{"id":13394,"date":"2015-02-14T16:26:25","date_gmt":"2015-02-14T16:26:25","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=13394"},"modified":"2015-02-14T16:26:25","modified_gmt":"2015-02-14T16:26:25","slug":"how-many-water-molecules-does-it-take-to-ionise-hcl","status":"publish","type":"post","link":"https:\/\/rzepa.net\/blog\/2015\/02\/14\/how-many-water-molecules-does-it-take-to-ionise-hcl\/","title":{"rendered":"How many water molecules does it take to ionise HCl?"},"content":{"rendered":"

According to Guggemos, Slavicek and Kresin, about 5-6![cite]10.1103\/PhysRevLett.114.043401[\/cite]. 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 moment occurring at\u00a0<\/em><\/strong>n<\/span>\u2248<\/span>5<\/span><\/span>\u2013<\/span><\/em><\/strong>6<\/em><\/strong>“.<\/span><\/span><\/span><\/span><\/span><\/span><\/p>\n

Modelling the structures takes little time. So here are some \u03c9B97XD\/6-311++G(2d,2p) gas phase models. I state at the outset that these are not dynamic-stochastic models, averaged over many conformations, but a static picture of individual poses. As usual, click on individual images to obtain an interactive 3D model (Java required).‡<\/sup><\/p>\n

n=1.[cite]10.14469\/ch\/189758[\/cite] Dipole moment 3.7D<\/p>\n

\"hcl+1h2o\"<\/p>\n

n=2.[cite]10.14469\/ch\/189760[\/cite] Dipole moment\u00a02.4D. Note how the O…H bond becomes shorter.<\/p>\n

\"hcl+2h2o\"<\/p>\n

n=3.[cite]10.14469\/ch\/189759[\/cite] Dipole moment\u00a02.5D. Note how the key O..H bond is contracting rapidly, as are the other H-bond interactions. This is the cyclic polarisation effect, where each bond influences the others. We are starting to approach the formation of H3<\/sub>O+<\/sup> and Cl–<\/sup>!<\/p>\n

\"hcl+3h2o\"<\/p>\n

n=4.[cite]10.14469\/ch\/189763[\/cite] Dipole moment\u00a02.3 D, We have two ways to add the next water molecule, firstly to try to stabilise the H3<\/sub>O+<\/sup>.\u00a0Nope.<\/p>\n

\"hcl+4h2o\"<\/p>\n

n=4,[cite]10.14469\/ch\/189763[\/cite] Dipole moment\u00a01.1 D. Better by solvating the\u00a0Cl–<\/sup>!\u00a0The proton originally attached to the Cl is now starting its transfer to the water to form that hydronium cation, but the dipole moment is not yet large.<\/p>\n

\"hcl+4h2o1\"<\/p>\n

n=5.[cite]10.14469\/ch\/189756[\/cite] Dipole moment\u00a04.7D. The ionisation is almost complete\u00a0and the dipole moment is on the increase.<\/p>\n

\"5\"<\/p>\n

n=6.[cite]10.14469\/ch\/189761[\/cite] The dipole moment is up to 8.2D and the three H-O bonds of the hydronium cation are almost all equal in length.<\/p>\n

\"6\"<\/p>\n

A cautionary observation though. The isomer below for n=6[cite]10.14469\/ch\/189764[\/cite] is lower in energy by \u0394G -1.2 kcal\/mol, and its dipole moment is only 2.5D! The charges (summed onto heavy atoms) show the chloride to have -0.88 and the hydronium cation +0.88, so it is a true ion-pair, despite its dipole moment.<\/p>\n

\"6a\"<\/p>\n

So these calculations do indeed appear\u00a0to confirm that 5-6 water molecules are required to ionise\u00a0HCl.\u00a0But it does raise the interesting issue that even for n=6, there are poses for the assembly which have low dipole moments. Clearly of course the observed dipole moment is a dynamic average over many conformations of similar energy but the prediction that some of these\u00a0may have low dipole moments should be noted.<\/p>\n

\n

‡<\/sup> If you right-click in the 3D model area, you can bring down a list of vibrational modes for each complex from the first item of the pop-up menu that appears (labelled model). You might wish to e.g. explore how the H-Cl stretch vibration changes as the ionisation increases.<\/p>\n

\n

Acknowledgments<\/h4>\n

This post has been cross-posted in PDF format at Authorea<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"

According to Guggemos, Slavicek and Kresin, about 5-6![cite]10.1103\/PhysRevLett.114.043401[\/cite]. 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 moment occurring at\u00a0n\u22485\u20136“. Modelling the […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[11,13],"tags":[902,1072,1353,1879,2238],"class_list":["post-13394","post","type-post","status-publish","format-standard","hentry","category-interesting-chemistry","category-reaction-mechanism-2","tag-energy","tag-gas-phase-models","tag-java","tag-pence","tag-similar-energy"],"_links":{"self":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/13394","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/comments?post=13394"}],"version-history":[{"count":0,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/posts\/13394\/revisions"}],"wp:attachment":[{"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/media?parent=13394"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/categories?post=13394"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/rzepa.net\/blog\/wp-json\/wp\/v2\/tags?post=13394"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}