Spectroscopy: IR

Renee Y. Becker

Valencia Community College

CHM 2011

• Radiant energy is proportional to its frequency (cycles/s = Hz) as a wave (Amplitude is its height)

• Different types are classified by frequency or wavelength ranges

• γ-rays (gamma rays)- greatest energy and highest frequency. Emitted from some radioactive nuclei. Can cause biological damage.

• X-rays- lower in energy than γ -rays. Can cause biological damage in high doses.

• Ultraviolet (UV) light- can cause sunburn and even skin cancer.

• Visible light- speaks for itself !

• Infrared radiation (IR) - heat!

• Microwaves- we cook with them and also used in radar.

• Radio waves- lowest frequency. Radio and TV transmissions and NMR spectroscopy.

• The mode of propagation of electromagnetic radiation is the wave.

• The wave is characterized by its wavelength (), frequency (), and amplitude.

• Frequency () units: s-1 or hertz (Hz)

• The intensity of a wave is proportional to the square of its amplitude.

• Electromagnetic radiation travels at constant velocity in a vacuum: 3.00 x 1010 cm/s (speed of light).

• Wavelength x Frequency = Speed

(cm) x (s-1) = c (cm/s)

c

c

• Thanks to Max Planck and Al Einstein:

hchE

E = Energy of 1 photon (1 quantum)h = Planck’s constant (6.62 x 10-34 J·s = 1.58 x 10-34cal·s) = Frequency (s-1) = Wavelength (cm)c = Speed of light (3.00 x 1010 cm/s)

• The energy of a photon varies directly with the frequency and inversely with the wavelength

• High frequencies and short wavelengths = higher energies

• Low frequencies and long wavelengths = lower energies

Infrared Spectroscopy of Organic Molecules

• IR region lower energy than visible light (below red – produces heating as with a heat lamp)

• 2.5 x 10-4 cm to 2.5 x 10-3 cm region used by organic chemists for structural analysis

• IR energy in a spectrum is usually measured as wavenumber (cm-1), the inverse of wavelength and proportional to frequency

• Specific IR absorbed by organic molecule related to its structure

• The IR spectrum covers the range from 4000 cm-1 to 400 cm-1

• This represents energy ranges from 48.0 - 4.80 kJ/mol (11.5 - 1.15 kcal/mol).

• IR energy absorption corresponds to specific modes, corresponding to combinations of atomic movements, such as bending and stretching of bonds between groups of atoms called “normal modes”

• Energy is characteristic of the atoms in the group and their bonding

• Corresponds to vibrations and rotations

Interpreting Infrared Spectra

• Most functional groups absorb at about the same energy and intensity independent of the molecule they are in

• Characteristic higher energy IR absorptions in Table 12.1 can be used to confirm the existence of the presence of a functional group in a molecule

• IR spectrum has lower energy region characteristic of molecule as a whole (“fingerprint” region; 1300 to 625 cm-1)

• See samples in Figure 12-13

Regions of the Infrared Spectrum

• 4000-2500 cm-1 N-H, C-H, O-H (stretching)– 3300-3600 N-H, O-H– 3000 C-H

• 2500-2000 cm-1 CC and C N (stretching)

• 2000-1500 cm-1 double bonds (stretching)– C=O 1680-1750– C=C 1640-1680 cm-1

• Below 1500 cm-1 “fingerprint” region

Differences in Infrared Absorptions

• Molecules vibrate and rotate in normal modes, which are combinations of motions (relates to force constants)

• Bond stretching dominates higher energy modes

• Light objects connected to heavy objects vibrate fastest: C-H, N-H, O-H

• For two heavy atoms, stronger bond requires more energy: C C, C N > C=C, C=O, C=N > C-C, C-O, C-N, C-halogen

Infrared Spectra of Hydrocarbons

• C-H, C-C, C=C, C C have characteristic peaks– absence helps rule out C=C or C C

IR: Alcohols and Amines

• O–H 3400 to 3650 cm1 – Usually broad and intense

• N–H 3300 to 3500 cm1

– Sharper and less intense than an O–H

1-butanol

butylamine

IR: Aromatic Compounds

• Weak C–H stretch at 3030 cm1

• Weak absorptions 1660 - 2000 cm1 range

• Medium-intensity absorptions 1450 to 1600 cm1

• See spectrum of phenylacetylene, Figure 12.15

IR: Carbonyl Compounds

• Strong, sharp C=O peak 1670 to 1780 cm1

• Exact absorption characteristic of type of carbonyl compound– 1730 cm1 in saturated aldehydes

– 1705 cm1 in aldehydes next to double bond or aromatic ring

C=O in Ketones• 1715 cm1 in six-membered ring and acyclic ketones

• 1750 cm1 in 5-membered ring ketones

• 1690 cm1 in ketones next to a double bond or an aromatic ring

3-hexanone

C=O in Esters

• 1735 cm1 in saturated esters

• 1715 cm1 in esters next to aromatic ring or a double bond

Carboxylic Acids

R OH

O

1700 - 1725 cm-1

(strong)

2500 - 3300 cm-1

(very broad and intense)

Pentanoic acid

Identify the functional groups in compounds that are responsible for the following

absorptions:

• A compound with a strong absorption at 1710 cm-1

• A compound with a strong absorption at 1540 cm-1

• A compound with a strong absorption at 1720 cm-1

and at 2500-3100 cm-1

How might you use IR spectroscopy to distinguish between the following pairs of

isomers?

• CH3CH2OH and CH3OCH3

• Cyclohexane and 1- hexene