Questions tagged [molecular-orbital-theory]

Use of molecular orbital (MO) theory allows for an understanding of the observed properties (shape, reactivity) of molecules. The tag should be applied to all questions related to MO theory, from questions about the qualitative use of the theory to questions about the underlying quantum mechanics and applications in computational chemistry.

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What does Molecular Orbital Theory get wrong?

I am learning about MO theory in my advanced inorganic chemistry course and am starting to realize that it is truly the most accurate representation of how molecular orbitals look like, where they are ...
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mechanism in Birch reduction

at first I wondered why 2 H are added to opposite side of benzene like reaction below. because I could imagine 2 H added right next to each other or 1 C away from each other. so, looking for ...
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What is the criterion for filling up of electrons in molecular orbitals of increasing energy in simple diatomic species?

So far I have found different ways in different books. For e.g. One says "In MO electronic configuration of diatomic species up to 14 electrons, $\sigma2p_z$ orbital (considering $z$ as the ...
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Orbital and state correlation diagram for ring-closure of pentadienyl to a cyclic allyl

This is an exam question. I am to construct the orbital and state correlation diagram for one of the reaction below (disrotation or conratation). Also, I should explain whether the reaction is ...
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What does a molecules color have to do with its bond/orbital energies?

For example, elemental iodine is deep violet. Its sigma bond or perhaps the lone pairs are capable of absorbing all visible light frequencies except violet which is why we see it as that color. ...
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Why is tin(IV) iodide bright orange while elemental iodine is violet? [duplicate]

Does it have anything to with iodines polarizability? Is there a way to explain this using sigma bonding or an MO diagram?
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Confusion about atomic/molecular orbital terminology

I have come across several different orbital terms: atomic orbitals, natural orbitals, split-localized orbitals, molecule-intrinsic orbitals, quasi-atomic orbitals. I don't know what the difference ...
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Why is He2- unstable?

$\ce{He2-}$ according to MO theory should be possible with one electron in 2s sigma, but my professor said only at very low temperatures for some reason. Why only at low temperatures? What is the ...
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Do bonding and anti-bonding orbitals exist for ionic bonds?

Is there any concept of bonding and anti-bonding orbitals for ionic bonds, as there are in case of covalent bonds? OR is it possible to break an ionic bond by pushing electrons into the antibonding ...
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How are the hydrogens attached to the nitrogen atom in the ammonium ion?

Naively attaching hydrogens to a nitrogen atom's electron orbitals results in a picture like this: However, there are two problems with this drawing that I can see. First, I have heard that the ...
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Ordinary differential equation for molecule collision

I have no background in chemistry. But, I have a good background in mathematics and computer programming. I have planned to simulate the collision of two simple molecules $\ce{Br2}$ and $\ce{H2}$ ...
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Why HF is said to be formed from H-1s and F-2pz overlap?

According to Wikipedia, HF is formed by the interaction between hydrogen's $1\mathrm{s}$ orbital and fluoride’s $2\mathrm{p}_z$ orbital: In hydrogen fluoride HF overlap between the H 1s and F 2s ...
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Why does AsH5 not exist?

Why does $\ce{AsH5}$ not exist? Is it because the s-orbital of H doesn't and can't overlap with the d-orbital of As after As makes $\rm sp^{3}d$ hybridization?Theoretically the bond can occur because ...
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How to compare the energies of sigma and pi bonds in molecular orbitals of diatomic heteronuclear species?

There is a rule for diatomic homo-nuclear species that says that if $Z<8$ then $$E(\sigma_{2p})>[E(\pi_{2p_y})=E(\pi_{2p_z})]$$ and the inverse if $Z\geq8$ and in both case we have energy of $$E(...
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Why is conjugation not possible if there is not present alternate $\pi$ molecules according to MO theory?

I was reading this page at Chemwiki; there they were discussing conjugated-$\pi$ bond. In order to show that conjugation requires alternate double bonding, they cited the example of 1,3,6-...
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Jahn-Teller Distortions in Square Planar Complexes?

A Jahn-Teller distortion is predicted whenever a non-linear symmetric molecule has degenerate orbitals and has unequal electron occupation in those degenerate orbitals. Of course, this most often is ...
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Is it possible to have accumulation of electron density in the inter-nuclear region even if the overlap integral is zero?

Recap: Lately, I've been studying Hückel theory where I learned about the approximation of neglecting all overlaps and making overlap matrix an identity matrix that is $S_{AB}= 0\;_; \;S_{AA}= 1$ or $...
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How can off-diagonal Hamiltonian matrix elements be nonzero when the overlap matrix is diagonal?

While studying about Hückel theory, I got accustomed to the approximation of making the overlap matrix an identity matrix; that is making the off-diagonal elements zero as $S_{AB}= S_{BA}= 0\;;$ this ...
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What is the reason behind neglecting & putting the overlap integrals $0$ in Simple Hückel Theory?

This is an excerpt about Simple Hückel Theory from Elements of Physical Chemistry by Peter Atkins: The first step that Hückel took was to ignore the $\sigma$-bonding framework and focus solely on ...
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How to determine gerade & ungerade symmetry of a MO orbital?

J.D.Lee writes in his book Concise Inorganic Chemistry: [...] An alternative method for determining the symmetry of the molecular orbital is to rotate the orbital about the line joining the two ...
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'In MO theory, electrons are treated as spreading throughout the entire molecule'- isn't it exhibited in VB theory also?

First statement: In the molecular orbital theory, the valency electrons are considered to be associated with all the nuclei in the molecule. - Concise Inorganic Chemistry by J.D.Lee. Second ...
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Does light absorption result in a momentarily weaker bond?

I may have just misunderstood the topic, but light absorption by a molecule is a result of a promotion of an electron into an empty antibonding orbital; hence, does that mean that the strength of the ...
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Why does the PH3 geometry deviate more from the trigonal planar one, than does NH3?

$\ce{PH3}$ has a more bent structure than $\ce{NH3}$. The HOMO-LUMO gap for $\ce{PH3}$ is smaller than for $\ce{NH3}$, and so the distortion from the trigonal planar geometry is said to be larger. ...
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Successful predictions of MO software?

There have been many successful predictions of MO software. Is there a website or journal dedicated to recording them all?
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How derive g and u symmetry labels for orbitals?

When asked whether a molecule has an inversion center, we "invert" the coordinates of all atoms; i.e. move each atom from its position through the center of symmetry and to a new position equidistant ...
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Transition of electron from HOMO to LUMO?

Molecules usually have certain number of (doubly) occupied molecular orbitals and an infinite number of unoccupied molecular orbitals but the probability of electron makes the orbitals shapes so, how ...
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Why does the mixing of sigma 2s and 2p orbitals lower the energy of the sigma 2s orbital and raise the energy of the sigma 2p orbital?

My general chemistry textbook (General Chemistry Principles and Modern Applications, Tenth Edition) says that for bonding in diatomic molecules with Z ≤ 7, orbital mixing occurs between the $\sigma_{...
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How does one calculate chemical shift in a gauge independent manner?

The literature on ab initio calculations of chemical shift in NMR experiments usually provide Lamb's and Ramsey's formulae as the solution. Yet the expressions explicitly depend on vector potential ...
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Why is [PdCl4]2- square planar whereas [NiCl4]2- is tetrahedral?

The molecule $\ce{[PdCl4]^2-}$ is diamagnetic, which indicates a square planar geometry as all eight d electrons are paired in the lower-energy orbitals. However, $\ce{[NiCl4]^2-}$ is also $\mathrm{d^...
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MO shapes for dx2-y2 sigma bond

How do the Bonding and Anti-bonding MOs of dx2-y2 sigma bonding with another dx2-y2 look like ? I mean to ask that if the sigma bond is along the x-axis then is the shape of the lobes along y-axis ...
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In molecular orbital theory when two different atoms are combined, how do you tell which one is higher energy? [duplicate]

So in the picture above, see that there is an Atom A and Atom B and they combine. Notice that Atom A is higher in energy than Atom B (as indicated by how the 2s and 2p lines are drawn higher). Suppose ...
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Discrepancy in MO diagrams for Period 2 homonuclear diatomics

I've recently noticed that different sources on the internet and in books have different ideas of whcih atomic orbital influences which molecular orbital. In the diagram, where the lines connect the ...
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Point Groups and MO Theory

I am a little confused about how point groups give information about MO of polyatomic molecules. Is it true that all molecules of a given point group such as, $\mathrm{C_{2v}}$, will have the same MO ...
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How can we find the eV value of molecular orbital diagrams for a given atom?

It seems like sometimes the electron volt value of the molecular orbital is given but most of the time it just shows relative values of s p d orbitals. Is there some lookup table for such values?
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Hybrid orbitals forming molecular orbitals

My teacher showed me this diagram on how the hybrid orbitals of two atoms combine to form molecular orbitals. I was confused by this because I thought that VB and MO theories were two separate ...
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Closed shell molecules can have degenerate states?

I read a paper which said that $\ce{CO2}$ despite having orbital degeneracy is stable and not subject to the Jahn-Teller effect because it is a linear molecule. But $\ce{CO2}$ is a closed shell ...
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Orbital conservation in Molecular orbital theory

MOT is really confusing me. I was taught that the no. of orbitals remains conserved.That means, the number of atomic orbitals used is equal in number to the molecular orbitals formed. But how is it ...
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How is the Felkin-Anh model of stereoinduction correctly explained with MO theory?

One method to predict stereoinduction in asymmetric reactions is the Felkin-Anh model (derived from the more basic Cram model) that applies to nucleophile attacks on α-chiral aldehydes. In a ...
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What's the difference between HOMO-LUMO and valence-conduction bands?

From Wikipedia: Roughly, the HOMO level is to organic semiconductors what the valence band maximum is to inorganic semiconductors and quantum dots. The same analogy exists between the LUMO ...
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How to determine bond order for special cases?

In AP Chemistry, we learned two ways to determine bond order. The first method is: Bond order = (bonding electrons - antibonding electrons)/2. The second method is: number of bonds / number of bond ...
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How can the intense color of potassium permanganate be explained with molecular orbital theory?

In the permanganate ion, manganese is in the $+7$ oxidation state, therefore it is a $\ce{d^0}$ ion. $\ce{d^0}$ and $\ce{d^1^0}$ ions don't absorb visible spectrum radiation because there are no ...
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Formation of Bonding and Anti-bonding Molecular Orbitals

If two orbitals combine in-phase, a bonding molecular orbital is formed. When they combine out-of-phase, an anti-bonding molecular orbital is formed. For a single molecule, how are both orbitals ...
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Why do closo-boranes have a 2− charge?

In a question I was solving, it was mentioned that: [...] closo-boranes are compounds having the general formula $\ce{B_nH_n^2-}$ [...] an example is $\ce{B6H6^2-}$. Why do these boranes possess a ...
Chinmay Chandak's user avatar
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Physical Meaning of the Difference between MO and VB Wave Function of H2?

What would be the physical meaning of the difference between the MO wave function and VB wave function of $\ce{H2}$?
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Alternative Program for Orbital Population Analysis

Gaussian has Orbital-by-Orbital Population Analysis. From the manual: ...
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Destabilization of antibonding MO vs stabilization of bonding MO

My textbook writes that The net energy stabilization due to the occupation of a bonding MO is equal to the net energy destabilization due to the occupation of the corresponding antibonding MO. ...
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Creating Pi Molecular Orbitals from P Atomic Orbitals

As shown in the image, we have two free p orbitals in ethene but we need two p orbitals each for the bonding and anti-bonding orbital. That is a total of four. What is the concept behind this?
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Hybridization, MOT and Paramagnetism

In what way can hybridization or molecular orbital theory be used to explain paramagnetism? For instance, when something is hybridized to make enough bonding electrons, do all the electrons end up ...
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How can the gauche-effect be explained?

Generally, anti-conformations are more stable. But in the case of 2-fluoroethanol, the gauche confirmation is more stable. It's given in March's advanced organic chemistry book that intramolecular ...
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How is a molecular orbital a 'quantum superposition' of the atomic orbitals?

Let me quote from Feynman's lectures the concept of superposition: (1)The probability of an event in an ideal experiment is given by the square of the absolute value of a complex number $ϕ$ which ...
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