fluorescence - 29-5-2016 at 05:42
Hi,
J.E. House - Inorganic Chemistry:
Mentions that the solvatochromism of Iodine in different solvents is caused by an interaction of the solvent with the pi -> pi* Transition in
Iodine. I know about solvatochromism, I had some lections on that but I was a bit confused that the dark color of Iodine seems to be caused by
pi-Transitions ?
So we did some electronic vibrational spectroscopy on Iodine once and I thought you excite the bonding electrons in Iodine for the transition and not
some pi-Orbitals. In fact the bonding and anti-bonding pi-molecular orbitals in Iodine are already full of electrons I can't see any space left for a
transiion there. So it will probably be a sigma - sigma* Transition (?).
Also I know that the Chlorine + Hydrogen explosion is caused by yellow-green light or anything stronger. So the color that activates the reaction
seems to be in the visible spectrum and probably the energy difference between the Cl-Cl-sigma -> simga* molecular orbital. And the green light
will cause the Bond to split for a fraction of a time which is enough to react with the hydrogen.
So I think that the solvent actually interacts with the energy gap between the two sigmas.
Any suggestions ?
[Edited on 29-5-2016 by fluorescence]
PHILOU Zrealone - 31-5-2016 at 12:30
The colour of I2 is essentially:
-violet in apolar solvents
-dark brown into polar solvents
The I2 molecule being constitued of large atoms (by definition external electron layer is thus polarisable by influence of a dipole momentum (thus a
polar solvent for example))... the solvatochromism must then be due to some extand to the polarisability of the I atom...
About H2 and Cl2, the weakest link is Cl-Cl --> 2 Cl° (about half the energy required for H2 dissociation).
What is strange is that Cl2 absorbs maximally in the 325 nm regio...what is more UV like, not visible yellow-green light...
fluorescence - 1-6-2016 at 03:53
Yeah well Iodine is kinda special with it's bond-no-bond situations. But the question is pretty much on the
actual transition which causes the color. It could be pi-pi* only if it was like Nitrogen where the sigma* is under the pi* Molecular Orbitals but I
think this is not the case in Iodine. It's a regular single bond and there is only one electron missing for each atom so its like Hydrogen....they
dont form double or tripple bonds so it should be a sigma-sigma* transition of you ask me.