01:160:307 Lecture Notes - Lecture 12: Infrared Spectroscopy, Radiant Energy, Picometre
1 May 2018
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OC 307 – Chapter 12 – IR Spectroscopy – H. D. Roth
1
Chapter 12 – IR Spectrscopy
Electromagnetic Radiation
Electromagnetic radiation: Light and other forms of radiant energy can be described as waves
having wavelength, l, frequency, n, and velocity, c. Alternatively, it can be considered as a
particle with an energy, E.
Picometer (pm) 1 pm = 10-12 m
Angstrom (Å) 1 Å = 10-10 m
Relation
to Meter
Unit
1 mm = 10-3 m
1 nm = 10-9 m
1 µm = 10-6 m
Nanometer (nm)
Micrometer (µm)
Millimeter (mm)
Meter (m) ----
The human eye can detect a narrow range of
wavelengths and also distinguish between
them (true at least for most of us).
The human eye has small, medium and large
cones, which have color sensitivity as shown;
combined, they can capture an infinite number
of color nuances.
Wavelength (l): The distance between
consecutive peaks in a wave.
Frequency (n): The number of full cycles of a wave that pass a given point in one
second; the unit is hertz, Hz; 1 Hz = 1 s–1 (read “per second”).
λ
OC 307 – Chapter 12 – IR Spectroscopy – H. D. Roth
2
Molecular spectroscopy
The relationship between E and l, n, and c is given by
DE = hn
energy frequency
n = c/l
DE = hc/l
UV spectry wavelength
1/l =
IR spectry wave number
Wavenumber: The number of waves per
centimeter, with units cm–1 (read reciprocal
centimeters).
Electromagnetic radiation causes a transition to a
higher state; the energy difference, DE, provides
information about the molecule being probed.
The various types of radiation differ in
wavelength, l, and frequency, n.
Spectroscopy is the study of which energies of
radiation are absorbed or emitted by a particular
substance and the correlation of these energies
with details of molecular structure.
DE
OC 307 – Chapter 12 – IR Spectroscopy – H. D. Roth
3
The vibrational InfraRed region ranges from 2.5 mm (4000 cm–1) to 25 mm (400 cm–1)
Conversion of wavelength to wavenumber: = 1/l
Molecular Vibration
Atoms joined by covalent bonds undergo continual periodic movements (vibrations) relative to
each other.
The energies associated with transitions between vibrational energy levels correspond to
(frequencies?) wavenumbers 4000 to 400 cm-1 in the IR.
Only vibrations of polar covalent bonds, which cause periodic changes in the bond dipole
moment, absorb IR radiation.
Covalent bonds which do not meet these criteria do not absorb IR radiation; they are said to be IR
inactive.
ν = = 400 cm-1
= 4000 cm-1
ν = 10-2 m•cm-1
2.5 x 10-6 m
10-2 m•cm-1
2.5 x 10-5 m
Document Summary
Electromagnetic radiation: light and other forms of radiant energy can be described as waves having wavelength, l, frequency, n, and velocity, c. alternatively, it can be considered as a particle with an energy, e. The human eye can detect a narrow range of wavelengths and also distinguish between them (true at least for most of us). The human eye has small, medium and large cones, which have color sensitivity as shown; combined, they can capture an infinite number of color nuances. Wavelength (l): the distance between consecutive peaks in a wave. Frequency (n): the number of full cycles of a wave that pass a given point in one second; the unit is hertz, hz; 1 hz = 1 s 1 (read per second ). Electromagnetic radiation causes a transition to a higher state; the energy difference, de, provides information about the molecule being probed. The various types of radiation differ in wavelength, l, and frequency, n.
