC2O4.(2+x)H2O\", Acta Crystallographica<\/i>, vol. 18, pp. 917-921, 1965. https:\/\/doi.org\/10.1107\/s0365110x65002219<\/a>\n\n<\/li>\n
  3. G.A. Jeffrey, and G.S. Parry, \"The Crystal Structure of Sodium Oxalate\", Journal of the American Chemical Society<\/i>, vol. 76, pp. 5283-5286, 1954. https:\/\/doi.org\/10.1021\/ja01650a007<\/a>\n\n<\/li>\n
  4. Dinnebier, R.E.., Vensky, S.., Panthofer, M.., and Jansen, M.., \"CCDC 192180: Experimental Crystal Structure Determination\", 2003. https:\/\/doi.org\/10.5517\/cc6fzcy<\/a>\n\n<\/li>\n
  5. Dinnebier, R.E.., Vensky, S.., Panthofer, M.., and Jansen, M.., \"CCDC 192182: Experimental Crystal Structure Determination\", 2003. https:\/\/doi.org\/10.5517\/cc6fzf0<\/a>\n\n<\/li>\n
  6. F.E. Freeberg, K.O. Hartman, I.C. Hisatsune, and J.M. Schempf, \"Kinetics of calcium oxalate pyrolysis\", The Journal of Physical Chemistry<\/i>, vol. 71, pp. 397-402, 1967. https:\/\/doi.org\/10.1021\/j100861a029<\/a>\n\n<\/li>\n
  7. D. Dollimore, and D. Tinsley, \"The thermal decomposition of oxalates. Part XII. The thermal decomposition of lithium oxalate\", Journal of the Chemical Society A: Inorganic, Physical, Theoretical<\/i>, pp. 3043, 1971. https:\/\/doi.org\/10.1039\/j19710003043<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    A pyrophoric metal is one that burns spontaneously in oxygen; I came across this phenomenon as a teenager doing experiments at home. Pyrophoric iron for example is prepared by heating anhydrous iron (II) oxalate in a sealed test tube (i.e. to 600\u00b0 or higher). When the tube is broken open and the contents released, a […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":true,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":5,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"","footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[1745,1086],"tags":[2079,169,2077,1432,1395,2080,2078,1431,157,2081,1832,2082,2076,1834],"ppma_author":[2661],"class_list":["post-16619","post","type-post","status-publish","format-standard","hentry","category-crystal_structure_mining","category-reaction-mechanism-2","tag-aluminium","tag-calculated-free-energy-barrier","tag-carbon-monoxide","tag-chemical-elements","tag-chemistry","tag-higher-activation-energy","tag-iron","tag-matter","tag-metal","tag-metal-oxalates","tag-oxide","tag-pyrophoric-metal","tag-pyrophoricity","tag-reducing-agents"],"yoast_head":"\nPyrophoric metals + the mechanism of thermal decomposition of magnesium oxalate. - Henry Rzepa's Blog<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16619\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Pyrophoric metals + the mechanism of thermal decomposition of magnesium oxalate. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"A pyrophoric metal is one that burns spontaneously in oxygen; I came across this phenomenon as a teenager doing experiments at home. 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