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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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81 votes
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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
66 votes
9 answers
41k views

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
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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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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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^...
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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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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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34 votes
3 answers
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Why is the bond length of CO+ less than that of CO?

According to molecular orbital theory, the bond order of $\ce{CO}$ is 3. When $\ce{CO+}$ is formed, the bond order decreases to 2.5, and thus the bond length should increase. However, the bond length ...
Papul's user avatar
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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
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
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30 votes
1 answer
630 views

Imaginary Bonding Interactions

Usually in chemistry, we deal with bonding interactions. That is, if I have the diatomic A-A molecule or A-B molecule, there's a favorable interaction (i.e., a bond) and a prototypical MO diagram like ...
Geoff Hutchison's user avatar
29 votes
1 answer
105k views

What are t2g and eg in CFT?

In the crystal field theory (CFT), when the splitting of the d-orbital occurs, it gets divided into two parts. The upper part with higher energy is the $\mathrm{e_g}$ and the lower part with lower ...
NeilRoy's user avatar
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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
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
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
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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
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26 votes
1 answer
5k views

Canonical MOs vs. Localized MOs: Do both represent reality in the same way?

In my understanding localized molecular orbitals (LMOs) are equivalent to "standard" molecular orbitals, often called canonical orbitals (CMOs—by the way, why are they called canonical?). We can ...
DSVA's user avatar
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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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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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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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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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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
22 votes
3 answers
20k views

When are two orbitals orthogonal?

When are two orbitals considered to be orthogonal? It will be helpful if you can provide an example molecule (or molecular orbitals in a molecule) since I can't really think of a scenario where ...
CPak's user avatar
  • 451
22 votes
2 answers
2k views

Why do n AOs only form n MOs? [duplicate]

The textbooks I have read introduce LCAO by considering the H2 molecule. In this example, there are only two possible combinations of the two 1s orbitals (in phase and out of phase). When there are ...
Adroit's user avatar
  • 780
22 votes
1 answer
49k views

Why is the bond order of CO+ 3.5?

According to my books the bond order of $\ce{CO+}$ is $3.5$. But shouldn't it be $2.5$? On googling this, I found the following answer that is on Stack Exchange but its only talks about the bond ...
Chloritone_360's user avatar
22 votes
1 answer
578 views

Is the Springborg 6D phase space model used in modern molecular orbital modeling?

In various papers written in the 1980s, Michael Springborg explored an interpretation of the Wigner phase space function as an electron density in a six-dimensional $(q,p)$ phase space. He applied it ...
Terry Bollinger's user avatar
22 votes
2 answers
6k views

Why is the endo product the major product in a Diels-Alder reaction?

I understand that the endo product is, at least in all cases in my textbook, the major product in a Diels-Alder reaction. However, I don't understand why this is the case. The explanation in my ...
user1's user avatar
  • 571
21 votes
2 answers
6k views

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 ...
Geoff Hutchison's user avatar
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
20 votes
1 answer
314 views

Similarities and Differences between Resonance and MCSCF treatments

Recently, there has been a question by Voldemort concerning different resonance structures of $\ce{NCO-}$, requesting an explanation why one resonance structure would be more preferred than another. ...
pH13 - Yet another Philipp's user avatar
19 votes
4 answers
1k views

Does a neutral dimercury molecule exist?

Is a neutral $\ce{Hg2}$ molecule possible, as a gas under extremely low (partial) pressures? What is the enthalpy of formation or similar? It is my impression that usual mercury vapors are monatomic? ...
Jeppe Stig Nielsen's user avatar
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
  • 2,420
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
  • 7,816
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
18 votes
1 answer
2k views

Orbital correlation diagram for the cleavage of cyclopropylmethylene carbene

In the last exam we were asked to rationalize the mechanism of the cyclopropylmethylene carbene cleavage into acetylene and ethylene. In the lecture we discussed pericyclic reactions (cycloadditions, ...
laminin's user avatar
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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
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17 votes
1 answer
29k views

Molecular orbital diagram for BF3

I'm trying to build a molecular orbital diagram for BF3 and I'm running into problems with irreducible representations on the F side. 2s for B has an irreducible representation of A1. 2p for B has ...
Billqaz3's user avatar
  • 525
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
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
  • 171
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
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17 votes
2 answers
94k views

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 ...
user avatar
16 votes
4 answers
1k views

How accurate is this polar mechanism for the Diels-Alder reaction?

So the prof got to Diels-Alder without discussing it in the context of molecular orbital theory. Instead we got a resonance picture of what happens... this is what he told us: 1) 1,3-butadiene has ...
Dissenter's user avatar
  • 19k
16 votes
4 answers
542 views

Hierarchy of electronic wavefunctions

The previous question contained too much unnecessary information and was edited. I am wondering about the "hierarchy" of wavefunctions. If one can combine atomic orbitals (AO) into molecular orbitals ...
CHM's user avatar
  • 2,638
16 votes
2 answers
515 views

Do MO's span a bond or the whole molecule?

I'm reading Organic Chemistry by David Klein (2nd Ed), and when discussing the causes of torsional strain, he writes this Based on recent quantum mechanical calculations, it is now believed that the ...
lightweaver's user avatar
  • 1,529
16 votes
1 answer
1k views

How is spiro[4.4]nonatetraene antiaromatic

How is spiro[4.4]nonatetraene antiaromatic? I know that spiro compounds exhibit homoaromaticity, but how do they exhibit antiaromaticity?
EJC's user avatar
  • 14.4k
16 votes
1 answer
723 views

Why is an energy cost associated with splitting the HOMO spins in singlet oxygen?

The molecular orbital schemes for both forms of singlet oxygen ($\mathrm{^1\Delta_g}$ and $\mathrm{^1\Sigma_g^+}$) and triplet oxygen ($\mathrm{^3\Sigma_g^-}$) are typically given as shown in the ...
Jan's user avatar
  • 68.2k
16 votes
1 answer
508 views

Classifying some molecular orbitals after Hartree-Fock calculation

I have done a Hartree-Fock (HF) calculation on a dioxetane molecule with one methyl group on one of the C-atoms (Fig 1), with minimal basis (using Molcas). I want to identify the active space that ...
Yoda's user avatar
  • 4,733
16 votes
1 answer
3k views

Is azide paramagnetic?

Is the azide ion ($\ce{N3-}$) paramagnetic? How do I use molecular orbital theory to show that it is or isn't? I drew a sine wave diagram and filled up the $1\pi$ and $2\pi$ MOs, and since there ...
kane9530's user avatar
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