Subject: Relative heat of stabilization differences

From: "Park, Tae-Yun" <[email-masked]>
Date: Tue, 26 Aug 1997 10:30:14 +0200 (DFT)

Dear all,

Please allow me to ask you concering relative heat of stabilization
differences of surface bonded carbenium ions related to the corresponding
gas-phase carbenium ions.  I need your insight on this problem VERY
urgently, and certainly I will summerize if there is even slight demand.

Perhaps, the question I want to ask you has been already addressed in the
computational chemistry field, but I can not reach the information.

Consider the following surface bounded carbenium ions,

          C-C-C                C-C-C-C
            |                    |
            O                    O    
           /+\                  /+\
          Al Si                Al Si

      [ C-C(+)-C ]s        [ C-C(+)-C-C ]s

which are usually formed by the protonation of propylene or
butene, i.e.,

C=C-C + [H(+)]s  --->  [ C-C(+)-C ]s
C=C-C-C + [H(+)]s  --->  [ C-C(+)-C-C ]s

where the "[ ]s" means the species on the catalyst surface.  The catalyst
considered here would be any kind of zeolites, say HZSM5, HY...etc, which
are activated by Bronsted acidic site, [H(+)]s. I'm doing some kinetic
modelling work for reactions containing those surface carbenium ions by
relating to their gas-phase carbenium ions, since the thermochemical data
of gas-phase ones are available, while surface ones are dependent on
catalysts.

If one consider the amount of heat stabilization from the gas-phase
carbenim ions to the surface bonded ones due to the cartalyst, i.e.,

q1 = Delta_H_f( C-C(+)-C, gas-phase ) 
     - Delta_H_f( C-C(+)-C, surface bonded)

q2 = Delta_H_f( C-C(+)-C-C, gas-phase ) 
     - Delta_H_f( C-C(+)-C-C, surface bonded)

Obviously the q1 and q2 should be positive, since the surface carbenium
ions are more stable than gas-phase ones.  Here the Delta_H_f means the
heat of formation. It is also expected that the value of the q1 and q2
will vary with different catalysts, depends on the acidic strenth of
catalyst.

My question is, if the catalyst and reaction temperature are fixed, are
the values of the q1 and q2 would be the same?  In other words, are the
values of the amount of heat stabilization from the gas-phase carbenim
ions to the surface bonded ones due to the cartalyst also vary depending
on the structure or molecular weight(or whatever) of the carbenum ions?  

So far, some researchers in our field have assumed that the the value of
the amount of heat stabilization would be more or less independent on the
surface carbenium ions, so that q1 equals to q2 in the example of this
message.  But it seems this assumption does not work in my case.

If the amount of heat stabilization varies with different carbenium ions,
what would be the major factor to make this difference you think?? 

Any suggestion/advice/information(articles, books...) would be GREATLY
appreciated.

Thank you very much in advance

				Sincerely,

				     Park, TAE-YUN    
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State University of Ghent
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: [email-masked]
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