Chapter 6: UV-Vis

Question 1

What is the region for UV?

Answer 1

200 nm-400 nm

Question 2

What is the region for visible light

Answer 2

400 nm-700 nm

Question 3

HOMO

Answer 3

Highest Occupied Molecular Orbital

Question 4

LUMO

Answer 4

Lowest Unoccupied Molecular Orbital

Question 5

Chromophore

Answer 5

Molecule responsible for absorption

Question 6

Oxochrome

Answer 6

Alters absorption, but does NOT absorb itself

Question 7

Bathochromic Shift

Answer 7

Lambda max shifts to lower energy (Red Shift)

Question 8

Hypsochromic

Answer 8

Lambda max shifts to higher energy (Blue Shift)

Question 9

What is more useful?
a) sigma to sigma*
b) n to sigma*
c) n to pi*
d) pi to pi*

Answer 9

D) pi to pi*

They are the most favorable to UV-Vis due to conjugation.

Question 10

What makes a better solvent: alkanes or alkenes?

Answer 10

Alkanes

Question 11

Spin Selection Rule

Answer 11

Electronic transition can NOT change net spin (M=2S+1)

Question 12

Frank-Condon Selection Rule

Answer 12

Symmetry of ground and excited states must overlap

Question 13

LaPorte Rule

Answer 13

For centrosymmetric molecule transition between orbitals of same party (symmetry) are forbidden.

Question 14

Ungerada

Answer 14

Changed by inversion

Question 15

Gerada

Answer 15

Unchanged by inversion

Question 16

Can you go from:
a) u to g
b) g to u
Both

Answer 16

a) u to g ONLY

Question 17

What is A in Beer’s Law? And what is its units?

Answer 17

Absorbance, no units.

Question 18

What is epsilon in Beer’s Law? And what is its units?

Answer 18

Molar Absorbtivity. M^-1*cm^-1

Question 19

What is b in Beer’s Law? And what is its units?

Answer 19

Path length. cm.

Question 20

What is c in Beer’s Law? And what is its units?

Answer 20

Concentration. M.

Question 21

What are some assumptions of UV-Vis?

Answer 21

1. Light is monochromatic (not true)
2. Absorbing species art independently of each other, problematic if H-bonding intermolecular (solvent) attraction.
3. There is no scattered light->minimize $$

Question 22

Are absorbances addititive?

Answer 22

Yes.

Question 23

What is advanced mode?

Answer 23

Determines epsilon at 1 nm intervals for all wavelengths in range from standards.

Question 24

Limitations with UV-Vis

Answer 24

1) Problems at high concentrations
2) Deviations due to mismatched or dirty cells. (Avoid this by looking at where A=0, baseline, all readings go to the same baseline).
3) Need to keep A<1
4) Roll over can occur due to
-high concentration due to internal screening
-Detector
-Index of refraction could change

Question 25

What is stray light?

Answer 25

Light that strikes the detector NOT at the nominal wavelength selected.

Question 26

What can be used as the radiant source in UV-Vis?

Answer 26

-Deuterium lamps
-Tungsten lamps
-Xenon lamps

Question 27

Double Beam

Answer 27

2 “cell holders” in sample compartment, one for “blank”, one for sample.
Essentially it “zeros on the fly” scans. It corrects for drift in light source.

Question 28

Single Beam

Answer 28

Very simple and cheap. Need to change wavelength if scanning. (Remember T=P/P0).

Question 29

Diode Array Polychromator

Answer 29

Removes exit slit and replaces with linear array of detectors.

Question 30

Types of cuvettes

Answer 30

-Acrylic Plastic
-Glass
-Quartz

Question 31

Sources of instrument noise

Answer 31

-Source
-Detector
-Signal processor/read out

Question 32

What can you do to lower noise? (Hint: Think temperature)

Answer 32

Cool detectors.