Sorry, but you will not be able to find such tables. Such numbers are
actually called molar extinction coefficients. In real practical life
they are applicable to only UV and visible light range. The reason for
this is a fundamental difference between absorption of electromagnetic
energy (i.e. light) by molecules depending upon the energy of photons
(quanta, whatever you want to call it). I am assuming you are familiar
with wavelengths scale and corresponding energy of em-waves. Shorter
wavelength corresponds to higher energy. You could imagine a similar
energy scale of molecules (or chemical bonds) by a property being able
to absorb energy. A molecule can only absorb light (or other
em-radiation) if there is a resonating frequency which is an intrinsic
property of a molecule or chemical bond. Now, imagine screening
through levels of energy (i.e. wavelength) going from high (UV-light)
through medium (regular visible light) to low (heat energy or IR) to
even lower (radio waves). You need to consider what is it in a
molecule that lets it absorb a particular band of em-radiation. There
are several things that do (bear in mind that this is very simplistic
explanation):
- chemical bonds can vibrate along with direction of a bond (i.e. UV-vis spectra);
- chemical bonds can rotate around an axis of a bond (i.e. IR-specra);
- electron shells can rotate (as a whole thing) around a nuclei within
a molecule (NMR spectra, when interact with certain nuclei on magnetic
level);
- etc.
Each of those "things" have specific most favorable frequencies, which
depend on chemical composition and structure of molecules.
When a molecule is irradiated with em-energy, a particular band of
energy that matches most favorable for a molecule, or a bond in a
molecule, will be absorbed by that molecule, just because it can.
Everything else will pass through without any change. This is
(simplistically) a basis for spectrophotometry as a method for
identification of molecules or chemical bonds within molecules.
Now, let's go through energy spectrum. UV and visible light (by
energy) happens to correspond to resonative vibrational energies or
frequencies of chemical bonds in most common molecules. Then, there is
a certain energetical gap where em-energy is not high enough to
correspond to vibrational energy of any bond, but it's not low enough
to correspond to any resonative rotational energy of any chemical
bond. Then, energy becomes low enough to be absorbed by many organic
(not "organic food" kind) molecules due to rotational properties of
chemical bonds. This is the energy band that actually represents IR
spectral properties, and this is the nature of your question. The
amount of energy that can be absorbed by one mole of molecules is
called "molar extinction coefficient". To fully understand why there
are tables and defined values for molar extinction coefficients for
visible and UV light, but not for IR, you need to consider a
temperature factor. Temperature (i.e. heat) is also energy. Resonative
vibrational energies of chemical bonds do not change much with
relatively broad changes of temperatures. A life example is: green
light is still green, regardless if you stop at a stop light in
Montana in January (say 0 degrees) or in August in Arizona (say 115
degrees). Not so for IR. IR is also heat after all. Little changes in
temperatures change the amount of energy that bonds are capable to
absorb dramatically. Few degrees matter a lot. The longer wavelength
you go the bigger the change. At some point, a degree or two may
change absorption properties several fold. At such point it becomes
very impractical to go through all trouble quantitatively measuring
energy that can be absorbed by a mole (or certain number of
molecules). Even if somebody did go through all such trouble, you
would not be able to reproduce it within necessary precision of
measuring (or rather controlling) the temperature in your lab. That's
the reason. It still works very well on a relative scale. IR spectra
are measured very quickly, so you can identify wavelength(s) that
correspond to resonative frequencies of molecules. And because there
are so many more quantum levels of rotational energy levels of
molecules, IR spectra are much more informative than UV-vis. They are
still not quantitative at all (unlike UV-vis spectra which are
quantitative), but they may be more informative for molecular
identification.
If you are familiar with NMR (radio waves band spectra), think about
why is that NMR spectra must be measured at least in liquid nitrogen
to be a little useful, but usually it takes liguid helium to match
modern requirements :-)
Hope this answers your question, or at least helps a little. |
