Subject: HOMO-LUMO vs. lambda max

From: "Yeoh Hak Koon" <[email-masked]>
Date: Wed, 26 Apr 2000 11:35:54 +0800

Dear Colleagues,

Something not addressed in all past postings :

What are the reservations or objections of equating the HOMO-LUMO energy gap
to the maximum wavelength peak (lambda max) in a UV-Vis spectra?

Are these the reasons?
(a) the HOMO-LUMO transition might be forbidden, hence does not show up in
the spectra
(b) the lambda max has vibrational contributions, so it is usually more
energetic than the 0-0 transition predicted.

For (a), is there a way to predict whether the transition is forbidden or
not?  (I am using a commercial software PC Spartan Pro).

For (b), since at room temperature, the vibrational energy is relatively
small, I could still say the approximation is acceptable, right?

My apologies if these insult your intelligence; I am not trained as a
chemist, but a chemical engineer.  Thank you very much.

Best regards,
H. K. Yeoh
26 April 2000

---

From: Deepak Singh <[email-masked]>
Date: Wed, 26 Apr 2000 14:47:17 -0400

A simple HOMO-LUMO approach to calculating lambda max is fundamentally incorrect.
Spectroscopic properties require a good knowledge of the excited state surface,
which ground state geometry optimization methods, like  HF, will not give you.
For an excitation to be visible, (i) it must be allowed, (ii) it should not have
an oscillator strength close to 0.  To calculate abosrption maxima one has to
apply some method which includes electron correlation and can be applied to
excited states

The easiest way to calculate absorption maxima is to use a CI method like ZINDO.
Of course, ZINDO fails in cases like polyenes which have low lying forbidden
states, but it is a start.

Deepak.

Yeoh Hak Koon wrote:

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