[ifuller]

Hello all,
I know that microwave ovens work on the principle of sending microwaves of a certain wavelength into water molecules to make the vibrate and therefore heat them up. How does one determine the wavelength of electromagnetic energy needed to accomplish this? Is it possible to change the wavelength and therefore energize different elements? Thank you

[enahs]

E = hυ = hc/λ

E energy.
h, Planck's constant
υ, frequency
c, speed of light
λ, wavelength


Yes it is possible to change the wavelength and causes different things to happen to different molecules.

This is the fundamental principal of all spectroscopy.

Electromagnetic radiation in the Radio wavelength is used for NMR.
Electromagnetic radiation in the Microwave region is used for Rotational Spectroscopy
"               "            "           "  Infrared region is used for Vibrational Spectroscopy
"             "              "           "  Visible to UV region is used for various Electronic Spectroscopy

Those are just broad classifications, the point is that yes, changing the wavelength changes the effect greatly. Not all compounds respond to every type of Spectroscopy either.

To learn more, continue in your chemistry career and take Physical Chemistry, this type of stuff is typically covered in great detail in Quantum Physical Chemistry.

[ifuller]

Thank you very much for your answer. It explains allot to me. In the equation you gave me, how do you take into account the different molecules that you might want to energize? How do you come up with a wavelength for water that would be different than, lets just say.... H2 or NaCl or H2O2 ect? This probably is all covered in the chemistry course you told me about but is there a way to determine the energy of different molecules or of their bonds so that that value can be plugged in for E?

[enahs]

You can not really. It can only be done for Hydrogen. We can use the theory and method of doing it for hydrogen, and make assumptions, estimations, and make approximations for other molecules. Sometimes those approximations are pretty accurate, but as you get more and more complicated they get less and less accurate (and it gets infinitely hard to do it for more and more complicated molecules).

Ultimately it comes through measurements. Once you begin to cover spectroscopy you will find that while it is impossible to accurately account for every aspect of a molecule, it is quite easy to build up a data base of the common energy levels with respect to wavelength and frequency, or visa versa. You can then use those known data to infer the presence of a something like say OH group in a organic compound,etc.


The person who develops a mathematical approach that can do what we can for hydrogen, as well as it does (which, it is not even 100% spot on), but for any molecule, if possible; will simply put, change the entire scientific field like nothing else in modern history.



You also have to be careful by saying energize, energize how? That changes things.
Probably the easiest thing to start with, in this respect, would be to look at Spectral lines, using something like the Rydberg Formula/Equation.

[ifuller]

Thanks again,
When I said energize I was talking about the effect that the common microwave oven has on water molecules in that it makes them vibrate so that they heat up. The reason for all of these questions is that I am trying to figure out if I could use electromagnetic radiation to help proceed, almost catalyze chemical reactions. By adding energy of a certain wavelength I was thinking that it was possible to help reactions occur by "energizing" certain bonds.

[enahs]

Ohh yes, by all means.

You have to be careful when storing chemicals because light of various types either produced or catalysis a decomposition reaction.

Various forms of light and electromagnetic radiation are used all of the time for catalysis, or to better regulate a traditional chemical or physical catalyst activity, etc.

You can find more information then you could possibly understand right now by reading up on Photochemistry and/or photochemical reactions. It should be possible to find some general guidelines on developing something like this, but it will take experimentation, if at all possible. I have had very little experience to this type of chemistry.

[ifuller]

Thank you very much

[Sam (NG)]

The difference must be stressed between vibration and rotation.  Microwave ovens cause dielectric heating by inducing rotation in polar molecules.  Infra Red radiation causes vibrations within a molecule, as already stated by enahs.

[ifuller]

Which type of radiation do you think would help a reaction proceed?

[ifuller]

I have done alittle more research I think I need to determine the frequency at which the different bonds vibrate. Does anyone know where I might a table of values for this? Or at least how I might go about determining this? Thank you very much again

[Sam (NG)]

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

The units are wavenumbers, you can convert that to wavelength/frequency using c=λν (λ= wavelength in metres, ν= frequency in Hz) and wavenumber=1/ν where c is the speed of light in a vacuum (constant) and remembering that to do the conversion you must first change wavenumbers into m^-1.

If you want to find the exact frequencies, then run an IR spectrum of the reactant in the solvent that the reaction is going to be carried out in (because solvent can have an effect on absorption frequencies), and use the table i've given you to assign the peaks then you have the exact wavelength you need to excite that bond.

The IR data only works for bonds with a dipole, for bonds which do not have a dipole moment then you need to consider using Raman spectroscopy.

[Nick.Sparrow]

Hello, everyone!

I invite you all to discuss this topic on a new Microwave Chemistry forum

http://forum.microwavetec.com

The forum already works. The website itself

http://microwavetec.com

will start working very soon. Hope it will help you to answer many of your questions!