Vibrational Spectra
Molecules are not Static
Vibration of bonds occurs in the liquid, solid and gaseous phase
Vibrating  Energy  Frequency (and the appropriate frequencies
for molecular vibrations are in the Infrared region of the
electromagnetic spectrum
Vibrations form therefore, a fundamental basis for spectroscopy in
chemistry--the bonds are what makes the chemistry work in structure
and function
For Organic Chemistry the most important uses of these vibrations is
for analysis of:
The IR spectra in this
•functional groups
evenings talk are from
•structural identity, “fingerprinting” the SDBS data base.
Tonight’s lecture
Infrared Spectroscopy for Structure Determination
A little theory
Some notes on sampling
Defer discussion of instrumentation
Defer discussion of solids analysis
Lots of examples, working through trends in
related structure classes
How to interpret, use data.
Some Perspective
IR’s are not as thoroughly interpretable as NMR, Mass Spec
Lacks the quantitative character on atom-atom basis that NMR
has. (all the chromophores are not equal)
Not used as much for identification as NMR and MS have
become more accessible
Still very useful for confirmation of structure cf. Reference
spectra.
Diagnostic for functional groups that may be silent or
ambiguous in the NMR
Quite sensitive, and can measure in all sorts of strange matrix
e.g on surfaces, extremely useful for solid state
characterization.
Infrared vs. Raman
These two spectroscopies measure the same thing, vibrations,
in different ways.
IR is a absorption measurement, while Raman measures
scattered light from a laser source, that in being scattered, is
superimposed with the vibrational structure of the molecule.
The selection rules are different--IR bands are active if in the
act of the vibration, the dipole moment of the molecule
changes. Raman band are active if the polarizability of the
molecule changes.
Often these two are complementary to each other
Molecules of high symmetry frequently will not show IR
activity
IR Measurement
Neat film, or melt between two sodium chloride plates
Solid solution in KBr, ground together and pressed into a
transparent pellet
Solution with appropriate blank region of solvent. Solution
IR can be used to minimize broadening from self-association,
H-bonding.
Salt plates of CsCl2 for lower frequency window
transparency (down to 200 cm-1)
Mulling (grinding with mineral oil as dispersion), spread on
salt plate
Many different reflectance techniques, light must pass into
the sample and reflect out.
Vibration levels are quantized,
like everything else
Like a harmonic oscillator
With L-H, L moves most easily
From Skoog and West
Can couple to other springs but heavy atoms can
block or minimize this effect
What Kind of vibrations are These?
Bonds can…….
These can number into the
hundreds.
Bend
Stretch
Some are symmetrical, some
antisymmetrical and many are
coupled across the molecule
Can be calculated. One easy
approximation is:
  5.3 10
Wag
(rock)

12
m1m2
m1  m2
k

k is the “force constant”, like the
Hookes Law restoring force for a
spring. Known and tabulated for
different vibrations
The “reduced mass” where m1, m2
are the masses on either side of
vibration
Regions of Frequencies
Wavenumber(cm-1)
Spectral Region
Frequency(Hz)
Near -to
visible- IR
(NIR)
Combination
bands
3.8 x 1014 to 1.2
x 1014
12800 to 4000
0.78 to 2.5
Mid Infrared
Fundmental
bands for
organic
molecules
1.2 x 1014 to 6.0
x 1012
4000 to 200
2.5 to 50
200 to 10
50 to 1000
Far IR
6.0 x 1012 to 3.0
x 1011
Inorganics
organometallics
Wavelength (,m)
After Table 16-1 of Skoog and West, et al. (Chapter 16)
What kinds of Bonds Absorb in
which Regions?
Bending is easier than stretching--
happens at lower energy
(lower wavenumber)
Bond Order is reflected in ordering-- triple>double>single (energy)
with single bonds easier than double
easier than triple
The k in the frequency
equation is in mDyne/Å of
Heavier atoms move slower than
displacement
lighter ones
Single bond str 3-6 mD/Å
Double bond str. 10-12 mD/Å
Triple Bond 15-18 mD/Å
The Fundamentals
antisymmetric
symmetric
R
R
R
R
R
H
H
R
H
R
H
R
scissoring
R
R
H
These oscillating electric
dipoles match in frequency
the incoming e-field
oscillations of IR light.
stretching
H
H
H
in-plane
bending
All the simple possibilities.
For n atoms in a molecule;
– Linear: 3n – 5 modes
– Non-linear: 3n – 6
modes
– Example for a
methylene,given n=3
rocking
H
R
H
wagging
R
H
H
twisting
out-of-plane
bending
While useful, this
oversimplifies, since
molecular orbital picture
requires that atoms can’t
vibrate without affecting
the rest of the molecule.
Calculated IR bands for CH2 in
formaldehyde
Formaldehyde spectrum from: http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm#ir2
Results generated using B3LYP//6-31G(d) in Gaussian 03W.
Looking at a Spectrum
Divide the spectrum in to two regions:
4000 cm-1 1600 cm-1 most of the stretching bands,
specific functional groups
(specific atom pairs). This is the
“functional group” region.
1600 cm-1  400 cm-1 Many band of mixed origin. Some
prominent bands are reliable. This
is the “fingerprint” region. Use
for comparison with literature
spectra.
Wavenumber is cm-1=104/()
Nujol (mineral oil)
Some IR
Media are
better than
others
CHCl3
Nujol, for example
wipes out the
hydrocarbon region
for transparency..
CDCl3
Note that some older IR
spectra while they are linear in
frequency, the wavelength
scale compresses the higher
we go, Affects the appearance
of spectra, not the line
positions.
Reaction Monitoring
Gas phase IR
Pivaldehyde +
methylamine
Note loss of C=O
Thank to Drs. Dalton and Mascavage
A Functional Group Chart
4000
3600
3200
2800
2400
2000
1600
1200
800
group
O-H str
NH str
COO-H
=C-H str
Csp3-H
C-H
-(C=O)-H
CN
CC
C=O
-C=N
-C=C
phenyl
C-O
C-N
F
Cl C-X
Br
I
The hydrogen stretching region
Tip-Draw a line straight up from
3000 cm-1. Intensity on left is
Csp2-H, to the right is Csp3-H
Arom- H
H
H
3000cm-1
Amines
R-NH2
3500, 3300 cm-1 doublet, frequently
(without, with H-bonding effect) NH
stretch
1600 cm-1 NH2 scissoring - broad
700-900 cm-1 NH2 wagging - broad, strong
1080 cm-1 C–N str. --weak for alkyl
1300 cm-1 Ar–N str. strong
R–NH–R
3400 cm-1 singlet str.
Weak C–N 1125 cm-1
R–NR–R
No good IR bands, adj CH2 will shift to 2800 cm-1.
A tert amine salt NH strong at 2500 cm-1
Amine Salts
R-NH3+ like methyl, but broad
NH str. centered at 3000, br.
deformation 1520-1570 br.
R2NH2+ 3000 br, spikes at 2200,2500 cm-1
NH2 scissor at 1600 cm-1, broad
Alcohols
C–O–H stretch 3600 cm-1 in dilute
solution
phenethanol
Typically H-bonding and at lower
frequency ~3400 cm-1
C–O stretch in same region as C–C
but much more intense
Position is sensitive to subs. pattern
1-phenylethanol
2-piperidinylethanol
RCH2–OH
1050 cm-1
R2CH–OH
1110 cm-1
R3C–OH
1175 cm-1
Bands that should appear together
SERVE AS CROSS CHECK
e.g.
see a triple bond? Check for C–H str.
see C=O, check for OH, C–O
I will point a few of these out as we go...
Alkynes
6-methyl5-hepten-1-yne
3,5dimethyl-1hexyn-3-ol
phenylacetylene
Also wk
overtones at
1820,1790 cm-1
benzonitrile
Phenyl-1-butyne
Functional Groups can be “NMR
Silent” or ambiguous. IR can
play a key role in Identification
O
O
O
R
O
R
R
1800, 1750
cm-1 (cyclic, C-O-C vs 1180has more intense 1220 cm-1
1750, acyclic,
more instense
1800
1800 cm-1 doublet, Fermi
resonance
Cl
Poor resonance for
2p-3p, but strong
inductive effect
More NMR silent groups-Nitro groups
Analogous to
Carboxylate ion.
Strong bands
+
N
O
1520, 1350 cm-1
O
R
+
N
O
Aromatic nitro
O
Aliphatic nitro
1550, 1370 cm-1
Double Bonds
1640 cm-1 is double bond stretch
not seen for symmetrical molecules
lower freq by conjugation, more intense
=C–X lowers to 1590 cm-1
Ca. 890-910 cm-1 and 985cm-1 are o.o.p bendings for
terminal =CH2
Cis vs Trans?
Disubstituted –HC=CH960-970 cm-1 trans o.o.p bend
675-730
cm-1
cis o.o.p. bend
Medium intensity
Methylenes
1460 cm-1 CH2 scissoring
725 cm-1 characteristic rocking for 4 or
more CH2’s in a row (non-cyclic)
The Carbonyl Stretch is our
friend…
Carbonyl stretch changes its
position for variation in
specific structure
THIS BAND IS ALWAYS
STRONG!!!
Good rules to remember…
C=O conjugated to double bond goes lower in frequency
With electronegative substituent (O, Cl) goes to higher frequency
C=O in strained ring, goes to higher frequency
C=O…(H hydrogen bonds lower the frequency)
Ketones--sensitive to strain
O
O
O
1715 cm-1
1750 cm-1
1780 cm-1
Ca. 30 cm-1 higher for every C atom removed
-diketones, str-str for open chain, IR inactive; in ring, 1720,1740
-haloketones--can see second band from rotamer populations
(1720, 1745)
Ketones--Sensitive to
conjugation
O
O
H
O
1660-1700 cm-1
rotational isomers cause
doubling. S-trans 1674,
S-cis 1699
1650-1700 cm-1
O
1580-1640 cm-1
for enol
1715 cm-1
for the
keto bond
Along with br. OH str.
Aldehydes
Doublet straddles 2800 cm-1
(Fermi resonance)
O
Fundamental
at ca. 2800
H
R
Bending at
1400, gives
overtone at
R
2800
O
H
cyclohexylcarboxaldehyde
Ethyl-2-butenal
Carboxylic Acids
1715 cm-1
br OH stretch
Hept-3-enoic acid
Good
example of
the
broadening
from Hbonding
Cyclohexanecarboxylic acid
Also C—O 1280 cm-1,
often a doublet
O—H o.o.p bend br
920 cm-1
Phenylbutyric acid
Salts have1600,1350
cm-1 broad!
Esters and Lactones
O 1735 cm-1
O
Butyl acetate
Cyclohexyl acetate
1300-1100
intense, often
doublet
R
Effects of conjugation
O
Lowers to
1715 cm-1
O
O
R
O
Raises to
1770 cm-1
Similar, to
1715 cm-1
O
Weakens DB
character
R
O
O
:
O
Strengthens DB
character (inductive
over resonance)
Lactones, similar effects
O
O
1735 cm-1
1770 cm-1
O
O
O
O
1765 cm-1
O
1715 cm-1
O
amides
NH str 3300 cm-1
C=O 1650 cm-1
NH bend 1640 cm-1
phenylacetamide
phenylacetanilide
Moves to 1550 for
R-C(=O)-NHR’
N-benzylbenzamide
L--aspartyl-Lphenylalanine 1-methyl
ester
Methyl Groups
CH3 “umbrella”
Ca. 1375 cm-1
“t-butyl split”
“isopropyl split”
isopropylcyclohexane
t-butylcyclohexane
O
R
CH3
Moves
lower by
20 cm-1
But…
methylcyclopentane
OCH3, NCH3 do
not give this band
Benzene rings--substitution patterns
Ring pucker at ca 700 cm-1is IR active
for mono, 1,3-di-, 1,3,5-tri-, 1,2,3-trisubs rings.
Complements the
out-of plane
bendings, related
to the number of
adjacent H…
R
750,700
R
750
From Crewes,
Rodriguez and
Jaspars, ch 8
R
5
750
4
750
3
780
2
820
1
850
R
850,780,
700
R
820
R
R
Unreliable with NO2, CO2H subs
Out-of-plane bending combinations,
quite small, but in a normally clean
region of IR. Reliable even with nitro or
carboxyl substitution
Putting the S in Silent; Sulfur
containing groups
O
R
wk SH at
2580
S
H
R
O
S
1050
S
Extremely wk at
590-700 cm-1
S
S
R
O
cm-1
R O S OH
O
O
R
O
O
OR
1360, 1180 cm-1
R
S
S
1390, 1200 cm-1
O
R
1320, 1140
cm-1
Induction strengthens D.B. resonance not significant
R
S
O
NH2
1340, 1160 cm-1
Silence is Deafening
(the IR guys think “silence is
golden”
R-N=C=O strong at 2250-2290 cm-1
R-N=C=S strong 2000-2190 cm-1
R-N=N=N strong 2100-2200 cm-1
R-N=C=N-R strong 2150 cm-1
R-CN 2250 cm-1
R-N+C:- 2130-80 cm-1
R-C=C=C-R’ (allenes) 1900-2000 cm-1 with very
strong 850 wag if terminal