{"id":16308,"date":"2016-04-22T17:05:07","date_gmt":"2016-04-22T16:05:07","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=16308"},"modified":"2023-09-17T07:29:49","modified_gmt":"2023-09-17T06:29:49","slug":"deuteronium-deuteroxide-the-why-of-pd-7-435","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16308","title":{"rendered":"Deuteronium deuteroxide. The why of pD 7.435."},"content":{"rendered":"
\n

\n\tEarlier<\/a>, I constructed a possible model of hydronium hydroxide, or H3<\/sub>O+<\/sup>.OH– <\/sup>One way of assessing the quality of the model is to calculate the free energy difference between it and two normal water molecules and compare the result to the measured difference. Here I apply a further test of the model using isotopes.\n<\/p>\n

\n\tPure water has pH 7, which means equal concentrations for both [H3<\/sub>O+<\/sup>] and  [OH–<\/sup>] of 10-7<\/sup>M. Converting this to a free energy one gets ΔG298<\/sub> 19.088 kcal\/mol. Now the pD of pure deuterium oxide is reported<\/a> as 7.435, equivalent to ΔG298<\/sub> 20.274, an isotope effect on the free energy of ΔΔG298 <\/sub>=1.186<\/span><\/strong> kcal\/mol. How does the theoretical model (ωB97XD\/Def2-TZVPPD\/SCRF=water‡<\/sup>) previously reported[1]<\/a><\/span>,[2]<\/a><\/span> do? The value obtained is 1.215<\/span><\/strong>,[3]<\/a><\/span> an apparent error of only 0.029 kcal\/mol. I am quite pleased with the close correspondence; at least the model is capable of reporting good isotope effects on the ionisation equilibrium of pure water!\n<\/p>\n

\n\tFinally, with some confidence assured, one might apply this to tritonium tritoxide. Tritiated water is so radioactive it would boil in an instant, probably well before its pT could be measured. ΔΔG298<\/sub> is calculated as 1.798<\/span><\/strong> kcal\/mol. Will this estimate ever be challenged by experiment?\n<\/p>\n

\n

\n\t‡ It is assumed no isotope effect acts on the dielectric constant of water and hence the continuum model used here to model it. In fact the isotope effect on this property is modest; ε298<\/sub> = 77.94, compared with 78.36 for normal water.[4]<\/a><\/span>\n<\/p>\n

\n

Acknowledgments<\/h4>\n

This post has been cross-posted in PDF format at Authorea<\/a>.<\/p>\n

References<\/h2>\n
  1. H.S. Rzepa, \"H 22 O 11\", 2016. https:\/\/doi.org\/10.14469\/ch\/191999<\/a>\n\n<\/li>\n
  2. H.S. Rzepa, \"H 22 O 11\", 2016. https:\/\/doi.org\/10.14469\/ch\/191998<\/a>\n\n<\/li>\n
  3. H. Rzepa, \"Deuteronium deuteroxide; free energy differences.\", 2016. https:\/\/doi.org\/10.14469\/hpc\/407<\/a>\n\n<\/li>\n
  4. C. Malmberg, \"Dielectric constant of deuterium oxide\", Journal of Research of the National Bureau of Standards<\/i>, vol. 60, pp. 609, 1958. https:\/\/doi.org\/10.6028\/jres.060.060<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    Earlier, I constructed a possible model of hydronium hydroxide, or H3O+.OH– One way of assessing the quality of the model is to calculate the free energy difference between it and two normal water molecules and compare the result to the measured difference. Here I apply a further test of the model using isotopes. Pure water has pH 7, which means equal concentrations for both […]<\/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":[431,24,40,1808,1810,1809,1812,1715,1813,506,1811],"ppma_author":[2661],"class_list":["post-16308","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","tag-dielectric","tag-energy","tag-free-energy","tag-heat-transfer","tag-heavy-water","tag-kilocalorie-per-mole","tag-model-is-to-calculate-the-free-energy-difference","tag-properties-of-water","tag-the-free-energy","tag-thermodynamics","tag-tritiated-water"],"yoast_head":"\nDeuteronium deuteroxide. The why of pD 7.435. - 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=16308\" \/>\n<meta property=\"og:locale\" content=\"en_GB\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Deuteronium deuteroxide. The why of pD 7.435. - Henry Rzepa's Blog\" \/>\n<meta property=\"og:description\" content=\"Earlier, I constructed a possible model of hydronium hydroxide, or H3O+.OH– One way of assessing the quality of the model is to calculate the free energy difference between it and two normal water molecules and compare the result to the measured difference. Here I apply a further test of the model using isotopes. 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These calculations involved optimising the positions of around 250-254 atoms, for d(CGCG)2 and d(ATAT)2, an undertaking which has taken about two months of computer time! The geometries are finally optimised to the\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":15395,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=15395","url_meta":{"origin":16308,"position":3},"title":"I\u2019ve started so I\u2019ll finish. Kinetic isotope effect models for a general acid as a catalyst in the protiodecarboxylation of indoles.","author":"Henry Rzepa","date":"January 10, 2016","format":false,"excerpt":"Earlier I explored models for the heteroaromatic electrophilic protiodecarboxylation of an 3-substituted indole, focusing on the role of water as the proton transfer and delivery agent. 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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":13899,"url":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=13899","url_meta":{"origin":16308,"position":5},"title":"The mechanism of borohydride reductions. Part 1: ethanal.","author":"Henry Rzepa","date":"April 12, 2015","format":false,"excerpt":"Sodium borohydride is the tamer cousin of lithium aluminium hydride (LAH). It is used in aqueous solution to e.g. reduce aldehydes and ketones, but it leaves acids, amides and esters alone. Here I start an exploration of why it is such a different reducing agent. 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