Nuclear Magnetic Resonance (NMR) Spectroscopy Theory of NMR

Theory of NMR
Nuclear Magnetic Resonance
(NMR) Spectroscopy
• Our department’s NMR spectrometer (in Dobo 245)
has a superconducting magnet with a field strength
of 9.4 Tesla. On this instrument, 1H nuclei absorb
(resonate) near a radiofrequency of 400 MHz;
13C nuclei absorb around 100 MHz.
e-
Part 1
Carbon 13 NMR
• Nuclei are surrounded by electrons.
The strong applied magnetic field (Bo)
induces the electrons to circulate
around the nucleus (left hand rule).
Bo
(9.4 T)
Theory of NMR
Theory of NMR
• The positively charged nuclei of certain elements (e.g.,
13C and 1H) behave as tiny magnets.
• 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).
• 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.
Bi
e-
Bo
• The latter (opposed) is slightly higher in energy than
aligned with the field.
Energy
ΔE is very small
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
Theory of NMR
• The small energy difference between the two alignments
of magnetic spin corresponds to the energy of radio
waves according to Einstein’s equation E=hν.
• 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 the nucleus is less shielded, or is deshielded
by the electronegative atoms.
• 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.
hν
• 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.
H
H C H
H
H
δ
H C Cl
H
C and H are deshielded
H
δ
H C Cl
Cl
C and H are more deshielded
1
Chemical Shift
CMR Spectra
• We define the relative position of absorption in the NMR
spectrum the chemical shift. It is a unitless number
(actually a ratio, in which the units cancel), but we assign
‘units’ of ppm or δ (Greek letter delta) units.
• For 1H, the usual scale of NMR spectra is 0 to 10 (or 12)
ppm (or δ).
• 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):
CH3
• This standard has only one type
CH3 Si CH3
of C and only one type of H.
CH3
• Each unique C in a structure gives a single peak in the
spectrum; there is rarely any overlap.
– The carbon spectrum spans over 200 ppm; chemical shifts only
0.001 ppm apart can be distinguished; this allows for over 2x105
possible chemical shifts for carbon.
• 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:
– Peaks from carbon atoms that have attached hydrogen atoms
are bigger than those that don’t have hydrogens attached.
• Carbon chemical shifts are usually reported as downfield
from the carbon signal of tetramethylsilane (TMS).
13C
Chemical Shifts
Chemical Shifts
C13 Chemical Shift (δ ) vs. Electronegativity
CH3 F
90
CH3
CH2
C13 Chemical Shift
80
CH3 O
70
60
CH
CH3 N
50
CH3 C
40
C O
C C
CH3 Si
10
220
200
180
160
0
-10 1.5
2
C N
Aromatic C
30
20
C X (halogen)
C N
2.5
3
3.5
4
140
13C
downfield
C O
TMS
C C
120
100
Chemical shift (δ)
80
60
40
20
0
upfield
Electronegativity
Chemical Shifts
• Both 1H and 13C Chemical shifts are related to three
major factors:
– The hybridization (of carbon)
– Presence of electronegative atoms or electron attracting groups
– The degree of substitution (1º, 2º or 3º). These latter effects
are most important in 13C NMR, and in that context are usually
called ‘steric’ effects.
• First we’ll focus on Carbon NMR spectra
(they are simpler)
2
Predicting 13C Spectra
• Problem 13.6 Predict the number carbon resonance
lines in the 13C spectra of the following:
CH3
CH3
CH3
C
C
C
4 lines
C C
plane of symmetry
CH3
CH3
CH3
CH3
C
CH3
O
O
C
c
CH3
5 lines
O
CH3
CH3
C
C
CH3
5 lines
H
Predicting 13C Spectra
• Predicted the number of carbon resonance lines in the
13C spectra of the products 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
1
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
2 lines
CH3
Symmetry Simplifies Spectra!!!
O
CH3CCH3
CH3
O
C
CDCl3 (solvent)
2
OCH3
CH3
O
CH3COCH3
CDCl3 (solvent)
O
C
CH3
OCH2
O
CH3COCH2CH3
CH3
CDCl3 (solvent)
O
C
3
CH3
O
OCH2
CH3
CH2
CH3CH2COCH2CH3
CDCl3 (solvent)
O
C
O
O22COCH2CH3
CH3C
CH
CH
6H212
ethyl butanoate
CDCl3 (solvent)
OCH2
CH2
CH3
CH2
CH3
O
C
4
O
CH2
CH3
CH3CCH2CH3
CH3
CDCl3 (solvent)
O
C
H
C
H
O
C
C
C
C
C
C
and
C
H
H
CH3
C
H
CDCl3 (solvent)
C
C
H
H
O
CH3C
H
H
H
5
CH2
CH3
CH2
CH3CH2CH2Br
Br
CDCl3 (solvent)
CH3CH2CH2OH
CH2
CH2
CH3
OH
CDCl3 (solvent)
6
CH2
CH2
CH3
CH3CH2CH2CH2OH
CH2
OH
CDCl3 (solvent)
CH2
CH2
CH3CH2CH2CH2CH2OH
CH2
OH
CH2
CH3
CDCl3 (solvent)
7
CH3
C
C
H
C
CDCl3 (solvent)
C
.
2-methyl-1-hexene
CH3
and
CH2
CDCl3 (solvent)
CH2
CH2
CH2
CH3
C
CH2
CH3
C
CH2CH2CH2CH3
8

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