Chemical shielding & chemical shift Flashcards
Symbol for chemical shift
delta
How is the chemical shift of a nuclear environment defined?
With respect to the nuclei in a reference compound, whose chemical shift is usually given a value of 0
Calculating chemical shift
(vL-vLref)/vSpec (in Hz) x 10^6
or (vL-vLref)/vSpec (in MHz)
where vL = resonant frequency
Gives a value in ppm
Deshielded nuclei
High frequency
High ppm
Downfield/low field
Shielded nuclei
Low frequency
Low ppm
Upfield/high field
What does the resonant frequency (vL) of a nucleus depend on?
The magnetic field the nucleus experiences
What affects the magnetic field experienced by a nucleus?
The chemical/electronic environment around the nucleus
i.e. besides the applied field B0, the nuclei in a sample also experience additional magnetic fields set up by the electrons in the sample as they move within molecular orbitals
Beff =
B0 - b0
Where B0 = applied (external) magnetic field and b0 = the magnetic field generated by the surrounding electrons
Why does each nucleus resonate at a different frequency?
Because each nucleus is in a different chemical environment
vL = yBeff/2pi
Where Beff = B0(1-sigma)
Sigma
Shielding constant
Can be described as the summation of many terms
Diamagnetic shielding term (sigmadia)
Electrons in s-orbitals surrounding the nucleus circulate, which produces a second magnetic field at the nucleus that opposes the applied field
The nucleus is therefore shielded from the spectrometer field so resonates upfield
This term dominates 1H chemical shifts
Why are variations in the diamagnetic shielding term only of relevance for 1H, 6Li and 7Li?
Because this term only uses spherical electron distribution (mainly s-orbitals) - it arises from the core electrons
For heavier nuclei, sigmadia contributes a constant value
Paramagnetic shielding term (sigmapara)
The circulation of electrons between the ground and excited states of p-orbitals induced by the external magnetic field gives rise to local magnetic fields that support the spectrometer field
The nucleus therefore experiences a larger net field and is deshielded from the spectrometer field, therefore resonates downfield
The paramagnetic shielding term is proportional to
DeltaE^-1
DeltaE = the electronic excitation energy (i.e. the energy spacing between ground and excited states)
Why is the paramagnetic shielding term small for 1H?
For 1H, DeltaE is large, therefore DeltaE^-1 is small, therefore sigmapara is small
When does sigmapara begin to dominate over sigmadia?
For heavier nuclei in molecules with low lying electronically excited states
e.g. 31P, 195Pt
Normal chemical shift range for 1H
+20 to -20 ppm
Normal chemical shift range for 11B
+100 to -100 ppm
Normal chemical shift range for 205Tl
5500 ppm range
Why is the chemical shift range larger for heavier/larger nuclei?
Heavier/larger nuclei have more electrons so have larger values for their paramagnetic shielding terms
Ramsay-Karplus-Pople equation
Describes the size of the paramagnetic shielding term
sigmapara = -constant(r^-3)^-1(DeltaE)^-1(c^2)
r in Ramsay-Karplus-Pople equation
Average distance between the nucleus and its valence electrons
DeltaE in Ramsay-Karplus-Pople equation
HOMO-LUMO gap
Smaller HOMO-LUMO gap means a greater paramagnetic effect
c in Ramsay-Karplus-Pople equation
A measure of the covalence of the bond(s) to adjacent atoms
LCAO coefficient
What is paramagnetic electron circulation?
Mixing between ground and excited states
When might a diamagnetic compound have a paramagnetic shielding term contribution?
If the compound (e.g. a transition metal) has a paramagnetic excited state
e.g. Octahedral low-spin complexes with a d6 configuration i.e. Co3+
SigmaN
Nuclear anisotropy shielding term
Anisotropy
The property of being directionally dependent - i.e. different properties in different directions
Anisotropy in NMR
Chemical bonds are regions of high electron density with associated magnetic fields
However, the fields are stronger in some directions than in others - i.e. an abnormal electron distribution affects the applied magnetic field and changes the observed chemical shift
The effect of the field on the chemical shift of nearby nuclei is dependent on the orientation of the nucleus in question with respect to the bond
Particularly important for pi-bonds
SigmaR
Ring current shielding term
An anisotropic effect produced by the pi-systems of aromatic rings - the circulating electrons create a magnetic field that opposes the applied field at the centre of the ring but reinforces the applied field outside the ring
Sigmae
Electronic shielding term (paramagnetic NMR)
Electronic shielding term
For molecules that are paramagnetic in the ground state, huge chemical shift contributions can arise from the interaction with the large magnetic moment of the unpaired electron
This has 2 effects:
1. Line broadening (several 1000 Hz) :(
2. Paramagnetic shift (several hundred ppm) :)
The usefulness of the NMR will depend on the relative magnitude of these 2 effects
Methods for quantitative analysis of paramagnetic NMR spectra
- Effect on chemical shift
2. Line broadening effect
Quantitative analysis of the effect of paramagnetism on chemical shift
delta-delta(dia) = const.(A) + ‘const.(3cos^2(theta)-1)/r^3
First term = Fermi contact shift, where A = hyperfine coupling constant (through bond interaction, similar to J coupling)
Second term = dipolar shift (pseudo-contact shift), where theta = orientation and r = distance from the metal
The ‘const depends on magnetic anisotropy and thus vanishes for isotropic spin distribution
Isotropy
Uniformity in all orientations
Quantitative analysis of the line-broadening effect of paramagnetism
W = const.(r^-6)tauC + ‘const.(A^2)(tauS)
First term = dipolar contribution, shows that broadening depends on distance from the paramagnetic centre
Second term = contact contribution
Tau = correlation times (depend on the molecular tumbling i.e. the viscosity)
Fermi contact shift/interaction
The magnetic interaction between an electron and an atomic nucleus
Responsible for hyperfine coupling
Typical paramagnetic shift reagents
Paramagnetic transition metal ions e.g. Ni, Co, Fe often lead to broadening of peaks
However, paramagnetic lanthanide ions produce large shifts in signals but do not significantly broaden spectra, so can be used to separate peaks that otherwise sit on top of one another (such as a long alkyl chain of multiple CH2 groups)
Forumla for paramagnetic shift
Deltadelta = K[(3cos^2theta - 1)/r^3]
Chiral lanthanide shift reagents
Can be used to calculate ee
Enantiomers are indistinguishable by NMR (unless they form diastereomers) - adding a chiral LSR separates the enantiomer peaks in the NMR, allowing the relative ratio of each, and hence the ee, to be calculated
Avoids the use of chiral HPLC
Main factors influencing chemical shift
Electronegativity
Charge
Oxidation state
Each of these affects the electron density around the observed nucleus, thus affecting the chemical shift
Factors that usually result in a downfield shift
Electronegative substituents
Positive charges
Increase in oxidation state
As electron density at the nucleus decreases…
…shielding becomes small so chemical shift increases (downfield shift)
Transition metal NMR
Need to take Crystal Field Theory/Ligand Field Theory into account because the excitation energy (DeltaE) is directly affected by the identity of the ligands, according to the spectrochemical series
Relationship between chemical shift and the spectrochemical series
The paramagnetic shielding term (sigmapara) is proportional to (DeltaE)^-1
Weak field ligands are therefore deshielding (downfield shift) because DeltaE is small, meaning sigmapara is large
Strong field ligands are shielding (upfield shift) because DeltaE is large, therefore sigmapara is small
Sigmai
Isotopic shielding term
Isotope effects on chemical shift
Isotopes of an element have approximately the same electronic environments
Within the Born-Oppenheimer approximation, we can assume that the electronic potential of an X-Ym bond is the same as that of an X-Yn bond (i.e. the only important difference between isotopes is their mass difference)
Due to their different masses, isotopes occupy different vibrational energy levels within the same electronic potential well of the X-Y bond (where Y is either the Ym or Yn isotope)
This leads to a shorter bond length for the heavier isotope, Yn
Shielding of X is greater when bound to the heavier isotope
Why do heavier isotopes form stronger (shorter) bonds?
Heavy atoms vibrate more slowly
Lighter atoms vibrate more which leads to more lengthening of the bond
When is isotope shift the largest?
When isotope substitution causes the greatest fractional change in mass (i.e. H/D substitution)
Additive effect of isotopic substitution on shielding
The more isotopic substitutions made, the greater the shielding
e.g. 18O-N-18O is more shielded that 18O-N-O which is more shielded than O-N-O
