From hdc@ctrhdc1.ct.ornl.gov Thu May 21 07:32:02 1998 Received: from ctrhdc1.ct.ornl.gov (ctrhdc1.ct.ornl.gov [128.219.18.173]) by www.ccl.net (8.8.3/8.8.6/OSC/CCL 1.0) with ESMTP id HAA00793 Thu, 21 May 1998 07:32:02 -0400 (EDT) Received: (from hdc@localhost) by ctrhdc1.ct.ornl.gov (8.8.5/8.8.5) id HAA13237 for CHEMISTRY@www.ccl.net; Thu, 21 May 1998 07:32:01 -0400 (EDT) From: "Hank D. Cochran" Message-Id: <9805210732.ZM13235@ctrhdc1.ct.ornl.gov> Date: Thu, 21 May 1998 07:32:01 -0400 In-Reply-To: "Donald E. Williams" "CCL:Electrostatic model for ordinary hydrogen bonds" (May 19, 2:43pm) References: <9805191443.ZM12041@xray5.chem.louisville.edu> X-Mailer: Z-Mail (3.2.3 08feb96 MediaMail) To: CHEMISTRY@www.ccl.net Subject: Re: CCL:Electrostatic model for ordinary hydrogen bonds Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Dr. Williams: This subject has received considerable attention since 1991 when we published results of classical molecular dynamics simulations of water using the Simple Point Charge (SPC) P. T. Cummings, H. D. Cochran, J. M. Simonson, R. E. Mesmer, S. Karaborni, "Simulation of Supercritical Water and of Supercritical Aqueous Solutions," J. Chem. Phys., 94, 5606-5621 (1991). which tested the ability of the model to extrapolate from ambient conditions, for which it was developed, to supercritical conditions. Hydrogen bonding in these simulations was explicitly discussed in a subsequent paper H. D. Cochran, P. T. Cummings, and S. Karaborni, "Solvation in Supercritical Water," Fluid Phase Equilib., 71, 1 (1992). In 1993, a European group challenged the reliability of the electrostatic model based on their new neutron scattering experiments at supercritical conditions using isotope substitution P. Postorino, R. H. Tromp, M. A. Ricci, A. K. Soper, and G. W. Neilson, "The Interatomic Structure of Water at Supercritical Temperatures," Nature, 366, 668 (1993). This paper provoked some controversy, and in 1997, a revised interpretation of the experimental results was published by the original authors A. K. Soper, F. Bruni, and M. A. Ricci, "Site-site pair correlation functions of water from 25 to 400 degrees C: Revised analysis of new and old diffraction data," J. Chem. Phys., 106, 247 (1997) based on a different approach to inelasticity corrections of the data. The revised scattering results are in semi-quantitative agreement with our original simulations. The erroneous inelasticity corrections affected results from prior scattering studies of ambient water and the electrostatic models based on them. Some early scattering experiments were vindicated by the new new analyses of old data. This interesting story, with results of simulations with some new electrostatic models, will appear shortly A. A. Chialvo, P. T. Cummings, J. M. Simonson, R. E. Mesmer, and H. D. Cochran, "The Interplay Between Molecular Simulation and Neutron Scattering in Developing New Insights into the Structure of Water ", Ind. & Eng. Chem. Res., accepted for publication (1998). Regards, Hank Cochran On May 19, 2:43pm, Donald E. Williams wrote: > Subject: CCL:Electrostatic model for ordinary hydrogen bonds > > There seems to be a widespread misunderstanding that an ordinary > hydrogen bond (say O-H...O) can be modeled with a purely electrostatic > potential by "dumping everything into electrostatic parameters". If anyone > knows of such a model, I would like to hear about it. > There is a parameterization called the "disappearing hydrogen" model of > the OHB. In this model, electrostatics are included, but also (and very > importantly) the repulsion of the hydrogen atom is set to zero, quite a drastic > change from a normal hydrogen atom. It is easily shown that the effect of > eliminating hydrogen repulsion is by far more important than the > electrostatics. > > >decompose an HB into its "electrostatic" and "covalent" contributions. An > >"ordinary" HB (OHB) has both electrostatic and covalent components, although > >it can be reasonably modelled by classical force fields by dumping everything > >into electrostatic parameters. An OHB can be neutral or charged. A charged > > -Donald Williams > > -- > Dr. Donald E. Williams email:dew01@xray5.chem.louisville.edu > Department of Chemistry > University of Louisville phone:502-852-5975 > Louisville, KY 40292 fax: 502-852-8149 > > > --- >-- End of excerpt from Donald E. Williams -- H. D. Cochran ------------------------------------------------------------------------ Chemical Technology Division tel 423/574-6821 Oak Ridge National Laboratory fax 423/241-4829 Bethel Valley Road email hdc@ornl.gov Oak Ridge, TN 37831-6224 (USA) http://www.ornl.gov/people/HDCochran ------------------------------------------------------------------------ "Beyond the mountain there are mountains." ------------------------------------------------------------------------