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Chemical shielding & chemical shift Flashcards

1
Q

Symbol for chemical shift

A

delta

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2
Q

How is the chemical shift of a nuclear environment defined?

A

With respect to the nuclei in a reference compound, whose chemical shift is usually given a value of 0

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3
Q

Calculating chemical shift

A

(vL-vLref)/vSpec (in Hz) x 10^6
or (vL-vLref)/vSpec (in MHz)
where vL = resonant frequency
Gives a value in ppm

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4
Q

Deshielded nuclei

A

High frequency
High ppm
Downfield/low field

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5
Q

Shielded nuclei

A

Low frequency
Low ppm
Upfield/high field

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6
Q

What does the resonant frequency (vL) of a nucleus depend on?

A

The magnetic field the nucleus experiences

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7
Q

What affects the magnetic field experienced by a nucleus?

A

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

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8
Q

Beff =

A

B0 - b0

Where B0 = applied (external) magnetic field and b0 = the magnetic field generated by the surrounding electrons

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9
Q

Why does each nucleus resonate at a different frequency?

A

Because each nucleus is in a different chemical environment

vL = yBeff/2pi
Where Beff = B0(1-sigma)

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10
Q

Sigma

A

Shielding constant

Can be described as the summation of many terms

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11
Q

Diamagnetic shielding term (sigmadia)

A

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

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12
Q

Why are variations in the diamagnetic shielding term only of relevance for 1H, 6Li and 7Li?

A

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

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13
Q

Paramagnetic shielding term (sigmapara)

A

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

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14
Q

The paramagnetic shielding term is proportional to

A

DeltaE^-1

DeltaE = the electronic excitation energy (i.e. the energy spacing between ground and excited states)

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15
Q

Why is the paramagnetic shielding term small for 1H?

A

For 1H, DeltaE is large, therefore DeltaE^-1 is small, therefore sigmapara is small

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16
Q

When does sigmapara begin to dominate over sigmadia?

A

For heavier nuclei in molecules with low lying electronically excited states
e.g. 31P, 195Pt

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17
Q

Normal chemical shift range for 1H

A

+20 to -20 ppm

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18
Q

Normal chemical shift range for 11B

A

+100 to -100 ppm

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19
Q

Normal chemical shift range for 205Tl

A

5500 ppm range

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20
Q

Why is the chemical shift range larger for heavier/larger nuclei?

A

Heavier/larger nuclei have more electrons so have larger values for their paramagnetic shielding terms

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21
Q

Ramsay-Karplus-Pople equation

A

Describes the size of the paramagnetic shielding term

sigmapara = -constant(r^-3)^-1(DeltaE)^-1(c^2)

22
Q

r in Ramsay-Karplus-Pople equation

A

Average distance between the nucleus and its valence electrons

23
Q

DeltaE in Ramsay-Karplus-Pople equation

A

HOMO-LUMO gap

Smaller HOMO-LUMO gap means a greater paramagnetic effect

24
Q

c in Ramsay-Karplus-Pople equation

A

A measure of the covalence of the bond(s) to adjacent atoms

LCAO coefficient

25
Q

What is paramagnetic electron circulation?

A

Mixing between ground and excited states

26
Q

When might a diamagnetic compound have a paramagnetic shielding term contribution?

A

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+

27
Q

SigmaN

A

Nuclear anisotropy shielding term

28
Q

Anisotropy

A

The property of being directionally dependent - i.e. different properties in different directions

29
Q

Anisotropy in NMR

A

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

30
Q

SigmaR

A

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

31
Q

Sigmae

A

Electronic shielding term (paramagnetic NMR)

32
Q

Electronic shielding term

A

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

33
Q

Methods for quantitative analysis of paramagnetic NMR spectra

A
  1. Effect on chemical shift

2. Line broadening effect

34
Q

Quantitative analysis of the effect of paramagnetism on chemical shift

A

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

35
Q

Isotropy

A

Uniformity in all orientations

36
Q

Quantitative analysis of the line-broadening effect of paramagnetism

A

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)

37
Q

Fermi contact shift/interaction

A

The magnetic interaction between an electron and an atomic nucleus
Responsible for hyperfine coupling

38
Q

Typical paramagnetic shift reagents

A

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)

39
Q

Forumla for paramagnetic shift

A

Deltadelta = K[(3cos^2theta - 1)/r^3]

40
Q

Chiral lanthanide shift reagents

A

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

41
Q

Main factors influencing chemical shift

A

Electronegativity
Charge
Oxidation state

Each of these affects the electron density around the observed nucleus, thus affecting the chemical shift

42
Q

Factors that usually result in a downfield shift

A

Electronegative substituents
Positive charges
Increase in oxidation state

43
Q

As electron density at the nucleus decreases…

A

…shielding becomes small so chemical shift increases (downfield shift)

44
Q

Transition metal NMR

A

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

45
Q

Relationship between chemical shift and the spectrochemical series

A

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

46
Q

Sigmai

A

Isotopic shielding term

47
Q

Isotope effects on chemical shift

A

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

48
Q

Why do heavier isotopes form stronger (shorter) bonds?

A

Heavy atoms vibrate more slowly

Lighter atoms vibrate more which leads to more lengthening of the bond

49
Q

When is isotope shift the largest?

A

When isotope substitution causes the greatest fractional change in mass (i.e. H/D substitution)

50
Q

Additive effect of isotopic substitution on shielding

A

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

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