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Author Topic: UV-Visible Spectroscopy: Electronic Transitions  (Read 8481 times)

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Winga

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UV-Visible Spectroscopy: Electronic Transitions
« on: April 04, 2005, 04:07:13 AM »

http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/spectrum.htm

Quote
As a rule, energetically favored electron promotion will be from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), and the resulting species is called an excited state.

I think there is a problem in this statement.
It stated that the e- transition from HOMO to LUMO is energetically favoured. What's that mean? It means that the lower energy gap between two states, the easier for the transition?

Just consider a situation of n->pi* transition, is it more favoured for the transition than that of pi->pi*?

If I made any mistake, please point it out, maybe I misunderstood the statement.
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Donaldson Tan

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Re:UV-Visible Spectroscopy: Electronic Transitions
« Reply #1 on: April 05, 2005, 06:11:20 PM »

there can be only 1 HOMO, either n or pi. you can't consider both in the same molecular system.

unless you have a molecule whose HOMO is n and its LUMO is pi*, then n->pi* is the most probable outcome.
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Winga

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Re:UV-Visible Spectroscopy: Electronic Transitions
« Reply #2 on: April 06, 2005, 02:54:43 AM »

I mean if the HOMO is pi orbital (so, LUMO is n orbital), it is much easier for pi to pi* transition than that of pi to n transition due to the symmetry of orbitals between two states.

However, the statement mentioned that it is energically favoured for HOMO to LUMO transition because of smaller energy gap between two states.
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Re:UV-Visible Spectroscopy: Electronic Transitions
« Reply #3 on: April 06, 2005, 10:00:39 AM »

If the HOMO is a pi then the LUMO will be a pi*

I can't think of any examples of a pi -> n transition.
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Winga

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Re:UV-Visible Spectroscopy: Electronic Transitions
« Reply #4 on: April 06, 2005, 10:55:45 AM »

Sorry, I made a mistake.

Just consider there are 2 situations:

1. n -> pi*
2. pi -> pi*

The energy gap between n & pi* is lower than that of between pi & pi* in most cases.

If electronic transitions are energetically favoured, this implies that n -> pi* transitions are more favourable than that of pi -> pi*  transitions due to less energy is required.

However, the intensity of transitions is depend on molar absorptivity (molar extinction coefficient), epsilon, and independent to delta E.
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Re:UV-Visible Spectroscopy: Electronic Transitions
« Reply #5 on: April 06, 2005, 12:08:18 PM »

If electronic transitions are energetically favoured, this implies that n -> pi* transitions are more favourable than that of pi -> pi*  transitions due to less energy is required.

However, the intensity of transitions is depend on molar absorptivity (molar extinction coefficient), epsilon, and independent to delta E.

Yes, exactly, less energy is necessary so it's easier to do.

The intensity is based on other things, as you said.  The energy necessary to effect the transition will dictate what wavelength of light will be necessary to cause the transition.  What you will see in the UV spectrum then is that the peak will be in a different place, then intensity, however, might vary drastically based on the other effects you mentioned.
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