24: UV and MO

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UV-VIs Theory and Application

Light in the UV region is known to excite electrons from one MO to higher MOs. Since MOs are understood as wavefunctions their behavior can be modeled as waves.

What is the relationship between energy and frequency?

There are 4 common types of transitions (n-π*, π-π*, σ-σ*, n-σ*) for organic compounds

What structural features must be present for each type of transition?
- σ -> σ*
- π -> π*
- n -> π*
- n -> σ*
Propose transition types for the list of functional groups below:
- Alkane
- Alkene
- Alkyne
- Alcoholo
- Carbonyl
- Amine
- Nitrile

Table of wavelengths and extinction coefficients (emax) for those transitions

Transition	~λ_max (nm)	ε_max
σ -> σ*	135	v. small
π -> π*	160-280	2,400-25,000
n -> π*	279-320	10-50
n -> σ*	160-250	100-7,000

Why are the λ_max values ranges and not discrete numbers?
Using the MO energy diagram below, explain the relative energies of the transitions.

Screen Shot 2019-03-22 at 9.12.21 PM.png

The extinction coefficient tells you how many electrons actually moved between those orbitals.

σ -> σ* has a small extinction coefficient. Why?
n -> π* are the lowest energy of any transitions we observe and consequently have the longest wavelength. However, they typically have the smallest extinction coefficient. Why is the n -> π* transition so difficult even though the energy gap is small?

Solvents used for UV spectroscopy samples.

What solvents (list solvents with structures) would interfere with UV analysis?
Not everything is soluble in hexane/octane, what range of wavelengths can you measure using methanol as your solvent?

UV Application Problem

You have isolated a previously unknown protein in your laboratory experiment. Some chemistry experiments have allowed you to determine that this protein contains no tryptophan and no phenylalanine. You need to know if there is any tyrosine present in the protein sequence. A UV spectrum of the pure sample of the protein is shown below. Is there tyrosine? How can you tell?

Screen Shot 2019-03-22 at 9.16.46 PM.png