IR Spectroscopy

What are bond vibrations?

bonds in organic molecules are constantly vibrating around a certain equilibrium distance - stretching and bending

What are stretching modes of vibration?

either lengthen or shorten bonds to longer or shorter bond lengths

What are bending modes of vibration?

increase and decrease in bond angles in a molecules

How does the number of atoms influence stretching and bending?

the greater the number of atoms, the more stretching and bending

What do vibrating bonds produce

changes in bond dipoles - creating an oscillating electric field

What does the IR spectrum tell?

functional groups in an organic molecule

Where are H-bonds seen?

3600-2700 stretch

Where are triple bonds seen?

in the 2700-1900 stretch

Where are double bonds seen?

in the 1900-1500 stretch

Where is the fingerprint region?

1500-500 stretch

What is the dividing line between sp3 hybridized carbons and those that are not?

3000

what does a C-H bond of CH3 bond look like?

sharp tooth around 3000 - towards right of spectrum

sharp tooth for all sp3 C-H

What bond is shown to the left of 3000? - is a single sharp peak

sp2 or sp hybridized C-H bonds

What does a broad signal around 3300 region mean?

O-H - further to left compared to C-H - like a tongue

How are NH bonds shown?

near 3300 area, but not strong -

NH2 - can show up as two teeth

• if 1 N - MW will be an odd number

How do nitriles show up?

very strong - swords - in 2200 - 2000 region

How do alkynes show up?

weaker swords - 2200-2000 region - a little more to right than nitriles

Do symmetrical alkynes show up?

no - they sometimes do not show up at all - nonpolar

Where does an aldehyde C-H bond show up?

2800-2700

How do carbonyls show up?

very strong sword in 1800-1600 region

How do alkenes show up?

weaker swords - lower dipole moment

terminal alkenes are the strongest of all alkenes

How do aromatic rings show up?

swords around 1600 - most to the right before fingerprint region

conjugation lowers wave numbers by 25

How do aldehydes show up?

two signals - 2700, 2800 - Fermi resonance