KOT 222 ORGANIC CHEMISTRY II
CHAPTER 12
INFRARED SPECTROSCOPY
and
MASS SPECTROSCOPY
Part I
Infrared Spectroscopy
What is Spectroscopy?
¾ Spectroscopy is the study of the interaction of
matter and electromagnetic radiation.
¾ It is an analytical technique used to determine
the organic structures.
¾ It works with small samples and destroy little or
no sample.
Absorption Spectroscopy:
¾ Involves the measurement of the amount of light
absorbed by a compound as a function of the
wavelength of light.
Spectroscopic Techniques
¾ Infrared (IR) spectroscopy
measures the bond vibration frequencies in a
molecule and is used to determine the functional
group.
¾ Mass spectrometry (MS)
fragments the molecule and measures the
masses.
¾ Nuclear magnetic resonance (NMR)
spectroscopy
detects signals from hydrogen /carbon atoms
and can be used to distinguish isomers.
¾ Ultraviolet (UV) spectroscopy
uses electron transitions to determine bonding
patterns.
Electromagnetic Radiation
¾ Radiant energy that displays wave properties.
Frequency and wavelength are inversely proportional.
Electromagnetic radiation has wave-like properties
which travel as photons.
c = speed of light (3x1010 cm/sec)
h = Planck’s constant, 6.62 x 10-37 kJ•sec
ν= frequency (Hz)
λ = wavelength (cm)
The Infrared Region
¾ Just below red in the
visible region.
¾ The infrared spectrometers usually operate at the
wavelengths between 2.5 – 25 μm.
¾ More common units are wavenumbers ( ν), or cm-1,
the reciprocal of the wavelength in centimeters.
¾ Wavenumbers are proportional to frequency and
energy.
Molecular Vibrations
¾ The covalent bonds in molecules are constantly
vibrating.
¾ A bond vibrates with both stretching and
bending motions.
¾ Each stretching and bending vibration of a bond
occurs with a characteristic frequency.
¾ Factors affecting the stretching vibration:
™ Masses of the atoms
™ Stiffness of the bond
Masses of the atoms:
¾Heavier atoms vibrate slowly than lighter ones.
¾Frequency decreases with the increasing atomic
weight.
Stiffness of the bonds:
¾Stronger bonds usually vibrate faster than weaker bonds.
¾Frequency increases with increasing bond energy.
Vibrational Modes
Nonlinear molecule with n atoms usually has
3n - 6 fundamental vibrational modes.
Water: H2O
3(3) – 6 = 3
Methanol: CH3OH
3(6) – 6 = 12
Ethanol: CH3CH2OH
3(9) – 6 = 21
No. of vibrational modes ≠ no. of peaks in IR spectrum
IR Spectrum of Methanol
1430 – 4000 cm-1, where the
functional groups absorb.
Fingerprint Region (600 – 1400 cm-1):
•contains many absorptions caused by
complex vibrations.
•unique for different compounds
IR-Active and IR-Inactive
¾ IR absorptions only happen when the vibrations
of bonds caused changes in their dipole
moments.
¾ A polar bond is usually IR-active.
¾ A nonpolar bond in a symmetrical molecule
(internal alkyne) will absorb weakly or not at all –
IR inactive.
Infrared Spectrometer
¾ It measures the frequencies of infrared light
absorbed by a compound.
Types: 1) Dispersive infrared spectrometer.
2) Fourier transform infrared spectrometer
(FT-IR).
Dispersive infrared spectrometer
Allow only one frequency of light
to enter the detector at a time
Reference
beam
Detect the
difference in
the intensity of
light in both
beams
Sample beam
100 % = no absorption
0%
= absorb of the light
Fourier transform infrared spectrometer, FT-IR
¾ Has better sensitivity.
¾ Less energy is needed
from source.
¾ Completes a scan in 1-2
seconds.
¾ Takes several scans
and averages them.
¾ Has a laser beam that
keeps the instrument
accurately calibrated.
Interferogram
The interferogram at the right
displays the interference pattern and
contains all of the spectrum
information.
A Fourier transform converts the time
domain to the frequency domain with
absorption as a function of frequency.
IR Spectroscopy of Hydrocarbons
¾ Two common bondings in hydrocarbons:
™ Carbon – carbon bond.
™ Carbon – hydrogen bond.
H
H
H
H
H
H
C
C
C
C
C
H
H
H
pentene
H
Carbon-Carbon Bond Stretching
¾ Three types of carbon-carbon bonds:
• C – C single bond
• C = C double bond
• C ≡ C triple bond
¾ Each type gives different absorption band at
different wavenumber depends on the bond’s
stiffness.
ƒ C-C
ƒ C=C
ƒ C≡C
1200 cm-1 (weak)
1660 cm-1
<2200 cm-1 (weak or absent for internal
alkyne)
C = C double bond
¾ Observable stretching absorptions in the region
of 1600 – 1680 cm-1.
¾ Specific frequency depends on whether there is
another double bond nearby.
Overlapping of pi
bonds
Isolated; 1645 cm-1
1600 cm-1
Conjugated; 1620 cm-1
isolated C=C
1640-1680 cm-1
conjugated C=C 1620-1640 cm-1
aromatic C=C
approx. 1600
cm-1
Carbon-Hydrogen Bond Stretching
¾ Alkanes, alkenes, and alkynes also have
characteristic C-H stretching frequencies.
¾ Amount of s character in the carbon orbital used
to form the C-H bond determine the absorption
frequencies.
√
√
√
IR Spectrum of Alkane
C-H bending
Absorption band for C-C (1200 cm-1) is weak
due to small dipole moment.
IR Spectrum of Alkene
The most important absorptions in the 1-hexene
are the C=C stretch at 1642 cm-1, and the
unsaturated =C-H stretch at 3080 cm-1.
IR Spectrum of Alkyne
Internal alkyne:
No ≡C-H bond.
Disubstituted C≡C
has very small dipole
moment.