All Questions
Tagged with molecular-orbital-theory organic-chemistry
119 questions
0
votes
2
answers
63
views
MO energy of p-orbitals with different substituents
Why is the HOMO of reaction 3 raised in energy, compared to the others, if the carbocation has partial sp2 character due to resonance? Wouldn't the donation into the p-orbital be stabilizing thus ...
- 157
0
votes
0
answers
39
views
Excited states: S0, S1, S2 interpretation
Good morning, everyone. I have a question about molecular excitation. I'm studying photochemistry, and I came across the terms excited states S₀, S₁, and S₂. Initially, I thought that S₁ represented ...
0
votes
0
answers
28
views
Non-radiative decay rates computation
What is the most straightforward computational method to estimate the non-radiative decay rates in organic molecules? I have done TD-DFT calculations and really struggling to compute the rates.
- 31
0
votes
0
answers
80
views
Pi MO Diagram of Guanidinium Cation
My organic chemistry professor has asked us to draw the pi MO diagram of the guanidinium cation.
I only got as far as the lowest and highest energy orbitals (with zero and three nodal planes ...
- 11
0
votes
0
answers
24
views
Reconciling bonding and antibonding MOs with how reactions proceed
Take the reaction between MeI and some nucleophile (Nu:) to give Me-Nu. Does the approach of the Nu, provide the energy input necessary to promote the Me-I electrons to the antibonding MO? Do the Me-I ...
- 31
1
vote
0
answers
53
views
What do the pi antibonding orbitals of 1,5-hexadiene look like?
Hi so right now I'm learning about the Cope and Claisen rearrangements, and often something is shown where the antibonding pi orbitals of the two alkenes have to overlap in a certain way, like in the ...
- 19
0
votes
0
answers
24
views
Bonding and antibinding orbital energy difference
Reading Claydens organic chemistry, in chapter 5 it states that Br-Br makes a good electrophile while the C-C in ethane doesnt because the energy difference between bromines bonding and anti bonding ...
- 1
0
votes
0
answers
44
views
Why hyperconjugation is called as σ to π* donation for alkenes and alkynes?
Why hyperconjugation is called as σ to π* donation for alkenes and alkynes if for C2 lowest unoccupied molecular orbital is σ2p not π*2p?
- 139
1
vote
0
answers
37
views
Why do bonds interact with other bonds and exhibit overlapping effect?
In carbocations, alkenes and carbon free radicals during hyperconjugation, overlapping of molecular orbitals take place. So how and why does this happen?
- 19
-1
votes
1
answer
196
views
How does MO theory explain the effect of directing groups on aromatic compounds?
From what I could find, this is usually explained using resonance. For example,
from Wade's Organic chemistry, 9th edition, p. 823.
but I can't figure out how to see this through the MOs of the ...
4
votes
0
answers
129
views
What is the driving force of Bartoli indole synthesis?
The Bartoli indole synthesis mechanism begins with a series of attacks on the nitroarene regent by the Grignard reagents. But the atom $\ce{O}$ in $\ce{-NO2}$ should be negative, and Grignard reagent ...
- 49
0
votes
1
answer
91
views
Shouldn't the negative charge be on carbon in this scenario according to molecular orbital theory [closed]
I am sorry in advance for posting an image in this question. I know it makes searchability difficult, but I don't see any other way
The above screenshot is from Claydens organic chemistry. We can see ...
- 11
0
votes
0
answers
65
views
Orbitals in benzene and other delocalised structures
I've read that molecular orbital theory demands that if there are $n$ orbitals prior to some event like bond formation, there should be $n$ orbitals after this event.
Does this hold even in structures ...
- 101
3
votes
1
answer
822
views
Why is this diagram depicting the molecular orbital (MO) basis for a back-side attack the way it is?
Consider:
The description of this image in my textbook is as follows:
In order to form a bond, the HOMO (the highest occupied molecular orbital) of one species must interact with the LUMO (the ...
- 31
0
votes
0
answers
291
views
Molecular orbital theory and Lewis structure for dioxygen
I am a bit confused about how molecular bond theory predicts pairings of unbounded electrons. For example, in $\ce{O2}$ there are two pairs of electrons on each oxygen along with the double bond. ...
7
votes
4
answers
652
views
When pigments absorb light only around a single particular wavelength, why aren't they still white?
I'm trying to understand how organic molecules get their colour.
One major factor are conjugated double bonds which create delocalized pi-orbitals (e.g. https://en.wikipedia.org/wiki/...
- 199
2
votes
1
answer
114
views
How can Phthalocyanine be aromatic when it is not planar? [closed]
Even as Hückel's rule is not applicable in every scenario, especially in metallic clusters, it seems that organic structure need to be planar to show aromaticity. That being the case, Pubchem's ...
- 739
1
vote
0
answers
180
views
$\pi$ Molecular orbital diagram of compounds like urea, guanidine, acetone etc
In my organic chemistry course, we've only been taught about how the $\pi$ MO diagrams of compounds like butan-1,3 diene, hexan-1,3,5 triene and similar single chain compounds look like. I was ...
2
votes
1
answer
743
views
What is the LUMO of a molecule with an alkene and a carbonyl?
In a molecule that has both an alkene (C=C) and carbonyl (C=O) that are NOT conjugated, such as pent-4-en-2-one, how do you determine which π* anti-bonding orbital is the LUMO?
Essentially, I'm trying ...
- 21
1
vote
0
answers
74
views
A contradiction in the nucleophilic attack of hydride ion based on charge densities
On page 130 of Organic Chemistry by Clayden, it is stated that
Nucleophilic attack by the hydride ion, $\ce{H-}$, is an almost unknown reaction. This species, which is present in the salt sodium ...
- 805
5
votes
0
answers
523
views
Orbital Interaction for electrophile and nucleophile
Clearly, the above diagrams depict the interaction between empty orbital of electrophile with filled orbital of nucleophile. We may assume that the HOMO of nucleophile being more electronegative will ...
- 805
2
votes
0
answers
87
views
What does Molecular Orbital Theory allow you to predict?
Molecular Orbital theory reveals the outcome when two atomic orbitals overlap. Almost all of the time we draw a Y axis with the notation of energy that can be described using electron volt units (eV). ...
2
votes
0
answers
46
views
Interpreting data on molecular orbitals in GaussView 6.0 [duplicate]
Introduction
I have been trying to use computational software to gain more insight into how molecular orbitals are formed, as well as their shapes and sizes in accordance with Molecular Orbital Theory....
-1
votes
2
answers
98
views
Confusion regarding orbital, electron and Quantum no’s [closed]
Now , In some textbook I have read that orbital is nothing but the shape of electron . s,p,d orbitals etc.
So , after knowing shape of an orbital . I got to know that inside the orbital is an electron ...
- 190
4
votes
0
answers
275
views
What are "Secondary Orbital Interactions" in Organic Chemistry? [closed]
We used these to analyze the stability of molecules in an Organic Chemistry Course.
My teacher analyses the stability of compounds on 3 levels:
Atom level by studying the hybridization of the main ...
- 67
10
votes
1
answer
556
views
Frontier orbital analysis of zinc-catalysed carbonyl-ene reaction
Q1. Is the $\sigma_\ce{C-H}$ bond a donor in this reaction? If not, then can we take the alternate combination (HOMO of alkene, LUMO of the $\ce{C-H}$ bond and, HOMO of the carbonyl)?
Q2. We can also ...
- 175
3
votes
0
answers
295
views
Can cycloaddition reactions be antarafacial?
Cycloaddition reactions of the form [4+2] and [2+2] are always suprafacial, as the geometry is too constrained to allow for antarafacial interaction between the FMO's. However, for cycloaddition of ...
- 6,338
6
votes
0
answers
144
views
How do bonds rotate in pericyclic reactions?
In Pericyclic reactions, it is shown that the terminal bonds rotate (conrotation or disrotation) to allow for allignment of the FMO's, thereby forming the sigma bond
However, a detail that most of my ...
- 6,338
8
votes
1
answer
5k
views
Why are hydrogen bonds in an antiparallel beta sheet stronger than those in parallel beta sheets?
Beta sheets are illustrated as such in most diagrams, where:
In an antiparallel β-sheet, the polypeptide strands are arranged such that a $\ce{C=O}$ and an $\ce{NH}$ from adjacent strands face each ...
- 380
1
vote
1
answer
146
views
Chemical reaction of radicals with different multiplicity
I am studying the chemical reaction of radicals by computational chemistry software.
I just want to know what is the rules for multiplicity.
I have two radicals for which, the ground state of one is a ...
- 11
2
votes
0
answers
48
views
Focusing on HOMO when discussing thermal electrocyclic reactions
When taking into consideration the outcome of a thermal electrocyclic reaction, why do we focus specifically on the HOMO of the conjugated polyene? Why does it seemingly have the highest contribution ...
3
votes
0
answers
57
views
Why is 4-t-butyl aci-nitro cyclohexane stable whereas aci-nitro cyclohexane is not?
During my PhD, about fifty years ago, I had reason to use 4-t-butyl aci-nitro cyclohexane and found it to be remarkably stable. Usually aci- nitro compounds revert to nitro compounds very quickly ...
1
vote
0
answers
991
views
Why is the top SN2 reaction faster? (Molecular Orbital Theory)
I'm having trouble understanding why the top SN2 reaction proceeds faster. I know that benzylic and allylic halides react faster in SN2 reactions than non-conjugated halides. I think it's because the ...
-2
votes
2
answers
2k
views
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 ?
2
votes
0
answers
63
views
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?
- 203
2
votes
1
answer
1k
views
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 $\...
- 1,246
9
votes
0
answers
206
views
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 ...
- 732
2
votes
1
answer
750
views
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 ...
- 757
7
votes
1
answer
2k
views
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 $\pi$ ...
- 10.8k
13
votes
1
answer
305
views
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 ...
- 455
3
votes
1
answer
2k
views
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 ...
- 187
3
votes
0
answers
302
views
Protonation of pyridine
Recently, I have been reading the Nobel Lecture given by Kenichi Fukui when he won the Nobel Prize in 1981, for his contributions to the development of Frontier Molecular Orbital Theory. One of the ...
- 10.8k
2
votes
1
answer
607
views
How does hyperconjugation in alkenes affect bond lengths?
I understand how the diagram below results in hyperconjugation stabilising alkenes.
Essentially, the C-H sigma bonding orbital interacts with the pi bonding orbitals creating two molecular orbitals. ...
1
vote
0
answers
133
views
Antiaromaticity can be explained by Hückel method?
Pentalene is one of the most famous antiaromatic molecules. But I obtained its energy of $\pi$ system as $8\alpha+10.46\beta$ by Hückel method (with a bit complicated but straightforward calculation). ...
- 121
1
vote
0
answers
176
views
What are the pi-MOs in anisole like?
The $\pi$-orbitals of benzene are well known and given in any standard undergraduate textbook. When considering anisole, phenol, etc, a lone pair in a p orbital of the O can delocalise into the $\pi$-...
- 239
3
votes
1
answer
6k
views
Why are pi antibonding orbitals lower in energy than sigma antibonding orbitals?
I know when a carbonyl group is attacked by a nucleophile, the electrons of the nucleophile enter the antibonding π orbital of the $\ce{C=O}$ bond, which breaks the π bond. My textbook says that ...
- 63
-1
votes
1
answer
1k
views
Why is a carbon to iodine pi-bond less stable than a carbon to fluorine pi-bond?
This question comes from a second semester undergraduate organic chemistry course. Please refer to the image below with regard to the question.
My understanding of the trend in section 4a is that "...
- 41
2
votes
1
answer
740
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 ...
- 10.8k
9
votes
1
answer
2k
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 ...
- 811
5
votes
1
answer
3k
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 ...
- 811