Transcript Chemistry 59-330 Lecture 8

Nuclear Magnetic Resonance
Requirements
Molecules must contain nuclei that have a nuclear angular momentum (or nuclear
spin) quantum number I >0 which results in (2I + 1) magnetic quantum numbers
(states) m = + I, I -1, I -2, …, - I +1,- I.
All nuclei with odd mass, odd atomic number or both are NMR active:
I = ½ for 11H,136C, 199F, 3115P and I = 1 for 21H, 147N
For NMR measurements magnetic nuclei must be
in a magnetic field as the eigen states of m are
degenerated in an non-magnetic environment.
hn = DE = g(h/2p)Bo
DE is the energy difference between the generated
states and depends on the applied external
magnetic field Bo and the nucleus specific gyromagnetic ratio g. The energy differences, however,
are very small for all nuclei and electromagnetic
radiation of radio frequency is sufficient (5-100
MHz at a field of 2.3 T).
Sample Preparation and Measurement
The resonance frequency no for protons in 1H-NMR are at 60 MHz in a
1.41 T magnetic field (100 MHz at 2.34 T, 300 MHz at 7.05 T, and 500
MHz at 11.7 T.
The sample is dissolved in a solvent that does not contain the magnetic
nucleus of interest and this is why deuterated solvents are used in 1HNMR: e.g. C6D6, CDCl3,or CCl4.
An NMR tube of 5 mm diameter is typically filled with 2 mg of compound
dissolved in 0.5 mL of solvent (1H-NMR) and spun in the magnetic field
to average out field inhomogeneities.
Chemical Shift
NMR would be useless if all protons in a sample had the same
resonance frequency. Fortunately, the resonance frequency is slightly
varied by the chemical environment of each proton. The frequency
difference n of a nucleus is measured relative to a standard
(tetramethylsilane (TMS) for 1H- and 13C-NMR) and have been named
chemical shift d. d is given in ppm.
d(X) = 106 Dn/n with d(TMS) = 0
Example: The protons of a methyl group have a resonance frequency
of 126 Hz lower than TMS at an observation frequency of 60 MHz.
dH(CH3) = 106 (126/60*106) = 2.10 ppm
H3C
Si
H3C
CH3
CH3
NMR Scale
The advantage of using the dimensionless ppm unit is that the scale is
independent of the external magnetic field.
Example for 1H-NMR with TMS as reference and 300 MHz resonance frequency.
3000 Hz
0 Hz
0 ppm
10 ppm
Field Strength Effect
Hb
Hx
Ha
CN
60 MHz
300 MHz
NMR Scales (1H-NMR)
0 Hz
3000 Hz
10 ppm
300 MHz, 7.05 T
0 ppm
(downfield)
higher frequency-less shielded
(upfield)
lower frequency-more shielded
6000 Hz
10 ppm
600 MHz, 14.1 T
0 Hz
0 ppm
Chemical Shift and Molecular Structure
The resonance position of a given nucleus is determined by its shielding constant
s. s is made up of four terms, the diamagnetic shielding sdia, the paramagnetic
shielding spara, shielding due to neighbouring groups sintra, and shielding due to
intermolecular effects sintra. s can possess positive and negative values.
neff = (g/2p)Bo(1-s)
Shielding and deshielding effects are produced by local magnetic fields generated
by circulating electron densities. Thus, changes in the local electron density will
influence the chemical shift.
A positive value of d is left of TMS, which as a high electron density around its
protons, and means the nuclei are deshielded relative to TMS.
For example, we can deduced from the increasing ppm values in 1HNMR given below that C is more electronegative than H:
R3CH>R2CH2>RCH3>CH4
1.6
1.2
0.8
Chemical Shift of 1H
DE = hn0 = gB0h/2p,
n0 = gB0/2p,
with Blocal = Bo(1-s)
s is the electronic shielding of a specific proton and its resonance frequency
with shielding is termed chemical shift d;
n0 = gB0(1-s)/2p
An increase in d means a 1H nucleus is deshielded relative to TMS and vice
versa;
As d depends on electron density around nuclei, the electronegativity of atoms
close to the nuclei will affect the chemical shifts;
d for H in H3CX with X = F, HO, H2N, H, Me3Si, or Li are 4.26, 3.38, 2.47, 0.23,
0.0, and -0.4 (very solvent depending), respectively;
The electron density and, consequently, d are not only influenced by the inductive
effect but also by resonance;
1.30
5.03
4.18
1.71
H
H
O
O
6.88
H
3.50
H
H
4.97
H
5.70
O
H
4.04
4.19
6.47
H
1.71
5.83
Hybridization of the carbon to which a proton is attached also influences the
electron density around the proton; As the proportion of s-character increases
form sp3 to sp hybridization, bonding electron move closer to the carbon and the
proton becomes more deshielded;
CH4
1.96
1.96
1.96
0.23
5.25
0.86
1.96
0.86
5.25
4.00