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INFRARED SPECTROSCOPY
Atoms, and the constituents of atoms, interact with electromagnetic
radiation. For example, atoms absorb energy in the ultraviolet and visible
regions of the electromagnetic spectrum. This energy is the right magnitude to
move electrons from low energy to higher energy orbitals. Infrared radiation
(that is, heat radiation) is of lower energy; it is not energetic enough to affect
electrons; instead, it is absorbed by molecules and sets atoms to vibrating and
twisting along their bonds:
Figure 1: 3 Vibrational modes of the water (H-O-H)
Molecule.
The frequency of electromagnetic radiation required to
produce each type of motion is in units of cm-1. (From
Silverstein et al., Spectrophotometric Identification of
Organic Compounds, 5th ed., John Wiley 1991).
Oxygen atom
Hydrogen atom
Direction the bond is vibrating
H
Symmetrical stretch @
3652cm-1
asymmetrical stretch @
3756 cm-1
Scissoring @ 1596 cm-1
pageFigure
2 2: 4 Vibrational modes of carbon dioxide (O=C=O)
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The frequency of electromagnetic radiation required to
produce each type of motion is in units of cm-1. (From
Silverstein et al., Spectrophotometric Identification of
Organic Compounds, 5th ed., John Wiley 1991).
Carbon atom
oxygen atom
Direction the bond is vibrating.
Symmetrical stretch @ 1340 cm-1
Asymmetrical stretch @ 2350 cm-1
-
-
Scissoring (in(-) & out(+) of the
plane of the paper) @ 660 cm-1
Scissoring (up & down in the plane
of the paper) @ 660 cm-1
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VIBRATIONAL MODES OF A METHYLENE (-CH2-) GROUP.
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Carbon atom
Hydrogen atom
C-C bond coming out at you
C-C bond going in back of the
plane of the paper
Symmetrical C-H Stretches @ 2853 cm-1
Scissoring of C-H Bonds in the plane of the paper
@ 1465 cm-1
Asymmetrical C-H Stretches @ 2926 cm-1
In-plane rocking of C-H bond @ 720 cm-1
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Out of plane twisting of C-H bonds @ 1350-1150 cm-1
Wagging C-H bonds in & out of the plane of the
paper (+/-) @ 1350 – 1150 cm-1
Petalling Vibrational bonds of the –CH3 (Methyl) Group
The 3 C-H bonds are opening &
closing like the petals of a flower @
1375 cm-1
2 C-H bonds are opening and one is closing at 1450 cm -1
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Different absorbed energies result in different types of vibrations (stretching,
scissoring, wagging, rocking). Some intramolecular movements require more
energy than others.
The units of energy used in IR spectroscopy are expressed in cm-1 (reciprocal
centimeters) 1 cm-1 = 104/wavelength in M. Cm-1, also called wavenumbers,
are proportional to the energy of the radiation; the higher the wavenumber, the
higher the energy of the radiation. Stiffer bonds vibrate at higher energies than
less stiff bonds.
Hooke’s Law can model the behavior of atoms connected by bonds; this
describes oscillations between two particles connected by a spring:
-------------------
I.  (2c ) = [f/(MxMy/(Mx + My)]1/2
where:

ν is the vibrational frequency in cm-1;

c is the speed of light (2.99792 x 1010 cm/sec)

Mx and My are the masses, in grams, of the two atoms

f is the force constant ( a measure of the stiffness of the bond) in dyne/cm




BEFORE YOU COME TO LAB:

CALCULATE THE MASS OF 1 C ATOM
CALCULATE THE MASS OF 1 H ATOM
CALCULATE THE MASS OF 1 O ATOM
Solve Equation I for f. Hint: Call the term “(MxMy/(Mx + My)” Mr
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The correlation chart below relates intramolecular vibrations and stretching to
vibrational frequency:
CH2 rock
720 cm-1
C-H stretch
3330-3270 cm-1
O-H stretch
3650-3585 cm-1
C=C stretch
1665-1640 cm-1
C=C stretch
2260-2100 cm-1
C-O stretch
1200-1000 cm-1
C=O stretch
1870-1540 cm-1
C=N stretch
2260-2240 cm-1
C-X stretch
1000-1300 cm-1
(X=F,Cl,Br,I)
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The infrared spectrophotometer consists of:

an infrared radiation source (heat lamp!);

a beam of infrared light;

a sample holder (which must be transparent to IR radiation. Glass won’t
do. Usually a salt (NaCl or KBr) window is used);

a detector.
From Silverstein et al. Spectrophotometric Identification of Organic Compounds, 5th ed. John Wiley 1991
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IR spectrum of ethanol (CH3-CH2-OH) in the gas phase. From NIST.
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In this laboratory exercise you will work in pairs. You will obtain spectra of
2 compounds: For example, group I will obtain spectra of methanol and
and carbon dioxide.

You will identify and absorption band corresponding to a vibration between
two atoms: in this example, the C-O stretch. You will calculate the force
constant for the stretch, and compare force constants within your group.
Which bond is stiffest (has the largest force constant)?

Is there a correlation between force constant and atomic mass? number of
bonds?