Chapter 5: 5.2 Infrared Spectroscopy Flashcards
Define:
Wavenumber
The reciprocal of the wavelength
What is wavenumber measured in?
1/cm
Why are wavenumbers convenient?
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
When a molecule absorbs __ light of the proper wavelength, it is excited from the ______ state to an _______ state
IR
Ground
Excited
In IR spectroscopy, how are bonds excited?
Vibrational excitement
What are the two main types of molecular vibration?
Stretching
Bending
__________ requires more energy than _______
Stretching
Bending
What types of stretching are there?
Symmetric
Asymmetric
What types of bending are there?
Scissoring
Rocking
Wagging
Twisting
True or False:
In IR Spectroscopy, different bonds have the same absorption frequencies
False
What is the relationship between absorption and transmittance?
High absorption = Low transmittance
In UV Spectroscopy, __________ is the function of __________
Absorption
Wavelength
In IR Spectroscopy, _____________ is function of __________
Transmittance
Wavenumber
What is the region from about 1500-500 cm^-1 known as? Why?
Fingerprint region
Very complex region that is usually unique to any given compound
What is the region from 4000-1500 cm^-1 known as? Why?
Functional group region
Functional groups are readily identified in this region
What can molecular bonds be thought of?
Balls and springs that vibrate as a harmonic oscillator
What is k in frequency of vibration calculation?
A force constant proportional to the strength or “stiffness” of the bond
What is μ in frequency of vibration calculation?
The reduced mass of the molecule expressed in kilograms
What does bond strength affect?
k (strength or stiffness of bond)
What do stronger bonds result in?
Stronger bonds = Higher k = Higher frequency
What does hybridization result in?
Since s orbital is smaller:
More s character = Smaller orbital = Smaller bond length
What does mass difference affect?
μ (reduced mass of molecule)
What does mass result in?
Heavier atoms connected to spring = Lower frequency
A stretch below 3000 cm^-1 shows…
sp3 C-H
A stretch 3000-3300 cm^-1 shows…
sp2 C-H
A stretch around 3300 cm^-1 shows…
sp C-H
What are characteristics of Aromatic rings in IR absorptions?
sp2 C-H stretches
C=C stretches between 1400-1600 cm^-1
What are characteristics of alcohols in IR absorptions?
Strong, broad OH stretch at 3200-3500 cm^-1
What are the types of amines?
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)
What are characteristics of amines in IR absorptions?
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)
What are characteristics of carbonyls in IR absorptions?
Sharp, strong absorption in the region of 1600-1800 cm^-1
Are aldehydes more easily oxidized than ketones?
No, the opposite is true
What are characteristics of carboxylic acids in IR absorptions?
Strong C=O stretch at 1700 cm^-1
Very broad, offset O-H stretch between 2400-3400 cm^-1
What are characteristics of esters in IR absorptions?
Strong C=O stretch at around 1750 cm^-1
Lacks broad O-H stretch
Strong C-O stretches at around 1000-1250 cm^-1
