Electron Spin Resonance Spectroscopy ESR / EPR Absolute requirement: 1 or more unpaired electrons – Stable species: O 2 , NO, [Fe(CN) 6 ] 3– Transients: CH 3 , C 6 H 6 , Ge[N{Si(CH 3 ) 3 } 2 ] 3 – Lifetime > 10 ns High sensitivity: detect 10 11 spins easily MCWE region of spectrum: ca. 9.5 GHz (X- band) Magnetic fields: ca. 0.35 Tesla CW or FT
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Electron Spin Resonance Spectroscopy ESR / EPR Absolute requirement: 1 or more unpaired electrons –Stable species: O 2, NO, [Fe(CN) 6 ] 3 –Transients:
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Electron Spin Resonance Spectroscopy ESR / EPR
Absolute requirement: 1 or more unpaired electrons– Stable species: O2, NO, [Fe(CN)6]3
– Transients: CH3, C6H6, Ge[N{Si(CH3)3}2]3
– Lifetime > 10 ns High sensitivity: detect 1011 spins easily MCWE region of spectrum: ca. 9.5 GHz (X-band) Magnetic fields: ca. 0.35 TeslaCW or FT
Physics of electron spin The energy of an electron (mass m, charge -e) in a
magnetic field B is: E = (1/2) g B B
where B is the Bohr magneton = 9.27410-24 J T-1
B is defined as = (e h ) / (4 m)g is a dimensionless constant; the g-value (chemical shift) For electrons the spin quantum number S=1/2 with
components mS= (1/2) Thus an unpaired electron whose spin orientation is
opposite to that of the applied magnetic field has a lower energy (for a H-nucleus the converse is true)
ESR
Resonance equation h g BB
B Gauss)9.5 GigaHz
B = 9.274 10-24 J / T
S
K lys tro n D etec to r
N
E ne rg y M ag n e t i c fi e l d
B Z
Example
The species, AlH3 gives rise to a complex spectrum
centered at 329.48 mT with microwave radiation of frequency 9.235 GHz.
Origin of hyperfine coupling Dipole-dipole interaction (distant electron & magnetic nucleus)
– If electron in s-orbital or molecule rotates rapidly (as in gas phase or solution) then dipole field averages to zero. Non-zero solid state; anisotropic hyperfine coupling‡.
Fermi contact interaction– Depends upon electron density at nucleus– a(nucleus) { e N | (r=0)|2
Simulated ESR spectraWinSim, by Dave Duling (1996) National Institute of Environmental Health Sciences
Fire up, click on Simulations menu, parameters Toggle “Calculate this species?” on Enter appropriate values for:
– Set Hyperfine Spin Number Change Simple LW from 0.50 as required Now Simulate; green ESR display onscreen Pick Display menu, toggle FT Imaginary off, Update
& Close Use Edit menu & Copy data to paste into Excel & plot
Mapping unpaired e densityFor H aH = (+)50.68 mT 100% in 1s AO
But in CH3 aH = (-)2.3 mT so e only spends (2.3/50.68)
100% on 1s AO on a H 4.5% in a specific 1s H AO or 13.5% on all 3 Hs Therefore (100-13.5=86.5) % on CFor 2s e on 13C computed field is 111.5 mTIn 13CH3
aH = (+)4.1 mT so (4.1/111.5) 100=3.7%
So (100-3.7)=96.3% on 2p C AOs Hence methyl radical is planar
Spin polarisation
How come? Electron in 2p AO polarizes electrons so that neighbouring
e has same spin, the other e has opposite spin Near H atom bond 1s AO so e can interact with
magnetic nucleus Note sense of interaction is different (-ve coupling)
C H C H
Allyl radical Unpaired electron in non-
bonding MO = 0.707 - 0.707
So e interacts only with 4 terminal Hs not central H
But from ESR spectrum– a(terminal H) = () 1.39 mT– a(central H) = (+) 0.40 mT
Enhanced unpaired spin at terminals and at central H
C C C
C C C
1 2 3
Line shape analysis
Electron transfer reactions(TCNE) + TCNE TCNE + (TCNE)
(NC)2C=C(CN)2
4 equivalent Ns with I=1 Slow:
– (241 + 1) = 9 nonet Fast:
– collapse to single line Intermediate:
– Line fit kinetics of reaction
Spin labels
If compound of interest does not have an unpaired electron it has no ESR.
Attach (by reaction) a compound (spin label) that does; its ESR spectrum will hopefully change reflecting the new environment that the spin label is in.
Piperidinyloxy free radical
N
NH2
O
Spin traps The compound of interest
has a very small lifetime & is therefore invisible in the ESR spectrometer.
Trap it with a diamagnetic reactant to form a new more stable compound but still containing an unpaired electron.
CH3OH + CH2OH
CH3O not observed
N
O
C(CH3)3
N-tert-Butyl Phenyl-nitrone
N
O
5,5-Dimethyl-1-pyrroline N-oxide
Electron-Nuclear Double ResonanceUse ESR spectrometer Pick a specific B & Increase MCWE power until
that ESR signal disappears Irradiate with a second
transmitter from 0 to 50 MHz
ESR signal reappears momentarily at two frequencies– Each set of equivalent spin
1/2 nuclei gives rise to just 2 ENDOR lines
Anthraquinone radical anion has 4 -Hs & 4 -Hs and thus a 25 line ESR
ENDOR spectrum consists of just 4 lines at 13.12, 13.72, 15.26 & 15.86 MHz(ENDOR) = |H aH/2)|
Average to H = 14.49 MHz– a1 = 15.86 - 13.12 = 2.74 MHz
– a2 = 15.26 - 13.72 = 1.54 MHz
– Or a1 = 9.8 mT & a2 = 5.5 mT
ENDOR Example ENDOR of 1,4,5,8-tetrakis-(trimethylsiyl)--octalin
radical anion has 10 lines (all in MHz):4.07 13.27 14.31 14.43 14.73 14.86 15.73 15.90 21.53 33.23
In this spectrometer H14.6 MHz & Si2.9 MHzHow many sets of equivalent nuclei in this species?Pick pairs of lines which average to 14.6 MHz (2.9??) (14.73, 14.43) 14.580 hence a1=14.73-14.43=0.30 MHz (14.86, 14.31) 14.585, (15.73, 13.27) 14.50 no! (15.90, 13.27) 14.585, (33.23, 4.07) difference is 2H sum=a a4=37.30 MHz