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Topic: What is the number of bond stretching vibrations in CH3HgCN?  (Read 3591 times)

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Offline eddzzz

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What is the number of bond stretching vibrations in CH3HgCN?
« on: January 10, 2012, 09:12:35 PM »
So this is a question on a past paper. This would be N-1 (no = no molecules) so,

6-1 = 5. Is this correct? Does seem simple. Cheers,

Ed

Offline Jorriss

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Re: What is the number of bond stretching vibrations in CH3HgCN?
« Reply #1 on: January 10, 2012, 09:46:41 PM »
That's not quite right.

http://en.wikipedia.org/wiki/Molecular_vibration

Read the second paragraph of the introduction. The number of vibrational degrees of freedom is not quite as simple as n-1, but it's still fairly straightforward.

Offline eddzzz

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Re: What is the number of bond stretching vibrations in CH3HgCN?
« Reply #2 on: January 10, 2012, 10:06:24 PM »
Thanks for the speedy reply. Its a multiple choice question in which: Find the incorrect statement:

a) Number of bond stretching vibrations in CH3HgCN = 6

b) the first overtone of a vibrational mode is found at approx double the frequency of the fundamental peak

c) bending of CO2 is doubly degenerate

d) rule of mutual exclusion applies only to thouse molecules with a centre of symmetry

e) intensity of first overtone peak in an infrared spectrum is much greater than that of the corresponding fundamental peak.

So for a) the 3(6)-6 = 12 so A would be wrong.

Offline swati

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Re: What is the number of bond stretching vibrations in CH3HgCN?
« Reply #3 on: August 14, 2012, 02:01:17 AM »
http://books.google.co.in/books?id=z08q2SyROjoC&pg=PA185&lpg=PA185&dq=number+of+bond+stretching+vibrations+is+%22N-1%22&source=bl&ots=36Us855ppW&sig=5_r3SNb9K53XpU4ZZqYxSv7Uuek&sa=X&ei=XeMpUJqqEIjJrAfL-YBo&ved=0CCYQ6AEwBDgK#v=onepage&q=number%20of%20bond%20stretching%20vibrations%20is%20%22N-1%22&f=false

Read the paragraph "Vibrations of Polyatomic molecules" in the above link

For 'N' number of molecules
Total degrees of freedom = 3N

No. of translational movements = 3

No. of rotational movements (non-linear molecule) = 3
No. of rotational movements (linear molecule) = 3 ( because it cant rotate along its bond axis)

No. of bond stretching vibrations = N-1
No. of bond bending vibrations (non-linear molecule) =(3N-6)-(N-1) = 2N-5 
No. of bond bending vibrations (linear molecule) =(3N-5)-(N-1) = 2N-4

Offline Jorriss

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Re: What is the number of bond stretching vibrations in CH3HgCN?
« Reply #4 on: August 14, 2012, 02:09:10 AM »
Oh damn, I was wrong.

It is N-1.

Wish you posted that 7 months ago.

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