Organic Spectra
Infra Red Spectroscopy
H. D. Roth
THEORY and INTERPRETATION of ORGANIC SPECTRA
H. D. Roth
Infra Red Spectroscopy
Infrared spectroscopy (IR) is an analytical technique concerned with
molecular vibrations and rotations; we will be concerned mostly with
vibrations such as stretching of bonds or bending of groups. The nature of the
bond or group determines the vibrational frequency. To observe an IR band, a
change in dipole moment must accompany the motion. Therefore polar
covalent bonds (C=O, O–H, C–Cl) have strong IR bands, whereas
symmetrically substituted bonds or functions do not absorb at all.
R
C≡C
O
R
C
O
The dipole moment of a two-atomic entity or along a bond connecting
two groups can be formulated as
+
–
µ = (| δ | + | δ |) × d
The charge of the electron is 4.8×10–10 esu and atomic distances fall into
the range of 10–8 cm (Å); typical dipole moments fall into the order of 10–18
esu. For convenience this quantity is defined as 1 D (debye).
Applications of IR Spectry
1.
Identify groups by characteristic frequencies.
2.
Positive identification by comparison with known spectra (Aldrich IR
Library).
3.
Fourier transform (FT)-IR very sensitive, used for trace analysis –
GC-FT-IR even more sensitive.
4.
Quantitative Analysis - Beer's Law cannot be used easily. In special
cases calibration curves can be set up using known concentrations.
Calibrations checked with polystyrene film (strong bands at 1603, 1946 cm-1)
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Organic Spectra
Infra Red Spectroscopy
H. D. Roth
Four types of samples are used routinely in IR spectroscopy
Films
Solutions
(NaCl cell) CCl4, CS2, CH3Cl (why not H2O?)
Mulls
fine suspension in Nujol
Pellets
pressed from mixture with finely ground KBr
What kind of vibrations give rise to IR transitions (and can be observed)?
1.
Stretching
"A–B" (self-explanatory; stretching frequencies
are most useful for structure identification)
Molecules with two identical atoms (H, Cl) or groups (CH3, CN) on the
same C (or N) may have two “coordinated” stretching frequencies, i.e.,
symmetrical and antisymmetrical, illustrated below for a CH2 group:
C
C
H
H
Symmetrical
2.
H
H
Antisymmetrical
Bending, a deformation of one or more bond angles; for example a methyl
group has a symmetrical bending mode, increasing/decreasing all three R–C–H
angles equally, and an unsymmetrical mode where one R–C–H angle is changed in
the opposite sense.
H
R
C
H
R
H
H
Symmetrical
C
H
H
Unsymmetrical
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Organic Spectra
3.
Infra Red Spectroscopy
H. D. Roth
Additional types of molecular motions, giving rise to IR bands, include
Scissoring
C
H
H
Wagging
Twisting
C
C
H H
H H
Rocking
C
H
H
In these drawings the vibrations are illustrated for a CH2 group;
analogous vibrations occur also for groups such as C(CH3)2, CCl2, CF2, etc.
Number of vibrations to be expected
Non-linear molecules of N atoms have 3 N – 6 "normal” vibrational modes
H 2O
HCHO
HCOCl
NH3
3 atoms
3 modes
4 atoms
6 modes
CH4
5 atoms
9 modes
ethanol
9 atoms
21 modes
acetone
10 atoms
24 modes
2-chlorobutane
14 atoms
36 modes
pentose
pinene
20 atoms
54 modes
26 atoms
72 modes
terpenes
Obviously, in complex molecules there will be many similar and
overlapping bands, reducing the overall number of bands, and rendering
certain regions of the IR spectrum less useful. For example bands in the alkyl
C–H region, although different from alkenyl C–H and alkyne C–H bands (vide
infra), does not lend itself to specific assignments.
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Organic Spectra
Infra Red Spectroscopy
H. D. Roth
Identification of functional groups
4000–1300 cm
1300–910 cm
910–650 cm
–1
–1
–1
Spectral Range
Characteristic frequencies of individual groups.
"Fingerprint" region; allows comparison with known
spectra
Differently substituted aromatics; various bending
frequencies
3800–2700 cm
2300–2000 cm
1900–1500 cm
1300– 800 cm
Overview
–1
C–H, O–H, N–H
–1
C≡C, C≡N
–1
C=C, C=O, C=N, N=O
–1
C–C, C–O, C–N
For example, we can follow an esterification,
R–COOH + R'–OH → R–COO–R' + H2O
(would you use an NaCl cell?)
or the reaction of acetic anhydride with an alcohol
(CH3–CO)2O + ROH → CH3–COOR + CH3COOH
by observing characteristic C–O, O–H, and C–O–H frequencies.
Characteristic frequencies
Free O–H
(sharp)
H–bonded O–H
(broad)
R–O–H
bend
C–O
stretch
3650–3590 cm–1
3550–3200 cm–1
1200–1050 cm–1
1410–1260 cm–1
Alkenes
The characteristic frequencies for various types of alkenes are not very
different. 1H and 13C NMR will be of much greater value for the proper
assignment of alkenes.
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Organic Spectra
Infra Red Spectroscopy
H. D. Roth
1645 cm–1
1655 cm–1
R-CH=CH2
R2C=CH2
1660 cm–1
1675 cm–1
cis-R-CH=CH-R
trans-R-CH=CH-R
1670 cm–1
R2C=CR2
Alkynes: A) Internal
Compare
R–C≡C–R'
2260-2100 cm–1
R–C≡C–R
–
R–C≡N
2260-2225 cm–1
symmetrical
This type of carbon has a very characteristic 13C frequency
B) Terminal
R–C≡C–H
Compare
or
C≡C
2140–2100 cm–1
C–H
3320–3270 cm–1
RCH=CH2
C–H 3040–3010 cm–1
C–H 2900 cm–1
R–CH3
C–H frequencies reflect bond strengths
R C ≡C
H
H
R C C
H
R CH2
H
sp2
sp
sp3
strongest
weakest
C–H most acidic
3320–3270 cm–1 3040–3010 cm–1
R
C ≡N
N
sp
intermediate
base
least acidic
2900 cm–1
N
2
sp
weakest
H
3
sp
strongest
base
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Organic Spectra
Infra Red Spectroscopy
H. D. Roth
The carbonyl (>C=O) stretching frequency (1690-1750 cm–1) is used to
illustrate subtle substituent effects on the frequency; substituents change
frequencies in a predictable manner. Three different effects illustrated:
a)
Inductive effects
b)
Resonance
c)
Strain effects
Example A: inductive effects
O
O
O
O
C
C
C
C
CH3
H3C
CF3
H3C
CF3
F3C
F
F
1769
1928 cm–1
1801
1724
The electron withdrawing effect of the CF3 or F functions cause changes in
the dipole moment relative to CH3 or alkyl substituted ketones.
–
–
O
O
+
C+
C
F3C
CF3
Example B: effect of resonance
O–
O–
CH3
+
H
X
+
OCH3
O +
N
CH3
O
1677
X = NH2
CH3
+
O–
1700 cm–1
1691
1683
Example C: effects of ring strain
O
O
O
1718
1746
1
7
+
O–
1788 cm–1
27 kcal mol-1
Stretching frequency increases with increasing strain energy
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Organic Spectra
Infra Red Spectroscopy
O–
O
O
H. D. Roth
O–
+
+
1725-1705
1670-1635
Aldehydes
O
R
C
H
α,β-unsaturated aldehydes
1705 - 1680
C=O stretch
C–H stretch
1740 - 1720
2830 - 2820
and
2775 - 2700
actually the first overtone of the carbonyl–C–H bending frequency (~1390).
[Also unistakeable 1H (>9.5 ppm) and 13C chemical shift (>200 ppm).]
Esters
Lactones (cyclic esters)
δ,ε...
1750-1735
Acids
1750-1735
γ
β
1780-1760
1820 cm–1
(cf., cyclic ketones)
1725-1700 cm–1
3550-3500 cm–1
C=O
O–H
This frequency only in very dilute solution, 3300–2500 dimers, etc.
1610-1550 cm–1 antisym stretch
Carboxylate ions
R-COO–
(2 bands)
1400-1300 cm–1 sym stretch
1870-1790 cm–1 antisym stretch
Anhydrides
(2 bands)
1765-1725 cm–1 sym stretch
Acid chlorides
1785-1765cm–1
Aroyl chlorides have a weak second band at 1750-1735 cm–1
Amides
1680-1630 cm–1
The amide C=O frequency is lowest among carbonyls because of resonance
O
–
+
N
C
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Organic Spectra
Infra Red Spectroscopy
H. D. Roth
Harmonic Oscillator
Evib = h c ϖ (vq + 1/2)
νvib = 1
2π
K 1/2
µ
( )
mA x mB
µ =
mA + mB
Evib
vibrational energy
ω
harmonic wavenumber (cm-1) related to vibrational
frequency and to the potential energy function
vq
vibrational quantum number
(v = 0,1,2,3...n)
energy levels are evenly spaced.
νvib
vibrational frequency
K
force constant (millidynes/Å)
µ
reduced mass (a measure of the total mass that is
vibrating).
Note:
do not mistake ν (vibrational frequency, νvib) for vq
(vibrational quantum number) and vice versa
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Organic Spectra
Infra Red Spectroscopy
H. D. Roth
The role of isotopic substitution on IR stretching frequencies
The effect is largest for the lightest atom; we compare the
vibrational frequencies of C–D vs. C–H or O–D vs. O–H bonds
C–H vs C–D Stretching Frequency
νvibC-D
(µC-H)1/2
0.964377
=
=
νvibC-H
(µC-D)1/2
1.313454
12 x 2
µC-D = 12 + 2 = 1.725161; (µC-D)1/2 = 1.313454
12 x 1
µC-H = 12 + 1 = 0.930023;
(µC-H)1/2 = 0.964377
νC-D
0.964377
=
νC-H
1.313454 = 0.7342
Accordingly, the typical C–H frequency (3,000 cm–1) is
reduced to 2200 cm–1 upon D-substitution.
O–H vs O–D Stretching Frequency
νO-D
(µO-H)1/2
νO-H = (µO-D)1/2 = 0.7280
15.9994 x 2.0140
= 1.7888
15.9994 + 2.0140
15.9994 x 1.007825
µO-H = 15.9994 + 1.007825 = 0.9481
νO-H = 3650 cm-1
µO-D =
ν O-D = 2657 cm-1
Δν
= 1000 cm-1
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Organic Spectra
Infra Red Spectroscopy
H. D. Roth
Isotope substitution in bonds between two “heavy” atoms results in
less dramatic changes. An example involving two "heavy" elements:
ν13C-14N
ν12C-14N = 0.97900
µ13C-N = 6.74452
(µ13C-N)1/2 = 2.59702
µ12C-N = 6.46426
(µ12C-N)1/2 = 2.54249
ν12C-N = 2100 cm-1
ν13C-N = 2056 cm-1
Δν =
44 cm-1
The Color of Water
Water has the three vibrational frequencies:
H
O
-1
ν1 3657cm
H
a stretching mode
H
O
-1
ν2 1595cm
H
a bending mode
H
O
-1
ν3 3756cm
H
a combination mode
The fourth overtone of ν3 would occur at a wavenumber
of ~15,000 cm–1, i.e., a wavelength of ~665nm.
Although very weak, this overtone can be observed, if the
"cell" is longer than the usual thin film (IR) or 10 cm
(UV/VIS).
Is this the reason why algae are green?
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Organic Spectra
Infra Red Spectroscopy
H. D. Roth
FOOD FOR THOUGHT
What would the color of water be on a planet where D,
and not H, is the predominant isotope of hydrogen?
More on overtones
Optical fibers for telecommunications used to be
manufactured from the thermal reaction of SiCl4 with
molecular oxygen.
SiCl4 + O2 —> SiO2 + 2 Cl2
SiCl4 is hygroscopic and reacts with water by hydrolysis
SiCl4 + H2O —> SiCl3OH + HCl
Even minor OH impurities would seriously affect the
performance of the optical fibers, since the fourth overtone
of the Si–O–H stretching frequency, at a wavenumber of
~15,000 cm-1, corresponding to a wavelength of ~665nm,
would absorb the light of the diode lasers used for the
transmission of optical data. Although the overtone is very
weak, it becomes prohibitive for cables that are many miles
long, i.e., for a "cell" many kilometers long.
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Organic Spectra
Infra Red Spectroscopy
Specific EXAMPLES
H
1)
–1
3085 cm
1600 cm–1
C=C
C=C
2)
3350 cm–1
O–H
3)
1700 cm
–1
O=C
4)
1690 cm–1
O=C
5)
–1
1720 cm
H
OR
O=C
3480 cm–1
6)
3395 cm
–1
H
N
H
1816 cm–1
7)
1768 cm
8)
–1
2250 cm–1
C=O
O
C=O
C≡N
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H. D. Roth