IR Spectroscopy

Infrared Spectroscopy (IR)

an analytical technique used to provide information about the structure of molecules

technique

radiating a sample with infrared light and measuring the absorbance vs frequency

y-axis

transmittance % of the sample

x-axis

wavenumber between 4000 cm-1 and 1500cm-1

fingerprint region

region below 1500cm-1

measurement

measures the vibration of atoms and bonds which vibrate at different energies

criteria impacting frequency

hybridization, resonance, hydrogen-bonding, and mass

stronger bonds (double and triple bonds)

vibrate at higher frequencies than weaker bonds because it takes more energy to stretch and compress them

smaller atoms (hydrogen)

vibrate faster at higher energies, so bonds with hydrogen will vibrate at higher frequencies while heavy atoms vibrate at low frequencies

“s” character

the more of this the bond has, the stronger the bond and it takes more energy to vibrate, sp bonds vibrate at higher frequencies than sp3

sp3 hybridized C-H bonds (alkanes)

jagged peak around 2900 cm-1

c=c double bonds

pointy and shallow peak around 1650cm-1

alkene with sp2 C-Hs

jagged peak right of 3000 cm-1

c≡c triple bonds

pointy and shallow peak around 2150 cm-1

alkynes with sp C-Hs

sharp narrow peak around 3300 cm-1

alcohol

broad U peak beginning and ending above 3000 cm-1

carboxylic acid

big broad U spanning across 3000 cm-1 and a narrow peak stretching almost all the way to the bottom at 1700 cm-1

ketone/aldehyde

narrow peak stretching almost all the way to the bottom at 1725-1710 cm-1

primary amine 2Hs

W shaped peak at 3400cm-1

secondary amine 1H

V shaped peak at 3400cm-1

tertiary amine

no peak