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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12
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2answers
59k 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 ...
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2answers
3k views

How does a Frost diagram reproduce the solutions to the wave equation?

I came across a Frost diagram for cyclic compounds in my book, and all my book had to offer was that it geometrically reproduces the solutions of the wave equation, and can therefore determine the ...
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2answers
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How can off-diagonal Hamiltonian matrix elements be nonzero when the overlap matrix is diagonal?

While studying about Hückel theory, I got accustomed to the approximation of making the overlap matrix an identity matrix; that is making the off-diagonal elements zero as $S_{AB}= S_{BA}= 0\;;$ this ...
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1answer
9k views

Why is there a difference between O2 and B2 sigma 2p molecular orbitals in diagrams?

The molecular orbital diagram for $\ce{O2}$ says that the sigma 2p bonding molecular orbital is lower in energy than the pi 2p bonding molecular orbital. Why is this not the case in the $\ce{B2}$ MO ...
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4answers
897 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 ...
11
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1answer
5k views

Diatomic halogens - bond strength

Why is the $\ce{F-F}$ bond relatively weak? And why does bond strength decrease from $\ce{Cl-Cl}$ to $\ce{I-I}$? Does this have anything to do with "lone-pair" repulsion on the rather small $\ce{F-F}...
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2answers
3k views

Can a s orbital overlap with any p orbital to form a sigma bond?

In internet resources, only $p_z$ orbitals are shown to overlap with $s$ orbitals to form sigma bond. But my teacher says that according to Valence Bond Theory, $s$ orbital overlaps with all three $...
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2answers
11k views

Where does the 9th electron go in a N=O bond?

In the first resonating structure you can see 5 unpaired electrons and 4 shared electrons on nitrogen, then isn't this a extended octet? If it is so, then in which orbital that 9th electron is ...
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1answer
563 views

What is the dipole moment direction in the nitrosonium ion?

In the recent question about the molecular orbitals of $\ce{NO^{$x$}};\ x\in\{-1,0,+1\}$ the follow-up title question arose: Just a random side-question: What’s the dipole moment’s negative and ...
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3answers
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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 ...
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2answers
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Counting Nodal Planes in cyclopropane

The energy of molecule orbitals increases with more nodal planes. W1 (in the attached picture) has no nodal plane. I'd like to know how to draw the nodal planes in cyclopropane molecule orbitals but ...
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1answer
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Molecular orbital diagram for nitrogen monoxide, the nitrosyl cation and the nitrosyl anion

As I understand it, all of them should have same order of energy of molecular orbitals as a model for $\ce{NO}$ should withstand ionization and adding an electron. JD Lee Pg 109 shows the same ...
5
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1answer
583 views

How are the electrons of benzene actually configurated?

I found this diagram: According to it, it is not true that the six electrons of carbons in the pi-orbitals above the ring are all identical. But, how can it be the case, seeing that this molecule ...
5
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1answer
85 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 ...
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3answers
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Precise definition of atomic orbital

In school I was taught that an atomic orbital is the 3-dimensional region in which the electron is located with a probability of 90%. However, aren't there infinitely many regions of space in which ...
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2answers
1k views

Asymmetry in trigonal bipyramidal geometry

I teach an MCAT course in chemistry. I like to explain VSEPR by saying.. first, imagine arranging electron pairs around the central atom so they are maximally distant from each other, and uniformly ...
24
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1answer
2k 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 ...
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2answers
4k views

Hybrid orbitals forming molecular orbitals

My teacher showed me this diagram on how the hybrid orbitals of two atoms combine to form molecular orbitals. I was confused by this because I thought that VB and MO theories were two separate ...
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2answers
309 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 ...
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2answers
36k views

Is s-p mixing referring to hybridization or is it the mixing of one atoms s orbital with the other's p orbital?

According to molecular orbital theory s and p orbitals can mix if they are close enough in energy to each other. For period 2 diatomics, this occurs for $\ce{Li}$, $\ce{Be}$, $\ce{B}$, $\ce{C}$ and $\...
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1answer
6k views

Using group theory to make molecular orbital diagrams

Because of Orthocresols' meta post I ask this question about application of group theory to molecular orbital theory. In my undergraduate lecturers, we've seen many examples of making qualitative MO ...
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2answers
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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 ...
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2answers
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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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1answer
15k views

Why a higher s character increases a carbon atom's electronegativity?

To make us understand certain phenomenon like the higher acidity of alkynes than alkanes and alkenes, our teacher told us to learn this as a general rule: Higher s character leads to a more ...
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1answer
2k views

Is the hybridization only related to atomic orbitals but not to the molecular orbitals?

What is Hybridization? (Spelling: Hybridisation (British English) / Hybridization (American English)) The hybridization is a concept that describes atomic orbitals. In other words: Hybridization ...
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4answers
379 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 ...
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2answers
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Which one, Mulliken charge distribution and NBO, is more reliable?

Sometimes the Mulliken and NBO turn out to be so different that I can't decide which one I can trust. I've heard that Mulliken is inaccurate, but is NBO always accurate? And should I use Gaussian or ...
6
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1answer
176 views

Electronic model with highest prediction rate

Amongst many models, including the valence bond model (VB) or the molecular orbital (MO) model, which are the ones with best predictive power? (e.g. the MO is thought to predict spectroscopic ...
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3answers
2k views

Do core electrons have molecular orbitals?

Valence electrons are associated with molecular orbitals and hybridizations. Do core electrons have molecular/hybridized orbitals, or the original atomic orbitals?
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1answer
4k views

Why do two atomic orbitals form two molecular orbitals? [duplicate]

According to molecular orbital theory, two atomic orbitals form two molecular orbitals analogous to waves combining constructively or destructively but how can a wave combine destructively and ...
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2answers
9k views

Difference between backbonding and hyperconjugation and conjugation [closed]

What is the difference between backbonding and hyperconjugation and conjugation? I mean the basic differences as all are concerned with donation of electrons. What is the most striking effect of each ...
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1answer
250 views

Confusion about atomic/molecular orbital terminology

I have come across several different orbital terms: atomic orbitals, natural orbitals, split-localized orbitals, molecule-intrinsic orbitals, quasi-atomic orbitals. I don't know what the difference ...
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4answers
36k 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 ...
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1answer
10k views

Why/how is blood red? (colours of hemoglobin)

Oxyhaemoglobin is red, deoxyhaemoglobin is bluish-purple, and carboxyhaemoglobin is a cherry red colour. Evidently in the porphyrin around the iron molecule in heme there is a conjugated $\pi$-system ...
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1answer
4k views

Significance of phase of atomic orbitals

I am learning about orbitals and bonding and antibonding MOs. So far, I know that when you combine s orbitals, you form both bonding and antibonding MOs- bonding when the wavefunctions are added in ...
5
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1answer
817 views

Formation of Bands in Semi-Conductors

At school, we are learning about semi-conductors and their applications in modern electronics. One of the features of semi-conductors is that there is a small energy gap between the valence and ...
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2answers
8k views

How derive g and u symmetry labels for orbitals?

When asked whether a molecule has an inversion center, we "invert" the coordinates of all atoms; i.e. move each atom from its position through the center of symmetry and to a new position equidistant ...
12
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1answer
415 views

What's the reaction type of the Hydrogen shift?

Rearrangement of cycloheptatrienyl carbene yields heptafulvene via a hydrogen shift. I see that the reaction is favorable because there is a conjugative effect with the $\pi$ system of the ring in ...
12
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3answers
409 views

What is the usage of orbitals more complex than f orbitals?

Every high school learner, in each corner of the world, faces the lesson History of Atom during his courses, just as I did. We learned about s, p, d and f orbitals, though there were no signs of ...
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2answers
582 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 ...
8
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1answer
7k views

Why does the mixing of sigma 2s and 2p orbitals lower the energy of the sigma 2s orbital and raise the energy of the sigma 2p orbital?

My general chemistry textbook (General Chemistry Principles and Modern Applications, Tenth Edition) says that for bonding in diatomic molecules with Z ≤ 7, orbital mixing occurs between the $\sigma_{...
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1answer
1k views

Why is the thione tautomeric form of 2-Mercaptopyridine favoured?

During some organic chemistry studies (through an old online course material) I ran into a doubt about the equilibrium in tautomeric forms of some heteroaromatics. First, the context: On the course ...
7
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1answer
255 views

Geometries of methyl and silyl radicals

The $\ce{.CH3}$ radical is planar but the $\ce{.SiH3}$ radical is a trigonal pyramid. Why are they different? After all, $\ce{C}$ and $\ce{Si}$ are in the same group.
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2answers
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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 ...
6
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1answer
3k views

Hybridization, MOT and Paramagnetism

In what way can hybridization or molecular orbital theory be used to explain paramagnetism? For instance, when something is hybridized to make enough bonding electrons, do all the electrons end up ...
5
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1answer
812 views

Torquoselectivity: Disrotatory ring opening

In organic chemistry lecture we discussed the beginning of the Woodward Hoffmann Rules with a key step of the vitamin B12 synthesis. (The initial paper from their work can be found here: http://pubs....
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2answers
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Hydrogen bonding relating to molecular orbital theory

In [1, p. 232] here is an explanation of how alcohols hydrogen bond to each other and I do understand how the alcohols form hydrogen bonds to each other: FIGURE 6.15 Water and alcohols are ...
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1answer
4k views

Structure of Buckminsterfullerene

In several pictures that I have seen of buckminsterfullerene and graphite (such as below), each carbon is bonded to only 3 other carbons, so it only forms 3 covalent bonds. I asked my teacher about ...
4
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1answer
143 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 $\...
4
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1answer
914 views

Can I predict the preferred resonance structure of carbon monoxide from its molecular orbital scheme?

Because of the 8-valence electron rule I assume in CO the left resonance structure to be favored . $$\ce{:\!\overset{\ominus}{C}#\overset{\oplus}{O}\!: ~<->~ :\!C=O\!:: ~<->~:\!\overset{\...