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 cm/s (speed of light).

Wavelength x Frequency = Speed (cm) x (s-1) = c (cm/s)

Thanks to Max Planck and Al Einstein: E = Energy of 1 photon (1 quantum) h = Planck’s constant (6.62 x J·s = 1.58 x cal·s) = Frequency (s -1 ) = Wavelength (cm) c = Speed of light (3.00 x 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 cm to 2.5 x 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 kJ/mol ( 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 cm -1 N-H, C- H, O-H (stretching) – N-H, O-H –3000 C-H cm -1 C  C and C  N (stretching) cm -1 double bonds (stretching) –C=O –C=C 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 cm  1 –Usually broad and intense N–H 3300 to 3500 cm  1 –Sharper and less intense than an O–H

1-butanol

butylamine

IR: Aromatic Compounds Weak C–H stretch at 3030 cm  1 Weak absorptions cm  1 range Medium-intensity absorptions 1450 to 1600 cm  1 See spectrum of phenylacetylene, Figure 12.15

IR: Carbonyl Compounds Strong, sharp C=O peak 1670 to 1780 cm  1 Exact absorption characteristic of type of carbonyl compound –1730 cm  1 in saturated aldehydes –1705 cm  1 in aldehydes next to double bond or aromatic ring

C=O in Ketones 1715 cm  1 in six-membered ring and acyclic ketones 1750 cm  1 in 5-membered ring ketones 1690 cm  1 in ketones next to a double bond or an aromatic ring

3-hexanone

C=O in Esters 1735 cm  1 in saturated esters 1715 cm  1 in esters next to aromatic ring or a double bond

Carboxylic Acids

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 cm -1

How might you use IR spectroscopy to distinguish between the following pairs of isomers? CH 3 CH 2 OH and CH 3 OCH 3 Cyclohexane and 1- hexene