Re: resonance

On 2005-06-29, Brian Salter-Duke <b_duke@xxxxxxxxxxxxxxxxxxxx> wrote:
> On 2005-06-29, monika hohlmeier <monika_hohlmeier@xxxxxxxx> wrote:
>> Brian Salter-Duke wrote:
>>
>>> So it is wrong. If you assume that the C ring is a square then you do
>>> indeed get a triplet. That is the ground state is degenerate. Since it
>>> is non-linear, the Jahn-Teller theorem kicks in and the molecule
>>> distorts to remove the degeneracy. The ground state of cyclobutadiene is
>>> a singlet. It is a real di-ene with two opposite short bonds and two
>>> opposite longer bonds. It is indeed highly reactive. This is a case
>>> where simple theories are wrong.
>>
>> High precision calculations predict the ground state of cyclobutadiene for
>> the square geometry to be a singlet. To explain this in terms of MO theory
>> I remember something called "dynamical polarization", a term which
>> certainly cannot be found in any introductory chemistry text either and
>> which goes beyond Hartree Fock.
>> The same holds true for Cyclooctatetraene.
>> So simple MO theory does not even predict the correct multiplicity of the
>> ground state of antiaromatic molecules.
>> Really not a good test bed for oversimplified theories, whether MO or VB.
>
> Agreed. Cyclobutadiene is not a good test bed for any simple theory.
> Therefore it is not a good idea to use it to say VB or MO is wrong.
>
> On you first point, I am not sure, but I doubt that the singlet state
> you describe is at a stationary point. The molecule would distort to
> gave double and single bonds. I think straight Hartree-Fock theory
> would give a singlet rectangle structure if you optimised the geometry
> as a singlet and that energy would be lower than the triplet structure.
> I do not have time to try that as I have to get ready to leave for a
> conference at crack of dawn tomorrow.
>
> I recall that the first experimental evidence for CBD was a
> Ni(CO)n(CBD) complex and that the crystal structure showed it was not
> square. If the complex was dissociated the CBD was unstable so I doubt
> there is an experimental geometry for the free molecule.
>
> Why do you keep on about introductory chemistry texts? Any chemist,
> including your synthetic organic chemist, should have gone well beyond
> that.

Sorry, I got confused (early in the morning here!). I thought I was
replying to the other poster.

> There are several cases where HF theory even with large basis sets gives
> the wrong structure and dynamic correlation (that is the term you want)
> is needed to predict the correct structure. For example, what is the
> structure of CH2=C=C=O?
>
> Actually there is a simple theory for CBD:-
>
> Huckel --> square --> triplet --> therefore degenerate --> Jahn-Teller
> --> distorts --> singlet.
>
>


--
Brian Salter-Duke Spotswood, Melbourne, Australia
.

Relevant Pages

  • Re: resonance
    ... If you assume that the C ring is a square then you do ... That is the ground state is degenerate. ... > the square geometry to be a singlet. ... Why do you keep on about introductory chemistry texts? ...
    (sci.chem)
  • Re: resonance
    ... If you assume that the C ring is a square then you do ... That is the ground state is degenerate. ... > opposite longer bonds. ... Really not a good test bed for oversimplified theories, ...
    (sci.chem)