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### Topic: About sp-mixing!  (Read 23992 times)

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#### Winga

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« on: January 03, 2005, 05:19:02 AM »
Use N2 for example, after sp-mixing, the sigma*2s orbital is lower in energy level than before, while sigma2pz orbital is higher in energy than before. Why?

From textbook, it said both MO has both s & p characters, if yes, sigma*2s should rise in energy level and sigma2pz orbital should drop in energy level.

#### Demotivator

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« Reply #1 on: January 03, 2005, 10:29:21 AM »
I don't know why you mention the sigma*2s orbital.

After sp-mixing, the sigma2p orbital interacts with the sigma2s orbital because they are close enough in energy and have the same symmetry. The result is that the sigma2s drops further and the sigma2p is raised in energy.

Although both MOs have sp character they don't have equal character. The sigma2s has more s character than the sigma2p.
The sigma2s orbital drops in energy because it more closely resembles the lower energy interacting 2s orbital.

#### Winga

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« Reply #2 on: January 04, 2005, 12:48:34 AM »
e.g. N2
Does sp-mixing mean that the 2s atomic orbital of one N interacts with 2p atomic orbital of another N, vice versa?

So, why we use the interaction between sigma2s & sigma2p molecular orbitals? Because, using this approach will give a new sigma2s MO & sigma2p MO.

From textbook, it indicates that
sigma1 = sigma2s
sigma2 = sigma*2s
sigma3 = sigma2p
sigma4 = sigma*2p

and it says the MO sigma2 & sigma3 have both p and s character.
So, which one is correct?
sigma2s + sigma2p or sigma*2s + sigma2p?

#### Demotivator

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« Reply #3 on: January 04, 2005, 12:00:08 PM »
1) No. sp mixing is mixing within one atom's orbitals (like hybridization) That's where the s and p character come from. They combine with another atom's mixed orbitals to form a MO.

2) By interaction, I don't mean they combine to produce more orbitals. I mean the orbitals that are already formed, namely the sigma2s and sigma2p, are close enough energetically (in the case of N2) and in symmetry to cause a  "repulsive" effect where one goes further lower in energy than normal and the other (the sigma2p) is further raised in energy than normal. I was trying to explain why the sigma2p goes up in energy. It doesn't happen in all cases but it does for N2.

3) They all have s and p character.
I don't know why the sigma*2s goes down in energy if it does.

#### Winga

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« Reply #4 on: January 05, 2005, 10:40:59 AM »
One more question:

sigma2p should be closed to sigma*2s than that of sigma2s?
So, the repulsion is between sigma2p and sigma*2s?
« Last Edit: January 05, 2005, 10:41:17 AM by Winga »

#### Demotivator

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« Reply #5 on: January 05, 2005, 05:06:04 PM »
sigma2p should be closed to sigma*2s than that of sigma2s?"
yes

"So, the repulsion is between sigma2p and sigma*2s? "
I doubt it because sigma2p and sigma*2s don't have the same symmetry.

#### Demotivator

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« Reply #6 on: January 26, 2005, 11:46:49 AM »
The  sigma*2s is lower in energy than before because of interaction with the sigma*2p (not sigma2p). They both have the same symmetry. The sigma*2p goes higher than before.

#### Winga

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« Reply #7 on: January 26, 2005, 12:45:34 PM »
The  sigma*2s is lower in energy than before because of interaction with the sigma*2p (not sigma2p). They both have the same symmetry. The sigma*2p goes higher than before.
So,
the interaction (repulsion) between sigma2s and sigma2p results in lower sigma2s energy but sigma2p raised in energy
&
the interaction (repulsion) between sigma*2s and sigma*2p results in lower sigma*2s energy but sigma*2p raised in energy.
2 interactions co-existed, right?

One more thing I want to make sure is that is s-p mixing a mixing of one atom's s orbital with another atom's p orbitals, vice versa, (according to a textbook) or just as you said form one atom's?

#### Demotivator

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« Reply #8 on: January 26, 2005, 01:13:51 PM »
1)right.

2) I'll amend what I said. The mixing occurs (from MO theory point of view) when the MOs form between the two atoms. Since they have the same symmetry and are close enough in energy, the s based MO mixes to some extent with the p based MO and this interaction alters their energies.
As I pointed out here:
« Last Edit: January 26, 2005, 01:26:07 PM by Demotivator »

#### Winga

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