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# Molecular Orbital Theory watch

1. I understand the main concept of MO theory but there are certain things that confuses me. When 2 atmoic orbitals combine, do they make 1 MO or 2 MO's. At first I thought it was just 1 as they both combine to make 1 new orbital, but then I saw a video on Khanacadamy saying it created 2: 1 bonding and 1 antibonding.

Which one is right?
2. (Original post by mrdoovde1)
I understand the main concept of MO theory but there are certain things that confuses me. When 2 atmoic orbitals combine, do they make 1 MO or 2 MO's. At first I thought it was just 1 as they both combine to make 1 new orbital, but then I saw a video on Khanacadamy saying it created 2: 1 bonding and 1 antibonding.

Which one is right?
2 AOs combine/interact to form 2 MOs - one bonding and one anti-bonding

3AOs combine/interact to form 3 MOs - one bonding, one non-bonding and one anti-bonding

(but of course, that is if they interact due to correct symmetry - more in Group theory, i suppose, but what i stated should be sufficient for the time being). one source does not tell you a whole story, you can always read from other sources to confirm.
3. (Original post by mrdoovde1)
I understand the main concept of MO theory but there are certain things that confuses me. When 2 atmoic orbitals combine, do they make 1 MO or 2 MO's. At first I thought it was just 1 as they both combine to make 1 new orbital, but then I saw a video on Khanacadamy saying it created 2: 1 bonding and 1 antibonding.

Which one is right?
The number of orbitals in the basis set = number of molecular orbitals
4. I was hoping it wasn't true as it confuses me even more!

Take H2 for example. When the 2 1s orbitals combine, how can they bond in-phase AND out-of-phase? Creating 1 bonding and 1 antibonding, wouldn't that require 4 AO's (2 in the same phase and 2 in different phases)?
5. (Original post by shengoc)
3AOs combine/interact to form 3 MOs - one bonding, one non-bonding and one anti-bonding
A reasonable general statement but I'm afraid it isn't quite true - take the case of the pi orbitals in odd numbered cyclic polyenes, eg the cyclopropyl anion/radical/cation.
6. (Original post by mrdoovde1)
I was hoping it wasn't true as it confuses me even more!

Take H2 for example. When the 2 1s orbitals combine, how can they bond in-phase AND out-of-phase? Creating 1 bonding and 1 antibonding, wouldn't that require 4 AO's (2 in the same phase and 2 in different phases)?
When orbitals overlap you get an inphase and out-of-phase combination. Hence for a 2 orbital system.... 2 MO's
7. (Original post by illusionz)
A reasonable general statement but I'm afraid it isn't quite true - take the case of the pi orbitals in odd numbered cyclic polyenes, eg the cyclopropyl anion/radical/cation.
there are unfortunate amount of special cases/instances in chemistry, sigh...

just look at how organic lecturers always mention a general statement and then start showing you the caveats.
8. (Original post by shengoc)
there are unfortunate amount of special cases/instances in chemistry, sigh...
Too true! Funny how the only thing I can remember from theoretical chemistry these days is the MO diagrams for cyclic/linear polyenes
9. (Original post by shengoc)
there are unfortunate amount of special cases/instances in chemistry, sigh...

just look at how organic lecturers always mention a general statement and then start showing you the caveats.
How are the Pi MO's of cyclic polyenes an exception???? Still the same number of pi symmetry MO's as the number of pi atomic orbitals involved in the bonding.
10. (Original post by JMaydom)
How are the Pi MO's of cyclic polyenes an exception???? Still the same number of pi symmetry MO's as the number of pi atomic orbitals involved in the bonding.
ermm, i think during my time, it was briefly covered in aromaticity lecture in first year, then in second year, it was again briefly touched upon in i-don't-recall-what-series lecture, that is.

but definitely by third year, the cyclic polyene orbitals get very important in a lot of the organometallic complexes formed (esp the half sandwich/sandwiched ones).
11. (Original post by JMaydom)
How are the Pi MO's of cyclic polyenes an exception???? Still the same number of pi symmetry MO's as the number of pi atomic orbitals involved in the bonding.
He said that 3 AOs give you 3 MOs of which one is bonding, one non bonding and one antibonding. The bold part is the bit which isn't quite true, as in the cyclopropyl cation/radical/anion you get 2 antibonding orbitals and one bonding. If I recall correctly, in a cyclic polyene, you only get a non bonding orbital when the length of the polyene tends to infinity. (ie it is linear).
12. If i recall correctly, there is that policy of always fixing only one edge of a cyclic polyene to the lowest energy orbital when constructing the rough MO based on Huckel theory.

ie in cyclopropyl, 3 MOs, the shape is a triangle with one edge fixated to the lowest energy orbital

in cyclobutane or its analogue, 4 MOs, then the shape is a square, but again, you fix only the edge of the square to the one orbital of the lowest energy orbital

in cyclopropane, 5 MOs, again, fix only one edge of the pentagon to the lowest energy orbital

so it goes for cyclic polyene MOs (Huckel theory, etc, i dont know much after that, lol)
13. (Original post by illusionz)
He said that 3 AOs give you 3 MOs of which one is bonding, one non bonding and one antibonding. The bold part is the bit which isn't quite true, as in the cyclopropyl cation/radical/anion you get 2 antibonding orbitals and one bonding. If I recall correctly, in a cyclic polyene, you only get a non bonding orbital when the length of the polyene tends to infinity. (ie it is linear).
I'll check when I get that far with my revision.....

(Original post by shengoc)
If i recall correctly, there is that policy of always fixing only one edge of a cyclic polyene to the lowest energy orbital when constructing the rough MO based on Huckel theory.

ie in cyclopropyl, 3 MOs, the shape is a triangle with one edge fixated to the lowest energy orbital

in cyclobutane or its analogue, 4 MOs, then the shape is a square, but again, you fix only the edge of the square to the one orbital of the lowest energy orbital

in cyclopropane, 5 MOs, again, fix only one edge of the pentagon to the lowest energy orbital

so it goes for cyclic polyene MOs (Huckel theory, etc, i dont know much after that, lol)
Will be revising Huckel theory soon..... That doesn't sound familiar though.
14. (Original post by JMaydom)
When orbitals overlap you get an inphase and out-of-phase combination. Hence for a 2 orbital system.... 2 MO's
So if 2 AO's combine in-phase and-out-of phase, that must mean every orbital has a positive AND negative wavefunction right?
15. (Original post by mrdoovde1)
So if 2 AO's combine in-phase and-out-of phase, that must mean every orbital has a positive AND negative wavefunction right?
my tutor likes to use + to indicate in-phase combinations of AOs - though some prefer shading, vice versa for out-of-phase combinations

when you say positive and negative wavefunction, correct me if i am wrong, those does not sound right at all. perhaps stick with in-phase and out-of-phase.
16. (Original post by JMaydom)
Will be revising Huckel theory soon..... That doesn't sound familiar though.
He's almost there. You place a point of the regular polyhedron on the lowest energy MO, and then the other points give the relative locations of the other MOs.
17. (Original post by illusionz)
He's almost there. You place a point of the regular polyhedron on the lowest energy MO, and then the other points give the relative locations of the other MOs.
better summarised than mine,
18. (Original post by illusionz)
He's almost there. You place a point of the regular polyhedron on the lowest energy MO, and then the other points give the relative locations of the other MOs.
Oh yes, I remember that. From organic lectures..... I was trying to think about Physical chem Huckel thoery.
19. (Original post by JMaydom)
Oh yes, I remember that. From organic lectures..... I was trying to think about Physical chem Huckel thoery.
no compartmentalisation, sorry, mate! it is always best if you can use all the knowledge to relate to certain topics which you target to answer in Finals! too bad, your lots would have to do more questions than my cohort had to.

I recalled being able to skip all the matter topics (polarisation, etc), electrochemistry and solution chemistry for physical and still get by great with mostly quantum, thermo and statistical mech.
20. (Original post by shengoc)
no compartmentalisation, sorry, mate! it is always best if you can use all the knowledge to relate to certain topics which you target to answer in Finals! too bad, your lots would have to do more questions than my cohort had to.

I recalled being able to skip all the matter topics (polarisation, etc), electrochemistry and solution chemistry for physical and still get by great with mostly quantum, thermo and statistical mech.
It's even better at cambridge! This year I've only done organic/biological and a little bit of main group organnometallics. No physical or theoretical Only had to do a little bit of huckel theory and high resolution spectroscopy last year too. No thermodynamics since 2nd year!

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