Typical Infrared Absorbance Bands of Common Functional Groups and Other Things to Know

Absorption numbers for Professor Richard's Class

O-H

3200-3600(strong and broad)

N-H

3200-3600(medium and broad, one for each NH)

O-H (RCO2H)

2500-3600 (strong and very broad)

C-H(sp)

3200-3300(strong)

C-H(sp2)

3000-3200(variable, depends on number of groups)

C-H(sp3)

2850-3000(variable, depends on number of groups)

O=C-H

2750 and 2850(two sharp peaks, may be masked)

C=-N

2220-2260(medium)

C=-C

2100-2260(medium, narrow)

C=C

1600-1700(medium, narrow)

C=C(Ar)

1500 and 1600(two narrow peaks)

C=O

1600-1800(very strong)

C-O

1050-1250(strong)

C-N

1020-1230(medium to strong)

Spectroscopy

study of the interaction of matter and electromagnetic radiaion(light)

Frecuency(v)

number of waves per unit time (number of waves per second, Hz)

Wavelengths

distance between wave crests

Wavenumber

number of waves per unit distance(# of waves per cm,cm^-1)

Which type of vibrations are we most interested in in IR Spectroscopy?

Stretching vibrations

How does IR spectroscopy work?

A light source is shined at a sample, some of the light is absorbed by the sample and some light is TRANSMITTED through the sample to the detector. The light that is transmitted is what we’re seeing.

What can the amount of light be quantified as?

absorbance or percent transmittance

Where do you spend most of your time in an IR Spectrum?

above 1400 (anything lower is the fingerprint region, above is the functional group region)

The higher the peak in IR…

the higher the absorbance

What can you use IR Spectroscopy for?

determine functional groups present

What determines the wavenumber of absorbance?

atomic mass, bond strength

What determines bond strength?

type of bond (C=-C, C=C, C-C), hybridization, and resonance

What factors determine intensity of absorbtion?

Polarity and number of bonds

What determines the broadness of absorbance?

H-bonding

What is H-Bonding?

When an electronegative atom (O,N,F) steals the H’s from other atoms.

Why is the O-H bond so broad?

Because of hydrogen bonding which allows interactions with neighboring atoms. The oxygen hydrogen bond stretching vibrations vary in frequencies vs one sharp frequency.