Chemistry - symmetry/point groups question
Have 2 molecules: Trans PtCl2(CO)2 and the Cis version (both square planar). I need to determine if the isomers can be distinguished using the observed C=O vibrations in either IR or Raman spectroscopy. Here is what I a have done:
Cis molecule: C2v point group, CO stretches span A1 and B1. Both are IR active, B1 is also Raman active.
Trans molecule: D2h point group. Reducible representation for both C=O basis points as follows:
E C2(z) C2(y) C2(x) i (xy mirror) (xz mirror) (yz mirror)
2. 0. 0. 2. 0. 2. 0. 2
Question: is my reducible correct? Assuming it is the main question is this: when applying the reducing formula, I'm getting some irrep characters to be fractions. I've previously only had them be 1 or 0. What do fractions mean? For the irrep B2g in D2h table for example, i get the irreducible representation of: -1/2
Cis molecule: C2v point group, CO stretches span A1 and B1. Both are IR active, B1 is also Raman active.
Trans molecule: D2h point group. Reducible representation for both C=O basis points as follows:
E C2(z) C2(y) C2(x) i (xy mirror) (xz mirror) (yz mirror)
2. 0. 0. 2. 0. 2. 0. 2
Question: is my reducible correct? Assuming it is the main question is this: when applying the reducing formula, I'm getting some irrep characters to be fractions. I've previously only had them be 1 or 0. What do fractions mean? For the irrep B2g in D2h table for example, i get the irreducible representation of: -1/2
(edited 1 year ago)
Hi,
I tried answering the question, I'll explain what I did below, it might not be right however, as I am just studying this concept now- so don't completely rely on what I say!
For the trans molecule I got a different reducible representation to you:
In the same order you did I got: 2 0 0 0 0 2 0 0
The C2(x) rotation wouldn't keep the CO's in the same place I think, and if you did a reflection in the yz mirror plane they would flip positions, I'll put a diagram below.
Edit: I think I drew the C2(x) rotation wrong ( it should rotate about the x axis so C1 flips with the Cl to its left)
So for my irrep I got 0.5Ag 0.5B1g 0.5B2u 0.5B3u (this might be wrong I did it quickly lol but if you try doing it yourself you can see if it matches mine?)
So I got 2 peaks in the IR spec due to B2u and B3u and one peak in my raman due to Ag.
Let me know what you get if you redo the question, as as I said I'm still studying this so don't completely understand everything/ might have done something wrong!
As for the fractions I think they're possible as you're just expressing characters of some representations as linear combinations of characters of a smaller representation- again don't quote me on that as I haven't had to deal with fractions either, maybe someone more expert can weigh their opinion in lol
I tried answering the question, I'll explain what I did below, it might not be right however, as I am just studying this concept now- so don't completely rely on what I say!
For the trans molecule I got a different reducible representation to you:
In the same order you did I got: 2 0 0 0 0 2 0 0
The C2(x) rotation wouldn't keep the CO's in the same place I think, and if you did a reflection in the yz mirror plane they would flip positions, I'll put a diagram below.
Edit: I think I drew the C2(x) rotation wrong ( it should rotate about the x axis so C1 flips with the Cl to its left)
So for my irrep I got 0.5Ag 0.5B1g 0.5B2u 0.5B3u (this might be wrong I did it quickly lol but if you try doing it yourself you can see if it matches mine?)
So I got 2 peaks in the IR spec due to B2u and B3u and one peak in my raman due to Ag.
Let me know what you get if you redo the question, as as I said I'm still studying this so don't completely understand everything/ might have done something wrong!
As for the fractions I think they're possible as you're just expressing characters of some representations as linear combinations of characters of a smaller representation- again don't quote me on that as I haven't had to deal with fractions either, maybe someone more expert can weigh their opinion in lol
(edited 1 year ago)
Original post by ghostyblop
Hi,
I tried answering the question, I'll explain what I did below, it might not be right however, as I am just studying this concept now- so don't completely rely on what I say!
For the trans molecule I got a different reducible representation to you:
In the same order you did I got: 2 0 0 0 0 2 0 0
The C2(x) rotation wouldn't keep the CO's in the same place I think, and if you did a reflection in the yz mirror plane they would flip positions, I'll put a diagram below.
Edit: I think I drew the C2(x) rotation wrong ( it should rotate about the x axis so C1 flips with the Cl to its left)
So for my irrep I got 0.5Ag 0.5B1g 0.5B2u 0.5B3u (this might be wrong I did it quickly lol but if you try doing it yourself you can see if it matches mine?)
So I got 2 peaks in the IR spec due to B2u and B3u and one peak in my raman due to Ag.
Let me know what you get if you redo the question, as as I said I'm still studying this so don't completely understand everything/ might have done something wrong!
As for the fractions I think they're possible as you're just expressing characters of some representations as linear combinations of characters of a smaller representation- again don't quote me on that as I haven't had to deal with fractions either, maybe someone more expert can weigh their opinion in lol
I tried answering the question, I'll explain what I did below, it might not be right however, as I am just studying this concept now- so don't completely rely on what I say!
For the trans molecule I got a different reducible representation to you:
In the same order you did I got: 2 0 0 0 0 2 0 0
The C2(x) rotation wouldn't keep the CO's in the same place I think, and if you did a reflection in the yz mirror plane they would flip positions, I'll put a diagram below.
Edit: I think I drew the C2(x) rotation wrong ( it should rotate about the x axis so C1 flips with the Cl to its left)
So for my irrep I got 0.5Ag 0.5B1g 0.5B2u 0.5B3u (this might be wrong I did it quickly lol but if you try doing it yourself you can see if it matches mine?)
So I got 2 peaks in the IR spec due to B2u and B3u and one peak in my raman due to Ag.
Let me know what you get if you redo the question, as as I said I'm still studying this so don't completely understand everything/ might have done something wrong!
As for the fractions I think they're possible as you're just expressing characters of some representations as linear combinations of characters of a smaller representation- again don't quote me on that as I haven't had to deal with fractions either, maybe someone more expert can weigh their opinion in lol
I don't tink we're supposed to get fractions aha
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