NMR + EPR

Question 1

deshielding

Answer 1

nucleus sees greater field than expected

nucleus = more exposed to magnetic field

EWG cause deshielding

signal moves downfield (more +ve)

Question 2

shielding

Answer 2

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)

Question 3

appearance of NMR spectra - electronegativity

Answer 3

shielding increases as electronegativity decreases

Question 4

appearance of NMR spectra - ligand effects

Answer 4

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

Question 5

appearance of NMR spectra - coordination number

Answer 5

upfield shift as coordination number increases

larger no. of ligands = more shielded nucleus

Question 6

appearance of NMR spectra - M oxn state

Answer 6

upfield shift as oxn state decreases

more e- rich metal = more shielded nucleus

Question 7

appearance of NMR spectra - nature of metal

Answer 7

decreased shielding relative to atomic size

larger atom -> start filling f orbitals = more shielding

Question 8

31P{1H}

Answer 8

proton decoupled

only observe P

Question 9

coupling constant - s-character

Answer 9

increases as s-character of bond increases

Question 10

coupling constant - coordination number

Answer 10

increasing coordination number decreases coupling constant

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

Question 11

coupling constant - hybridisation

Answer 11

sp > sp2 > sp3

more s-character = bigger coupling constant

Question 12

coupling constant - electronegativity

Answer 12

higher electronegativity = bigger coupling constant

Question 13

coupling constant - trans influence

Answer 13

π acidic ligands reduce coupling constant

groups that are trans to one another = larger coupling constant

Question 14

what does coupling relate to?re

Answer 14

polarisation

Question 15

coupling constant - oxn state

Answer 15

as oxn state increases, polarisation = more difficult

higher oxn state = smaller coupling constant

Question 16

coupling constant - lone pairs

Answer 16

coupling constant decreases when l.p. are coordinated

Question 17

coupling constant - bond angles

Answer 17

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

Question 18

receptivity

Answer 18

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

Question 19

satellites

Answer 19

occurs when active nucleus is not 100% abundant

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

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

Question 20

dynamic systems

Answer 20

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

Question 21

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

Answer 21

axial

Question 22

NMR vs EPR

Answer 22

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)

Question 23

EPR - g-values

Answer 23

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

Question 24

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

Answer 24

ge = 2.0023

ge = free-electron g factor

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

Question 25

what does the size of the hyperfine coupling depend on?

Answer 25

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