What really are resonance structures?

[Yashbhatt]

This is a concept I have never really understood. I mean to say is how can we include such a thing in a theory? How can we use them if we know that they don't actually exist? Are they some sort of calculation tool?

 

[Bystander]

some sort of calculation tool?

Head count on "valence shell" electrons, and a rough idea of how they contribute to various bonding.

 

[Yashbhatt]

Head count on "valence shell" electrons, and a rough idea of how they contribute to various bonding.

But why do we include them when they don't really exist?

 

[Bystander]

include them

"Why use them?" As tools to enhance understanding of molecular structure, they are crude. As a chemist, when working with anything more complex than hydrogen atoms or hydrogen like ions, I'm stuck with crude tools.

 

[Yashbhatt]

"Why use them?" As tools to enhance understanding of molecular structure, they are crude. As a chemist, when working with anything more complex than hydrogen atoms or hydrogen like ions, I'm stuck with crude tools.

I think of them as states. More number of resonance structures means more number of states in which the compound can exist thus making the compounds more stable. Is it correct to think of it in that way?

 

[Bystander]

Is it correct to think of it in that way?

I wouldn't call it incorrect. If you'll do a little reading on benzene, Dewar benzene resonance structures, aromaticity, and compare the bond energies of three C-C single bonds plus three C=C double bonds to the delocalization energy of a benzene or aromatic ring, you will find that there is a quantitative difference in the stabilization energy when compared to that of the Dewar resonance structures.

 

[DrDu]

There are different ways to expand a multi-electron wavefunction into a basis set. One approach is molecular orbital theory, where the building blocks are Slater determinants made up of single electron molecular orbitals. Another approach is valence bond theory where the molecular wavefunction is constructed from atomic orbitals. There are different ways of doing so and one approach uses a set of paired atomic orbitals wherea complete set of different independent pairing schemes are found from Rumer diagrams which are the mathematical construct corresponding to the different valence structures.
It often turns out that the electronic wavefunction can be approximated very well as a resonance structure, or, in modern terminology, a superposition of only a handful of valence structures. This makes the valence structures a valuable tool though they aren't unique.
The classic text to understand all this is still be book by Eyring, Walther and Kimball, Quantum chemistry.

 

[Yashbhatt]

I wouldn't call it incorrect. If you'll do a little reading on benzene, Dewar benzene resonance structures, aromaticity, and compare the bond energies of three C-C single bonds plus three C=C double bonds to the delocalization energy of a benzene or aromatic ring, you will find that there is a quantitative difference in the stabilization energy when compared to that of the Dewar resonance structures.

Yeah. That's the thing I have been studying these days.

 

[Yashbhatt]

I wouldn't call it incorrect. If you'll do a little reading on benzene, Dewar benzene resonance structures, aromaticity, and compare the bond energies of three C-C single bonds plus three C=C double bonds to the delocalization energy of a benzene or aromatic ring, you will find that there is a quantitative difference in the stabilization energy when compared to that of the Dewar resonance structures.

Yeah. That's the thing I am studying these days.

There are different ways to expand a multi-electron wavefunction into a basis set. One approach is molecular orbital theory, where the building blocks are Slater determinants made up of single electron molecular orbitals. Another approach is valence bond theory where the molecular wavefunction is constructed from atomic orbitals. There are different ways of doing so and one approach uses a set of paired atomic orbitals wherea complete set of different independent pairing schemes are found from Rumer diagrams which are the mathematical construct corresponding to the different valence structures.
It often turns out that the electronic wavefunction can be approximated very well as a resonance structure, or, in modern terminology, a superposition of only a handful of valence structures. This makes the valence structures a valuable tool though they aren't unique.
The classic text to understand all this is still be book by Eyring, Walther and Kimball, Quantum chemistry.

I have a question. Are resonance structures something like dynamic equilibrium(i.e. continuously changing states) or are they something which is in between both the states?

 

[James Woodward]

I have a question. Are resonance structures something like dynamic equilibrium(i.e. continuously changing states) or are they something which is in between both the states?

Good question. The reality is somewhere between all of the resonance structures. For example, benzene's C-C bond length is somewhere between the single and double bond lengths for carbon.