From:	"Park, Tae-Yun" <tp $#at#$ elptrs7.rug.ac.be>
Date:	Mon, 24 Jun 1996 12:24:21 +0200 (DFT)
Subject:	HELP!(urgent):theoretical discrimication & software

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 <-at-> elptrs7.rug.ac.be
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=






Similar Messages
08/01/1996:  Re: CCL:M:Heat of formation calculation using MOPAC.
08/26/1997:  Relative heat of stabilization differences
12/16/1994:  Spin contamination & AM1 "ROHF" versus UHF
08/01/1995:  Spin contamination, effect on energy and structure.
03/11/1996:  Law of conservation of difficulty: violations.
08/03/1995:  ACS Chicago - CINF Abstracts    - 29 pages document -
04/23/1992:   Huckel MO Theory software
06/08/1993:  undergrad computational chem
01/07/1998:  Chem Topic: Finding Transition Structures
10/01/1993:  torsion of conjugated systems -- summary


Raw Message Text