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Stretching frequency of dimethyl sulphoxide?

Hi! I know this post was a while ago but I've got a question about reduced mass too!

I'm calculating the stretching frequency of dimethyl sulphoxide (DMSO) from an IR spectrum and part of the equation requires the reduced mass.

Below is the structure of DMSO.

DMSO.png

Should I use the mass of all the atoms? all the hydrogens? only some?

I'm really confused and I can't seem to find a clear explanation anywhere!

Please help! :smile:
Reply 1
Avatar for shengoc
shengoc
12
Original post by gadiscymraes
Hi! I know this post was a while ago but I've got a question about reduced mass too!

I'm calculating the stretching frequency of dimethyl sulphoxide (DMSO) from an IR spectrum and part of the equation requires the reduced mass.

Below is the structure of DMSO.

DMSO.png

Should I use the mass of all the atoms? all the hydrogens? only some?

I'm really confused and I can't seem to find a clear explanation anywhere!

Please help! :smile:


There are many bonds in the molecule shown. and the simple harmonic vibration theory is based on ONE vibrating "bond", i.e. between two atoms.

say in DMSO, you are interested in the stretching frequency of S-O, then
you only use the mass of S and O atom to calculate the reduced mass for the vibrating system.
Original post by shengoc
There are many bonds in the molecule shown. and the simple harmonic vibration theory is based on ONE vibrating "bond", i.e. between two atoms.

say in DMSO, you are interested in the stretching frequency of S-O, then
you only use the mass of S and O atom to calculate the reduced mass for the vibrating system.



Original post by gadiscymraes
Hi! I know this post was a while ago but I've got a question about reduced mass too!

I'm calculating the stretching frequency of dimethyl sulphoxide (DMSO) from an IR spectrum and part of the equation requires the reduced mass.

Below is the structure of DMSO.

DMSO.png

Should I use the mass of all the atoms? all the hydrogens? only some?

I'm really confused and I can't seem to find a clear explanation anywhere!

Please help! :smile:


The IR spectrum will of the stretching modes, not the individual bonds. MUCH more complicated
Reply 3
Avatar for gadiscymraes
gadiscymraes
OP
Original post by shengoc
There are many bonds in the molecule shown. and the simple harmonic vibration theory is based on ONE vibrating "bond", i.e. between two atoms.

say in DMSO, you are interested in the stretching frequency of S-O, then
you only use the mass of S and O atom to calculate the reduced mass for the vibrating system.


Ahh I see! Thank you so much! :smile:

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