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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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66 votes
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Why do compounds like SF6 and SF4 exist but SH6 and SH4 don't?

Both $\ce{SF6}$ and $\ce{SH6}$ and $\ce{SF4}$ and $\ce{SH4}$ have the same central atom and the same hybridization, but my teacher specifically mentioned that $\ce{SH6}$ and $\ce{SH4}$ don't exist. I'...
sayantankhan's user avatar
42 votes
2 answers
25k views

How can the dipole moment of carbon monoxide be rationalised by molecular orbital theory?

Despite the fact that oxygen is much more electronegative than carbon, the bond in $\ce{CO}$ presents a weak dipole moment. This observation can easily be explained using the concept of "dative bond", ...
Isaiah G.'s user avatar
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41 votes
3 answers
33k views

Why is the bond angle H-P-H smaller than H-N-H?

$\ce{N}$ & $\ce{P}$ are in the same group. Both $\ce{NH3}$ and $\ce{PH3}$ have one lone pair and according to VSEPR theory, both the central atoms are predicted to be $\ce{sp^3}$ hybridized. But ...
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25 votes
1 answer
11k views

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 ...
Nilay Ghosh's user avatar
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81 votes
4 answers
42k views

Bonding in diatomic C2, a carbon-carbon quadruple bond?

Carbon is well known to form single, double, and triple $\ce{C-C}$ bonds in compounds. There is a recent report (2012) that carbon forms a quadruple bond in diatomic carbon, $\ce{C2}$. The excerpt ...
Janice DelMar's user avatar
23 votes
3 answers
11k views

Why is the inversion barrier larger in PH3 than it is in NH3?

The inversion barrier in $\ce{NH3}$ is approximately $5~\mathrm{kcal~mol^{-1}}$ and that of $\ce{PH3}$ is $35~\mathrm{kcal~mol^{-1}}$. This has well-known stereochemical consequences in that amines ...
wuschi's user avatar
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8 votes
3 answers
11k views

π Bonding in Carbon Dioxide

I am fairly sure the first diagram I drew for carbon dioxide is wrong in terms of showing π bonding. This is because we use a π orbital twice, which isn't possible. The second diagram corrects this by ...
Dissenter's user avatar
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56 votes
5 answers
10k views

How can antibonding orbitals be more antibonding than bonding orbitals are bonding?

In molecular orbital theory, the fact that a bonding and antibonding molecular orbital pair have different energies is accompanied by the fact that the energy by which the bonding is lowered is less ...
stochastic13's user avatar
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17 votes
2 answers
24k views

How to rationalise with MO theory that CO is a two-electron donor through carbon?

A question I am looking at is as follows: $\ce{CO}$ is isoelectronic with $\ce{N2}$. Sketch MO diagrams for $\ce{CO}$ and $\ce{N2}$. Point out key differences between the diagrams and use the ...
Adam's user avatar
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27 votes
2 answers
14k views

How to rationalise the resonance structures and hybridisation of the nitrogen in a conjugated amine?

I was given the first structure, and then drew the other 5 resonance structures: First of all, are they correct? ChemBioDraw had some complaints, but as far as I can see there's the same number of ...
user avatar
23 votes
2 answers
11k views

How can the Bonding in IF7 be explained using LCAO method?

I know that the VSEPR theory explains $\ce{IF_7}$ (iodine heptafluoride)'s structure as a pentagonal bi-pyramidal one. The valence bond theory can be used to say that it has $\mathrm{sp^3d^3}$ ...
scienceauror's user avatar
14 votes
1 answer
10k views

Delocalization of π-electrons in nitrate ion

Benzene and nitrate ion are given in my textbook as examples for the delocalization of π-electrons. Benzene, due to symmetry of its resonating structures, is simple enough. We assume that σ-electrons ...
Gerard's user avatar
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39 votes
1 answer
50k views

What is a non-classical carbocation?

What is a non-classical carbocation? How is it different from a classical carbocation? I am confused as I have come across this term many times on Chem.SE but there seems to be nothing for my level ...
Karan Singh's user avatar
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38 votes
1 answer
80k views

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^...
user avatar
37 votes
4 answers
23k views

What is actually the difference between valence bond theory and molecular orbital theory?

Recently I have read about both of the concepts in my book (Physical Chemistry by Atkins, Paula). It was literally a reading; though I could understand the language and superposition of orbitals, ...
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19 votes
1 answer
7k views

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 ...
Aditya Dev's user avatar
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13 votes
1 answer
3k views

How does spin flipping of triplet carbenes occur?

Below is presented a page from Clayden Organic Chemistry 2ed. How and why does the spin flipping during the collision with solvent molecules actually occur?
EJC's user avatar
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8 votes
3 answers
9k views

Does the shared electron in Covalent bonds revolve around nucleus?

We know that electrons are charges that revolve around the nucleus. Then, when in covalent bonds the electron is shared; does the electron obey the rule?
SPurushottama's user avatar
14 votes
1 answer
4k views

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. ...
Dissenter's user avatar
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34 votes
1 answer
10k views

When is it true that more nodes equals higher energy?

Consider all the MOs of some isolated molecule. (It could be a single atom too; I'll use MO to refer to AOs as well.) Number them in increasing order of the number of nodes (node = surface where the ...
Silvio Levy's user avatar
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26 votes
5 answers
18k views

Physical intuition behind negative values for wave function?

So a positive and a positive wave function create a bonding orbital where the probability of finding an electron is summed while a positive and a negative create an anti-bonding orbital with a lower ...
norlesh's user avatar
  • 543
25 votes
2 answers
5k views

Cyclobutadiene - Jahn–Teller effect or not?

In transition metal chemistry the Jahn–Teller effect arises when the configuration of the metal ion and d orbital splitting set up a doubly degenerate state, which is less stable than a state without ...
EJC's user avatar
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15 votes
2 answers
20k views

The effect of hyperconjugation on the stability of alkenes with MO theory

Hyperconjugation stabilizes carbocations and that makes sense because electrons are given to the empty p orbital. But how does it stabilize alkenes? Can molecular orbital theory be used to explain it?...
biogirl's user avatar
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14 votes
5 answers
13k views

Structure of NO2 compound?

What is the correct structure for the $\ce{NO2}$ compound (not ion)? I always thought it was like: since the negative charge would be residing the on the highly electronegative oxygen, but was ...
1110101001's user avatar
  • 3,276
11 votes
2 answers
2k views

How does localized bonding theory and hybridisation work?

So I understand molecular orbitals and how to do VSEPR models, but I seem to be struggling with understanding localized bonding theory and how to do hybridizations. After drawing the initial lewis ...
Lillie's user avatar
  • 121
9 votes
1 answer
8k views

Energetic Placement of Atomic Orbitals in the HCl Molecular Orbital Diagram

How are the 3p orbitals of chlorine lower in energy than the 1s orbital of hydrogen?
EJC's user avatar
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6 votes
1 answer
1k views

Physical reason for why delocalisation leads to stability?

Why does delocalization of electrons generally make compounds more stable (e.g. in carboxylate anions, where the lone pair on the negatively charged oxygen is delocalised into the C=O π* orbital)? Is ...
user avatar
33 votes
2 answers
16k views

Sp5 hybridization in cyclopropane?

I have never before heard/read about something as a $sp^5$ hybridization. Today, Henry Rzepa's blog post made me aware of the existance of such a bonding system. That made me search a little bit and I ...
pH13 - Yet another Philipp's user avatar
28 votes
4 answers
29k views

Carbon with 5 bonds?

I've heard that, even though according to Molecular Orbital Theory there is no chance of having nobel gases bonded to each other, it is not totally impossible. For example, under extreme conditions, ...
gunakkoc's user avatar
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27 votes
2 answers
136k views

Molecular orbital (MO) diagram for N2 and N2^-

I have been taught that the MO diagram is different for molecules with 14 or less electrons than the one used for molecules with 15 or more electrons. For $\ce{N2}$ the orbitals in increasing energy ...
user17987's user avatar
  • 271
19 votes
5 answers
37k views

What is the chemical structure / hybridization of sulfuric acid?

I was looking at the chemical structure of $\ce{H2SO4}$. Intuitively, I would have expected this molecule to be square planar in accordance with $p^2d^2$ or $sp^2d$ hybridization, but instead it is ...
bobthejoe's user avatar
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17 votes
1 answer
5k views

Why are antibonding molecular orbitals formed?

When two atomic orbitals make one molecular orbital there must be an antibonding orbital also. Why should they make also an antibonding molecular orbital, such as the $2\mathrm b_2$ orbital in the ...
ordinary chemistry student's user avatar
17 votes
2 answers
1k views

After a unitary transformation, is Koopmans' theorem still valid?

Koopmans' theorem says that the energy of the HOMO of the Hartree-Fock orbitals is equal to the first ionization energy of whatever system is being studied. This is only approximate because it assumes ...
jheindel's user avatar
  • 13.1k
13 votes
1 answer
6k views

The "rules" for LCAOs in Molecular Orbital Theory

In our course on physical chemistry, which involves MOT, we have been taught that in the LCAO approach, the wave function for a molecule … say hydrogen ion ($\ce{H2+}$), can be approximated by a ...
transistor's user avatar
13 votes
3 answers
2k views

How do atomic orbitals interfere both constructively AND destructively to give two molecular orbitals?

I am an organic chemistry student learning how atomic orbitals interfere to give rise to molecular obitals. The image below suggests that each of the hydrogens' atomic orbitals interfere both ...
silverbackgorilla's user avatar
12 votes
3 answers
4k views

LCAO (Linear Combination of Atomic Orbitals) and Phases

So when combining atomic orbitals to form molecular orbitals, you can either add the wave functions or subtract them. But at the same time, orbitals can exist in opposite phases (say one lobe of the p ...
Jake Ormseth's user avatar
30 votes
3 answers
6k views

Are the bonding orbitals in methane equivalent - photoelectron spectrum

The low energy portion (the part dealing with the $\ce{2s}$ and $\ce{2p}$ electrons) of the photoelectron (PE) spectrum of methane is reproduced below. (image source) The reaction being examined is ...
ron's user avatar
  • 85k
25 votes
3 answers
50k views

What is the origin of the differences between the MO schemes of O₂ and N₂?

Here are the MO schemes of $\ce{N2}$ (left) and $\ce{O2}$ (right). Why is the $\sigma$-MO formed by the $p$ AOs energetically above the $\pi$-MO for $\ce{N2}$ but not for $\ce{O2}$? Can it be ...
EJC's user avatar
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20 votes
2 answers
4k views

What does it mean when it is said that an sp³ orbital has 25% s character?

My textbook frequently mentions: $\mathrm{sp^3}$ hybrid orbital has 25% $\mathrm{s}$-character and 75% $\mathrm{p}$-character. What are these "characters"? And how do these characters influence ...
user avatar
18 votes
1 answer
698 views

Hyperlithiated Carbon Species [closed]

Below is replicated Question 1 from the Final Qualifying Exam of the Australian Chemistry Olympiad, 2004B, here. Question: The theory of promotion-hybridization is quite successful at explaining why ...
Chronocanth's user avatar
17 votes
2 answers
1k views

What utility does the tau bond model of orbital overlap have?

In his book on molecular orbital theory, Molecular Orbitals and Organic Chemical Reactions, Ian Fleming notes that Pauling formulated an early alternative model to Hückel theory for explaining the ...
Greg E.'s user avatar
  • 11.9k
14 votes
2 answers
3k views

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 ...
Jan's user avatar
  • 68.2k
13 votes
1 answer
2k views

Why is the benzyne triple bond distorted?

In many places I've seen the "extra" bond in benzyne being labelled as $\ce{sp^2-sp^2}$ overlap or distorted (not parallel) $\ce{p\pi-p\pi}$ overlap. But I've failed to see why we can't have a normal, ...
ManishEarth's user avatar
  • 15.3k
13 votes
2 answers
11k views

How to know whether s-p mixing will happen in heteronuclear molecules?

How do I know whether s-p mixing would occur in some diatomic heteronuclear molecules like CN, CO, NO etc?
EJC's user avatar
  • 14.4k
11 votes
2 answers
4k views

Pi molecular orbitals of polyenes

As an organic chemist, I'm comfortable deriving the pi molecular orbitals of linearly conjugate systems to give the following result: In a qualitative sense, these molecular orbitals are easily ...
NotEvans.'s user avatar
  • 17.2k
9 votes
2 answers
2k views

Why is the resonance concept not required in molecular orbital theory?

In valence bond theory, resonance plays a pivoting role; why isn't such concept needed in MO theory? Why is it told that "MO theory provides a global, delocalized perspective on chemical bonding"? (...
user avatar
7 votes
1 answer
428 views

Geometries of methyl and silyl radicals

The $\ce{.CH3}$ radical is planar but the $\ce{.SiH3}$ radical is a trigonal pyramid. Why are they different? After all, $\ce{C}$ and $\ce{Si}$ are in the same group.
Peter's user avatar
  • 71
7 votes
2 answers
23k views

Why does AlCl3 dimerise to Al2Cl6 at low temperatures?

I know that it has something to do with HOMOs and LUMOs (I can't what the HOMO/LUMO for AlCl3 is, though).
Mick Jenkins's user avatar
7 votes
1 answer
2k views

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
5 votes
1 answer
2k views

Why is oxygen more stable than oxygen dication?

According to molecular orbital theory (MOT), $\ce{O2^2+}$ has a greater bond order than $\ce{O2}$ and two less antibonding electrons. So why is molecular oxygen $\ce{O2}$ more stable than the ...
Vulgar Mechanick's user avatar