[davidenarb]

Hello!

We know that a pi bond is formed via an overlap of two p orbitals. The most stable MO of the pi bond is when the two phases of the p bond overlap constructively ( positive lobe with a positive lobe, and negative lobe with the negative lobe)

My question is: Can a MO of a pi bond adopt a conformation where the positive lobe overlap with a negative lobe, and a negative lobe with the positive lobe) ?

I know that this MO is called the anti-bonding orbital, but I am not sure if I can consider this conformation as a "normal" pi bond as the term "antibonding" misleads my perception.


Thank you

[davidenarb]

So, in other words, can these two p orbitals form a pi bond even though it is not the most stable state?

[Corribus]

This combination of p-orbitals would form an antibonding molecular orbital, which has a higher energy than an isolated atomic p-orbital. An electron placed in this molecular orbital would tend to push the two nuclei apart - that is, it would weaken the overall strength of the bond between the two nuclei (decrease the overall bond order). So no, strictly speaking this combination would not lead to formation of a pi-bond.

[davidenarb]

This combination of p-orbitals would form an antibonding molecular orbital, which has a higher energy than an isolated atomic p-orbital. An electron placed in this molecular orbital would tend to push the two nuclei apart - that is, it would weaken the overall strength of the bond between the two nuclei (decrease the overall bond order). So no, strictly speaking this combination would not lead to formation of a pi-bond.

So why it is happening here

[Jackson Murphy]

Is it worth considering what would make an electron shell an entity of stability. That is the  electron that takes you from 9 electrons to 10 electrons in a molecule could have been the one that took you from 8 to 9?

And the proton the same?

Why is it that electrons in outer shells feel the attraction of protons through the negative and thus repulsion of inner shells?

Is it because the electrons in inner shells while probably in a particular spot evens throughout the shell but instantaneously having particular location means electrons in outer shells can find the influence of a protons positive charge in the gaps like a back row spectator at the World Cup seeing the game by peering through the gaps left by the guys in front?

Could we therefore propose the electrons in outer shells will always have location driven by the momentary location of electrons in a inner shell?

Does that take us anywhere regards the current question?

Cheers

[Irlanur]

If the antibonding pi-MO is the only interaction (whatever this should mean), the term "Bond" would simply be wrong because nothing would bond. (The energy would only get higher for smaller distances).

What you show in the coloured picture is the product from a DA-Reaction. The Interaction between the to p-orbitals is indeed destabilizing, however it's obviously not the only one. There are also two sigma-bonding interaction within this MO, leading to an overall bonding character.

[Irlanur]

Is it because the electrons in inner shells while probably in a particular spot evens throughout the shell but instantaneously having particular location means electrons in outer shells can find the influence of a protons positive charge in the gaps like a back row spectator at the World Cup seeing the game by peering through the gaps left by the guys in front?

Could we therefore propose the electrons in outer shells will always have location driven by the momentary location of electrons in a inner shell?

I don't think we come very far when we only consider the classical picture. And then, an electron always "sees" the nucleus, this does not depend on the "inner electrons" (electrons are indistinguishable! ). This can be seen in the Hamiltonian. And also electrons do not have a "particular location" as in classical physiscs. There is, however, such a thing as instantaneous electron correlation, the holy grail of quantum chemistry. Also exchange interaction does not exist in the classical picture.

[davidenarb]

If the antibonding pi-MO is the only interaction (whatever this should mean), the term "Bond" would simply be wrong because nothing would bond. (The energy would only get higher for smaller distances).

What you show in the coloured picture is the product from a DA-Reaction. The Interaction between the to p-orbitals is indeed destabilizing, however it's obviously not the only one. There are also two sigma-bonding interaction within this MO, leading to an overall bonding character.

Do you mean that, in some cases, pi bonds can be formed through the overlap of 2 p orbitals with opposite signs?

[Enthalpy]

What if the two atoms have more bonds than the pi? One strong bond could keep the atoms together despite the pi is antibonding, perhaps momentarily.
To synthesize cyclobutane, one path is to excite one ethylene molecule by UV light so it reacts with the other. Isn't that exactly the case of an antibonding pi where an other bond keeps both carbons together?

[Enthalpy]

Why is it that electrons in outer shells feel the attraction of protons through the negative and thus repulsion of inner shells?

Is it because the electrons in inner shells while probably in a particular spot evens throughout the shell but instantaneously having particular location means electrons in outer shells can find the influence of a protons positive charge in the gaps like a back row spectator at the World Cup seeing the game by peering through the gaps left by the guys in front?

If an atom has, say, 9 protons, and if the orbitals were concentric separated shells (they're not, they overlap), then the "inner" 2+6 electrons would still let the 9th feel one positive charge.

"instantaneously having particular location" is not what orbitals do. Orbitals are stationary, that is time-independant, and I say more crudely: immobile (they can have an orbital momentum though). But by writing one single wavefunction for all particles, quantum mechanics permits a similar situation. With a Psi(R₁, R₂) not being Psi(R₁)*Psi(R₂), you can have some simple distribution to find one electron in a zone (say, with spherical symmetry) and have a smaller probability to find a second electron near the first one - all that independently of time. A mental picture of this is difficult.

Then, you must factor in that electrons are fermions, so that Psi(R₁, R₂) = -Psi(R₂, R₁). This constraints the function Psi, but leaves many possible functions, and the electrostatic repulsion among electrons guides the choice of the function.

[Jackson Murphy]

"instantaneously having particular location" is not what orbitals do. Orbitals are stationary, that is time-independant, and I say more crudely: immobile (they can have an orbital momentum though). But by writing one single wavefunction for all particles, quantum mechanics permits a similar situation. With a Psi(R₁, R₂) not being Psi(R₁)*Psi(R₂), you can have some simple distribution to find one electron in a zone (say, with spherical symmetry) and have a smaller probability to find a second electron near the first one - all that independently of time. A mental picture of this is difficult.

Then, you must factor in that electrons are fermions, so that Psi(R₁, R₂) = -Psi(R₂, R₁). This constraints the function Psi, but leaves many possible functions, and the electrostatic repulsion among electrons guides the choice of the function.

Can we say that the electrons in inner orbitals, to any extent that they stand between in a interactive sense the nucleus and outer electrons, would resolve their pattern or position before those in further out shells, and that if there is a effect on the positions or patterns of electrons between shells if both equally could occupy one pattern or position the electrons in the inner shells would take priority?

[Enthalpy]

Priority to inner shells: I don't know how accurate this is.

Deeper orbitals are smaller, their bonding energy bigger, and so is the repulsion among electrons. That would suggest that outer electrons influence them less.

I'd also expect a full level to be stiffer than a partially filled one.

Though, orbitals in one atom overlap very much. It's even necessary in order to be orthogonal, in the sense <Psi1|Psi2>=0: take for instance the 1s which is positive everywhere, all others are positive and negative both where 1s has a significant density.

With the orbitals occupying much the same volume, the mental image of concentric shells can't be accurate, hence my caution.

To some extent, numerical simulations of atoms and crystals do suppose that higher orbitals influence lower ones little - finite elements for any significant number of electrons must be too hard for computers.

[Jackson Murphy]

The thought process that led me to think about this idea of some kind of priority in instantaneous characteristics associated with electrons in  inner shells, whether those characteristics include spatial distance from a nucleus or not, taking your point, is that a sequence of determination of characteristics of electrons from inner to outer, outer being constrained in determined characteristics by the values just calculated for inner, would simplify any attempt at programming, as compared to electrons being equally determinative of the values of the characteristics of each other electron.

But I also take your point about the thought that x electrons exhibit a pattern of characteristics and x plus 1 electrons exhibit another pattern, and it may be to mask an opportunity to see a solution to "why" to only allow thinking in terms of concentric layered spatial zones of increasing distance from a central point.

Part of me though sees that as we are talking attraction and repulsion that a term in the function of attraction and repulsion must be a vector of the experience of that attraction and repulsion and connection and spatial proximity seems evident everywhere as a vector of such experience.

It's equally conceivable though that the arrival of another electron influences others from the outside then deeper as initially the change in spatial proximity is at the outer areas of the atomic structure as the new electron arrives?

[orgopete]

Let me see if I can get this right. If you draw the spin states for the MO’s of butadiene, they alternate, +-+-. This gives the lowest energy form, no nodes, in colors, all pink.  The next higher energy level would be +--+ and a node between C2 and C3. If drawn with colors, this would half pink and half blue for this Psi state. Post two could be as stated by Corribus. This would be equivalent to the LUMO of ethylene, no pi bond, anti-bonding. If colors, pink and blue.

Post four shows the colors. In this case, the HOMO of butadiene and the LUMO of ethylene. This is not showing the possible spin states, just the psi states. For bonds to form, the colors must match.

I have contributed to this confusion in the past by referring to the spin states with plus and minus signs and then used plus and minus signs for the psi-states. I don't know if this is exactly what the poster is asking about, but it looks as though it might be. The symmetry rules for electrocyclic reactions seem to work quite well in predicting the resulting stereochemistry. One should know this. Because spin states can have a mirror image, they are not used, or it may be additional reasons.

[Irlanur]

The thought process that led me to think about this idea of some kind of priority in instantaneous characteristics associated with electrons in  inner shells, whether those characteristics include spatial distance from a nucleus or not, taking your point, is that a sequence of determination of characteristics of electrons from inner to outer, outer being constrained in determined characteristics by the values just calculated for inner, would simplify any attempt at programming, as compared to electrons being equally determinative of the values of the characteristics of each other electron.

(Some more points please, hard to read )

This is, somehow, done in most calculations that use a split-valence basis. But you still need to say goodbye to the idea of different electrons. you can't distinguish them! Thinking in Quantum Mechanical Terms, which obviously fit atoms better than classical mechanics, there is no such thing as "one electron close to the nucleus than another". look at the Hamiltonian and Psi expressed as an (superposition of) slater-determinants.

Let me see if I can get this right. If you draw the spin states for the MO’s of butadiene, they alternate, +-+-. This gives the lowest energy form, no nodes, in colors, all pink.  The next higher energy level would be +--+ and a node between C2 and C3. If drawn with colors, this would half pink and half blue for this Psi state. Post two could be as stated by Corribus. This would be equivalent to the LUMO of ethylene, no pi bond, anti-bonding. If colors, pink and blue.

Post four shows the colors. In this case, the HOMO of butadiene and the LUMO of ethylene. This is not showing the possible spin states, just the psi states. For bonds to form, the colors must match.

I have contributed to this confusion in the past by referring to the spin states with plus and minus signs and then used plus and minus signs for the psi-states. I don't know if this is exactly what the poster is asking about, but it looks as though it might be. The symmetry rules for electrocyclic reactions seem to work quite well in predicting the resulting stereochemistry. One should know this. Because spin states can have a mirror image, they are not used, or it may be additional reasons.

What?