29
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What is the mathematical basis behind the Jahn-Teller effect?
1. Perturbations
As already mentioned, the Jahn–Teller effect has its roots in group theory. The essence of the argument is that the energy of the compound is stabilised upon distortion to a lower-...
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26
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Does trans-decalin really have a plane of symmetry?
It's not easy to see from a diagram, because it distorts bonds and angles. I recommend building it with a balls-and-sticks model set. You can also use a molecular viewer to model it; there are a ...
- 43.8k
20
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Theoretical basis behind orbital correlation diagram for pericyclic reactions
Introduction
Instead of the usual cycloadditions (which has some extra complications in terms of the reacting orbitals), let's consider the concerted reaction $\ce{H2 + D2 -> 2HD}$. We'll come ...
- 71.1k
18
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Symmetrize nearly symmetric molecule
I've done some work in both symmetry detection and in distance matrix methods. I think it's a great idea in concept, but the devil will be in the details for large, more complex molecules.
The first ...
- 27.7k
17
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Jahn-Teller Distortions in Square Planar Complexes?
Introduction
Thank you for prompting me to look for square planar complexes that are not $\mathrm{d^8}$; I learnt some valuable stuff while researching the answer to this question! Do take this ...
- 67.3k
16
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Why is 2-methyl-2,3-pentadiene achiral (allene)?
I'll start by linking to Symmetry @ Otterbein, as it's a very useful tool for looking at various structures and seeing planes an axes of symmetry. If you're having issues visualising rotations and ...
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16
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Relationship between the symmetry number of a molecule as used in rotational spectroscopy and point group
This is not in general true
Consider molecules a point group not containing inversion symmetry, e.g. $C_2$ hydrogen peroxide
The $C_2$ group has only two elements, $E$ and $C_2$, and the $C_2$ ...
- 1,909
15
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How are there two C3 rotation axes in ammonia?
In a character table for $C_{3v}$, $ 2C_3$, means that the element is actually 2 $C_3$ elements in the same conjugacy class. That means the two $C_3$ operations are related in some way.
In fact, it ...
- 1,909
14
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What constraints are imposed on a wavefunction by the symmetry of the system?
I use quantum chemistry about as much as a lumberjack uses math (that is, just enough not to fell a tree on one's own head), so my answer will be short and hopefully useful as a tl;dr to someone else'...
- 31k
14
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How do I show that a transition is magnetic dipole allowed with group theory/symmetry?
For a state-to-state transition coupled by an operator to be allowed, the direct product of the irreducible representations (irreps) of all three components must contain the completely symmetric irrep ...
- 7,428
14
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How are there two C3 rotation axes in ammonia?
The top row of the character table does not refer to symmetry elements, but rather symmetry operations. (For the difference between the two, see Wikipedia:Molecular symmetry.)
In this particular case, ...
- 71.1k
13
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How do I find the ground state term symbol for transition metal complexes?
To find the ground state term symbol, you should be using symmetry and group theory arguments, you shouldn't have to resort to searching Tanabe-Sugano diagrams to get the answer.
We'll start with ...
- 71.1k
13
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Derivation of the Orgel diagram for octahedral d2 complexes
1. Weak-field and strong-field limits
I will adopt the description used in Figgis and Hitchman's Ligand Field Theory and Its Applications (p 5), because I cannot really phrase it better:
It is ...
- 71.1k
11
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Why can anthracene, but not phenanthrene, take part in Diels–Alder reactions?
Have a look at the reaction mechanism of the Diels-Alder Reaction, e.g. at Wikipedia.
We have talked about this reaction before on this site, and concluded that a sufficient explanation is only ...
- 43.8k
11
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Is there a physical explanation why benzene's pi molecular orbitals in a Frost circle are degenerate but pyridine's are not?
My first thoughts are along the lines of your second-last paragraph. Here is a quick sketch of how to formalise it. From simple Hückel theory, you can obtain the coefficients of the AOs in the MOs:
$$...
- 71.1k
11
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Does 1,3-dichloroallene possess a C2 symmetry axis?
A $C_2$ axis is indeed here—although not the one that you are thinking of: your suggestion of a $C_4$ then $C_2$ rotation does not qualify as a $C_2$ symmetry operation. There is a true $C_2$ axis ...
- 37.4k
10
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What is difference between T1g and T2g?
Well, not quite. Here's the quick summary on subscripts in Mulliken notation for point group symmetry.
“g” stands for "gerade" and refers to the symmetry around an inversion center. (That is, the ...
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10
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AB5E type molecule
Why? Because there is no other choice. Starting from the AB6 octahedral configuration, all six vertices of the octahedron are symmetric, so it doesn't matter whichever one you “choose” to replace by ...
- 23.6k
9
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Why doesn't 1,3-dichloro-2,4-dimethylcyclobutane have a plane of symmetry?
Ivan already stated quite briefly in the comments, that the planes you drew would transform an upward facing chlorine to a downward facing chlorine and vice versa, and that the same holds for the ...
- 43.8k
9
votes
Rule of mutual exclusion: is the converse true?
If there is a molecule of $O$ (as opposed to $O_h$) or $I$ (as opposed to $I_h$) symmetry, it would be such a counter example (no mode that is both IR and raman active, and not centrosymmetric).
...
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9
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Rule of mutual exclusion: is the converse true?
Have a look at the point groups O (not $\ce{O_h}$) I, $\ce{D_{5h} and C_{5h}}$. A pic of an O point group molecule is shown below. The $\ce{c_2}$ axes are not shown. Taken from molecule-viewer.com web ...
- 30k
9
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Why does the 31P{1H} NMR spectrum of cis-[Mo(CO)2(dppe)2] show two signals?
In this complex there are two different 31P environments which are not related by symmetry:
The two green phosphorus nuclei can be interconverted by a $C_2$ rotation (the rotation axis bisects the OC–...
- 71.1k
9
votes
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What's the relation between two symmetry groups, if one has all the symmetry of the other and some more?
What mathematical term describes the relationship between $C_s$ and $C_{2v}$? Is $C_{2v}$ a subgroup of $C_s$? Is it rather a subset of it? Should we rather say that $C_{2v}$ implies $C_s$?
$C_\...
- 71.1k
8
votes
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How many peaks would there be in H1 NMR of ethene?
You need to consider whether the protons in ethene are chemically equivalent, magnetically equivalent, or both. The lewis structure of ethene:
Because of the symmetry of the planar ethene molecule, ...
- 4,703
8
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Identifying the C3, C4, S4, and S6 symmetry operations in the Oh point group
At first I thought the character table was counting both positive and negative rotations, but then the order for the $C_2'$ and $C_2$ rotations should be doubled if that were the case ($12C_2'$ and $...
- 67.3k
8
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Can a molecule be neither gerade nor ungerade?
g/u is not a property of a molecule; I assume you meant molecular orbital.
All functions can be written as a linear combination of even + odd functions: there is a short explanation on Wikipedia. ...
- 71.1k
8
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Why is CO2 not in the C∞v point group?
Both are linear with a $C_{\infty}$ axis, but $D_{\infty h}$ has a center of inversion and $C_{\infty v}$ does not.
- 22.3k
8
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Use of axis of symmetry
Alternating axes are not axes, much like a Guinea pig is not a pig.
Having axes of symmetry is irrelevant to chirality.
Having alternating axes is a necessary and sufficient condition of non-...
- 31k
8
votes
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Can the plane of symmetry for a compound be the plane of the paper?
In short: The presence of the $\sigma_h$ plane you see depends on the conformation of the methyl groups.
If your example were either benzene or 1,4-dichlorobenzene, then you were be right, the ...
- 28.4k
8
votes
Chromium cyanide complexes
This doesn't appear to be a rare case and there are plenty $\ce{Cr^2+}$ square planar complexes.
I found 15 square planar complexes with well-determined crystal structures $(R_\mathrm{int} < 10\%)$ ...
- 37.4k
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