Ammonia group theory example for IR

[@eamyhtak18]

How did you arrive at the numbers for "contributions per atom" besides memorizing them?

[@Chemistryuniversity]

I did just memorize them, but since you asked, I have found the real reason. If you take the unit vectors for x, y and z (around the central atom) and add them up after the operation it will equal the contribution per atom.

[@eamyhtak18]

thank you!!! i used this shortcut method for my exam~ it's XeO4, Td group. it really helped!!

[@aswathyt3627]

good for easy understanding

[@qiaoliu1891]

Thanks for the great video! I have a question, when you calculate the contributions per atom for C3 operation, if you take the unit vectors for x, y, and z for the central atom (which is N), and add them up, you get 1 since z vector doesn't shift under C3 operation, why in the video you have 0 for the contribution of C3?

[@Chemistryuniversity]

The x and y sum to -1 and that cancels out the z.

[@kaidenjorge2748]

i guess it's quite off topic but do anyone know of a good site to stream newly released movies online ?

[@noorh3608]

How do I get lectures on this topic

[@jericflores9044]

How to determine the contribution

[@Chemistryuniversity]

Take the unit vectors (x,y,z) around the central atom and add them up after the operation.

[@mithunisenadeera8600]

Could you explain this for trans CH2Cl2 please?

[@Chemistryuniversity]

I can do one for another molecule. Are you asking for dichloromethane (no cis/trans isomers in a tetrahedron) or did you mean trans-1,2-dichloroethene (C2H2Cl2)?

[@mithunisenadeera8600]

@@Chemistryuniversity trans-1,2-dichloroethene

[@Chemistryuniversity]

Here is the link: kzbin.info/www/bejne/nHOmp4dqa850eJo I also did the cis isomer. kzbin.info/www/bejne/oWbKk4iYoN-Njpo

[@mithunisenadeera8600]

@@Chemistryuniversity Thank you so much!

[@willh9104]

So I'm not quite sure why the vector contribution for N is 0 when looking at the C3 axis. Doesn't the z vector (the vector along the principle axis) stay the same when the axis is rotated?

[@Chemistryuniversity]

The z axis vector is not changed, but the x and y vectors sum to -1 so the net is 0

[@willh9104]

@@Chemistryuniversity so are you only able to determine the -½ through the matrix? Or is it because of a physical change in the x and y vectors? (In my inorganic class we're only looking at spatial changes for the vectors, so reversed = -1, no change = 1, and any other change = 0.)

[@Chemistryuniversity]

Will Holder the contribution per atom comes from the sum of the unit vectors after the symmetry operation. We would need to do some trigonometry to calculate the x and y vectors since it is not aligned with an axis when rotating 120 degrees.

[@willh9104]

@@Chemistryuniversity ah I see. Thanks for the explanation! Your videos are great, love your channel.

[@Ann-vf5xy]

can you explain the matrix representation c3v of the nh3 for character table?

[@deshengzheng4704]

Ammonia has 6 vibrations

[@Chemistryuniversity]

Ammonia has 2 A1 vibrations and 2 E vibrational representations (which are 2 dimensional so there are 6 total vibrations, but 2 of these are degenerate so you should only see 4 bands in the spectrum).

[@deshengzheng4704]

I am totally agree with your comments. Thanks