Chemical Forums
Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: Winga on December 05, 2004, 09:39:28 PM
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e.g. 2 H atoms, each 1s atomic orbital overlap to form binding & antibonding orbitals.
My question is why there are 2 combinations, 1s + 1s & 1s - 1s?
Why not the overlapping of two 1s orbitals either 1s + 1s or 1s - 1s?
(Two waves join together should either constructive interference or destructive interference?)
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I don't understand
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I mean once the two 1s orbitals overlap and form a bonding MO, this is a constructive interference of two waves, so, why the destructive interference also exists?
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It only exists if you try to add more electrons into the region between the nuclei.
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Do bonding and antibonding orbitals overlap each other?
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I mean once the two 1s orbitals overlap and form a bonding MO, this is a constructive interference of two waves, so, why the destructive interference also exists?
i remember once i told you not to think too much about matter waves.You just understand that their are two types of interferences in EM waves,this phenomenon has to do correspondingly with matter waves also,but in a very complex way.So with constructive interference their is also destructive interference.
and yes,how can you say they overlap each other if they have different symmetry.
And i advice you to check this site http://www.shef.ac.uk/chemistry/orbitron/index.html (http://www.shef.ac.uk/chemistry/orbitron/index.html).very good stuff.
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Sorry, I mean superimpose, not overlap.
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no, bonding and antibonding orbitals do not superimpose. Only the atomic orbitals superimpose in you example. At low energy the superimpose constructively(bonding), jack up the energy and they superimpose destructively(anti-bonding).
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How about H2-? (an extra e- added to the H2 molecule)
Do bonding & antibonding MO superimpose now?
How a single e- (wave) make a destructive interference itelf without overlap with other wave (another e-)? (bonding MO has 2 e-)
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"Do bonding & antibonding MO superimpose now?"
no.
This discussion is getting off the mark.
The question was
My question is why there are 2 combinations, 1s + 1s & 1s - 1s?
Why not the overlapping of two 1s orbitals either 1s + 1s or 1s - 1s?
(Two waves join together should either constructive interference or destructive interference?)
The schroedinger equation is exactly solvable for atoms. It is also exactly solvable for the simplest molecule H2+. Just as atomic wavefunctions having different numbers of nodes and shapes arise as solutions for the one atom, the analogous thing results as wavefunction solutions for the one eletron under the influence of binuclear H2. There is no linear combinations of atomic orbitals involved.
The linear combination of atomic orbitals (LCAO) is a mathematical method that is used to approximate a solution to the schroedinger equation for molecules. The only reason it's used is because the schroedinger equation gets too complex for molecules (other than H2+) to be solved directly and exactly.
Since LCAO is just a mathematical method that adds and subtracts orbitals, it is not necessarily a representation of what happens dynamically in reality (schroedinger equation is the more ideal representation); ie. when 2 s orbitals of two atoms approach, it makes no intuitive sense, as Winga says, to say they simultaneously add and subtract. What is important is that the end result correctly achieved by LCAO is 2 MOs, which if mathematically added together produce the same space as the original atomic orbitals.
That is the idea behind the LCAO method, to produce the correct end result consisting of bonding/antibonding pairs which are not superimposed in the molecule, but if mathematically superimposed all together, produce the original atomic orbitals. It is a manifestation of the conservation of matter and energy and orbitals.
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So, both addition & subtraction of orbitals existed simultaneously is just the definition of LCAO, right?
The linear combination of atomic orbitals (LCAO) is a mathematical method that is used to approximate a solution to the schroedinger equation for molecules. The only reason it's used is because the schroedinger equation gets too complex for molecules (other than H2+) to be solved directly and exactly.
Is it really too complex for Schrodinger equation to solve the molecules (or more than 1 e- system) or after solving Schrodinger equation cannot get the exact information of molecules?
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It is to complex.
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Is it MO = LCAO ?
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MOs are made from Linear Combinations of Atomic Orbitals.
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I confused...
:animatedfear:
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overlapping and superimposing are kinda the same thing, except the latter emphasises more on wave mechanics.
in a region where two orbitals of both + signs overlap, electron density is higher than the mere sum of the electron density of the two seperate orbitals. More electrons density is shared between two atoms. the attraction of both nuclei for these electrons is greater than the mutual repulsion of the nuclei, and a net attractive fore aka bonding intereaction occurs.
in regions where two orbitals of both - signs overlap, there is effectively no electrons between the nuclei, Hence, there is only repulsion force between the 2 nuclei, hence no bonding occurs.
in regions where two orbitals of opposite signs overlap, the electron density present between two nuclei might not be sufficient to produce an attractive force stronger than the repulsion force between the nuclei. The electron density between the two nuclei in this molecular orbital is less than the sum of individual electron density of each orbitals. Hence, no bonding occurs.