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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4
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0answers
41 views

Rhombohedral second-order Jahn–Teller effect in Au(II) complex?

I was recently listening to a talk about the following work[1] when the author mentioned that they explained the following. In this $\ce{Au(II)}$-porphyrin complex you observe two sets of $\ce{Au-N}$ ...
6
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1answer
286 views

Why do electrons in sigma bonds have zero orbital angular momentum about the internuclear axis?

As per the title, why do electrons in σ-bonds have zero orbital angular momentum about the internuclear axis? As far as I know, the outermost electrons participate in bonding. Suppose that the 3p ...
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127 views

Rotational barrier of biphenyl in ground and excited electronic states

I'm studying how the energy of biphenyl varies with its dihedral angle. I know how to describe the dependence quantitatively, but still, I don't know how to explain why the rotational barrier of ...
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174 views

Oscillator strength of a transition

I am trying to compute the oscillator strength in a molecule. Therefore I've got its Energy level values plus both wave functions $\Psi_i$ and $\Psi_j$ for the transition I want to calculate the ...
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121 views

Hückel method for biphenyl

I am trying to calculate the energy levels of biphenyl (see picture below) using Hückel method. Since this is my first time using Hückel I am a little confused. Biphenyl has 12 carbon atoms, ...
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61 views

Identifying principal quantum numbers in natural population analysis

Following a Hartree-Fock calculation, I would like my program to perform some kind of Natural Bond Orbital (NBO) analysis. Preceding this I would like to output a list of atomic orbitals, along with ...
2
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1answer
3k views

Why is CO ligand higher than CN- in the spectrochemical series?

I've drawn MO diagrams for both molecules and they are isoelectronic. The main difference is that $\ce{C}$ is closer in electronegativity to $\ce{N}$ than it is to $\ce{O}$ so there will be less ...
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732 views

Why is antimony pentafluoride a stronger Lewis acid than phosphorus pentafluoride or arsenic pentafluoride?

How can these relative reactivities as a Lewis acids be rationalized: $$\ce{SbF5 > AsF5 > PF5}$$ One simple argument could be the size of the $\ce{Sb}$-center: Because it is bigger than for ...
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1answer
231 views

Would an (NH3)2+ molecule be trigonal planar like BH3 rather than trigonal pyramidal?

I've been learning about using MO theory to explain why $\ce{BH3}$ and $\ce{NH3}$ have different geometries and by following the line of reasoning used to rationalise the differences in geometry I ...
2
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1answer
189 views

How to know about radical character of a diradical molecule?

I understand that the radical character is quantified by the occupation number of the lowest unoccupied natural orbital obtained from UHF calculations or from CASSCF calculations. My issue is that I ...
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105 views

Which natural orbitals are better for CASSCF calculations for organic diradicals?

I am trying to perform CASSCF calculations for a series of diradicals. As input orbitals I have used two types of natural orbitals. One is unrestricted natural orbitals and the other MP2-based natural ...
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How to calculate a molecule's oscillator strength?

I was wondering how to calculate a molecule's (e.g. biphenyl) oscillator strength using a very simplified model. If I want to calculate a molecule's excited states a good start is something like a ...
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1answer
133 views

Why are there as many molecular orbitals as atomic orbitals? [duplicate]

So molecular orbitals are represented as a weighted sum over atomic orbitals. Of course I reckon that the total sum of the weights per atomic orbital has to equal 1, when summed over all molecular ...
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3answers
230 views

De-excitation of a molecule

In our chemistry lecture today on UV/vis-spectroscopy, we discussed the origin of colour in conjugated molecules due to electronic transitions from lower-energy molecular orbitals to higher-energy ...
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1answer
2k views

Ionization energy of C2-, C2, and C2+

My textbook says that $\ce{C_2}^-$ has a lower ionization energy than both $\ce{C_2}$ and $\ce{C_2}^+$. I calculated that the bond orders of $\ce{C_2}^+$, $\ce{C_2}$, and $\ce{C_2}^-$ are 1.5, 2, and ...
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2answers
306 views

Why is dipositive dilithium more stable than neutral dilithium? [closed]

According to J.D Lee, compounds with fraction bond number are unstable. I calculated that the bond order of $\ce{Li2^2+}$ is 0.5 while that of $\ce{Li2}$ is 1. Hence, $\ce{Li2^2+}$ must be less stable ...
6
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1answer
276 views

Relationship between Hartree-Fock and Hückel coefficients

I am new in computational chemistry. I'm studying the Hartree–Fock–Roothaan (HFR) method and have some of questions about the HFR basis function coefficients. Do we have the condition on HFR ...
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60 views

Bond dissociation energy of the trihydrogen cation

Recently, in one of the questions in an assignment on the topic of energetics, we were asked to explain why the bond dissociation energy for the trihydrogen cation is much larger than what we would ...
3
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1answer
332 views

Why is Frost's Circle an extension of Huckel's 4n+2 rule? What physics leads to the emergent property that the circle can predict relative MOs?

The Frost Circle is a useful mnemonic to determine the relative energies for the molecular orbitals of an aromatic ring. One vertex of the polygon of the cycle is drawn pointing downwards and the ...
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1answer
188 views

Is there a simple way to predict the molecular pi orbitals in conjugated pi systems?

Particularly for longer alkyl chains. Or is predicting these molecular orbitals from quantum mechanics too complex to be able to be translated easily as a geometric pattern? If Frost Circles exist, I ...
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3answers
2k views

Is it possible to have negative bond orders?

In the MO theory, is it possible to have a molecule with bond order less than 0? The least bond order I have come across as of now is 0 (for di-atomic noble gases), indicating that the molecule cannot ...
6
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1answer
361 views

Calculating the density matrix from a WFN file

I want to calculate the density for the $\ce{HCN}$ molecule. This molecule has 14 electrons and thus 7 occupied molecular orbitals. I have a .wfn file which ...
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1answer
941 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. ...
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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 ...
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1answer
309 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 ...
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2answers
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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)...
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1answer
536 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 ...
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169 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
614 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 ...
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2answers
509 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, ...
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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 ...
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142 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 ...
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60 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 ...
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1answer
1k 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 ...
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1answer
99 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{...
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1answer
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{...
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401 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 ...
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129 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 ...
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2answers
540 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
511 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?
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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 ...
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1answer
369 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 ...
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1answer
171 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?
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1answer
483 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 (...
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1answer
145 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 ...
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1answer
96 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 $\sigma _{u1s}$* ...
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2answers
121 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 ...
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0answers
214 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 ...
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0answers
31 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: \...
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125 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 ...

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