Chapter 5: 5.2 Infrared Spectroscopy

Question 1

Define:
Wavenumber

Answer 1

The reciprocal of the wavelength

Question 2

What is wavenumber measured in?

Answer 2

1/cm

Question 3

Why are wavenumbers convenient?

Answer 3

Unlike energy units (joules), which are very small numbers, and frequency units (hertz), which are very large numbers, wavenumbers in 1/cm is very manageable for IR light

Question 4

When a molecule absorbs __ light of the proper wavelength, it is excited from the ______ state to an _______ state

Answer 4

IR
Ground
Excited

Question 5

In IR spectroscopy, how are bonds excited?

Answer 5

Vibrational excitement

Question 6

What are the two main types of molecular vibration?

Answer 6

Stretching
Bending

Question 7

__________ requires more energy than _______

Answer 7

Stretching
Bending

Question 8

What types of stretching are there?

Answer 8

Symmetric
Asymmetric

Question 9

What types of bending are there?

Answer 9

Scissoring
Rocking
Wagging
Twisting

Question 10

True or False:
In IR Spectroscopy, different bonds have the same absorption frequencies

Answer 10

False

Question 11

What is the relationship between absorption and transmittance?

Answer 11

High absorption = Low transmittance

Question 12

In UV Spectroscopy, __________ is the function of __________

Answer 12

Absorption
Wavelength

Question 13

In IR Spectroscopy, _____________ is function of __________

Answer 13

Transmittance
Wavenumber

Question 14

What is the region from about 1500-500 cm^-1 known as? Why?

Answer 14

Fingerprint region
Very complex region that is usually unique to any given compound

Question 15

What is the region from 4000-1500 cm^-1 known as? Why?

Answer 15

Functional group region
Functional groups are readily identified in this region

Question 16

What can molecular bonds be thought of?

Answer 16

Balls and springs that vibrate as a harmonic oscillator

Question 17

What is k in frequency of vibration calculation?

Answer 17

A force constant proportional to the strength or “stiffness” of the bond

Question 18

What is μ in frequency of vibration calculation?

Answer 18

The reduced mass of the molecule expressed in kilograms

Question 19

What does bond strength affect?

Answer 19

k (strength or stiffness of bond)

Question 20

What do stronger bonds result in?

Answer 20

Stronger bonds = Higher k = Higher frequency

Question 21

What does hybridization result in?

Answer 21

Since s orbital is smaller:
More s character = Smaller orbital = Smaller bond length

Question 22

What does mass difference affect?

Answer 22

μ (reduced mass of molecule)

Question 23

What does mass result in?

Answer 23

Heavier atoms connected to spring = Lower frequency

Question 24

A stretch below 3000 cm^-1 shows…

Answer 24

sp3 C-H

Question 25

A stretch 3000-3300 cm^-1 shows...

Answer 25

sp2 C-H

Question 26

A stretch around 3300 cm^-1 shows...

Answer 26

sp C-H

Question 27

What are characteristics of Aromatic rings in IR absorptions?

Answer 27

sp2 C-H stretches C=C stretches between 1400-1600 cm^-1

Question 28

What are characteristics of alcohols in IR absorptions?

Answer 28

Strong, broad OH stretch at 3200-3500 cm^-1

Question 29

What are the types of amines?

Answer 29

Primary amine (one carbon connected to N, 2 N-H) Secondary amine (two carbons connected to N, 1 N-H) Tertiary amine (three carbons connected to N (no N-H)

Question 30

What are characteristics of amines in IR absorptions?

Answer 30

Absorption bands in the 3100-3500 cm^-1 region Number of bands depends on the number H atoms connected to N (primary = 2 band, secondary = 1, tertiary = 0)

Question 31

What are characteristics of carbonyls in IR absorptions?

Answer 31

Sharp, strong absorption in the region of 1600-1800 cm^-1

Question 32

Are aldehydes more easily oxidized than ketones?

Answer 32

No, the opposite is true

Question 33

What are characteristics of carboxylic acids in IR absorptions?

Answer 33

Strong C=O stretch at 1700 cm^-1 Very broad, offset O-H stretch between 2400-3400 cm^-1

Question 34

What are characteristics of esters in IR absorptions?

Answer 34

Strong C=O stretch at around 1750 cm^-1 Lacks broad O-H stretch Strong C-O stretches at around 1000-1250 cm^-1