Nuclear Magnetic Resonance (NMR)
NMR arises from the factthat certain atomic nucleihave a property called “spin”
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Technically, spin arises from the
fact that some nuclei possess a
magnetic moment, μ →
, and angular
momentum, I→
In analogy with other formsof spectroscopy, like UV-VIS,for example, where the electron can occupy either a ground state or excited state, certaintypes of NMR spins can assumetwo possible possible orientations,aligned or opposed to the staticmagnetic field, Bo. Aligned state is designated , opposed state isdesignated .
Excitation of NMR Spin
ΔE
α
β
ΔE
α
βIrradiate with Frequency so as
to satisfy Planck'sLaw
ΔE=hυ
Frequency (Hz)
Energy
NMR Chemical Shifts
= physical constant for a given type of nucleus (ratio of magnetic moment and angular momentum)
h = Planck’s constant
Bo = static magnetic field strength€
ν ∝Bo
€
ΔE = γhBo 2π = hν
Predictions Do Not Match Reality
Bo
Bo(1-σa)Bo(1-σb)
Bo €
Beff = Bo(1−σ )
€
ν eff ∝ Bo 1−σ( )
€
ΔE = γhBo 1−σ( ) 2π = hν
σ = chemical shielding tensor
Frequency
+D3N
HO
O-CH
H3CCH2
CH3
Ile in D2O
12
1
HDO
1
2 1
(Acquisition time = 4 hr)
Chemical Shielding
Shielding arises from the various ways by which electrons“shield” the nuclear spin from the external magnetic field (Bo)
Physical mechanism relates to induced circulation of electrons that oppose static magnetic field (Lentz’ Law)
Shielding (tensors) can in principle be determined through ab initio calculations. This, however, is computationally expensive, and realistically not applicable to large molecules
Classic Approaches to Shielding Local electronic structure; electronegativity of attached groups, bond lengths, bond angles, and conformation (dihedral angles)
Anisotropy of local groups (circulating electrons from aromatic rings for example)
Hydrogen bonds
Electric field effects that polarize bonds
Chemical Shielding Trends for Protons
Functional Groups
Proteins
1234567891011
R
CH3
R
CH2
R'
R
CH
R''R'
C
C
H
R
RCH2C6H5
RCH2XRCH2OR'H
RC6H5RCHO
Frequency
1234567891011
H Aliphatic MethylAromaticAmide
Frequency
Chemical Shifts Can Change Dramatically with Changes in Conformation
8 M Urea
Chemical Shielding & Chemical Shifts
€
ν eff ∝ Bo 1−σ( )Recall
Bo field dependence of frequency makes comparison of spectradifficult from one instrument to another
Hence, report relative ν’s, not absolute ν’s
Chemical Shift (ppm) = =
€
ν peak −ν ref
ν ref
x 106
νpeak = frequency of signal of interestνref = frequency of reference signal
IUPAC-IUB Shift Standard for Proteins
Sodium-2,2-dimethyl-2-silapentane-5-sufonate (DSS)
CH3
SiH3C
CH3
SO3-
J-coupling
Ha
Ha R
Hb
R'' R'
Ha Hb+ =
Hb
Ha
+ =Ha
Ha
J-couplings in Ile
+D3N
HO
O-CH
H3CCH2
CH3
Ile in D2O
1
2
1
1
2 1
1
NMR Active Nuclei
Isotope Natural Abundance (r adHz T-1) spin
1H 99.985 % 26.75 x 107 1/22H 0.02 % 4.12 x 107 1
12C 98.9% nmr-inactive nmr-inactive13C 1.1 % 6.73 x 107 1/2
14N 99.63 % 1.93 x 107 115N 0.37 % -2.71 10x 7 1/2
16O 99.9 nmr-inactive nmr-inactive17O 0.04% -3.63 10x 7 -5/2
31P 100 % 10.83 x 107 1/2
Sensitivity of NMR
€
ΔE = γhBo 2π
& spin states willassume a Boltzman distribution
Implications: Highest sensitivity w/ higher & higher Bo€
Nα /Nβ = expΔE
kT
⎛
⎝ ⎜
⎞
⎠ ⎟= exp
hγBo /2π
kT
⎛
⎝ ⎜
⎞
⎠ ⎟=1.0018 (γ = 2.67x108, Bo =11.7 T)
1D 13C Natural Abundance Spectrum of Ile
+D3N
HO
O-CH
H3CCH2
CH3
Ile in D2O (1H Decoupled)
(Acquisition time = 4 hr)
1
2 1
13C ppm13C ppm
112
CO
1030507090110130150170190210
Aliphatic MethylAromaticCarbonyl C
13C ppm
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