o-chem exp 5: spectroscopy workshop & spectroscopy problem set

infrared spectroscopy

measures excitation of vibrational motion in molecules; mostly used to identify functional groups in molecules

their position in Cm-1, shape of absorption (broad, sharp...) and strength (depth of absorption along y axis)

infrared absorptions are characterized by

functional groups

infrared spectroscopy is mostly used to identify

nuclear magnetic resonance

NMR spectroscopy

NMR spectroscopy

only works for samples in magnetic field and nuclei with non-zero spin

NMR shows

the different types of H in a molecule, how many H's of each type, and information about H on adjacent carbons

proportional

size of NMR peak is ___________ to the number of H's of that type

Chemical Shift (𝛿)

The x-axis of an NMR spectrum, measured in parts per million (ppm).

less shielded H

downfield (higher ppm)

electron shielding

The repulsion between electrons in different inner shells. Shielding reduces the net attractive force from the positive nucleus on the outer-shell electrons.

more shielded H

upfield (lower PPM)

electronegative substituents (halogens, oxygen)

move chemical shift downfield (higher ppm)

aromatic ring

moves chemical shifts downfield (~6.5-8 ppm)

peak splitting in NMR

Caused by neighboring hydrogens that are not chemically equivalent

Add 1 to the number of neighboring, equivalent hydrogens

1:1

doublet splitting pattern

1:2:1

triplet splitting pattern

1:3:3:1

quartet splitting pattern

splitting is not observed

if the H are separated by more than 3 sigma bond

coupling constant (J)

measure of how much the peak is split

O-H stretch, alcohol

3525-3200 Cm-1, strong, broad, symmetric

O-H stretch, carboxylic acid

3400-2500 cm-1, weak to medium, very,very broad, roughly shaped

N-H stretch, amine

~3500-3000, weak to medium, symmetric, average shape, not as wide or strong as O-H

sp2 C-H stretch

3140-3040, weak to strong, sharp

sp3 C-H stretch

3000-2800, med to strong, sharp

C≡N stretch

2260-2210, med, very sharp

C≡C stretch

2260-2100, weak, sharp

C=O stretch

1750-1650, strong, medium, symmetric

C=C stretch, alkene

1670-1600, med to weak, medium

C=C stretch, aromatic

1600-1400, medium, medium, often 2

C-O stretch

1300-1000, strong, medium

0.9 ppm

-CH3

1.3 ppm

-CH2-

2.3 ppm

H on carbon attached to aromatic ring

2-2.3 ppm

H on carbon next to C=O

3.8 ppm

H on carbon next to oxygen

6.5-8 ppm

H on aromatic ring

H near electronegative atoms / groups

- have chemical shifts with larger ppm

- includes oxygen, halogens (F, Cl, Br, I) and being near an aromatic ring (but not directly on the ring)

- changes are usually 1-2 ppm larger chemical shift