UV-Vis Flashcards
How does conjugation affect UV-Vis?
Conjugation DECREASES the HOMO-LUMO energy gap. So,
Wavelength increases (red shift)
Energy decreases
As conjugation increases, Wavelength increases
As conjugation decreases, wavelength decreases
What is Red shift?
Redshift is an increase of wavelength. aka bathochromic shift
What is Blue shift?
Blueshift is a decrease of wavelength. aka hypochromic shift.
What effect does resonance have on UV-Vis?
Resonance does not necessarily affect wavelength. However, more often than not if there are more resonance structures, wavelength shortens (blue shift) (because conjugation length is longer)
What effect does pH have on UV-Vis?
Low pH causes lone pairs to get protonated which leads to shortened conjugation length. Which means, higher energy, lower wavelength (blue shift)
High pH allows lone pairs to remain, meaning they can participate in conjugation. Which means, lower energy, higher wavelength (red shift)
How do polar solvents effect UV-Vis?
Polar solvents create a smaller energy gap between pi-pi* energy levels, increasing wavelength for those transitions. (red shift)
At the same time, they increase the energy gap between n-pi* energy levels which decreases wavelength. (blue shift)
How do Non-polar solvents effect UV-Vis?
They don’t affect UV-Vis. In fact, they give super defined UV-Vis results.
What are allowed molar absorptivity values?
if e>10^4 it is a high intensity absorption. It is associated with pi-pi* bonds
10^5 would be pi-sigma*
10^6 would be sigma-sigma*
anything under 10^4 is considered low-intensity.
What are considered “forbidden” molar absorptivity levels.
0-1000 is considered “forbidden” and is associated with n-pi* transitions
