Subject: Help!(URGENT): Strange trends of delta_Hf data for carbenium ions

From: "Park, Tae-Yun" <[email-masked]>
Date: Mon, 5 Aug 1996 16:17:23 +0200 (DFT)

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Dear all, 

I've been used MOPAC93 for calculating delta_Hf(Heat of formation)
for many carbenium ions in gas-phase, and I found very strange trends
of these values today.

Comparing the values of heat of formation for various primary, secondary, 
and tertiary carbenium ions, I've got a reasonable results, i.e,
the heat of formation values of primary carbenium ions are greater
than those of secondary carbenium ions, and the values of secondary 
carbenuim ions are greater than those of tertiary carbenium ions, etc.

However, within secondary or tertiary carbenium ions with the same 
number of carbon atom, I saw the values are on opposit than I expect.  

Please consider the following carbenium ions:
        
              +          
  (a) C-C-C-C-C-C  [ H9C4-C(+)(CH3)2 ]
              |
              C                      

        C
        | +
  (b) C-C-C-C   [ (H3C)3C-C(+)(CH3)2 ]
        | |
        C C

Both are tertiary carbenium ions with carbon number of 7.
According to literature, the second carbenium ion (b) should be 
more stable than the first one (a) due to the stablization by the
branched carbon atom on the alfa-positioned carbon atom that
is connected to the (+)-charged carbon atom.

But the results of MOPAC calculation for heat of formation for
both carbenium ions are,

   (a) HEAT OF FORMATION = 151.93056 KCAL

   (b) HEAT OF FORMATION = 160.04209 KCAL

which means that the first carbenium ion is more stable than the
second one!  What happen??

Even I have many calculated results like this, which shows exactly
the opposit trend than I expect.  One of them is,

            +
        C-C-C-C-C               
            |           HEAT OF FORMATION = 160.35787 KCAL
            C

          C
          |
        C-C-C-C
          + |           HEAT OF FORMATION = 162.62280 KCAL
            C

The keywords I used for the calculation of heat of formation
are, "SYMMETRY CHARGE=1 AM1 GEO-OK EF PRECISE".  Did I make
a mistake in using keywords or in somewhere else?  If so,
how can I get an accurate and reliable results in MOPAC??

If not, does it mean that MOPAC is not accurate enough to 
predict the heat of formation values within same type of
carbenium ions with the same carbon number? 

I attached my input data files of the two carbenium ions (a)
and (b) written above for MOPAC calculation.

PLEASE inform me of ANY suggestions/advice on this strange
values, if you have any idea on this problem.

Thank you VERY MUCH in advance.

				Sincerely,

				     Park, TAE-YUN    
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
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]
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

--------------------------------------------------------------
Input data for carbenium ions (a) and (b)
--------------------------------------------------------------
SYMMETRY CHARGE=1 AM1 GEO-OK EF PRECISE
r7-8.dat

  C    0.000000  0    0.000000  0    0.000000  0    0   0   0
  C    1.523987  1    0.000000  0    0.000000  0    1   0   0
  H    1.112930  1  109.468369  1    0.000000  0    1   2   0
  H    1.112946  1  109.470108  1 -119.422729  1    1   2   3
  H    1.112991  1  109.470108  1  119.424484  1    1   2   3
  C    1.523926  1  119.999756  1 -119.356293  1    2   1   3
  C    1.523926  1  120.001495  1   61.217728  1    2   1   3
  C    1.522888  1  109.475357  1  179.999634  1    7   2   1
  H    1.112869  1  109.471863  1   60.070694  1    7   2   1
  H    1.112946  1  109.470108  1  -60.074188  1    7   2   1
  C    1.522980  1  109.499832  1  179.999634  1    8   7   2
  H    1.112808  1  109.405411  1   60.088181  1    8   7   2
  H    1.112869  1  109.403671  1  -60.093414  1    8   7   2
  C    1.522980  1  109.499832  1 -179.999634  1   11   8   7
  H    1.112991  1  109.408920  1   60.095169  1   11   8   7
  H    1.112991  1  109.410660  1  -60.093414  1   11   8   7
  H    1.112976  1  110.001663  1  179.999634  1   14  11   8
  H    1.112976  1  109.999924  1   59.922073  1   14  11   8
  H    1.112976  1  110.001663  1  -59.920319  1   14  11   8
  H    1.112991  1  109.470108  1 -179.999634  1    6   2   1
  H    1.112930  1  109.471863  1   60.579514  1    6   2   1
  H    1.112946  1  109.473618  1  -60.577759  1    6   2   1

-----------------------------------------------------------------
SYMMETRY CHARGE=1 AM1 GEO-OK EF PRECISE
r7-20.dat

  C    0.000000  0    0.000000  0    0.000000  0    0   0   0
  C    1.524002  1    0.000000  0    0.000000  0    1   0   0
  H    1.112808  1  109.464874  1    0.000000  0    1   2   0
  H    1.112839  1  109.466614  1 -119.419235  1    1   2   3
  H    1.112808  1  109.464874  1  119.424484  1    1   2   3
  C    1.523972  1  120.001495  1  179.162949  1    2   1   3
  C    1.523987  1  120.001495  1   -1.379593  1    2   1   3
  C    1.522964  1  109.471863  1 -179.999634  1    7   2   1
  C    1.522995  1  109.471863  1   60.000748  1    7   2   1
  C    1.522995  1  109.471863  1  -59.999008  1    7   2   1
  H    1.112915  1  110.006912  1  179.999634  1    8   7   2
  H    1.112976  1  109.998169  1   59.918564  1    8   7   2
  H    1.112976  1  109.996414  1  -59.920319  1    8   7   2
  H    1.112946  1  109.470108  1 -179.999634  1    6   2   1
  H    1.112946  1  109.470108  1   60.576019  1    6   2   1
  H    1.112946  1  109.470108  1  -60.577759  1    6   2   1
  H    1.112976  1  109.998169  1  179.999634  1   10   7   2
  H    1.112976  1  109.998169  1   59.920319  1   10   7   2
  H    1.112961  1  109.998169  1  -59.923813  1   10   7   2
  H    1.112991  1  109.999924  1 -179.999634  1    9   7   2
  H    1.112961  1  109.999924  1   59.923813  1    9   7   2
  H    1.112991  1  109.999924  1  -59.922073  1    9   7   2

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