NMR + EPR Flashcards
deshielding
nucleus sees greater field than expected
nucleus = more exposed to magnetic field
EWG cause deshielding
signal moves downfield (more +ve)
shielding
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)
appearance of NMR spectra - electronegativity
shielding increases as electronegativity decreases
appearance of NMR spectra - ligand effects
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
appearance of NMR spectra - coordination number
upfield shift as coordination number increases
larger no. of ligands = more shielded nucleus
appearance of NMR spectra - M oxn state
upfield shift as oxn state decreases
more e- rich metal = more shielded nucleus
appearance of NMR spectra - nature of metal
decreased shielding relative to atomic size
larger atom -> start filling f orbitals = more shielding
31P{1H}
proton decoupled
only observe P
coupling constant - s-character
increases as s-character of bond increases
coupling constant - coordination number
increasing coordination number decreases coupling constant
more bonds means hybridisation of central atom changes (s-character decreases as no. of ligands increases)
coupling constant - hybridisation
sp > sp2 > sp3
more s-character = bigger coupling constant
coupling constant - electronegativity
higher electronegativity = bigger coupling constant
coupling constant - trans influence
π acidic ligands reduce coupling constant
groups that are trans to one another = larger coupling constant
what does coupling relate to?re
polarisation
coupling constant - oxn state
as oxn state increases, polarisation = more difficult
higher oxn state = smaller coupling constant
coupling constant - lone pairs
coupling constant decreases when l.p. are coordinated
coupling constant - bond angles
increasing bond angle = increasing s-character = larger J value
receptivity
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
satellites
occurs when active nucleus is not 100% abundant
central peak = inactive isotope (e.g. 12C) -> no coupling
satellites = active isotope (e.g. 13C)
dynamic systems
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
which has a larger coupling constant - axial or equatorial ligands?
axial
NMR vs EPR
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)
EPR - g-values
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
how do we know if g-value is large or small?
ge = 2.0023
ge = free-electron g factor
= dimensionless factor that corrects magnetic moment of quantum electron from classical result
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
