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### Topic: Wavenumber Equation  (Read 667 times)

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#### berger

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##### Wavenumber Equation
« on: May 19, 2019, 07:42:05 PM »

The original question appears unattainable and beyond my current knowledge, however is there an equation by which I can calculate the wavenumber of organic molecules?

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[Original]
Does anyone know if there is an equation which can be used to calculate the infrared spectroscopy of organic chemistry molecules?

I am trying to write a school paper in which I will vary the chain length of an organic molecule such as ketones and then calculate the infrared spectroscopy and compare to literature values. I am not sure if there is an equation I can use for this use or even if such a thing is possible, should I be trying to calculate something like maximum transmittance?
« Last Edit: May 19, 2019, 09:55:02 PM by berger »

#### wildfyr

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##### Re: Infrared Spectroscopy Equation
« Reply #1 on: May 19, 2019, 09:07:18 PM »
A simple equation? No. A complicated set of code? Yes. It's a whole area of study in cutting edge computational chemistry.

Honestly, just from experienfe, the carbon chain length won't effect the location of a ketone carbonyl peak very much. Things like it being s small cyclic ketone or near another functional group are much more important.

#### berger

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##### Re: Infrared Spectroscopy Equation
« Reply #2 on: May 19, 2019, 09:38:25 PM »
Honestly, just from experienfe, the carbon chain length won't effect the location of a ketone carbonyl peak very much. Things like it being s small cyclic ketone or near another functional group are much more important.

Is there something similar I could use to compare differences between functional groups?

#### berger

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##### Re: Infrared Spectroscopy Equation
« Reply #3 on: May 19, 2019, 09:53:52 PM »
[EDIT] Instead of the original where I was asking to calculate the IR, is it possible to calculate the wavenumber from the chain length or functional group?

#### wildfyr

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##### Re: Wavenumber Equation
« Reply #4 on: May 20, 2019, 08:39:21 PM »
What do you mean "calculate the wave number?"

#### berger

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##### Re: Wavenumber Equation
« Reply #5 on: May 20, 2019, 10:40:17 PM »
What do you mean "calculate the wave number?"

Each functional group has a wavenumber, is this obtained through a calculation, an experiment or is it simply observed off the IR Spectrum?

#### spirochete

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##### Re: Wavenumber Equation
« Reply #6 on: May 21, 2019, 01:12:33 AM »
As others have explained, it is extremely difficult to calculate IR frequencies. Unless you have a background in physical/computational chemistry, this is probably a bad idea to hinge your paper on. Even then, it might be more like a long research project rather than a single paper to do in school in an undergraduate class.

Mostly, the IR frequencies are experimentally determined. After the fact, you could then rationalize things based on hybridization, resonance, induction, ring strain, etc.

#### Corribus

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##### Re: Wavenumber Equation
« Reply #7 on: May 21, 2019, 08:44:27 AM »
The problem is that large molecules have tons of vibrational modes and they exhibit complex coupling with each other that is very difficult to predict. Throw in intermolecular coupling and.. well maybe you see the scale of that challenge.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

#### berger

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##### Re: Wavenumber Equation
« Reply #8 on: May 21, 2019, 04:32:00 PM »
It gets very hard to then calculate mm I see, is there any observations or calculations which can be made off the wave number?

#### berger

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##### Re: Wavenumber Equation
« Reply #9 on: May 21, 2019, 06:01:28 PM »
Okay, new question again. Is there an equation which I can use to deduce what kinds of vibrations a molecule undergoes at a certain wavelength based upon its wave number?

#### mjc123

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##### Re: Wavenumber Equation
« Reply #10 on: May 22, 2019, 04:27:59 AM »
Your question is meaningless. Have you any idea what "wavenumber" means?