From tp@elptrs7.rug.ac.be Mon Jun 24 06:56:15 1996 Received: from elptrs7.rug.ac.be for tp@elptrs7.rug.ac.be by www.ccl.net (8.7.5/950822.1) id GAA10353; Mon, 24 Jun 1996 06:23:28 -0400 (EDT) Received: by elptrs7.rug.ac.be (AIX 4.1/UCB 5.64/4.03) id AA19138; Mon, 24 Jun 1996 12:24:21 +0200 Date: Mon, 24 Jun 1996 12:24:21 +0200 (DFT) From: "Park, Tae-Yun" To: Computational Chemistry List Subject: HELP!(urgent):theoretical discrimication & software Message-Id: Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Dear researchers in computational chemistry, I'm a ph.D. student of State University of Ghent, Belgium. I'm working with zeolite HZSM-5. The goal of my work is to develop a detailed kinetic model for hydrocarbon transformation process over HZSM-5 catalyst. I have some urgent questions for my ph.D. work, which have been discussed with many people who were in the field of chemical engineering/organic chemistry/electrical chemistry in my university or in the internet. So far, I could not get reliable solutions, which have stuck me for relatively long time. During searching for the solutions, I realized that the best people who can give me an advice are in the field of computational chemistry. Please take some time to read the following questions and give me some informaion/advice if it doesn't bother you too much. *** About the structure of surface carbenium ions and symmetry number *** First thing I have to calculate is the symmetry number of surface carbenium ions on the zeolite surface, which is formed by protonation of various olefins. 1. How can we describe the bonding of carbenuim ions to the surface? Is the bonding more ionic or covalent? I know the proton is surely covalently bound, but it is less clear for the case of hydrocarbons(more exactly olefins) adsorbed on zeolite. 2. What is the symmetry number for such surface carbenium ions comparing with corresponding gas phase carbenium ions? In most cases, I believe, the symmetry number of surface ions are identical with those in gas phase, if there is no 2-fold axes. Consider the following examples. C \ C-C+-C (secondary ion in gas phase) / C symmetry number=3*3*3=27 C \ C-C-C (secondary ion on the surface) / | C | --(+)-- symmetry number=3*3*3=27 If there is two-fold axes, however, C \ 2-fold axes.....C+-C-C (tertiary ion in gas phase) / C symmetry number=(3*3*3)*2=54 C \ Not a 2-fold axes(?)...C+-C-C (tertiary on the surface) /| C | --(+)-- symmetry number=3*3*3=27 I think the surface ion loose its 2-fold axes due to the bonding to the catalyst surface. I just want to know whether those approach to calculate the symmetry number of surface carbenium ions is theoretically correct. To check this point, I think, the bonding of carbenium ions to the surface should be defined first, in a certain way of theoretical approach. **** About the structure of activated complex ****** Next problem that I have to solve is concerning the structure of activated complex in transition state for beta-scission. That is, an elementary step representing beta-scission, R1 ---> (Transition State) ----> R2 + O where R1 and R2 are the reactant and product carbenium ion, respectively, and O is the product olefin. What I want to know is the structure of the activated complex in the transition state, as detailed as possible in a certain theoretical way; how many bonds are related, which bonds are to be broken, which bond will be formed, and what does the activated complex look like(is it more like carbenium ion or more like olefin?), etc. Is there any reference/software to describe the structure of activated complex for this reaction in a theoretical way? **** About heat of formation for gas phase carbenium ions *** My final question is about the calculation of thermodynamic data, more exactly, the heat of formation of gas-phase carbenium ions. The reason is the following: To develop a detailed kinetic model, a complex reaction network containing a lot of elementary steps has to be considered. Large number of different carbenium ions are involved in the elementary steps, which produces large number of kinetic parameters that have to be estimated by a certain optimization method. Recently, a solution to solve the problem associated with the large number of the parameters has been considered, i.e, by applying the "Polanyi relation". This relation needs only relative differences in heat of reactions in each elementary steps. This means that the heat of formation for the surface carbenium ions are not necessary, but, at least, heat of formation of gas-phase carbenium ions appearing in the reaction network should be given. Unfortunately, only limited amount of data can be found in the literature, so that some analytical method to calculate the heat of formation is necessary. Benson's group contribution method has been known as one of the most approprate way to calculate the thermodydamic properties. The group contribution value of positively-charged carbon atom involved in the carbenium ions is, however, not available at the moment. The only solution I found recently is using a certain quantum chemistry package such as "MOPAC". My question is that is this a suitable package (accurate enough) to calculate the heat of formation data I need? If so, how can I obtain a copy of this package? If not, is there any other software in the field of computational chemistry to calculate these data? *************** End of my questions ************************* Thank you very much for reading my long message. Since I'm not familar with computational chemistry field, some of my questions would make you feel boring or make you think that this is too basic/stupid questions. I'm really sorry if it is really so. Any suggestions/informations will be greatly appreciated. Particularly, infomation on some computational chemistry softwares (name of the software/how to get it) which give some theoretical point of view on these problems will give me a great help. Thanks again for your patience and I am highly looking forward to hearing any news from you soon. Sincerely, Park, TAE-YUN =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Laboratorium voor Petrochemische Techniek Krijgslaan 281, Blok S5 9000 Gent, Belgium TEL:+(32)-0(9)-264-4527 FAX:+(32)-0(9)-264-4999 e-mail: tp@elptrs7.rug.ac.be =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=