NMR + EPR Flashcards

1
Q

deshielding

A

nucleus sees greater field than expected

nucleus = more exposed to magnetic field

EWG cause deshielding

signal moves downfield (more +ve)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

shielding

A

electrons in ground state produces a field that opposes B0 + sees smaller field

more e- around nucleus => less exposed to magnetic field

EDG cause shielding

signal moves upfield (more -ve)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

appearance of NMR spectra - electronegativity

A

shielding increases as electronegativity decreases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

appearance of NMR spectra - ligand effects

A

more ED = better back donation into CO ligand = increase in chemical shift

pushes e- density into metal/anti-bonding orbital (offload via back bonding)

=> deshields C nucleus

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

appearance of NMR spectra - coordination number

A

upfield shift as coordination number increases

larger no. of ligands = more shielded nucleus

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

appearance of NMR spectra - M oxn state

A

upfield shift as oxn state decreases

more e- rich metal = more shielded nucleus

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

appearance of NMR spectra - nature of metal

A

decreased shielding relative to atomic size

larger atom -> start filling f orbitals = more shielding

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

31P{1H}

A

proton decoupled

only observe P

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

coupling constant - s-character

A

increases as s-character of bond increases

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

coupling constant - coordination number

A

increasing coordination number decreases coupling constant

more bonds means hybridisation of central atom changes (s-character decreases as no. of ligands increases)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

coupling constant - hybridisation

A

sp > sp2 > sp3

more s-character = bigger coupling constant

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

coupling constant - electronegativity

A

higher electronegativity = bigger coupling constant

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

coupling constant - trans influence

A

π acidic ligands reduce coupling constant

groups that are trans to one another = larger coupling constant

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what does coupling relate to?re

A

polarisation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

coupling constant - oxn state

A

as oxn state increases, polarisation = more difficult

higher oxn state = smaller coupling constant

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

coupling constant - lone pairs

A

coupling constant decreases when l.p. are coordinated

17
Q

coupling constant - bond angles

A

increasing bond angle = increasing s-character = larger J value

18
Q

receptivity

A

how good a nucleus is at NMR

sensitivity (S/N ratio) may be estimated in terms of receptivity

large gyromagnetic ratio + natural abundance = large receptivity

19
Q

satellites

A

occurs when active nucleus is not 100% abundant

central peak = inactive isotope (e.g. 12C) -> no coupling

satellites = active isotope (e.g. 13C)

20
Q

dynamic systems

A

e.g. PF5

[rtp]
-fluxional behaviour = v. rapid
-all F = equivalent
= BROAD SINGLET

[low temp.]
-freeze fluxional behaviour of Berry pseudorotation
-equatorial + axial not equal
= TRIPLET OF QUARTETS

21
Q

which has a larger coupling constant - axial or equatorial ligands?

22
Q

NMR vs EPR

A

paired vs unpaired electrons

nuclear vs electron spin

chemical shift vs g-value (field(B), Gauss or mT)

J value (coupling constant, Hz) vs hyperfine coupling (Gauss)

23
Q

EPR - g-values

A

value = important (helps identify radical)

size of g-value influenced by spin-orbital coupling (i.e. what else is near radical)

organic radicals = small g-value

main group + transition metal radicals = large g value

24
Q

how do we know if g-value is large or small?

A

ge = 2.0023

ge = free-electron g factor

= dimensionless factor that corrects magnetic moment of quantum electron from classical result

25
what does the size of the hyperfine coupling depend on?
distance radical is from what it's coupling to [bond angles] -β larger than α -due to hyperconjugation - overlap of p orbital bearing the unpaired e- with sp3-orbital of C-H bond at adjacent carbon -overlap = most efficient when dihedral angle θ between p-orbital (with unpaired e-) and C-H = 0