From chemistry-request@server.ccl.net Mon Oct 23 15:53:51 2000 Received: from email.nist.gov (email.nist.gov [129.6.2.7]) by server.ccl.net (8.8.7/8.8.7) with ESMTP id PAA31517 for ; Mon, 23 Oct 2000 15:53:50 -0400 Received: from bratsche (bratsche.nist.gov [129.6.194.154]) by email.nist.gov (8.9.3/8.9.3) with SMTP id PAA23074 for ; Mon, 23 Oct 2000 15:53:51 -0400 (EDT) Message-Id: <4.1.20001023150944.00a653d0@mailserver3.nist.gov> X-Sender: irikura@mailserver3.nist.gov X-Mailer: QUALCOMM Windows Eudora Pro Version 4.1 Date: Mon, 23 Oct 2000 15:52:12 -0400 To: chemistry@ccl.net From: Karl Irikura Subject: verify a far-out MM3 result? Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" I recently obtained the following enthalpy of formation (298 K) for bicyclo[2.1.0]pent-2-ene (C5H6; cyclobutene with a cyclopropane ring fused to the edge opposite the double bond): MM3 545 kJ/mol (expt 333 kJ/mol) It's so far off I wonder if there's some problem with my copy of MM3 (came with Tripos' "Alchemy2000" software). It reports missing bond-equivalent parameters for two bonds (the sp2-sp3 bonds)--could they be worth -106 kJ/mol each? The corresponding bonds in the bicyclopentane are worth +23 kJ/mol each. The results look great for these related molecules: cyclobutene MM3 = 158 expt = 157 kJ/mol cyclopropane MM3 = 54 expt = 53 bicyclo[2.1.0]pentane MM3 = 156 expt = 158 I don't think the experimental value is bad: from G2 I get 320 kJ/mol and from B3LYP//cc-pVTZ//6-31G* I get 331 kJ/mol. I'd be grateful to learn the MM3 result obtained from other software (or even from an independent installation of Alchemy2000). Thanks for any help! Karl ---------------------------------------------- Dr. Karl K. Irikura National Institute of Standards and Technology 100 Bureau Drive, Stop 8380 Gaithersburg, MD 20899-8380 voice: 301-975-2510 fax: 301-869-4020 e-mail: karl.irikura@nist.gov http://www.nist.gov/compchem/ ----------------------------------------------