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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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2
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1answer
143 views

Hyperconjugation and the stability of alkenes

Clearly, this issue has been touched on here and here. However, I would like to raise a new point on the issue. I have always thought that stabilisation of the $\ce {C=C}$ $\pi$ bond via ...
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0answers
43 views

Can an interaction be considered both hard-hard and soft-soft?

The $\ce {B-F}$ bond can be described as having both a large ionic and a large covalent character$\ce {^1}$, with the AIM charges on $\ce {F}$ and $\ce {B}$ being $\ce {-0.81}$ and $\ce {+2.43}$, ...
9
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1answer
415 views

Explanation for bond lengths in trans-hexatriene

Hexatriene is an unsaturated hydrocarbon with six carbon atoms and five carbon-carbon bonds, three of which are double bonds. However, the bond lengths of the $\ce{C=C}$ bonds are not the same. The ...
2
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1answer
253 views

Molecular orbital explanation of why the protonation of the oxygen atom makes a carbonyl group more electrophilic

I was working on nucleophilic addition reactions to the carbonyl group (namely, hemiacetal formation) when I came across the following explanation for the use of acid catalyst and how it increases ...
3
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2answers
244 views

When is donation into an anti-bonding MO stabilising?

This has been puzzling me for quite a while and I'm not sure if it could be a misconception on my part. When is interaction of a filled orbital with a antibonding MO stabilising and when is the ...
2
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0answers
62 views

Is the concept of frontier orbital interactions chiefly a kinetic concept?

Frontier molecular orbital theory has proved to be an immensely useful concept. The concept of HOMO-LUMO interactions are frequently employed in detailing the mechanistic descriptions of reactions in ...
-2
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1answer
88 views

Is 1-(1-Methyl cyclopropyl) carbocation [1] as stable as Cyclopropyl Methyl Carbocation[2]

Does the extra Methyl group interfere with the exceptional stability of Cyclopropyl Methyl Carbocation?
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0answers
57 views

How is occupation number of lowest unoccupied natural orbital defined?

I have run some calculations to analyse the occupation of natural orbitals. Since it is needed to run in triplet state, I get two orbitals with occupation number 1. Now, which orbital occupation ...
2
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0answers
37 views

Calculate molecular orbitals of high-lying or Rydberg states in GAMESS (US)

I am trying to calculate the high-lying or Rydberg orbitals (i.e. unoccupied orbitals with their energies just below the ionization energy) of a nitrogen molecule. I use GAMESS(US) with the following ...
2
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0answers
241 views

Could Be2- exist?

I understand that Be2 cannot exist, since it has as many electrons in the antibonding as in the bonding orbitals. But it seems to me that since the next electron would go into the πu orbital, which ...
2
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1answer
50 views

Calculate the transition dipole moments between the 1s inner shell and the high-lying states of a molecule

I am trying to calculate the transition dipole moments between the 1s inner shell and the high-lying states of N2, namely, $<\psi_{1s}|{\bf r}|\psi_{high-lying}>$. For this, I installed ...
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0answers
38 views

Can LCAO be seen as superposition?

In wave physics, when waves interact, the principle of superposition applies. It states that the displacement of the resultant wave is made up of the sum of the individual displacements of the ...
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0answers
212 views

How to understand MO diagrams for cationic octahedral transition metal complexes

I am having problems understanding the construction of MO diagrams in octahedral transition metal complexes within ligand field theory (LFT) when the metal center is asumed as an cation: I am ...
5
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1answer
57 views

What determines the coefficients in the wavefunction of a hybrid orbital?

I was studying the hybridization of carbon atoms and I came to figure out that the $sp$ hybridization wavefunction is given as $$|sp_{+}\rangle =\frac{1}{\sqrt{2}}\left(\psi_{2s}+\psi_{2p_x}\right)$$ ...
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1answer
534 views

Which has more enol content? [on hold]

According to me, resonance in {a} is more effective than {b}. And for monocyclic ketones, enol has very less content. I say {1} has lesser enol than {2}. Which is right?
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83 views

Is the 9H-fluoren-9-ylium cation aromatic?

In this cation, there are two individual benzene rings which are aromatic. But in the entire π system, there are 12 π-electrons which should make it antiromatic. Which is correct?
5
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1answer
318 views

Why does benzene have 3 π–π* transitions in the UV range, but ethene, butadiene, and hexatriene each have only 1?

Are the π and π* orbitals of benzene much closer together in energy which allows for multiple transitions? Why is this not the case for hexatriene if they both contain the same number of pi bonds?
4
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1answer
116 views

Molecular orbitals symmetry - states notation

I'm reading some papers and I'm repeatedly seeing the following notation of system states: Could you, please, explain to me the meaning of it? E.g. the state $B^2 \Sigma_u^+$ - I'm aware, that $\...
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0answers
89 views

Implementing the projected atomic orbital (PAO) localization in Gaussian

I am currently attempting to learn how to setup the active space for performing CASSCF calculations. The textbook I am using as a reference is: Roos, Björn O. "Multiconfigurational quantum ...
1
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1answer
48 views

UV Absorption of iodine with some solvent

$I_2$ in solution is typically absorbing around $525 \; nm$ this is why it appear to be magenta, I recently learn that with some solvent : Benzene Toluene $o$-Xylene $p$-Xylene The coordination ...
2
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0answers
27 views

GVP(PP) orbital energies?

Hartree-Fock orbital energies can be interpreted physically, e.g. in Koopmans theorem, while one cannot assign active MCSCF orbitals to energies in general. What about GVB, say for example GVB(PP) ...
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0answers
254 views

HOMO / LUMO energies in a qualitative view

I have a question regarding the HOMO and LUMO energies depending on various effects like substituents and electronegativity. It seems to be a somewhat known topic but I cannot really find a clear ...
4
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1answer
89 views

Are any other orthonormalizations schemes besides Lowdin normally used when solving SCF equations?

When solving equations of the form: $$F(C)C = SC\epsilon$$ (e.g. Hartree-Fock-Roothan or Kohn-Sham-Roothan) where $F$ is the fock matrix, $C$ is the coefficient matrix and $S$ the overlap matrix, ...
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1answer
59 views

According to MO theory, do 1s orbitals in dioxygen interfere?

According to MO theory, are the 1s orbitals in $\ce{O2}$ too radially contracted to interfere with each other and thus cannot form bonding and antibonding orbitals, or do they still do that?
4
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1answer
1k views

How to explain the excited states in the dinitrogen cation?

The $\ce{N2^+}$ molecule has the molecular orbitals $$\ce{\sigma(1s)^2\; \sigma^*(1s)^2\; \sigma(2s)^2\; \sigma^*(2s)^2\; \pi(2p_x)^2 \; \pi(2p_y)^2 \; \sigma(2p_z)^1,}$$ which can be seen on the ...
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1answer
953 views

According to MO theory, why is the dilithium cation more stable than the dilithium anion?

According to molecular orbital theory, $\ce{Li2}$ fills up two electrons in the $\sigma_\mathrm{g2s}$ molecular orbital; $\ce{Li2+}$ fills up one electron in the $\sigma_\mathrm{g2s}$ MO, and $\ce{...
3
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2answers
325 views

The intrinsic electron-withdrawing nature of alkyl groups

What you are about to read may be a very mind-boggling paragraph but please do not regard it as nonsense. Please think through it thoroughly. In Chapter 6 of Organic Chemistry (4th ed.) by Maitland ...
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2answers
553 views

The molecular orbitals of lithium hydride

So my understanding of molecular orbitals is as follows: the molecular orbitals of $\ce{LiH}$ can be thought of as being formed from the interaction/overlap of two atomic orbitals, one from lithium ...
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0answers
40 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
249 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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0answers
118 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 ...
3
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0answers
159 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 ...
2
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0answers
107 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, ...
5
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0answers
60 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
2k 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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0answers
623 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 ...
1
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1answer
183 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
162 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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0answers
99 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 ...
5
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0answers
93 views

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 ...
0
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1answer
113 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 ...
3
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3answers
185 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 ...
4
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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 ...
2
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2answers
272 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
225 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 ...
2
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0answers
56 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
232 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 ...
1
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1answer
163 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 ...
7
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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
324 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 ...