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My Packs: UWO (CHEM 1301) > Chapter 5: 5.2 Infrared Spectroscopy > Flashcards

Chapter 5: 5.2 Infrared Spectroscopy Flashcards

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1
Q

Define:
Wavenumber

A

The reciprocal of the wavelength

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2
Q

What is wavenumber measured in?

A

1/cm

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3
Q

Why are wavenumbers convenient?

A

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

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4
Q

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

A

IR
Ground
Excited

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5
Q

In IR spectroscopy, how are bonds excited?

A

Vibrational excitement

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6
Q

What are the two main types of molecular vibration?

A

Stretching
Bending

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7
Q

__________ requires more energy than _______

A

Stretching
Bending

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8
Q

What types of stretching are there?

A

Symmetric
Asymmetric

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9
Q

What types of bending are there?

A

Scissoring
Rocking
Wagging
Twisting

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10
Q

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

A

False

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11
Q

What is the relationship between absorption and transmittance?

A

High absorption = Low transmittance

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12
Q

In UV Spectroscopy, __________ is the function of __________

A

Absorption
Wavelength

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13
Q

In IR Spectroscopy, _____________ is function of __________

A

Transmittance
Wavenumber

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14
Q

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

A

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

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15
Q

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

A

Functional group region
Functional groups are readily identified in this region

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16
Q

What can molecular bonds be thought of?

A

Balls and springs that vibrate as a harmonic oscillator

17
Q

What is k in frequency of vibration calculation?

A

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

18
Q

What is μ in frequency of vibration calculation?

A

The reduced mass of the molecule expressed in kilograms

19
Q

What does bond strength affect?

A

k (strength or stiffness of bond)

20
Q

What do stronger bonds result in?

A

Stronger bonds = Higher k = Higher frequency

21
Q

What does hybridization result in?

A

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

22
Q

What does mass difference affect?

A

μ (reduced mass of molecule)

23
Q

What does mass result in?

A

Heavier atoms connected to spring = Lower frequency

24
Q

A stretch below 3000 cm^-1 shows…

A

sp3 C-H

25
Q

A stretch 3000-3300 cm^-1 shows…

A

sp2 C-H

26
Q

A stretch around 3300 cm^-1 shows…

A

sp C-H

27
Q

What are characteristics of Aromatic rings in IR absorptions?

A

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

28
Q

What are characteristics of alcohols in IR absorptions?

A

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

29
Q

What are the types of amines?

A

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)

30
Q

What are characteristics of amines in IR absorptions?

A

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)

31
Q

What are characteristics of carbonyls in IR absorptions?

A

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

32
Q

Are aldehydes more easily oxidized than ketones?

A

No, the opposite is true

33
Q

What are characteristics of carboxylic acids in IR absorptions?

A

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

34
Q

What are characteristics of esters in IR absorptions?

A

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

My Packs: UWO (CHEM 1301) (17 decks)

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