{"id":16320,"date":"2016-04-24T19:42:16","date_gmt":"2016-04-24T18:42:16","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=16320"},"modified":"2016-06-26T08:25:11","modified_gmt":"2016-06-26T07:25:11","slug":"autoionization-of-hydrogen-fluoride","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16320","title":{"rendered":"Autoionization of hydrogen fluoride."},"content":{"rendered":"
\n

\n\tThe autoionization of water involves two molecules transfering a proton to give hydronium hydroxide, a process for which the free energy of reaction is well known<\/a>. Here I ask what might happen with the next element along in the periodic table, F.\n<\/p>\n

\n\tI have been unable to find much about the autoionization of HF in the literature; the pH of neat HF appears unreported (unlike that of H2<\/sub>O, which of course is 7). Even the dielectric constant of liquid HF[1]<\/a><\/span>,[2]<\/a><\/span> seems to vary widely, the largest reported<\/a> being ~84. It is suggested that liquid HF is much less ordered than e.g.<\/em> water, and this suggests that a single static model is unlikely to be entirely realistic. Nonetheless, I thought it might be informative to take the model I previously constructed for water and try applying it to HF. Here is part of the geometry optimisation cycle[3]<\/a><\/span> from the original edited water model. I used ωB97XD\/Def2-TZVPPD\/SCRF=water for the model. Why continuum water as the solvation treatment? Well, standard parameters for liquid HF are not available (perhaps given the variation in dielectric) and since the upper bound might be similar to water, I decided to use that to see what I got. Clearly however an approximation.\n<\/p>\n

\n\t\"\"\n<\/p>\n

\n\tThe low energy final geometry corresponds to 10 HF molecules and lies about 16 kcal\/mol lower (in total energy) than the cyclic structure containing H2<\/sub>F+<\/sup>.F–<\/sup> species connected by two (HF)3<\/sub> bridges and two further non-bridge HF molecules hydrogen bonding to the H2<\/sub>F+ <\/sup>and the F–<\/sup>. In fact the ionic structure turns out to be a transition state<\/strong> for proton shifting along the chain to create (HF)10<\/sub>, with a free energy barrier of 9.2 kcal\/mol above the neutral form.[4]<\/a><\/span> This difference between ionic and non-ionic forms is considerably less than that for water as previously indicated<\/a>. Note also how much shorter the hydrogen bonding H…F distances are in the HF cluster.\n<\/p>\n

\n\t\"\"\n<\/p>\n

\n\t\"\"\n<\/p>\n

\n\tSo unlike water, where the hydronium hydroxide is a clear minimum in the potential with a small but distinct barrier (~3.5 kcal\/mol[5]<\/a><\/span>) to proton transfer, with HF at the same level of theory the barrier is zero. Perhaps the difference might be because whereas hydronium hydroxide can support three stabilizing (H2<\/sub>O)3<\/sub> bridges, only two (HF)3<\/sub> bridges are possible with H2<\/sub>F+<\/sup>.F–<\/sup>. It might also be higher levels of theory (or better\/larger models of the HF cluster) could well give a barrier for the process, but this does tend to suggest that the dynamics of HF liquid may suggest quite different lifetimes for autoionized forms of HF compared to water. Liquid HF is clearly just as complicated a liquid as is H2<\/sub>O, certainly much less is known about it.<\/p>\n

References<\/h2>\n
  1. R.H. Cole, \"Dielectric constant and association in liquid HF\", The Journal of Chemical Physics<\/i>, vol. 59, pp. 1545-1546, 1973. https:\/\/doi.org\/10.1063\/1.1680219<\/a>\n\n<\/li>\n
  2. P.H. Fries, and J. Richardi, \"The solution of the Wertheim association theory for molecular liquids: Application to hydrogen fluoride\", The Journal of Chemical Physics<\/i>, vol. 113, pp. 9169-9179, 2000. https:\/\/doi.org\/10.1063\/1.1319172<\/a>\n\n<\/li>\n
  3. H.S. Rzepa, \"H 10 F 10\", 2016. https:\/\/doi.org\/10.14469\/ch\/192032<\/a>\n\n<\/li>\n
  4. H.S. Rzepa, \"H 10 F 10\", 2016. https:\/\/doi.org\/10.14469\/ch\/192034<\/a>\n\n<\/li>\n
  5. H.S. Rzepa, \"H22O11\", 2016. https:\/\/doi.org\/10.14469\/ch\/192022<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The autoionization of water involves two molecules transfering a proton to give hydronium hydroxide, a process for which the free energy of reaction is well known. Here I ask what might happen with the next element along in the periodic table, F. I have been unable to find much about the autoionization of HF in […]<\/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":false,"_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":[4],"tags":[431,24,1622,1799,40,206,1449,1801,1798,1806,1807,1800,1803,1715,1802,1805,1804],"ppma_author":[2661],"class_list":["post-16320","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-dielectric","tag-energy","tag-equilibrium-chemistry","tag-fluorides","tag-free-energy","tag-free-energy-barrier","tag-hydrogen-bond","tag-hydronium","tag-inorganic-solvents","tag-lithium-fluoride","tag-low-energy-final-geometry-corresponds","tag-oxides","tag-ph","tag-properties-of-water","tag-self-ionization-of-water","tag-water","tag-water-model"],"yoast_head":"\nAutoionization of hydrogen fluoride. - 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=16320\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Autoionization of hydrogen fluoride. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"The autoionization of water involves two molecules transfering a proton to give hydronium hydroxide, a process for which the free energy of reaction is well known. 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It is a small step from there to hydronium hydroxide, or H3O+...OH-. The measured concentrations [H3O+] \u2261 [OH-]\u00a0give\u00a0rise of course to the well-known\u00a0pH 7 of pure water, and converting this ionization constant to a\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":16308,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16308","url_meta":{"origin":16320,"position":1},"title":"Deuteronium deuteroxide. The why of pD 7.435.","author":"Henry Rzepa","date":"April 22, 2016","format":false,"excerpt":"Earlier, I constructed a possible model of hydronium hydroxide, or H3O+.OH-\u00a0One way of assessing the quality of\u00a0the model is\u00a0to\u00a0calculate\u00a0the free energy difference between it and two normal water molecules\u00a0and compare\u00a0the result to\u00a0the measured\u00a0difference. Here I apply a further test of the model using isotopes. Pure water has pH 7, which\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":16208,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16208","url_meta":{"origin":16320,"position":2},"title":"Azane oxide, a tautomer of hydroxylamine.","author":"Henry Rzepa","date":"April 15, 2016","format":false,"excerpt":"In the previous post I described how hydronium hydroxide or H3O+...HO-, an intermolecular tautomer of water, has recently been observed captured inside an organic cage and how the free-standing species in water can be captured computationally with the help of solvating water bridges. Here I explore azane oxide or H3N+-O-,\u2021\u2026","rel":"","context":"In "General"","block_context":{"text":"General","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":7100,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=7100","url_meta":{"origin":16320,"position":3},"title":"Dynamic effects in nucleophilic substitution at trigonal carbon.","author":"Henry Rzepa","date":"July 16, 2012","format":false,"excerpt":"Singleton and co-workers have produced some wonderful work showing how dynamic effects and not just transition states can control the outcome of reactions. Steve Bachrach's blog contains many examples, including this recent one. This shows that tolyl thiolate (X=Na)\u00a0reacts with the dichlorobutenone to give two substitution products in a 81:19\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2012\/07\/singleton.svg","width":350,"height":200},"classes":[]},{"id":13394,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=13394","url_meta":{"origin":16320,"position":4},"title":"How many water molecules does it take to ionise HCl?","author":"Henry Rzepa","date":"February 14, 2015","format":false,"excerpt":"According to Guggemos, Slavicek and Kresin, about 5-6!. This is one of those simple ideas, which is probably quite tough to do experimentally. It involved blasting water vapour through a pinhole, adding HCl and\u00a0measuring the dipole-moment induced deflection by an electric field. They\u00a0found\u00a0\"evidence for a noticeable rise in the dipole\u2026","rel":"","context":"In "Interesting chemistry"","block_context":{"text":"Interesting chemistry","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=4"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":25043,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=25043","url_meta":{"origin":16320,"position":5},"title":"Geometries of proton transfers: modelled using total energy or free energy?","author":"Henry Rzepa","date":"April 18, 2022","format":false,"excerpt":"Proton transfers are amongst the most common of all chemical reactions. They are often thought of as \"trivial\" and even may not feature in many mechanistic schemes, other than perhaps the notation \"PT\".\u00a0The types with the lowest energy barriers for transfer often involve heteroatoms such as N and O, and\u2026","rel":"","context":"In "crystal_structure_mining"","block_context":{"text":"crystal_structure_mining","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?cat=1745"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/www.ch.ic.ac.uk\/rzepa\/blog\/wp-content\/uploads\/2022\/04\/plot-1024x734.png?resize=350%2C200&ssl=1","width":350,"height":200},"classes":[]}],"jetpack_likes_enabled":false,"authors":[{"term_id":2661,"user_id":1,"is_guest":0,"slug":"admin","display_name":"Henry Rzepa","avatar_url":"https:\/\/secure.gravatar.com\/avatar\/897b6740f7f599bca7942cdf7d7914af5988937ae0e3869ab09aebb87f26a731?s=96&d=blank&r=g","0":null,"1":"","2":"","3":"","4":"","5":"","6":"","7":"","8":""}],"_links":{"self":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/16320","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=16320"}],"version-history":[{"count":36,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/16320\/revisions"}],"predecessor-version":[{"id":16359,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=\/wp\/v2\/posts\/16320\/revisions\/16359"}],"wp:attachment":[{"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=16320"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=16320"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=16320"},{"taxonomy":"author","embeddable":true,"href":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fppma_author&post=16320"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}