UV-Vis Absorption

Question 1

What happens when a molecule absorbs a certain quantum of energy?

Answer 1

Causes the molecule to move to a higher energy level

Question 2

What is the difference between an atom vs. a molecule moving to a higher energy level?

Answer 2

• ATOM - movement of electrons to a higher energy level
• MOLECULE - movement to a higher vibrational, rotational and spin

Question 3

What is an orbital?

Answer 3

A region of space around the nucleus where electrons are likely to be found

Question 4

Explain hybridisation of orbitals

Answer 4

Combination of atomic orbitals formed when atoms covalently bond

Question 5

Order the following orbitals with respect to energy level:
3s, 1s, 2p, 3p, 2s

Answer 5

1s, 2s, 2p, 3s, 3p

Question 6

Explain how orbital hybridisation enables carbon to have four unpaired electrons that can have four equal energy bonds

Answer 6

• In Carbon, one of the electrons in the 2s orbital gets excited to a 2p orbital. This results in a configuration of 1s2, 2s1, 2p3, with four unpaired electrons
• The one 2s and three 2p orbitals then hybridise to form four new equivalent orbitals known as sp3 hybrid orbitals. Each of these hybrid orbitals has one unpaired electron

Question 7

Explain why hydrogen spontaneously forms diatomic molecules

Answer 7

Sigma bond allows both hydrogens to fill their valence 1s orbital creating a more stable molecule

Question 8

What is the difference between σ (or bonding orbital) vs. σ* (or anti-bonding orbital)?

Answer 8

• σ orbitals LOWER the energy of the system and stabilize the molecule
• σ* orbitals INCREASE the energy of the system and can destabilize the molecule

Question 9

What is the difference between a π and π* orbital?

Answer 9

• π bonds are BONDING orbitals
• π* bonds are NON-BONDING orbitals

Question 10

With respect to orbitals, how is a σ bond formed?

Answer 10

Formed by the overlap of orbitals in and END-TO-END fashion

Question 11

With respect to orbitals, how is a π bond formed?

Answer 11

Formed by the overlap of orbitals in a SIDE-TO-SIDE fashion

Question 12

What is the relationship between σ and π bonds, and single, double, and triple bonds?

Answer 12

• SINGLE BONDS are always σ bonds
• DOUBLE BONDS comprise of one σ and one π bond
• TRIPLE BONDS comprise of one σ and two π bonds

Question 13

Explain bonding and anti bonding orbitals and list the characteristics of each

Answer 13

BONDING
* when atomic orbitals combine in-phase or constructively
* electron density increased between nuclei
* lower energy state

ANTI-BONDING
* when atomic orbitals combine out of phase or destructively
* electron density decreased between nuclei
* higher energy state

Question 14

Why are σ to σ* transition not seen using standard UV spectroscopy?

Answer 14

Energy produced is too high and transition cannot be detected

Question 15

What is HOMO?

Answer 15

Highest Occupied Molecular Orbital

Question 16

What is LUMO?

Answer 16

Lowest Unoccupied Molecular Orbital

Question 17

What is the difference between HOMO and LUMO?

Answer 17

• HOMO donates electrons
• LUMO accepts electrons

Question 18

What energy is required to promote an electron from the π to the π*?

Answer 18

170 nm
(UV region)

Question 19

What is a Conjugated System?

Answer 19

When a molecule has alternating single and double bonds

Question 20

What happens to the energy gap as the number of double bonds that are conjugated increases?

Answer 20

Energy gap DECREASES therefore energy to move electrons between energy levels is DECREASED

Question 21

What is the relevance of conjugated double bonds to chromophores?

Answer 21

Molecule more likely to be detectable using UV-Vis

Question 22

What happens when the wavelength of light needed to promote and electron from the HOMO to the LUMO moves into the visible region?

Answer 22

Molecule more likely to be detectable using UV-Vis

Question 23

Give an example of a chromophore

Answer 23

Lycopene
(Abs = 505 nm)

Question 24

What is the relationship between the conjugated regions of chromophores and wavelength absorption?

Answer 24

The longer the conjugated regions, the longer the wavelength