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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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9
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1answer
825 views

How does conjugation affect reactivity?

From this topic on MOs of butadiene vs ethene, it is clear that when two ethene molecules are combined to extend the conjugate chain, the HOMO is raised in energy and the LUMO is lowered in energy. ...
6
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2answers
1k views

Hyperconjugation in ethane conformers

I refer you to this set of presentation slides. On slide 8, the author explains the preference for the staggered conformation of ethane by saying that there is a favourable hyperconjugative ...
2
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1answer
283 views

ligand pi system irreducible representation for trigonal bipyramidal transition metal complex

I am considering a D$_{3h}$ trigonal bipyramidal transition metal complex MX$_5$. I have found that the irreducible representation for the ligand s and $\sigma$ systems is 2A$_1'$+A$_2''$+E$'$. I ...
7
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2answers
1k views

Does computational chemistry get the molecular orbitals of dioxygen wrong?

The molecular orbitals of $\ce{O2}$ are typically shown as follows, with every orbital filled by a spin alpha (or up) and spin beta (or down) electron (except the homo levels which are singly occupied)...
-1
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1answer
472 views

How to find the bond order in delocalized pi bonding of more than two atoms? [duplicate]

J.D Lee has written down some rules for calculating the bond order. I have no difficulty following step 1 and 2. It's easier to work with an example so I'll highlight my issues with step 3 , 4 ...
2
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0answers
158 views

Total irreducible representation of a quantum system - Slater determinant?

I'm beginning to study quantum chemistry and I became pretty confused about the total state of the molecule. When we describe the single orbitals using irreducible representations like in the picture ...
2
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2answers
520 views

Thermodynamic stability of benzene derivatives

Q1. How can we comment on the thermodynamic stability of the following benzene derivatives? Q2. What does thermodynamic stability mean, in general? What does it mean to compare the thermodynamic ...
0
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2answers
393 views

Resemblance of molecular orbital formed by mixing of s and p orbitals to hybridised sp orbital

See the following two images. I think that the two orbitals should not resemble each other because s and p orbitals overlap in different fashions in the two cases. In the case of sp hybrid orbital, ...
13
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2answers
2k views

Molecular orbital diagram and irreducible representations for dinitrogen

I'm trying to understand how to draw molecular orbital diagrams, since I'd like to use the molecular orbitals to determine the total state of the molecule (i.e. $A_1$, $B_1$ etc.), so I prefer the ...
1
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0answers
134 views

Why must the ground state of hydrogen molecule be gerade?

As we know, the electronic ground state of hydrogen molecule is $\Sigma_g^+$. My question is, how do we know it from the theoretical perspective? The nuclear potential in the hamiltonian of hydrogen ...
2
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0answers
59 views

Atomic and molecular orbital

I have read in my book that ,energy of atomic orbital is greater than that of molecular orbital . I know the fact that lower energy corresponds to higher stability . But I can't figure out the ...
3
votes
1answer
834 views

Why do halogens have odd numbers as oxidation number?

Halogens like $\ce{Cl}$ always exhibit $+7,+5,+3,+1,-1$ as their oxidation numbers. I found the following answer after checking several sources: Halogen atoms have $7$ electrons in their valence ...
0
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1answer
95 views

Is a ring intermediate formed in hydroboration?

This was the mechanism taught to us for hydro-boration Something really strange in that mechanism is that in step 2 hydrogen has two bonds. Also , the the doubly bonded $\ce{H}$ , has one bond $\ce{...
1
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0answers
2k views

Why does [Co(NO2)6]4- ion have 3 unpaired electron as opposed to 1?

Why does $\ce{[Co(NO2)6]4-}$ ion have 3 unpaired electron as opposed to 1? The book says that it has 3 unpaired electrons. I thought $\ce{NO2-}$ was a strong field ligand, making $\ce{Co^2+}$ a $\ce{...
0
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0answers
370 views

Sigma and pi coordination in complexes (symmetry)

According to the Miessler Tarr and many molecular orbital diagrams for octahedral complexes on the internet will σ-donors/acceptors only influence the $e_g$-orbital, while π-donors/acceptors influence ...
1
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0answers
119 views

Order of filling of molecular orbitals in heteronuclear diatomic molecules such as CO [duplicate]

Currently studying for possibly my last Chemistry exam in my life, can't understand why physics undergrads have to learn chemistry as high level as this, but oh well. So I'm currently dealing with ...
10
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2answers
463 views

Comparing the Hückel and extended Hückel methods

I'm very confused about the differences between these methods. From my textbook, it states that the Hückel method only takes into account the π bonding interactions, while the extended Hückel method ...
3
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1answer
486 views

Can a molecule be neither gerade nor ungerade?

Or can any molecular orbit always be written a linear combination of gerade and ungerade basis states?
2
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1answer
5k views

Bond order of H2O and NH3 using group theory to construct MOs

The bond order of H2O is 2, and that of NH3 is 3, which makes sense, when considering the number of bonds they have. However, if using basic group theory, you get the MO diagrams as presented in ...
1
vote
1answer
319 views

Which orbital lobes are occupied?

In the p-orbitals, there are 3 dumbbell-shaped lobes, each of which can contain 2 electrons. In the diagrams of the d- and f-orbitals, there are two or more lobes (or orbital shapes) or each value of ...
0
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1answer
156 views

Structure of PH4Br [closed]

What is the structure of PH4Br? I wanna know which atom is bonded to which. Does all five atoms are directly bonded to phosphorus?
8
votes
1answer
448 views

Why do bonding and anti-bonding states split in energy (mathematically)?

I am a physicist, and when I hear about the splitting of energy levels for MO antibonding and bonding states, I have a rough intuition that tells me this is reasonable to expect, but I can't think of (...
9
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1answer
138 views

Justifying the tremendous -I effect of -[NMe₃]⁺ by molecular orbital theory

The $\ce{-[NMe3]+}$ group is ranked almost at the first position in the -I effect series, which is often given at the very beginning of the teaching of general organic chemistry. I know this can be ...
-1
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1answer
90 views

1s orbitals and 2nd period diatomic elements

a general chemistry textbook I am reading says this: "In second-period diatomic molecules, the 1s orbitals of the two atoms barely overlap. Because the $\sigma _{g1s}$ bonding and $\sigma _{u1s}$* ...
10
votes
2answers
112 views

Is there a single MO that represents the π system of a Möbius aromatic system? If so, what does it look like?

I recently answered the question of whether twisted polyene chains were known by pointing towards Möbius aromatic system. While in conventional Hückel aromatic systems the lowest-lying π-orbital ...
2
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0answers
163 views

What is the valence molecular orbital of the ions of a fluorine molecule?

I have the following assignment: Draw the molecular orbital diagram of $\ce{F2}$, $\ce{F2+}$ and $\ce{F2-}$. Calculate the bond order and say, if there is a bond, which compound has the strongest ...
2
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0answers
30 views

Trend in the infrared stretching of molecular hydrogen to group 8 transitions metals?

While going through old notes, in relation to the Dewar-Chatt-Duncanson model, it states that the stretching frequency for $\ce{N2}$ to the group 8 transition metals ($\nu(\ce{NN})$) proceeds as: \...
1
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0answers
123 views

Molecular Orbitals. Relative phase vs Bonding Anti-Bonding

Most intro explanations of molecular orbital formation go something like this: as two 1s orbitals of H atoms approach each other, their wave functions begin to overlap, and due to constructive and ...
1
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0answers
1k views

How does the dx2-y2 orbital bond with four ligands in square planar metal-metal complexes

I understand that the complex [Re2Cl8]2- has a bond order of 4 between the ruthenium ions. The bond comprises of sigma interactions between Dz2 orbitals, two sets of pi interactions between dxz and d ...
8
votes
1answer
484 views

The shapes of molecules

According to this question/answer, isolated atoms are spherical – and don't have, for example, p-orbital lobes sticking out. The reason is that each complete set of p-, d- and f-orbitals taken ...
8
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0answers
243 views

Bond orders in the 3c-4e bond and 3c-2e bond

According to D. P. M. Mingos, in his book Essential Trends in Inorganic Chemistry, the bond order of the three-centre four-electron bond is approximately 0.717, which is one divided by the square root ...
4
votes
1answer
359 views

Does Bent's Rule only apply to molecules where there is hybridisation?

I would just like to ask if the pre-requisite for using Bent's Rule is that the bonding in the molecule involves hybridised orbitals. I was thinking of why the bond angle in hydrogen sulfide (~ 92 ...
6
votes
1answer
793 views

What is the number of nodes in the molecular orbitals of cyclic H3?

A quick google search does reveal the molecular orbitals of this cyclic H3. However, one source mentions there are are two nodes; while two of my professors said there's one node. Which is correct? ...
10
votes
2answers
509 views

Can orbitals in a molecule be occupied via excitation like they can in atomic orbitals?

I am facing a conundrum in the way I am viewing molecule orbitals due to the existence of excited states. What I wanted to know is, since atomic orbitals can exist in excited states (i.e. you get ...
3
votes
1answer
291 views

Phases of Atomic or Molecular Orbitals

What does phase mean in orbitals? I know that phases are separated by nodes. They are in some way related to wavefunctions, I can't understand, how? How can wavefunctions be negative, since they are ...
2
votes
0answers
148 views

Field Keyword and Molecular Orbitals--Gaussian 09

I am using Gaussian 09 to run calculations. I am applying a 0.1 eV field in the positive Z direction to a molecule. I am also using the %chk keyword so that I may analyze the Molecular Orbitals after ...
5
votes
1answer
744 views

Explaining why CN- is a soft base

So, I am having difficulty with the following explanation/question. (Based on your MO diagram and pi bonding theory) explain why $\ce{CN-}$ is a soft base and prefers to bind to soft acids. Now, I ...
1
vote
1answer
69 views

Isn't the dual behaviour of subatomic particles contradictory?

I recently came across Molecular Orbital Theory. In this, if two electrons are bonding, their waves merge and they form a proper bond. But this is directly opposite to the particle theory, according ...
5
votes
0answers
348 views

Platt's Notation/Nomenclature for transitions

How do you read Platt's Notation / Nomenclature? Where can I learn this? Some examples where I've seen this mentioned: In Platt's notation the transitions are designated as $\ce{A -> B}$ and $\...
6
votes
2answers
401 views

HOMO Odd, LUMO Even: Why does the affected orbital change depending on the carbon? How are carbons numbered?

While looking at how substituents affect the absorption spectra of molecules, I came across this: Substituents alter the emitted wavelengths of these molecules by affecting the electron densities. ...
2
votes
0answers
114 views

How to obtain the distance of maximum probability of finding an electron on x axis

I have the radial component of the wavefunction $R_{nl}(r)=r^2e^{-2r/3a_{0}}$ so to obtain the maximum distance on x axis I do not know if I have to compute the second derivative of $R_{nl}^2(r)$ or $...
1
vote
0answers
256 views

two questions about character tables

I have learnt about group theory before I learnt MO theory, so I have a mathematical understanding of those orthogonality theorems and character tables. I will use the character table of $D_{4h}$ to ...
15
votes
1answer
426 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 ...
8
votes
3answers
2k views

HCH bond angle in cyclopropane and cyclobutane

I understand why the $\ce{CCC}$ bond angles in cyclobutane and cyclopropane are less than ideal $109.5^\circ$ due to the geometric restrictions they encounter in a cyclic structure. However, it is not ...
6
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1answer
1k views

The electron configuration and electron density distribution in singlet oxygen

The molecular orbital schemes for singlet ($\mathrm{^1\Delta_g}$) and triplet oxygen ($\mathrm{^3\Sigma_g^-}$) are typically given as shown in the image below. Figure 1: Molecular orbital schemes of ...
8
votes
2answers
627 views

How is molecular orbital theory used in drug research?

I was reading up on MO theory and came across a particular section: Application: Computational Chemistry In Drug Design While the descriptions of bonding described in this chapter involve many ...
5
votes
1answer
79 views

Predicting the generation of singlet oxygen

On Wikipedia, I saw the reaction $\ce{H2O2 + NaOCl -> O2 + NaCl + H2O}$, where the $\ce{O2}$ is in the $^1\Delta_g$ (singlet) state. Can anyone offer an explanation for why it isn't in the usual ...
5
votes
2answers
831 views

Platinum Orbitals in Cisplatin Forming Bonds with LCAOs of Chlorine

My inorganic chemistry professor placed two questions on the exam review that he did not cover in class; I haven't been able to figure out how to answer them and could really use some help in learning ...
22
votes
2answers
1k 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 ...
5
votes
1answer
319 views

How does the state of iodine affect its HOMO–LUMO gap?

My teacher told me that the HOMO–LUMO gap in solid iodine is smaller than that in gaseous iodine. When I asked about the reason, he told me that it is due to higher interactions in solid iodine as ...