CCL: UV-vis in ORCA

 Sent to CCL by: Mariusz Radon [mariusz.radon%%gmail.com]
 On 11/05/2014 11:01 AM, Henrique Junior henriquecsj:+:gmail.com wrote:
 > Hi, my name is Henrique and I am an undergraduate student in Chemistry
 > doing some theoretical research about Fe(II) in water as a requirement
 > to finish my graduation.
 >
 > I’ve been using ORCA to perform some calculations and to generate
 UV-vis
 > spectra of [Fe(OH2)6]2+ and [Fe(OH2)6]3+.
 >
 > The result for [Fe(OH2)6]2+ is pretty much what I was expecting, but for
 > [Fe(OH2)6]3+ it seems a little weird to me:
 >
 > Since the d5 Iron(III) have only spin forbidden transitions, I
 wasn’t
 > expecting it to have more intense Abs than iron(II) (see attachment).
 >
 Dear Henrique:
 You are correct that for high-spin iron(III) complex all the d-d
 transitions are spin-forbidden, therefore will have exactly zero
 intensity (unless you include spin-orbit coupling in your calculations).
 Moreover, for centro-symmetric complexes, such as [Fe(H2O)6](3+), all
 the d-d transtions are also parity-forbidden (as gerade-to-gerade
 transitions they are forbidden by the Laporte rule). Hence, even if d-d
 the transitions were spin-allowed, the intensities calculated in TD-DFT
 will be zero for the ideal, centro-symmetric structure. To calculate
 intensities of such parity-forbidden transition correctly, you will need
 to consider coupling with molecular vibrations (of ungerade symmetry),
 which is far from being trivial. Therefore, I am afraid you cannot
 obtain correct intensities for such parity-forbidden d-d transitions
 > from single-point TD-DFT calculations.
 The intense absorption you have for Fe(III) at ca. 400 nm is almost
 certainly due to ligand-to-metal charge transfer transitions. Such
 transitions can be spin- and parity- allowed, thereby they will generate
 intense absorption bands.
 Best regards,
 Mariusz Radoń
 --
 Dr Mariusz Radon, Ph.D.
 Coordination Chemistry Group
 Faculty of Chemistry
 Jagiellonian University
 ul. Ingardena 3, 30-060 Krakow, Poland
 http://www2.chemia.uj.edu.pl/~mradon