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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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37 views

Mathematical relation between bond angle and fraction of s or p character

In Concise Inorganic Chemistry by J.D.Lee (Adapted by Sudarsan Guha, Fourth Edition), on page 75, under the topic "Effect of Electronegativity - When the surrounding atom is same with different ...
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Stabilizing donor-acceptor interaction between orthogonal orbitals in the SN2 transition state?

For the $\mathrm{S_N2}$ reaction between chloroacetone and iodide, my professor has drawn a donor-acceptor interaction between the iodine lone pair attacking the Cl-substituted carbon and the π ...
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diborane fragment orbitals energy of dihydrogen

I want to ask a question about the fragment orbitals of the $\ce{B2H6}$ system, specifically when I combine the $\ce{B2H4}$ with the $\ce{H2}$ fragments. I am halfway through my inorganic chemistry ...
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Varying sizes in LUMO of acrolein

I want to ask a question about the varying sizes in the LUMO of acrolein during a 1,2- or 1,4-addition. Today in our introductory lesson to Organic Synthesis with 1,2- and 1,4-addition we were ...
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How to deal with non-symmorphic space groups for orbital/vibrational analysis?

I am learning about how to apply group theory analysis of MO diagrams and vibrational in the solid state context (not isolated molecules). I think the strategy for calculating representations and ...
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Sulfur can violate the octet rule because it has an “empty D orbital” is there any other information the periodic table isnt telling us?

In sulfur or any atom with an expanded octet on the 3rd row, where does the idea that they have an empty D orbital come from ?
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Calculating the Molecular Orbitals of a Molecular State [closed]

I am performing ab initio calculations on a heteronuclear diatomic compound using the MRCI method on the MOLPRO quantum chemistry package. I obtained the molecular states of the compound but I was ...
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What is the purpose of phase matching when drawing a MO diagram?

During the lecture, we were learning how to draw the MO diagram for $\mathrm{BF_3}$. I understand that only orbitals with the same symmetry labels can interact and form molecular orbitals. My ...
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Why N=N is much stronger than N—N but C=C isn't

Why N=N bond is much stronger than N—N bond (about 2.5 times) but C=C is not (less than 2 times)? IMO, the radius of nitrogen and carbon atoms play a big role here. Since rN < rC, the AOs creating ...
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Properties that probe electron kinetic energy

This post is inspired by a question regarding the meaning of off-diagonal elements of the KE matrix (in some AO basis). One answer suggests that a diagonalized KE matrix might not be very useful. I ...
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Molecular orbital energies prediction with ML algorithms

In the recent years, computational chemistry community has focused on Machine Learning algorithms to predict molecular properties. Unfortunately, many of the authors of such papers are not chemists, ...
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Based on Molecular Orbital Theory , how is it decided that the double bond in O2 molecule has one sigma bond and one pi bond?

My Chemistry textbook says this in regard to a C2 molecule: It is important to note that double bond in C2 consists of both pi bonds because of the presence of four electrons in two pi ...
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Is Octet Expansion appropriate when regular Octet bonding is possible (such as using dative bonds)?

I was taught to use Occam's razor whenever in doubt over an octet expansion, being told it never happened when, for example, dative bonds could resolve the issue with a regular octet. This seems to ...
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Two questions about MO diagrams [duplicate]

In $\ce{HF},$ the $\mathrm{1s}$ orbital of $\ce{H}$ matches the $\mathrm{2p}$ orbital of $\ce{F}.$ However, in $\ce{LiF},$ the $\mathrm{2s}$ orbital matches the $\mathrm{2p}$ orbital. I knew that the ...
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Finding out more stable conformer via molecular orbital interaction

Out of these two, which one is more stable? I'm unable to imagine the orientation of the lone pairs of O and ABMO of C and Cl bond. Any tips to tackle these kind of problems?
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Explanation of SN2 Mechanism using MOT

SN2 is usually explained by MOT, where we state that the incoming nucleophile starts donating the electron density into the σ∗ orbital and the carbon-leaving group bond weakens and finally breaks and ...
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Why are sulfur trioxide and nitrate not isoelectronic, even though both have the same number of outer electrons?

Though $\ce{SO3}$ has the same number of outer electrons as $\ce{NO3-}$, the two are not isoelectronic. This statement is from JD Lee, but I could not understand why is he calling these two molecules ...
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Why CO is a stronger and more common ligand than N2?

Both N2 and CO are considered sigma-donor and pi-acceptor. Their MO diagram is similar, so I wonder why CO binds generally more strongly and it is a more common ligand.
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Cycloaddition vs Electrocyclic Pathway [closed]

Is there a molecular orbital/Woodward-Hoffmann explanation as to why Diels-Alder reactants don't commonly undergo electrocyclization instead of cycloaddition?
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1answer
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Molecular Orbitals (only pi system) of Allyl system using group theory

I have just started reading group theory and their applications in the MO theory. I tried to get the equations of the pi molecular orbitals of allyl system using group theory, but I am stuck at the ...
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1answer
69 views

Antibonding, Nonbonding and Bonding orbitals

Recently I came across nonbonding orbitals. I know something about bonding and antibonding orbitals and their formation, but I have no idea about nonbonding orbitals. Could someone explain their ...
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Why is triatomic hydrogen relatively short-lasting?

The first chapter of Fleming's "Molecular Orbitals and Organic Interactions," describes the formation of triatomic hydrogen. In summary, the sigma MO of diatomic hydrogen interacts in phase with the ...
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Crystal field splitting energy of hexaaqua copper(II) and hexaaqua iron(II)

I am performing an experiment for the IB diploma program to determine the crystal field splitting energy of d-orbitals of different transition metal ions when they are complexed with ligands. In my ...
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How can I obtain the wave function of the dihydrogen system, by hand using Self Consistent Field DFT calculations? [duplicate]

After doing some energy calculations for the $\ce{H2}$ system in Gaussian, using ωB97X-D/STO-3G level of theory, and generating a .wfn file, I wanted to understand better how the computer does ...
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Adding cyanide vs nitrite for nucleophilic substitution and why does it change when we use silver salts for it

On adding $\ce{NaCN}$ to $\ce{RX}$, we get $$\ce{NaCN + RX -> RCN + NaX}$$ And not $\ce{RNC}$. Clayden gives the explanation as follows: Although linear cyanide (which is isoelectronic with $\...
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Gauche conformations for 14-electron AH2BH2 systems

The hydrazine molecule, and the similar sulfonium/Oxonium ylides, all have a gauche conformation that is more stable than the anti conformation. Is there any particular reason for this pattern? ...
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Electronic States of the (unstable) tetrahedral CrCl4

I was probing for first order Jahn-Teller instability in the coordination complex CrCl4 (2 electrons in E set). Taking the direct product yields the states A1 + A2 + E, so I took the symmetric and ...
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Is any symmetry element conserved during a supra-antarafacial cyclo addition of two cis-butadiene units

I tried to construct a correlation diagram for the [4+4] cyclo addition of two cis-butadiene units. But I failed to construct any meaning full orbitals for the "educt" side where the two butadiene ...
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Is there a reliable chemical theory that predicts pKa based on structure?

Obviously, there are general stability arguments that can be made to estimate relative pKas, such as evaluating the stability of the conjugate base of an acid, or thinking about how polarized the bond ...
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Is fluorine also hybridised in carbon tetrafluoride?

I had read that when an atom forms multiple bonds, it hybridises it's orbitals to minimise the repulsion and decrease the overall energy. But do the terminal atoms also exist in hybridised form? E.g. ...
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Ionization energy and charge localization: π-bonds/systems and ionization energy

Page 35 of my textbook, Mass Spectrometry by Jürgen H. Gross, says the following in a section on ionization energy (IE) and charge localization: Molecules with π-bonds have lower IEs than those ...
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HOMO orbitals of B-N from an MO diagram

I want to ask a question about the MO diagram of BN. I was provided with the following question and answer and describe the situation occurring: After performing the s-p mixing for BN, I found this ...
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Why do solutions of electron in a box (and in a ring) predict coefficients for LCAO (linear combination of atomic orbitals) in 1D systems?

The solutions to the 1D particle in a box quantum mechanic system are standing waves (zero at both ends of the box) with 0,1,2... nodes for increasing energy (zero for the ground state). If I look ...
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1answer
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Band MO of Sodium Filling of electrons confusion

I wanted to ask a question about metallic bond - band electron theory. Consider the diagram of Sodium below: Circled are orbitals both containing $2$ electrons each which combine using LCAO to give ...
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1answer
50 views

Ab Initio and Molecular Orbitals [closed]

What is the difference between ab initio calculations and molecular orbital theory?
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1answer
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FMO analysis of the [3,3] Sigmatropic reaction regarding geometry. Is it accurate?

While studying the reasons behind the stereospecificity of [3,3] Sigmatropic reactions, I ran into an image portraying both possible transition states for the reaction: The Chair TS and Boat TS. The ...
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1answer
98 views

In the Lewis structure for barium fluoride, why does barium lose its valence electrons?

I searched for $\ce{BaF2}$ Lewis structure, and found out that the Barium atom loses its 2 valence electrons to form $\ce{F- Ba^2+F-}$. Before seeing it I thought each of $\ce{Ba}$'s valence ...
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50 views

Group theory and the orbitals of benzene

Can I apply group theory to the $\sigma$ orbitals of benzene? I have already seen it be applied to the $\pi$ orbitals, but I can't find an application to the $\sigma$ in literature. In this case, ...
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Is dihydrogen the only example of overlap between two s orbitals?

I heard that $\ce{H2}$ is the only example of s–s orbital overlap. Can anyone give an example which contradicts this statement?
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Molecular Orbital Hybridization vs Delocalization for positive charge stabilization

While studying Molecular Orbital Theory, I ran into a comparison of basicity between pyridine and piperidine. The latter was concluded to be more basic because of the sp3 hybridization of the ...
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Can 2 dz2 orbitals form a π bond?

Usually , d orbitals overlap sideways along a common internuclear axis, but drawing and checking for dz2 , it always seems to form a sigma bond or null overlap. Is this because it doesn't have any ...
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Determine HOMO and LUMO of compounds

I have the reaction below and am asked to determine the HOMO and LUMO of the two reactants. I know that the cyanide anion will be the nucleophile since it is donating an electron pair to the ...
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How does hyperconjugation explain stability of partial positive charges?

Recently I was reading this answer to the question Why does boron add to the less substituted carbon in the hydroboration of an alkene?. See the transition state given in the above answer, and the ...
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1answer
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What is the explanation of the structure of H2XeO4?

Using the obsolete VSEPR theory I had a hard time predicting the structure of $\ce{H2XeO4}$. So I looked it up. I found the molecule via chemapps.stolaf.edu: It looks weird to me. I can't explain the ...
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1answer
299 views

Does peroxide ion have any unpaired electrons?

According to the MOT diagram there are no unpaired electrons but in an exam I gave the answer key said it has 2 unpaired electrons. I can also find some images on google where it has 2 unpaired ...
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Molecular orbitals of the enolate ion

The reactivity of the enolate ion is most frequent explained with reference to its system of $\pi$ molecular orbitals. The image below taken from Wikimedia Commons depicts this system of $\pi$ ...
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Why is the HOMO always lower in energy than the LUMO?

We say that increasing the energy of the $\ce {HOMO}$ of molecules increases their reactivity towards electrophiles while decreasing the energy of the $\ce {LUMO}$ increases their reactivity towards ...
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What is the physical origin of conjugation?

I had a question about the origins of conjugation in conjugated molecules. Take two ethylenes as compared to butadiene. Huckel seemed to argue that conjugation was the result of pi-pi stabilization ...
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1answer
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Why does Methyl Carbocation have p orbital

For a carbocation to form, it needs to lose an electron. In the example below the leaving group is Br and the products are a methyl carbocation and a bromide ion. What I am unsure about is why the ...
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Why is a LCAO necessary within Hartree Fock?

As I understand it, the electronic Schrödinger equation cannot be solved for polyelectronic systems. To circumvent this problem in the Hartree-Fock method, it is assumed that the polyelectronic ...