Transcript C-13 NMR Spectroscopy

C-13 NMR
Spectroscopy
Manolito G. Ybañez Jr.
BSCT 2013
Theory of NMR
• The positively charged nuclei of certain
elements (e.g., 13C and 1H) behave as tiny
magnets.
• In the presence of a strong external magnetic
field (Bo), these nuclear magnets align either
with ( ) the applied field or opposed to ( ) the
applied field.
Bo
Theory of NMR
• The latter (opposed) is slightly higher in energy
than aligned with the field.
Energy
DE is very small
• The small energy difference between the two
alignments of magnetic spin corresponds to the
energy of radio waves according to Einstein’s
equation E=hn.
Theory of NMR
• Application of just the right radiofrequency (n)
causes the nucleus to “flip” to the higher energy
spin state
• Not all nuclei require the same amount of energy
for the quantized spin ‘flip’ to take place.
• The exact amount of energy required depends
on the chemical identity (H, C, or other element)
and the chemical environment of the particular
nucleus.
Theory of NMR
• The induced circulation of electrons sets up a
secondary (induced) magnetic field (Bi) that
opposes the applied field (Bo) at the nucleus
(right hand rule).
Bi
e-
Bo
• We say that nuclei are shielded from the full
applied magnetic field by the surrounding
electrons because the secondary field
diminishes the field at the nuclei.
Theory of NMR
• The electron density surrounding a given nucleus
depends on the electronegativity of the attached
atoms.
• The more electronegative the attached atoms,
the less the electron density around the nucleus
in question.
• We say that that nucleus is less shielded, or is
deshielded by the electronegative atoms.
Theory of NMR
• Deshielding effects are generally additive. That
is, two highly electronegative atoms (2 Cl atoms,
for example) would cause more deshielding than
only 1 Cl atom.
H
H C H
H

H C Cl
H

H C Cl
H
H
Cl
C and H are deshielded
C and H are more deshielded
Chemical Shift
• We call the relative position of absorption
in the NMR spectrum (which is related to
the amount of deshielding) the chemical
shift. It is a unitless number (actually a
ratio, in which the units cancel), but we
assign ‘units’ of ppm or  (Greek letter
delta) units.
• For 1H, the usual scale of NMR spectra is
0 to 10 (or 12) ppm (or ).
Chemical Shift
• The usual 13C scale goes from 0 to about 220
ppm.
• The zero point is defined as the position of
absorption of a standard, tetramethylsilane
(TMS):
• This standard has only one type of C and only
one type of H.
CH3
CH3 Si CH3
CH3
Chemical Shifts
C13 Chemical Shift ( ) vs. Electronegativity
CH3 F
90
CH3 Si
C13 Chemical Shift
80
70
CH3 O
60
50
CH3 N
40
30
CH3 C
20
10
0
-10 1.5
2
2.5
3
Electronegativity
3.5
4
CMR Spectra
• Each unique C in a structure gives a single peak
in the spectrum; there is rarely any overlap.
• The intensity (size) of each peak is NOT directly
related to the number of that type of carbon.
Other factors contribute to the size of a peak:
• Carbon chemical shifts are usually reported as
downfield from the carbon signal of
tetramethylsilane (TMS).
13C
Chemical Shifts
CH3
CH2
CH
C X (halogen)
C N
C N
Arom atic C
C O
220
200
180
C C
160
C O
T MS
C C
140
120
100
80
60
13C C he m i cal sh ift
) (
downfield
upfield
40
20
0
Predicting 13C Spectra
• Problem 1: Predict the number of carbon resonance
lines in the 13C spectra of the following (= # unique Cs):
CH3
CH3
CH3
C
C
C
4 lines
C C
plane of symmetry
CH3
CH3
CH3
CH3
CH3
O
O
CH3
C
H
c
C
O
CH3
C
C
CH3
5 lines
CH3
5 lines
Predicting 13C Spectra
• Predict the number of carbon resonance lines in the 13C
spectra of the major product of the following reaction:
CH3
CH3
Cl
CH2
KOH
or
???
ethanol, heat
CH3
CH2
CH3
C
c
C
c
C
C
CH2
CH2
C
7 lines
C
C
c
C
C
5 lines
plane of symmetry
Predicting 13C Spectra
CH3
H3C
CH3
H3C
CH3
CH3
C C
C
C
4 lines
C C
CH3
CH3
CH3
C C
CH3
CH3
C C
CH3
CH3
Symmetry Simplifies Spectra!!!
2 lines
CH3
O
CH3CCH3
CH3
O
C
CDCl3 (solvent)
CMR Spectra
• Each unique C in a structure gives a single peak
in the spectrum; there is rarely any overlap.
• The intensity (size) of each peak is NOT directly
related to the number of that type of carbon.
Other factors contribute to the size of a peak:
• Carbon chemical shifts are usually reported as
downfield from the carbon signal of
tetramethylsilane (TMS).
Uses of 13C NMR
Spectroscopy
13C
NMR spectroscopy provides information about:
•The number of nonequivalent carbons atoms in a
molecule
•The electronic environment of each carbon
•How many protons are bonded to each carbon
Uses of 13C NMR Spectroscopy
13C
NMR spectroscopy can verify that E2 elimination of an
alkyl halide gives the more substituted alkene (Zaitsev’s
rule)
• 1-Methylcyclohexene has five sp3-carbon resonances in the 20 to 50 
range and two sp2-carbon resonances in the 100 to 150  range
• Methylenecyclohexene, due to symmetry, has only three sp3-carbon
resonance peaks and two sp2-carbon resonance peaks
Uses of 13C NMR Spectroscopy
The 13C NMR spectrum of the E2 reaction product from the
treatment of 1-chloro-1-methylcyclohexane with a base. The
product is clearly identified as 1-methylcyclohexene.
Thank you!!!