From owner-chemistry@ccl.net Sun Jun 2 11:51:17 1996 Received: from bedrock.ccl.net for owner-chemistry@ccl.net by www.ccl.net (8.7.5/950822.1) id LAA25854; Sun, 2 Jun 1996 11:02:32 -0400 (EDT) Received: from AWIUNI11.EDVZ.UniVie.AC.AT for jahangir@sun.iust.ac.ir by bedrock.ccl.net (8.7.5/950822.1) id LAA08179; Sun, 2 Jun 1996 11:02:27 -0400 (EDT) From: Received: from pars.ipm.ac.ir by AWIUNI11.EDVZ.UniVie.AC.AT (IBM VM SMTP V2R2) with TCP; Sun, 02 Jun 96 17:02:07 MEZ Received: from [194.165.8.3] by pars.ipm.ac.ir with SMTP (5.65/1.2-eef) id AA10529; Sat, 1 Jun 96 15:24:03 +0305 Date: Sat, 1 Jun 1996 14:15:20 -2359 Message-Id: <96060114152026@sun.iust.ac.ir> To: chemistry@ccl.net Subject: help X-Vms-To: SMTP%"chemistry@ccl.net" Hi, If you have any information about GYPSUM ( any type of GYPSUM ) or about cement please send me your message. THANK YOU. From jerry@dft.chem.cuhk.hk Sun Jun 2 22:51:22 1996 Received: from iris.chem.cuhk.hk for jerry@dft.chem.cuhk.hk by www.ccl.net (8.7.5/950822.1) id VAA27434; Sun, 2 Jun 1996 21:58:09 -0400 (EDT) Received: from dft.chem.cuhk.hk by iris.chem.cuhk.hk via SMTP (931110.SGI/940406.SGI.AUTO) for CHEMISTRY@www.ccl.net id AA29560; Mon, 3 Jun 96 09:57:28 +0800 Received: by dft (950511.SGI.8.6.12.PATCH526/930416.SGI.AUTO) id JAA03880; Mon, 3 Jun 1996 09:57:22 +0800 Date: Mon, 3 Jun 1996 09:57:22 +0800 (HKT) From: Jerry C C Chan To: CHEMISTRY@www.ccl.net Subject: Experimental or Optimized Message-Id: Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Dear Netters, Two weeks ago I posted a question on the following issues: 1. Absolute shielding is considered as the best measure of the quality of calculated shielding tensor components. Why not chemical shift anisotropy and eta? 2. Since the experimental CSA and eta "contain" the effects of the environment, should one choose the experimental geometry rather than trying to optimized the geometry of the species when the absolute shielding scale is not available. The enlightening responses from Robert and Georg are greatly appreciated. Below please find the responses received. Jerry, The Chinese University of Hong Kong ***************** My bet is that most will tell you to use the experimental geometry but I would advocate a high level ab initio optimized geometry. X-ray results are influenced by crystal packing which distort the geometry to some extent. You usually do the NMR shielding calc in vacume so why not have a vacume geometry. Thsi should be obtained from a decent search and minimization at a theoretical level comparable to that at which you will do the NMR shielding calc at. Thank-you Robert A. Kirby BioChem Therapeutiques Inc. 275 Armand-Frappier Blvd. Laval, Quebec, H7V 4A7 kirbyr@biochem-pharma.com ***************** > 1. Absolute shielding is considered as the best measure of the quality of > calculated shielding tensor components. Why not chemical shift anisotropy > and eta? Those two are undoubtely also good measures for the quality of your calculation -- if you have the experimental results. Shieldings are often used because there is always the possibility that some systematic error remains hidden in relative shift if it happens to cancel out. > 2. Since the experimental CSA and eta "contain" the effects of the > environment, should one choose the experimental geometry rather than > trying to optimized the geometry of the species when the absolute > shielding scale is not available. If you have a GOOD experimental structure, than it is probably preferable. I stress the word "good" here since the shielding is extremely sensitive to small changes in the geometry (this in turn is just one of the reasons why experimentalists do NMR measurements). However, in many cases of interest, you don't have such structures, or not for the whole set of molecules that you are studying. In these cases, calculated structures are your choice. Some people use exclusively optimized structures since this provides a consistent picture. Yours, Georg P.S. I am looking forward to your summary. ============================================================================== Georg Schreckenbach Tel: (Canada)-403-220 8204 Department of Chemistry FAX: (Canada)-403-289 9488 University of Calgary Email: schrecke@zinc.chem.ucalgary.ca 2500 University Drive N.W., Calgary, Alberta, Canada, T2N 1N4 ==============================================================================