CCL:G: Carbocation calculations, an approach

Your reaction is:

A+B -> C

Therefore the reaction energy is:

Energy(C) -Energy(A+B), which (neglecting BSSE) is equal to

energy (C) -(energy (a) + energy (b) ) =

-390.92315 - (-195.60813 + (-195.27260) ) = -0.04242 hartree = -26.6 kcal.mol-1

"Exploring Chemistry with electronic structure Methods" is a good book to start understanding computational chemistry. It used to be distributed along with Gaussian, so probably there is a copy somewhere in your lab :-)

Pedro S.

On Tue, Mar 22, 2011 at 10:15 PM, Miguel Quiliano Meza <owner-chemistry[*]> wrote:
Dear colleagues.y(C)

My apologies if I disturb you with a couple of questions.

I have never worked with carbocations, but now I have a good case to start

I would like to answer some chemicals questions using computational chemistry, but unfortunately I obtained rare energy values.

Could you tell me please, what should I do? or what kind of chemistry model should I use?.

I want to answer some questions:

*Which reaction is the most probable to happen?
*What energy is necessary to reach transitional compounds before obtained products?

I have performed some calculations using this:

# opt=tight freq b3lyp/6-31+g scf=qc  with the respect spin multiplicity, but my calculations told me that compound A is most stable than B, besides compound C is most stable than A and B. I stopped here because I think that is impossible.

A: is a carbocation very similar in structure like B
 B is not a carbocation
C is a carbocation product of A+B

Final energy for A = -195.60813

Final energy for B = -195.27260

Final energy for C = -390.92315

Thank in advance for your time. Maybe if you don´t have time please recommen me one good paper or good tutorial.

Best regards.

Pedro J. Silva
Associate Professor
Universidade Fernando Pessoa
Porto - Portugal