Lecture 10 - IR Spectroscopy

Question 1

spectroscopy:

Answer 1

field of study of the interaction of matter and light

Question 2

what can spectroscopy be used to do?

Answer 2

spectroscopy can be used to determine unknown molecular structures

Question 3

what is light?

Answer 3

electromagnetic radiation that comes in various waves

Question 4

types of electromagnetic radiation:

Answer 4

visible, x-ray and ultraviolet light

Question 5

what does electromagnetic radiation propagate as and what is it categorised by?

Answer 5

EM radiation propagates as a wave motion through space and is categorized by its wavelength (l) or frequency (n)

Question 6

light particles are called ________

Answer 6

photons

Question 7

how can light exist?

Answer 7

although light can be described as a wave it also exhibits particle-like behaviour

Question 8

the most common type of spectroscopy used for structure determination:

Answer 8

absorption spectroscopy

Question 9

the amount of electromagnetic radiation absorbed by a sample is determined by?

Answer 9

a spectrometer or spectrophotometer

Question 10

what does infrared spectroscopy provide information about?

Answer 10

infrared provides information about what functional groups are present

Question 11

what does nuclear magnetic resonance provide information about?

Answer 11

nuclear magnetic resonance provides information on the number, connectivity and functional group environment of C & H

Question 12

what information does UV-visible spectroscopy provide information about?

Answer 12

UV-visible provides information about the types of p-electron systems present

Question 13

how is the light absorbed by a substance is recorded?

Answer 13

as a function of wavelength

Question 14

the horizontal axis is the __________ the ________ of the wavelength

Answer 14

wave number

inverse

Question 15

what are the conventional units of the infrared spectrum?

Answer 15

reciprocal cm or inverse cm (cm^-1)

Question 16

what is the y axis of infrared spectroscopy?

Answer 16

percent transmittance

Question 17

what are absorptions in the IR spectrum the result of?

Answer 17

they absorbances are as a result of bond vibrations

Question 18

what must the radiation exactly match?

Answer 18

the frequency of radiation must exactly match the frequency of bond vibration

Question 19

what do peaks correspond to in an IR spectrometry graph?

Answer 19

peaks corresponds to absorption of energy by the vibration of a particular bond or group

not all peaks can be interpreted but many are diagnostic

Question 20

wavenumber for O-H, N-H & C-H:

Answer 20

3400-2800

Question 21

wavenumber for C///N, C///C:

Answer 21

2250-2100

Question 22

wavenumber for C=O, C=N, C=C:

Answer 22

1850-1600

Question 23

wavenumber for C—C, C—O, C—N:

Answer 23

1600-1000

Question 24

wavenumber for C—H:

Answer 24

1000-600

Question 25

fingerprint region:

Answer 25

absorptions in this region are not usually interpreted, but can serve as a valuable molecular fingerprint

Question 26

Factors That Determine IR Absorption Position:

Answer 26

(1) strength of bond: stronger bond = higher frequencies

(2) masses of atoms:lighter masses = higher frequencies

(3) type of vibration: stretching vibrations = higher frequencies than bending ones

Question 27

three types of normal vibrational nodes:

Answer 27

symmetrical stretch, unsymmetrical stretch, unsymmetrical in-plan bend (rock)

Question 28

Factors That Determine IR Absorption Intensity:

Answer 28

• overall peak intensity is related to the concentration of the sample

• relative peak intensity is additive: A large number of similar groups (e.g., alkyl) will increase the intensity of a given peak

• relative peak intensity is also due to the dipole moment

Question 29

dipole moment is related to:

Answer 29

the charge separation and distance between two atoms

Question 30

as the bond stretches the dipole…

Answer 30

… dipole increases

Question 31

as the bond compresses the dipole….

Answer 31

… decreases

Question 32

The electric field of a light wave cannot interact with a bond that…

Answer 32

has no dipole

bonds with no dipole will not absorb in IR

Question 33

how do groups with large dipole interact with IR?

Answer 33

groups with large dipoles provide intense absorptions

Question 34

Molecular vibrations that occur but do not give rise to IR absorption are said to be…

Answer 34

infrared-inactive

Question 35

Any vibration that does give rise to an absorption is said to be…

Answer 35

infrared-active

Question 36

IR Spectra of Alcohols and Ethers:

Answer 36

•O-H stretch (H-bonded): 3200-3400 cm-1
•O-H stretch (not H-bonded): 3600 cm-1
•C-O stretch: 1050-1200 cm-1 (ROH and ethers)

Question 37

IR Spectroscopy of Aldehydes and Ketones:

Answer 37

Strong C=O stretch: 1700 cm-1

Question 38

Conjugation with a p bond…

Answer 38

lowers the absorption frequency (typically by 20-30 cm-1)

Question 39

The Infrared Spectrometer Machine:

Answer 39

•Most modern IR spectrometers are Fourier-transform spectrometers

•Liquid samples can be analyzed undiluted (neat), as a mineral oil dispersion (mull), or as a solution (CHCl3 or CH2Cl2 as solvent)

•Solid samples can be analyzed as a fused KBr pellet

•Our instruments are attenuated total reflectance (ATR) instruments

Question 40

spectroscopy deals with:

Answer 40

the interaction of matter and electromagnetic radiation

Question 41

IR spectroscopy deals with:

Answer 41

molecular vibrations

Question 42

there needs to be a change in ______ ______ on vibration

Answer 42

dipole moment