Comments on: UV-Vis Spectroscopy: Absorbance of Carbonyls

By: David

David — Fri, 17 Sep 2021 02:30:47 +0000

Very good. Made the learning fun.

By: James Ashenhurst

James Ashenhurst — Thu, 06 May 2021 17:50:35 +0000

In reply to Kelcey.

Like I said I could not find the original, and I linked to a page which was not my intellectual property.

Another source could be here, on the NIST webpage:
https://webbook.nist.gov/cgi/cbook.cgi?ID=C67641&Units=SI&Mask=400#UV-Vis-Spec

By: Kelcey

Kelcey — Thu, 06 May 2021 16:12:38 +0000

The link for the acetone spectrum showing lambda max at 195nm does not seem exist. It goes to a general page. Do you have an updated link or any link that shows absorbance at 195nm?

By: Pakiza

Pakiza — Thu, 18 Mar 2021 05:39:09 +0000

Hello
i have a Question plx…in the uv vis soectrum of acetobe we only consider two types of transition which are pi to pi* or n to pi*. My question is that sigma bonds are also present in carbonyls, similarly there may be transition between n to sigma*. Then why we consider only two possible transitions? why we don’t consider other transitions due to sigma bonds?

By: James Ashenhurst

James Ashenhurst — Fri, 11 Dec 2020 02:56:31 +0000

In reply to Musaib Yousuf.

See previous post in the series: https://www.masterorganicchemistry.com/2016/09/16/introduction-to-uv-vis-spectroscopy/

By: Musaib Yousuf

Musaib Yousuf — Tue, 08 Dec 2020 05:43:36 +0000

Nice explanation.
Related question;
How do conjugation helps in increasing the lambda max of conjugated compounds?

By: Ron Swisher

Ron Swisher — Sun, 12 Apr 2020 03:54:14 +0000

Great discussion of the topic!!
Question on Azo yellow dye. When I look up the absorption spectrum of cis azo yellow on the internet, I see that the absorption peak at ~420 is much weaker than the peak further into the UV, which would be expected for an n->pi star transition for the reasons you explained, but the lambda max for the stronger shorter wavelength absorption of the cis is actually at shorter wavelength than the trans. I would expect the opposite given the steric strain with the two phenyl groups in the cis conformation and poorer overlap of the p orbitals

Related question: I presume that the n > pi star transition results in weakening of the N=N double bond just like a pi>pi star transition, thus allowing the cis trans photo isomerization?
Thanks!

By: James Ashenhurst

James Ashenhurst — Mon, 07 Oct 2019 15:44:01 +0000

In reply to Tyson. Glad to hear it Tyson. Thanks!

By: Tyson

Tyson — Sat, 05 Oct 2019 09:27:30 +0000

Very helpful in my revision on organic spectroscopy…

By: James Ashenhurst

James Ashenhurst — Tue, 14 May 2019 05:54:12 +0000

In reply to Steve. Awesome Steve! Thanks for letting me know!