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UV-Visible Spectroscopy Fundamentals
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UV-Visible Spectroscopy Fundamentals
UV-Visible Spectroscopy Fundamentals
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1
Question
What wavelength ranges define the ultraviolet and visible regions in UV-Visible spectroscopy?
Page 2
Answer
Ultraviolet: 10–400 nm. Visible: 400–800 nm.
2
Question
Why is UV and visible absorption discussed together as one technique?
Page 2
Answer
Both are based on the same principle and lead to the same type of molecular electronic excitation.
3
Question
What alternative name is commonly used for UV-Visible spectroscopy?
Page 2
Answer
Electronic spectroscopy.
4
Question
What molecular energy levels are involved when UV/visible radiation is absorbed?
Page 2
Answer
Electronic energy levels of molecules are involved in the transitions.
5
Question
What is the vacuum ultraviolet region and why is it named that way?
Page 3
Answer
The region below 200 nm is the vacuum ultraviolet region because oxygen in air absorbs there so measurements need vacuum conditions.
6
Question
How can the vacuum UV region be studied despite oxygen absorption?
Page 3
Answer
By evacuating the instrument so the path is under vacuum and oxygen is removed.
7
Question
Why is studying wavelengths below 200 nm usually unnecessary for structure elucidation?
Page 3
Answer
Because most structural information is obtained from the ordinary UV region and visible region, so vacuum UV is not usually essential.
8
Question
What name is given to the ultraviolet region from 200 to 400 nm and why?
Page 3
Answer
The ordinary or quartz ultraviolet region because the atmosphere is transparent there and in the visible region.
9
Question
What happens to valence electrons when visible or ultraviolet light is absorbed by a molecule?
Page 4
Answer
Valence electrons are promoted from their ground states to higher energy excited states.
10
Question
Why are electronic absorption peaks usually broad instead of sharp?
Page 4
Answer
Because vibrational and rotational energy levels accompany electronic transitions, producing broadened peaks.
11
Question
Why are absorption energies quantized in molecules?
Page 4
Answer
Because the energies of the orbitals involved in electronic transitions have fixed discrete values.
12
Question
From which molecular orbital to which orbital do electronic transitions generally occur?
Page 5
Answer
From the highest occupied molecular orbital (bonding or nonbonding) to the lowest unoccupied molecular orbital (antibonding).
13
Question
Name the four main types of electronic transitions important in UV/VIS spectroscopy.
Page 6
Answer
σ → σ*, n → σ*, π → π*, and n → π*.
14
Question
What does the notation σ → σ* represent in electronic transitions?
Page 6
Answer
An electron promoted from a bonding sigma orbital to its antibonding sigma-star orbital.
15
Question
What kind of molecules primarily show σ → σ* transitions?
Page 7
Answer
Saturated hydrocarbons such as CH3—CH3 primarily show σ → σ* transitions.
16
Question
Why do σ → σ* transitions absorb at very short wavelengths?
Page 7
Answer
Because exciting sigma electrons needs a large amount of energy, so absorption occurs at vacuum UV wavelengths below about 150 nm.
17
Question
At approximately what wavelength does a typical C–C σ → σ* absorption occur?
Page 7
Answer
About 135 nm for a C–C bond.
18
Question
At approximately what wavelength does a typical C–H σ → σ* absorption occur?
Page 7
Answer
About 125 nm for a C–H bond.
19
Question
What is the cyclopropane exception mentioned for σ → σ* absorptions?
Page 7
Answer
Cyclopropane absorbs at about 190 nm, probably due to ring strain.
20
Question
Why are σ → σ* transitions usually not useful for routine UV/VIS analysis?
Page 7
Answer
Because they absorb in the vacuum UV below 200 nm, which most spectrophotometers cannot detect.
21
Question
In what kinds of molecules do n → σ* transitions occur?
Page 8
Answer
Saturated molecules that contain heteroatoms like oxygen, nitrogen, sulfur, and halogens.
22
Question
Describe the electronic change in an n → σ* transition.
Page 8
Answer
An electron moves from a nonbonding orbital on a heteroatom to an antibonding sigma orbital.
23
Question
What wavelength range is typical for n → σ* transitions?
Page 8
Answer
Between about 150 and 250 nm.
24
Question
What n → σ* absorption band does methyl alcohol show and at what wavelength?
Page 8
Answer
Methyl alcohol shows an n → σ* absorption band at 183 nm.
25
Question
What n → σ* absorption band does trimethylamine show and at what wavelength?
Page 8
Answer
Trimethylamine shows an n → σ* absorption band at 227 nm.
26
Question
Why does trimethylamine show no n → σ* absorption in aqueous acid?
Page 8
Answer
Because protonation removes nonbonding electrons, so the protonated amine has no nonbonding electrons to excite.
27
Question
Are many n → σ* transitions observable with ordinary UV/VIS spectrophotometers?
Page 8
Answer
Yes, many n → σ* transitions are observable with ordinary UV/VIS spectrophotometers.
28
Question
What structural features lead to π → π* transitions in molecules?
Page 9
Answer
Presence of double bonds, triple bonds, or aromatic rings leads to π → π* transitions.
29
Question
What is the basic orbital change described by a π → π* transition?
Page 9
Answer
An electron is promoted from a π bonding orbital to a π* antibonding orbital.
30
Question
What wavelength range is typical for simple π → π* transitions like ethylene?
Page 9
Answer
Generally around 160–190 nm, for example ethylene at 171 nm.