the spectroscopy that deals with
the infrared region of
the electromagnetic spectrum, that is
light with a longer wavelength and
lower frequency than visible light.
http://www.nasa.gov/images/content/56638main_Infrared_Region.jpg
approximately 14000-4000 cm−1
(0.8–2.5 μm wavelength)
Can excite overtone or harmonic
vibrations.
Has high energy
approximately 4000-400 cm−1
(2.5–25 μm)
used to study the fundamental
vibrations and associated rotationalvibrational structure.
approximately 400–10 cm−1
(25–1000 μm)
lying adjacent to
the microwave region
has low energy and may be used
for rotational spectroscopy.
2.5x10-6 to 2.5x10-5 m
Vibrational infared region – the portion
of the infrared region that extends from
4000 to 400 cm-1
Wavenumber (v) – radiation in the
vibrational infrared region. The number
of waves per centimeter.
Atoms joined by covalent bonds are not
permanently fixed in one position, but instead
undergo continual vibrations relative to each
other
The bond undergoing vibration must be polar.
It’s vibration must cause a periodic change in
the bond dipole.
The greater the polarity of the bond, the
more intense is the absorption.
3n – 6 (fundamental vibrations)
Example:
Ethanol CH3CH2OH
Hexanoic acid CH3(CH2)4COOH
Change in band length
• Change in band angle.
Data on absorption patterns of selected
functional groups are collected.
Characteristic IR Absorptions of Selected Functional Groups
Bond
Frequency (cm-1)
Intensity
O-H
3200-3500
Strong and broad
N-H
3100-3500
Medium
C-H
2850-3100
Medium to strong
C≡C
2100-2260
Weak
C=O
1630-1800
Strong
C=C
1600-1680
Weak
C-O
1050-1250
Strong
Example:
Determine the functional group that is likely
present if a compound shown IR absorption at
1705 cm-1
2950 cm-1
absorption of IR radiation only occurs if
there is a change in dipole moment
the larger the change in dipole moment, the
more intense the absorption band
the greater the electronegativity differences
between atoms in the molecule, the more
polarized, the larger change in dipole moment
Transmittance
Wavenumber (cm-1)
The sum of the number of rings and pi
bonds in a molecule.
Compare the number of hydrogens in
the molecular formula of a compound
of unknown structure with the number
of hydrogen in a reference compound
with the same number of carbon atoms
and with no rings or pi bonds.
CnH2n+2
Index of Hydrogen = (Hreference – Hmolecule)
2
Measures the attenuation of a beam of
light after it passes through a sample or
after reflection from a sample surface.
Spectral range of 190 nm to 900 nm
190 – 400 nm, UV
400 – 750 nm, Vis
Vis = 36 – 72 kcal/mol
UV = >72 – 143kcal/mol
• identifiable part of the molecule
• responsible for any spectral feature like bands
• use to deduce presence of structural fragment or
element in the molecule
• corresponds to a functional group, an atom or group
of atoms
• Molar absorptivity > 200 for UV-Vis
• UV - electronic transition in 200 – 380 nm
• Vis - electronic transition in 380 – 800 nm
SCHEMATIC OF A WAVELENGTH-SELECTABLE, SINGLEBEAM UV-VIS SPECTROPHOTOMETER
•
http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/UV-Vis/spectrum.htm