[madscientist]

Hi all,

I am having a bit of trouble interpreting UV spectra I obtained of acetophenone and benzalacetophenone.  I think that i may have labeled my spectra the wrong way around, I ended up with a wavelength of maximum absorption (lambda max) of:

acetophenone: lamda max = 242nm

benzalacetophenone: lambda max = 210nm

The spectrum of acetophenone had two major absorption peaks whereas the spectrum of benzalacetophenone had only 1 major peak.  Looking at either compound it seems that benzalacetophenone has a higher level of conjugation than acetophenone. Benzalacetophenone should therefore have a lower energy gap between its HOMO and
LUMO and should therefore have a lambda max of a greater wavlength relative to that for acetophenone.  Longer wavelength = lower energy.  Looking at the spectra i have for both compounds it seems to have the opposite, i.e. the lambda max for benzalacetophenone is at lower wavelength (higher energy) than the lambda max for acetophenone???  Can anyone please help me understand why this has occurred or whether you think i just have my spectra labelled the wrong way around.

Cheers,

madscientist

acetophenone


benzalacetophenone

[madscientist]

Ive just checked with other students who have similar spectra and i dont think i have mine labelled wrong. So im still absolutely confused by the lambda max values...?

[Dolphinsiu]

http://en.wikipedia.org/wiki/UV_spectroscopy

I guess you have labelled them wrongly although you think not!

Or you have not cleaned the apparatus.

[DrCMS]

I think you have them labelled correctly.

What solvent did you run them in at what concentration was it the same for both?
Does the solvent have an absorption of ~210nm by any chance?
Can you subtract a solvent baseline from your results?

I think you'll find (http://webbook.nist.gov)

acetophenone: lambda max ~ 230nm

benzalacetophenone: lambda max ~ 300nm

[madscientist]

ethanol was used as a solvent but it was used as a reference, i.e used as the blank so its absorbance was already subtracted.

[Ψ×Ψ]

Lambda max doesn't refer to the highest intensity peak, it refers to the longest wavelength peak.  Take another look at your spectra.

[DrCMS]

Lambda max doesn't refer to the highest intensity peak, it refers to the longest wavelength peak.  Take another look at your spectra.

Rubbish!

Lambda max (λmax) is the wavelength at which the maximum absorption occurs.

I think you need to take another look at some textbooks.

[GSK]

Lambda max doesn't refer to the highest intensity peak, it refers to the longest wavelength peak.  Take another look at your spectra.

Rubbish!

Lambda max (λmax) is the wavelength at which the maximum absorption occurs.

I think you need to take another look at some textbooks.

You are correct. Lambda max gives you the wavelength of the strongest absorption, indeed

[Ψ×Ψ]

Huh?  I must have confused that with the absorption edge.  It's been a long week.   
Still, if looking for the HOMO-LUMO gap, the absorption edge (quick & dirty version, the peak at highest wavelength) is the band to use.  It corresponds to the lowest-energy electronic transition (unless that transition is highly forbidden and doesn't show).  This is what my lab uses along with reduction/oxidation potentials to get the HOMO & LUMO energy levels.

[madscientist]

lambda max (wavelength of maximum absorption) refers to the wavelength of the most intense absorption rather than the longest wavelenth absorbed, which reflects the energy of the lowest energy electronic transition (HOMO-LUMO gap).

[Ψ×Ψ]

Yes.  So you should be looking at the 278 nm peak in the acetophenone spectrum and the drawn-out peak around 350 nm for benzalacetophenone.  However, this suggests that your acetophenone spectrum is missing a low-intensity peak around 319 nm.  There is no sensible relationship between transition energy and intensity.

[ARGOS++]

Dear MadScientist,

In the moment I have NOT all relevant Data for your Problem ready, but I’m still able to give you a Lot of Information of similar Compounds and some "general rules" of UV/Vis in this region.

• A.) As a basic knowledge of UV/Vis it is a "quite general rules", that Lambda_max of the individual Bands are decreasing with increasing Wavelength, at least as long we are not taking about too high resolution spectroscopy (In Solutions?).
  This is true with only a few Exceptions in this Wavelength range.
• B.) In this very low Wavelength range (around ~200nm) you have to be aware, that you measure not only л -> л^* Transitions, you will also recognise different n -> л^* Transition of >C=O, -O-, -S-, or isolated >C=C<, and others with very strong Absorptions.
  But I have anyway some troubles to believe your Spectra below ~220nm, even if measured in Ethanol, Methanol or Water!  (Why?)
• C.) You are comparing, at least in my opinion the wrong Chromophores.
  As I’m missing the correct spectrum too, I would compare the Chromophore of your Product with “Cinnamic Acid” as one of the most like. To get your preferred Product you “have only to replace” the –OH Group by a Phenyl, what is meaning, that your Spectrum must be a little bit more bathochrome/”red” shifted as the Acid (But not only.)!
  I think you now know the cause for!
  One with a little “shorter” Chromophor is also “Styrene”, but anyway, both have at least an already  larger resulting Chromophore as your “Acetophenone”.
• D.) For your measurement technique, there is in my opinion a missing artefact, but about that later.

With all this in mind we may now start our discussion:
As your imagination may already tell you, IT COULD BE a “certain” possibility, that your Spectrum is to believe, but only: could, as I’m missing the correct data!
By all we are talking, we have to distinguish between the so called “E-Band” and the “B-Band”, indicating, that they are “caused” from the “Ethylene Part”, or the “Benzene Part” of the Molecule.

JAFFÉ et all reports for “Cinnamic Acid” a Lambda_max of ~275nm (ε ~21’000) in Methanol, for the E-Band, which in this special case will strongly overlap with its B-Band.
NIST reports for the same ~266nm (ε ~200’000; Solvent not known), but it seams, that the ε is not so really to be believed.
For Styrene the Data (from JAFFÉ et all) are ~248nm (ε ~14’000) in Methanol for the E-Band, ~282nm (ε ~750) for its B-Band, and for an other ~291nm (ε ~500). Remember, we have still to think, that at least in case of Styrene we have to do some Additions for the larger Conjugation, where I can’t tell you jet, if it will summaries to your observed ~350nm.
On the other hand, you will still be able to “recognise” something from “your” shifted “Acetophenone Part”-Chromophore.

As a conclusion:   I cannot find an argument jet, of course ONLY if your “Benzalacetophenone” is of enough purity, which makes me believing to not trust your presented spectrum, -at least not for all Data I have at the moment
You may see, the strong Absorption around ~200nm (Solvent dependent, too!) is not a real Argument against your measurements, even if it may not be so “strong shifted” or of the same Strength as for your Acetophenone!
(Try once to ignore this “Peak”, and rescale only the Absorption of your Spectrum!)

Maybe somebody is able to get more exact Data for Benzalacetophenone.

Good Luck!
                    ARGOS⁺⁺

P.S.:   I’m irritated about your light source, because the Peak near ~200nm seams to me to be toooo weak covered with noise. Were you measuring with a very high filter-constant, maybe?