qeezur
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designating electronic transitions in UV
Hi friends and experts.
I have a question. I synthesized a novel azo chromophore recently and took its UV/V. There are two absorption bands in it one centered at 290 and the
other exists at 510. I have assumed that the first band at ~290 nm with high ϵ is due to the secondary π -π* band of aromatic ring. The low ϵ
band at 510 nm is assigned to n-π* transition of –N=N- of azo.
My questions
1) How do we assign transitions (π-π* or n-π*) to bands in a UV/Vis spectra. Is the only way through the molar absorptivity ϵ that we know that
low ϵ and high wavelength absorption would always be n-π* ?
2) Same azo group also has π -π* in charge separated resonance form Ar+=N-N--Ar. Where is that π-π* in the UV spectra ?
Please guide me. I shall be extremely thankful.
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Metacelsus
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You might want to run some DFT calculations to look at the orbital energy levels. TD-DFT might also be useful.
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AvBaeyer
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You are correct in that the n->pi* will occur at longer wavelength and and with lower extinction than the pi->pi* transition. The
n->pi*transition should also show pH and solvent change shifts. If you have access to a good library, look at Jaffe and Orchin, "Theory and
Applications of Ultraviolet Spectroscopy, " (1962), especially pages 182ff, 276ff and page 430.
AvB
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Texium
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Thread Moved
7-11-2016 at 19:35 |
Texium
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Thread Moved
27-11-2023 at 10:55 |
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