From owner-chemistry@ccl.net Mon Sep 8 03:33:01 2008 From: "VITORGE Pierre 094605 Pierre.VITORGE^-^cea.fr" To: CCL Subject: CCL:G: Ligand substitution calculation Message-Id: <-37724-080907184214-2948-6dD2hHqUl/P2Cm92Acg5lQ=server.ccl.net> X-Original-From: "VITORGE Pierre 094605" Content-class: urn:content-classes:message Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="iso-8859-1" Date: Sun, 7 Sep 2008 23:27:13 +0200 MIME-Version: 1.0 Sent to CCL by: "VITORGE Pierre 094605" [Pierre.VITORGE,+,cea.fr] Delta-G (actually Delta_r-G, where _r is for "of the reaction) give the = relative stabilities of the reaction, only for balanced reactions, where = the species you add for balancing the reaction are "in the standard = state", namely 1atm for gas (when G is calculated in typically = Gaussian03 Section thermochemistry)... If you calculate Delta_r-G fro = Reaction MCl2 + H2 -> MH2 + Cl2 you can deduce K from Delta_r-G =3D -R T ln(K) (actually ln(K/K=B0) where K=B0=3D1 in the units you are using, these = units give the reference state your are using)=20 and K =3D |MH2| |Cl2| /(|MCL2| |H2|) where |X| is the activity of X, an approximation of |X| is P(X) its = partial pressure for reactions in the gas phase. To decide which species = is the most stable between |MH2| and |MCL2|, you can consider Ratio=20 r=3D|MH2| /|MCL2|=20 for r=3D1 you obtain the half point of your reaction, which give a = chemical meaning of K K =3D P(Cl2)_1/2 /P(H2)_1/2 where P(X)_1/2 is the (here "a") partial pressure of X at the half point = of the reaction. This chemical meaning of K is the same as pKa =3D pH_1/2=20 for acid-base reactions (see the example below). If typically K >> 1 (Delta_r-G << 0) you will need much more Cl2 (than H2) to shift the equilibrium to the = half point reaction which, of course means that MH2, the product is the most stable indeed corresponding to Delta_r-G << 0: You did not need all the above reasoning to obtain this conclusion. This because your case is very simple, since all the stoichiometric = coefficients are 1. For this reason K =3D P(Cl2)_1/2 /P(H2)_1/2 is dimensionless. Conversely, in the other case where you will not obtain dimensionless K, = Delta_r-G (or equivalently K) does not directly give you which species = is the most stable... despite what is written in text books. As an illustration I thought of the following (still very simple) = example lg(K) =3D -5 for Equilibrium UO2++(aq) + H2O(l) <=3D> UO2(OH)+(aq) + H+(aq) Conclusion here Delta_r-G > 0 Hence?! UO2++(aq) is MORE stable than UO2(OH)+(aq) Now just write the above equilibrium UO2++(aq) + OH-(aq) <=3D> UO2(OH)+(aq) Its equilibrium constant is now lg(K1) =3D lg(K) - lg(Kw) =3D +9 where Kw is the ionic product of water (lg(Kw) =3D -14 at 25=B0C) Now Delta_r-G1 < 0 Hence?! UO2++(aq) is now LESS stable than UO2(OH)+(aq) Where is the error? Which secrecies is the most stable? Do I not need to give the answer her?! --=20 Pierre Vitorge http://www.vitorge.name -----Message d'origine----- De=A0: owner-chemistry[A]ccl.net [mailto:owner-chemistry[A]ccl.net]=20 Envoy=E9=A0: dimanche 7 septembre 2008 15:50 =C0=A0: VITORGE Pierre 094605 Objet=A0: CCL: Ligand substitution calculation Sent to CCL by: "David Eisenberg" [david.eisen]![gmail.com] Dear CCL users, I've got a really basic and silly question, and would appreciate any = help.=20 I want to compare the relative stabilities of two transition-metal=20 complexes: one with ligands that include two chlorides (-Cl), and the = other=20 with two hydrides (-H) instead of the chlorides. But to get the Delta-G for the ligand-substitution reaction, the = reaction=20 must be balanced. What is the most correct way of balancing it? With=20 molecular Cl2 and H2, or with ions of Cl- and H-, or maybe with Cl* and = H*=20 radicals?! Thank you in advance for any help, D. Eisenberg -=3D This is automatically added to each message by the mailing script = =3D-http://www.ccl.net/cgi-bin/ccl/send_ccl_messageSubscribe/Unsubscribe:=20Job: http://www.ccl.net/jobs=20Search Messages: http://www.ccl.net/htdig (login: ccl, Password: = search)http://www.ccl.net/spammers.txt From owner-chemistry@ccl.net Mon Sep 8 16:12:00 2008 From: "Jana Precechtelova janap..ncbr.chemi.muni.cz" To: CCL Subject: CCL: the performance of HF, MP2, CCSD(T) Message-Id: <-37725-080908152210-15908-MTkjJNU8Q9bONDTWrN6TPA a server.ccl.net> X-Original-From: Jana Precechtelova Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Date: Mon, 8 Sep 2008 20:25:16 +0200 (MEST) MIME-Version: 1.0 Sent to CCL by: Jana Precechtelova [janap[-]ncbr.chemi.muni.cz] Hi everybody, could anybody give me a hint what is the up-to-date performance of the HF, MP2, and CCSD(T) methods expressed in terms of the maximum number of CH2 fragments (or number of basis functions etc. ...) that can be handled by these methods. Thanks, Jana ============================================================= Jana Precechtelova ------------------------------------------------------------- NMR Laboratory National Centre for Biomolecular Research (NCBR) Faculty of Science, Masaryk University Kamenice 3, 625 00 Brno, Czech Republic ============================================================= email: janap+/-ncbr.chemi.muni.cz =============================================================