Unit 4

Question 1

Red wavelengths?

Answer 1

650-700nm

Question 2

Orange wavelengths?

Answer 2

600-650nm

Question 3

Yellow wavelengths?

Answer 3

550-600nm

Question 4

Green wavelengths?

Answer 4

500-550nm

Question 5

Blue wavelengths?

Answer 5

450-500nm

Question 6

Purple wavelengths?

Answer 6

400-450nm

Question 7

Resonance

Answer 7

radiation must have the same energy as a transition between two states

Question 8

What kind of atomic/molecular transition is involved in UV-visible spectroscopy?

Answer 8

transitions of valence electrons

Question 9

What kind of atomic/molecular transition is involved in infrared spectroscopy?

Answer 9

vibration transitions

Question 10

What are ways in which an atom or molecule can become excited?

Answer 10

• thermal energy
• absorption of a photon
• chemical reaction

Question 11

Between which molecular orbitals do molecules energetically favour excitation?

Answer 11

HOMO to LUMO

Question 12

Ground singlet state (S0)

Answer 12

• totally spin quantum number = 0

- two electrons of opposite spin are in HOMO

Question 13

1st excited singlet state (S1)

Answer 13

• total spin quantum number = 0

- one electron is located in HOMO and a spin paired electron is in the LUMO

Question 14

How and why does an excited molecule return to ground state?

Answer 14

• excited state is transient

- via production of heat, light, or energy transfer to another molecule

Question 15

What are ground state to excited state transition energies dependent on?

Answer 15

• molecular structure
• intermolecular interactions
• local environment

Question 16

Vapour phase

Answer 16

• molecules can rotate and vibrate freely

- vibration and rotational fine structure observed

Question 17

Dissolved in hexane

Answer 17

• rotational freedom is lost

- collisions with solvent broaden the vibrational transitions

Question 18

Dissolved in water

Answer 18

• stronger interactions with solvent broaden transitions further
• obliteration of all fine structure

Question 19

Chromophore

Answer 19

part of a molecular entity responsible for an absorption band and the approximate location of the corresponding electronic transiition

Question 20

Provide an example of a chromophore

Answer 20

-aromatics
-alkenes
-alkynes
-carbonyls
(any unsaturated bonds)

Question 21

What can be done to decrease transition energies?

Answer 21

More conjugation of unsaturated bonds

Question 22

Effects of conjugation

Answer 22

• shifts absorption maximum to longer wavelengths

- absorptivity doubles with each conjugated double bond

Question 23

Bathochromic

Answer 23

shift to longer wavelength

Question 24

Hypsochromic

Answer 24

shift to shorter wavelength

Question 25

Hyperchromic

Answer 25

shift to greater absorbance

Question 26

Hypochromic

Answer 26

shift to lower absorbance

Question 27

Auxochrome

Answer 27

chemical group that is attached to a chromophore and modifies its light absorption properties by altering the energies of the MOs

Question 28

Transmittance

Answer 28

- the ration of the power (P) of the transmitted beam to the incident beam (P0) - often expressed as %

Question 29

Absorbance

Answer 29

- negative logarithm of transmittance | - number of orders of magnitude by which transmittance is decreased

Question 30

Blank measurement

Answer 30

- account for non-absorption losses of light | - measured using only the solvent and the same/matched cell

Question 31

Beer-Lambert Law

Answer 31

A=EbC (define!)

Question 32

Spectroscopy

Answer 32

the study of how physical systems interact with or produce electromagnetic (EM) radiation

Question 33

Spectrometry

Answer 33

the measurement of EM radiation to obtain information about systems and their components

Question 34

Spectrophotometry

Answer 34

describe measurements made with ultraviolet (UV), visible, and infrared (IR) light

Question 35

Molecular spectrophotometry

Answer 35

- method of molecular analysis | - samples comprise a collection molecules

Question 36

Atomic absorption spectrophotometry

Answer 36

- method of elemental analysis - isolated atoms do not usually exist in samples - must generate an atomic population with energy input

Question 37

Block diagram of spectrophotometer

Answer 37

light source > wavelength selector > sample cell > photodetector > electronics,computer

Question 38

Light sources

Answer 38

- tungsten halogen lamp - deuterium lamp - light emitting diode - laser

Question 39

Wavelength selectors

Answer 39

- diffraction grating - prism - filters

Question 40

Sample cell

Answer 40

- quartz, glass, plastic | - macro, micro, sub-micro

Question 41

Photodetector

Answer 41

- photomultiplier tube (PMT) - photodiode (array) - charge-coupled device (CCD)