Determining Symmetry Group of Cyclopentadienyl using Huckel Theory

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Discussion Overview

The discussion centers on determining the symmetry group of cyclopentadienyl using Huckel theory, with a focus on systematic methods and potential software tools for analysis. Participants explore the symmetry elements associated with the molecule and clarify the distinction between cyclopentadiene and the cyclopentadienyl anion.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants inquire about systematic methods to determine the symmetry group of cyclopentadienyl and the feasibility of using software for this purpose.
  • One participant suggests consulting specific textbooks, such as "Chemical Applications of Group Theory" and "Physical Chemistry," to find the point group by inspection.
  • Another participant identifies symmetry elements for the molecule, including the identity (E) and a C2 rotation of 180 degrees, but expresses uncertainty about finding additional symmetry axes.
  • A different participant challenges the initial structure drawn, asserting that the correct structure should be for the cyclopentadienyl anion (C5H5-) rather than cyclopentadiene (C5H6), and questions the validity of the symmetry elements proposed.
  • This participant also states that the symmetry elements for cyclopentadiene are E, C2, σv, and σv', categorizing it as a C2v molecule, while acknowledging that the symmetry group is D5h for the cyclopentadienyl anion if the correct structure is used.
  • Another participant shares links to resources they found helpful during their studies related to symmetry in chemistry.

Areas of Agreement / Disagreement

Participants express differing views on the correct structure and symmetry elements for cyclopentadienyl and cyclopentadiene, indicating a lack of consensus on the initial assumptions and interpretations of symmetry elements.

Contextual Notes

There is ambiguity regarding the definitions and interpretations of symmetry elements, particularly the mention of "C2 horizontal," which is not standard. The discussion also highlights the importance of starting with the correct molecular structure to accurately determine the symmetry group.

greisen
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Hi,

I have to determine the symmetry group of cyclopentadienyl(assumed to be planar and symmetrical) and I should use Huckel theory. Is there a systematic way to determine the symmetry group and is it possible to use software to determine the symmetry group.

Thanks in advance
 
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greisen said:
Hi,

I have to determine the symmetry group of cyclopentadienyl(assumed to be planar and symmetrical) and I should use Huckel theory. Is there a systematic way to determine the symmetry group and is it possible to use software to determine the symmetry group.

Thanks in advance
Yes, there is a systematic way to find the point group that the molecule belongs to.

See Chemical Applications of Group Theory by Albert Cotton, or Physical Chemistry by Atkins.

For the above complex, you can find the symmetry group by inspection.
 
I draw the molecule and determine the different groups;

E - the identity
C2 rotation of 180 degrees
C2 horizontal 180 degrees

I know the molecule should have D5h but I have some trouble finding the rest of the symmetri axis.

Any help appreciated thanks in advance
 
greisen said:
I draw the molecule and determine the different groups;

E - the identity
C2 rotation of 180 degrees
C2 horizontal 180 degrees
Looks like you've got the wrong structure to begin with. It is not the cyclopentadiene molecule (C5H6), but the (cyclopentadienyl) anionic ligand (C5H5-) that you need to draw the structure for.

Example: The cyclic groups depicted http://content.answers.com/main/content/wp/en-commons/thumb/9/93/200px-Zirconocene-dichloride-3D-sticks.png are cyclopentadienyl rings.

Furthermore, even if you were finding the symmetry group for the C5H6 molecule, your third "symmetry element" makes no sense. What does "C2 horizontal" mean? That is not a standard symmetry element. The symmetry elements of the C5H6 molecule are [itex]E,~C_2, ~\sigma _v,~ \sigma _{v'}[/itex], making it a [itex]C_{2v}[/itex] molecule.

I know the molecule should have D5h but I have some trouble finding the rest of the symmetri axis.
Yes, it is D5h, but you will get that only if you start with the right structure.
 
Last edited by a moderator:
greisen said:
Hi,

I have to determine the symmetry group of cyclopentadienyl(assumed to be planar and symmetrical) and I should use Huckel theory. Is there a systematic way to determine the symmetry group and is it possible to use software to determine the symmetry group.

Thanks in advance

The following are webpages that I've found useful during my undergraduate years and in the inorganic chemistry course.

http://chemistry.umeche.maine.edu/Modeling/symmetry.html

http://www.reciprocalnet.org/edumodules/symmetry/

http://www.ch.ic.ac.uk/vchemlab/symmetry/

http://www.phys.ncl.ac.uk/staff/njpg/symmetry/
 
Last edited by a moderator:

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