infrared spectroscopy
a method for characterizing the chemical composition of molecules, and it relies on how IR radiation interacts with chemical bonds
electromagnetic radiation
light and other forms of radiant energy
wavelength
the distance between consecutive peaks on a wave
frequency
the number of full cycles of a wave that pass a given point in a second
molecular spectroscopy
a measure of which frequencies of radiation are absorbed or emitted by a particular substance and the correlation of these frequencies to the molecular structure
vibrational spectrum
- IR spectroscopy probes stretching and bending vibrations of organic molecules
- wavenumbers refer to the frequency of electromagnetic radiation as waves per centimeter and is typically used as the x axis in IR spectroscopy
molecular vibrations
- atoms are constantly vibrating where the energy to transition between vibrational energy levels is typically in the IR region
- Since dipole moment is partially dependent on the distance between atoms, vibrations can lead to a change in dipole moment, which is what we observe in IR spectroscopy. Molecules that undergo a change in dipole moment upon absorption of IR are called IR active molecules, while symmetric vibrations that don’t lead to a change in dipole moment are IR inactive.
fingerprint region
vibrations from 1500 to 400 cm-1 are complex and difficult to analyze but are characteristic of diff moleculse
Alkanes (RC-CR)
C-H stretch about 3000 cm-1
alkenes
C-H stretch about 3000 cm-1, C=C about 1650 cm-1
alkynes
C-H about 3300 cm-1; C-C about 2200 cm-1
Alcohols
C-O about 1050 cm-1; O-H about 3300 cm-1
Ethers
C-O about 1100 cm-1
Amines
N-H about 3300cm-1
-broad due to H-Bonding
Aldehydes and Ketones
C=O about 1700cm-1
Carboxylic acids
C=O about 1700cm-1 (broad due to H-Bonding); O-H about 3300 cm-1
Esters
C=O 1750 cm-1; C-O 1200 cm-1
amides
C=O 1750 cm-1; N-H 3400cm-1
