Chemistry 59-330 Lecture 5

1
Vibrational Spectroscopy
A rough definition of spectroscopy is the study
of the interaction of matter with energy
(radiation in the electromagnetic spectrum). A
molecular vibration is a periodic distortion of a
molecule from its equilibrium geometry. The
energy required for a molecule to vibrate is
quantized (not continuous) and is generally in
the infrared region of the electromagnetic
spectrum.
For a diatomic molecule (A-B), the bond between
the two atoms can be approximated by a spring
that restores the distance between A and B to its
equilibrium value. The bond can be assigned a
force constant, k (in Nm-1 the stronger the
bond, the larger k) and the relationship between
the frequency of the vibration, ?, is given by
the relationship
DAB
or, more typically
where , c is the speed of light, ? is the
frequency in wave numbers (cm-1) and ? is the
reduced mass (in amu) of A and B given by the
equation
rAB
0
re
re equilibrium distance between A and B
DAB energy required to dissociate into A and B
atoms
2
Infrared Radiation
Portion of the electromagnetic spectrum between
visible light and microwaves full range for IR is
10000-400 cm-1 of importance here is 4000-400
cm-1 (wavenumbers) or 2.2-25 mm
(wavelength) Note cm-1 is proportional to
Energy cm-1 104/mm this energy is absorbed
by molecules and converted to molecular vibration
3
IR Absorption
IR absorptions are characteristic of entire
molecule or essentially a molecular
fingerprint vibration spectrum appear as bands
molecular vibration is not a single energy as
also depends on molecular rotation band
intensities expressed as either transmission
(T) or absorption (A) A log10(1/T)
4
Molecular Vibrations
Stretching is a rhythmical movement along a
bond Bending is a vibration that may consist of
a change in bond angle (twisting, rocking and
torsional vib.) Vibrations that result in change
of dipole moment give rise to IR
absorptions alternating electric field produced
by changing dipole couples the molecular
vibration to the oscillating electric field of
the radiation
5
Vibrations for H2O and CO2
3650 cm-1 3756 cm-1 1596 cm-1
Symmetrical asymmetrical scissoring
stretch stretch (inactive in IR) for
CO2 1340 cm-1 2350 cm-1 666 cm-1
-
6
Bending for CH2


-
Asymmetric symmetric in-plane
out-of-plane stretch stretch bend
bend 2926 cm-1 2853 cm-1 1465 cm-1
1350-1150 cm-1
-

In plane bend or rocking 1350-1150 cm-1
Out-of plane bend or twist 1350-1150 cm-1
7
Assignments of Bands
For a stretching frequency interruption based on
Hookes Law Frequency 1/2pc(k/(MxMy/MxMy)1
/2 where f force constant of bond and M is
mass f is about 5 x105 dyne/cm for single bond
2x that for double bond and 3x that for triple
bond C-H stretchcalc 3040 cm-1 actual CH3
2960-2850 cm-1 Note for C-D stretch is 21/2 x
that of C-H
8
Instrumentation
Requirements source of IR radiation, sample,
detector
9
IR Spectrophotometer
10
Sample Handling
IR spectra can be obtained for gases, liquids and
solids Liquids may be neat or in solution Neat
between to NaCl plates (0.01 mm film) (NaCl does
not absorb until 600 cm-1) thick samples absorb
too strongly poor spectrum Solution cells are
0.1-1 mm thick(0.1-1 mL in volume) requires
second cell of pure solvent to correct for
absorptions of solvent Solids usually as a
mull (supension) in nujol oil (free of IR
absorptions 4000-250 cm-1 or dispersed in KCl
pellet
11
Spectral Interpretation
Precise and complete interpretation is NOT
possible thus must use IR in conjunction with
other techniques but Functional group
region 4000-1300 cm-1 eg OH, NH, CO, S-H,
CºC Many functional groups exhibit
characteristic bands Fingerprint regions
1300-650 cm-1absorptions here are usually
complex some interpretation is possible similar
compounds give similar spectra but fingerprint is
unique
12
Organic Functional Groups
13
An Organic Example
CºN stretch 2226
Aromatic C-H bands
14
Nuclear Magnetic Resonance
Sample in a magnetic field absorbs radio
frequency radiation absorption depends on
certain nuclei in molecule initially we deal
with 1H (proton) NMR inspection of NMR provides
much more structural data than MS or IR
15
Magnetic Nuclei
Nuclei with odd mass, odd atomic number or both
have quantized spin angular momentum
eg 11H, 21H, 136C, 147N, 3115P spin quantum
number, I 0, 1/2, 1, 3/2 .. For
11H,136C,3115P I 1/2 For 21H, 147N I 1
(nonspherical charge distribution electric
quadrupole) number of states in magnetic field
2I1
16
In a Magnetic Field
DE(hn/2p)Bo
Bo is related to strength of magnetic field h is
Plancks Constant
DE is in the radio frequency range
17
Absorbance of RF
In magnetic field spinning nucleus precesses
about applied magnetic field (Larmor
Frequency) when same frequency RF is
applied electric field of radiation and electric
field of precessing nucleus couple E is
transferred and spin changes -Resonance
18
Relaxation
How is this energy dissipated? T1 spin-lattice
or longitudinal relaxation process transfer of E
from excited protons to surrounding protons T2
spin-spin transverse relaxation transfer of E
among precessing protons, result is line
broadening
19
Instrumentation
Magnetic field, radio frequency generator
20
Instrument
1945-46 at Stanford Professor Bloch Nobel
Prize 1952
21
Sample
Typically if want to observe 1H NMR need to
avoid solvent with protons used deuterated
solvent or solvent with no protons for example
C6D6, CDCl3 or CCl4 sample is held in a 5mm tube
typically 2 mg in 0.5 mL) sample is spun in the
magnetic field to average out field
inhomogeneities
22
Magnets
1953 1.41 Tessla or 60 MHz for proton
resonance Now 200-500 MHz magnets are common
as high as 900 MHz in some NMR research Labs
magnetic fields are large in the case of 500
MHz magnetic 5Gauss lines forma a 15 ft sphere
about the magnets
23
Modern Instrument
24
Chemical Shift
Electron density in a magnetic field circulates
generating a magnetic field in opposition to the
applied field thus shielding the nucleus. Since
electron density for each type of proton
environment is different get different
resonance absorption of RF neff (g/2p)Bo(1-s)
s is the shielding constant reference position
relative to the standard TMS tetramethylsilane
25
NMR Scale
Set TMS to zero Hz (300 MHz magnet) if we
use this scale must specify the strength of
magnet as frequency of resonance will change
with field better to use dimensionless units
d (ppm) freq/applied field x 106 d
0 Hz
3000 Hz
0 ppm
10 ppm
26
NMR Scales
3000 Hz
300 MHz
0 Hz
0 ppm
10 ppm
6000 Hz
0 Hz
600 MHz
0 ppm
10 ppm
27
Field Strength Effect
60 MHz
300 MHz
28
Chemical Shifts
As the shift depends somewhat on electron density
electronegativity may be a guide for chemical
shifts electron density around protons of TMS is
high positive d increases to left of TMS
increase d means deshielded relative to
TMS since C is more electronegative than C
expect R3CHgtR2CH2gtRCH3gtCH4 1.6
1.2 0.8
29
NMR Scales
3000 Hz
300 MHz
0 Hz
0 ppm
10 ppm
Higher frequency-less shielded
Lower frequency-more shielded
6000 Hz
0 Hz
600 MHz
0 ppm
10 ppm
30
Acetylene
based on electronegativity expect higher chemical
shift than ethylene Apparent anomaly H-CºC
chemical shift is 1.8 ppm WHY? linear
molecule if aligned with magnetic field then
p-electrons can circulate at right angles to
field and generate magnetic field in opposition
to applied field thus protons experience
diminished field and thus resonance at lower
frequency than expected 1.7-1.8 ppm
31
Aldehydes
Deshielded position of aldehyde proton observed
at 9.97 ppm (acetaldehyde)
32
Benzene
Ring current effect deshields aromatic protons
7.0-8.0 ppm (depending on substitution)
33
18Annulene
Outside protons are deshielded 9.3 ppm protons
on inside shielded -3.0 ppm
34
Acetophenone
All protons are deshielded due tp ring currents
Ortho-protons are further deshielded due to
carbonyl meta, para 7.40 ppm ortho 7.85 ppm
Ring current effect infer planarity and
aromaticity
35
General Regions of Chemical Shifts
alkyne
monosubstituted aliphatic
disubstituted aliphatic
alkene
Aromatic
aldehydic
ppm
10 9 8 7 6 5 4 3 2
1 0
36
Integration Benzyl Acetate
Integration 523
At high resolution see multiplet
37
Spin-spin Coupling
Chemically inequivalent protons field of one
proton affects the other normally only see up
to 3-bond coupling
-1/2
-1/2
1/2
1/2
38
Spin-spin Coupling
Each proton has a unique absorption but effected
by magnetic field of other proton
J is the coupling constant
39
Coupling
C-H sees CH2 protons CH2 sees C-H proton
(1, 0, -1) (1/2, -1/2)
40
Ethylbenzene
Typical ethyl pattern A2B3
triplet
quartet
41
Pascals Triangles
42
Isopropylbenzene
43
Ethanol in CDCl3
Rapid exchange of OH do not see coupling
CH3CH2OH
44
Ethanol in DMSO
No exchange
CH3CH2OH
45
Doublet of Quartets
CH3CH2OH
Can see J(CH2-OH) and J(CH3-CH2)
46
N-methylcarbamate
14N has I 1, if exchange is rapid no
coupling intermediate or slow --broad NH
47
H-C-N-H Coupling
In trifluoroacetic acid, amine is protonated see
methylene coupling to N-H protons
48
Fluoroacetone, CH3COCH2F
19F has I 1/2
J2
J4
49
Other Magnetic Heteroatoms
2H (Deuterium) I 1 simplifies
proton spectrum as H-D coupling is
small X-CH2-CH2-CH2-COY X-CH2-CH2-CD2-COY triplet
, quintet, triplet triplet, slightly broad
triplet 31P I 1/2 (100 natural
abundance) large coupling constants P-H 200-700
Hz 29Si I 1/2 (4.7 Natural abundance) Si-CH
6 Hz low intensity (satellites) 13C I 1/2
(1.1 Natural abundance) not seen unless enriched
with 13C
50
Chemical Shift Equivalence
Nuclei are chemical shift equivalent if they are
interchangeable through a symmetry operation or
by a rapid process. Rotation about a simple
axis (Cn) Reflection through a plane of symmetry
(s) Inversion through a center of symmetry (i)
51
Rotation and Reflection
C2 axis of rotation Environments are
indistinguishable
Reflection through a plane protons are mirror
images of each other (enantiotopes)
52
Enantiotopes and Diastereotopes
Enantiotopic by i
Methylenes are diastereotopic not equivalent
couple to each other
Chiral moelcule
53
Diastereotopic protons(achiral molecule)
Plane makes H1s and H2s equivalent no plane
through CH2s thus the protons are diastereotopic
Diastereotopic protons can not be placed in same
chemical environment
54
Rapid Exchange
At high T see an average spectrum
Equilibrium at low T
55
13C NMR Spectroscopy
12C not magnetically active but 13C has I 1/2
Natural abundance is 1.1 sensitivity is
1/5700 of 1H this problem is overcome with
Fourier Transform (FT) NMR instrumentation
(1970s) use broadband decoupling of protons so
see no coupling and get NOE enhancement in 13C
signal intensity
56
13C1H NMR
13C samples usually run in CDCl3 and chemical
shifts are reported relative to TMS 300 MHz for
1H NMR 75.5 MHz for 13CNMR 10 mg in 0.4 mL of
solvent in 5 mm tube
57
13C NMR of diethylphthalate
Proton coupled
58
13C1H NMR of diethylphthalate
Proton decoupled
59
13C1H NMR of diethylphthalate
Proton decoupled 10-s delay
60
Peak Intensity
in 13C NMR the relaxation times vary over a wide
range so peak areas do not integrate for the
correct number of nuclei long delays could work
but the time required is prohibitive NOE
response is not uniform for all C atom
environments C atoms without protons attached
give low intensity
61
Deuterium Substitution
Substitution of D for H results in decreased
intensity deuterium has I 1 so 13C is split
into 3 lines ratio 111 possible spin states
for D are -1, 0 1 thus CDCl3 exhibits a 111
triplet in 13C NMR
62
Chemical Shifts
Carbon chemical shifts parallel (generally)
proton shifts but with a much broader
range eg. Two substituents on a benzene
ring para three carbon peaks ortho three
peaks meta four peaks
63
t-butyl alcohol
64
2,2,4-trimethyl-1,3-pentanediol
65
Alkenes, Alkynes and Aromatics
Alkenes sp2 carbons seen in range 110-150
ppm Alkynes sp carbons seen in range 65-95
ppm Aromatic benzene 128.5 ppm substituted
/-35ppm substituted carbons decreased peak
height longer T1 and diminished NOE
66
Carbon based Functional Groups
Ketones R2CO 203.8 ppm(acetone) Aldehydes
RHCO 199.3 ppm (Acetylaldehyde) Carboxylic
acids RCO2H 150-185 ppm Nitriles RCN 150-185
ppm Oximes R2CN(OH) 145-165 ppm
67
Example
159.2
158.7
9.75
11.00
18.75
11.50
29.00
21.50
68
13C-1H Coupling
Coupling is less important than in 1H NMR since
routinely decoupled. One-bond C-H coupling
110-320 Hz two bond -5 to 60 Hz three bond
about same as two bond for sp3 C but for
aromatics three bond is often bigger than two
bond in Benzene 3JC-H 7.4 Hz, 2JC-H 1.0 Hz
69
Example Spectra 1 C5H10O
quartet
Singlet 211.8 ppm
doublet
70
Example Spectra 2 C4H10O
doublet
quartet
triplet
71
Example Spectra 3 C11H14O2
doublet
quartet
triplet
singlet
72
Other Nuclei for NMR
Nuclei Spin Nat. Abund. 2H (1) 0.015 6Li
(1) 7.42 15N (1/2) 0.37 19F
(1/2) 100 23Na (3/2) 100 29Si (1/2) 4.7 31
P (1/2) 100
73
19F NMR Spectrum of fluoracetone
74
19F NMR Fluoracetophenone
75
29Si NMR Spectrum of TMS
76
29Si NMRtriethylsilane
77
29Si NMR1,1,3,4-tetramethyldisiloxane
78
31P NMR Spectrum of H3PO4
79
31P NMR Spectrum
80
31P NMR
81
31P NMR
82
Diastereomers