[74Dan]

I was using a Smiths Detection Hazmat ID, and I noticed that both Chlorine and NaCl were listed in the onboard library. Chlorine being a pure element, and NaCl which is ionically bonded, I guess I'm just wondering how those two substances would create a measurable IR spectrum.

Thanks,
Dan

[Corribus]

Well, I would imagine that chlorine gas (Cl2) is FTIR silent, as most homonuclear diatomics are - because they have no change in dipole moment when they vibrate. It's possible "chlorine" is referring to hypochlorite (Cl-O), which is often colloquially known as "chlorine" in the context of disinfectants and swimming pool sterilizers. Chlorine would have a Raman spectrum, but I can't imagine why that would be in your FTIR library.

Likewise, NaCl is usually considered to be FTIR silent - they make optical windows out of it for this reason, after all. I suppose it probably has some kind of lattice vibration somewhere, but I don't imagine it's in the typical FTIR spectroscopic range.

So, off the top of my head I can't think of why these would be in your instrument's spectroscopic library. Does the library include spectra for these chemicals?

[74Dan]

Well, I would imagine that chlorine gas (Cl2) is FTIR silent, as most homonuclear diatomics are - because they have no change in dipole moment when they vibrate. It's possible "chlorine" is referring to hypochlorite (Cl-O), which is often colloquially known as "chlorine" in the context of disinfectants and swimming pool sterilizers. Chlorine would have a Raman spectrum, but I can't imagine why that would be in your FTIR library.

Likewise, NaCl is usually considered to be FTIR silent - they make optical windows out of it for this reason, after all. I suppose it probably has some kind of lattice vibration somewhere, but I don't imagine it's in the typical FTIR spectroscopic range.

So, off the top of my head I can't think of why these would be in your instrument's spectroscopic library. Does the library include spectra for these chemicals?

I feel a little bit ridiculous, because I was doing two things at once and clearly got jumbled when I wrote the original post. I was using an Ahura FirstDefender, which uses Raman. My question stands, though.

[Corribus]

Ah, well that's easier then. The library likely isn't referring to Cl the atom, but Cl₂ the molecule. This will vibrate with characteristic frequencies, and the energy differences between these levels is unique to the molecule. This can be detected by Raman (but not FTIR, because of the molecule's symmetry).

As for NaCl - anything that vibrates will have a characteristic vibration spectrum, and assuming the symmetry is right, Raman will be able to see it. For an ionic solid, as mentioned, these are usually lattice vibrations rather than vibrations of distinct chemical groups.
E.g., http://onlinelibrary.wiley.com/doi/10.1002/pssb.2220500119/abstract

I'm assuming since NaCl is FTIR-inactive but Raman-active (it must be, to have a library entry), then the vibrations must be relatively symmetric. But: I haven't done a lot of spectroscopy on inorganic solids like this, so I couldn't elaborate further without doing a bit of research. I do know that Raman can be used to detect the salt concentration in water, based on the way dissolved ions change the vibrational frequencies of water, but this probably isn't what the library entry is referring to.