The absorption maximum is around 800 nm, and its molar absorptivity is about 12-13 l/(mol*cm). Example may be found at http://www.nntu.ru/trudy/2013/02/252-256.pdf (Fig. 1, curve 1).

Do you really mean Cu(H2O)4+2 or rather the normal hexahydrate Cu(H2O)6+2.

In which solvent are you working ?

These publications deal with Cu++ hydrolysis in an aqueous environment, from theoretical and experimental point of view.

The 1st one provides theoretical calculations of the uv-vis-nir spectra as function of conformation. It clearly shows that the spectra are highly dependent on the conformation of the hydrated Cu++ ion.

[1] K.J. de Almeida, N.A. Murugan, Z. Rinkevicius, H.W. Hugosson, O. Vahtras, H. Agren, A. Cesar, Physical Chemistry Chemical Physics : PCCP 11 (2009) 508.

[2] V.S. Bryantsev, M.S. Diallo, W.A. Goddard, The Journal of Physical Chemistry. A 113 (2009) 9559.

[3] J. Chaboy, A. Muñoz-Páez, P.J. Merkling, E. Sánchez Marcos, The Journal of Chemical Physics 124 (2006) 64509.

Do you mean maximum wavelength at which the compound absorbs? In which well you can easily determine with the sensitivity of your UV/Visible spectrometer. Just dissolve the substance in your chosen solvent. After that, measure the absorbance from 400nm - 800nm. You can then determine the maximum wavelength. You could as well decide to vary the concentration of the substance, measure the absorbance. Plot your absorbance against concentration. From the slope you will know the molar absorptivity.

It has quite a complex spectrum.

First whether it should be represented as [Cu(OH2)6]^2 or [Cu(OH2)4]^2 is argued as follows:

With octahedral stereochemistry, its ground state [2Eg] is subjected to a considerable Jahn Teller effect .This effect becomes all the more pronounced because of its very high λ(cu^2)[-830.0 cm-1] value to make it tetragonal with four Cu(II)-OH2 lying in one (xy) plane while the two other Cu(II)-OH2 bonds lie above and below this plane along z-axis. These two bonds very largely elongated. So their effect may be neglected to make it practically [Cu(OH2)4]^2 .

As regards to its spectrum, in a tetragonal distortion where the two bonds are elongated,

the splitting of the five d orbitals may be represented as:

dxz=dyz, dxy , dz2, dx2-y2 (their order of energies may change depending upon the extent of distortion; the first four are completely filled and the dx2-y2 being half filled).

In fact, it gives three transitions [d xz= d yz to dx2-y2; dxy to dx2-y2 and dz2 to dx2-y2]. These three transitions do not differ much in their energies and thus can not be resolved by ordinary spectrometers. So they merge to give a broad band which, in fact, consists of a packet of three closely spaced bands. Again it is difficult to locate the exact value of its λ max, but is ≈ 800.0 nm with comparatively large molar absorptivity (≈15).

Manohar

Just a side-line I looked at your page at Researchgate and some of your articles do not have titles?

Then you say the absorptivity is large? compared to the calculated or estimated extinction?

I ask this becassue we use Cu2+ as reference absorber at 750 nm for our carbon black monitor

Humbly, I submit as:

[I] I have already made a mention that due to the presence of a very broad band which, in fact, contains three closely spaced bands, it is very difficult to locate the exact position of λ max as is indicated the ≈ sign. [II] Again absorptivity is also appxo. but this value is higher than As COMPARED TO most of the symmetry forbidden octahedral complexes of first transition series metal ions because of the presence of distortion brought about by Jahn Teller effect in Cu(II) complexes.[III] As regards to the discrepancies in the names of articles, BELIEVE me that TWICE I have brought it to the notice of Research Gate that I have only 9 papers to my credit[8 articles and one conference] and not 11 papers. Let me further add that I have only two articles in Polyhedron and not three. [IV] [IV] Lastly that at the age of 70, I have no career to further. I learnt the working on the laptop very recently and joined the Research Gate only to keep myself in touch with the subject even after10 years of my retirement simply for the love of the subject and to learn more from young scholars like your self. So thanks a lot my friend. I will have no regrets even if I lose.

Manohar

Thank you for your nice explanation of the absorption.

What I wanted to mention is the very odd titles of your publications. You can edit your contributions. Below the article you can chose remove, and then the option fully. Then you can enter your contribution in the way you want it. If you have a question with this just mail me via ResGate

addition

When you click "remove" there are 3 options

You have to click then

"Delete publication"