Physical methods of structural determination Flashcards
What does nuclear spin depend on and give examples of elements with nuclear spin I = 0, integar, half integar?
Nuclear spin depends on Z and the isotoic mass
evenevenE I = 0 e.g. 126C, 168O
evenoddE I = integar e.g. 147N, 21H
oddE I = half integar e.g. 136C I = 1/2, 199F I = 1/2
When I ≥ 1 the nucleus is quadropolar
How many spin states does any given element have?
2I + 1
How does isotope abundance affect the NMR?
When the isotopic abundance is below 100% only a fraction of the nuclei show up in the specra - less receptive
What is receptivity?
How sensitive the nucleus is
Proportional to γ3NI(I+1)
γ is the gyromagnetic ratio, N is the natural abundance, I is the nuclear spin
What is the Larmor frequency?
E = mIγBħ
Refers to 1H in a specific field (400 MHz)
γ is the gyromagnetic ratio which is unique to each isotope
How sensitive is NMR?
Very insensitive
What are the mechanisms of relaxation?
spin-lattice relaxation - T1 - energy of the excited state is dissipated to the surroundings
Spin-spin relaxation - T2 - energy of the excited state dissipated to other nuclear spins
In high resolution, solution state NMR in most cases T1 = T2
How does the Heisenberg uncertainty principle affect NMR
ΔEΔt = ħ/2 (constant)
radiatve decay gives a spectrum - when there is very fast decay there is no radiatve process and no specrum
ΔE = uncertainty in energy - line width
Δt =fast ΔE large which implies broad spectrum
For 1H Δt = long so ΔE is small = high resolution spectrum
How does the Heisenberg uncertainty principle affect Paramagnetic species?
Δt - short when there is an unpaired electron
γe (of the electron) ≈ 103γ1H
Relaxation α γ12 x γ22
Very fast relaxation = broad lines and change in chemical shift
What are quadropolar nuclei?
Nuclei with spin ≥ 1
In a nucleus where I = 1/2 the charge distribution is spherical
When I ≥ 1 there is a prolate distribution (non spherical) - there is an electric field gradient at the nucleus
Quadropole initiates relaxation - charge couples with the non spherical charge distribution in the molecule
What is the rate of quadropolar relaxation?
When qz does not equal 0 RQ does not equal 0 and realxation is fast
When qz = 0 RQR = 0 and relaxation is slow and a spectrum is observed
How many lines are seen for NMR actuve nuclei in the spectra?
2nI + 1 lines
For aluminium flouride (AlF4)
Al: I = 5/2
(2 x 1 x 5/2) = 6 lines
F: I = 1/2
(2 x 4 x 1/2) = 5 lines
How does couplingto quadropolar nuclei show up
Non binomial distibution
For CH2D2 the 1H and 13C{1H} spectra show 1:2:3:2:1 quintet
Why do we often decouple a spectra?
Decoupling can suppress magnetic inequivelence
What are the spins, isotopes and abundances of Pd, Rh and Pt?
Pd (22% 105Pd I = 5/2 (quadropolar) the rest is I = 0) - no coupling - quadropolar nucleus means relaxation is too fast
Rh (100% 103Rh, I = 1/2) - every Rh centre will couple
Pt (33% 195Pt, I = 1/2) The other are all I = 0
What are satellites?
When a nucleus such as phosphorus couples to Pt 66% of the nuclei have I = 0 and therefore do not couple and therefore the phosphorus environments show up as singlets
33% of the Pt nuclei couple and show up as a doublet either side of the singlet - The J value between them is JPt-P - These are satellites
Satellites show up in spin dilute systems
What is partial hydrogen decoupling?
When only some of the coupling to hydrogen is excluded
For example to hydrogens are decoupled leaving any hydrides
How does the transeffect affect the J coupling constant?
The substituents trans to each other are competeing for the same orbital
If one substituent outcompetes another the bond to that substituent will be stringer and the J value will be higher
The weaker the trans influence of the substituent transto an NMR active substituent the higher the J value
Water, Cl have weak trans influences
CO has string trans influence
How does oxidation state affect the J couplig constant?
Generally when oxidation state increases the J constant decreases
This is due to the hybidisation - coupling is larger with greater s character as the s penetrates the nucleus better than the p and d orbitals
How do we calculate timescale?
Timescale = 1/frequency of experiment
NMR - MHz - 106 s-1 so timescale is 10-6 s
IR - THz - 1012 s -1 so timescale is 10-12 s
Individual species involved in exchanges can be ‘seen’ if rate f exchange is less than the experimental timescale
Berry pseudo rotation happens too fast on the NMR timescale and cannot be seen - only 1 environment observed
IR happens on faster timescale so can show BPR
Is it ever possible to see Berry Pseudo rotation in NMR?
If we cool the solution down so that rotation is on an NMR timescale we can observe multiple environments
Have to cool to about -132°C
What is EPR?
Electron Paramagnetic Resonance
Akin to NMR spectroscopy but uses spin of unpaired electron (I = 1/2)
0.34
No liquid helium or nitrogen needed
Frequencies 9.5 —> 35 GHz
uses microwaves
What is the g value in EPR?
‘The chemical shift’ of EPR
typical organic radicals:
1.99 ≤ g ≤ 2.01
Transition metals 0 < g < 6 - spin orbit coupling
- > 1 unpaired electron
What is hyperfine coupling?
electron spin will interact wth the magnetic dipole of neighbouring spins
Energy of electron spin-state, E, is modified to
E = gμBB0Ms + aMsmI
a - hyperfine coupling (mT) (G) - gaus
mI - spin of the interacting nucleus
Δms - ±1
ΔmI = 0
What is observed an electron couples to a single nucleus with spin 1/2?
Same rules as NMR apply - doublet shows
Shape of peaks different
Can unpaired electrons couple to quadropolar nuclei?
Yes
The timescale of the EPR spectra is much faster (109 s-1) so rapid relaxation not a probelm
What can EPR tell us about the unpaired electron?
Can give us informaton about which atom/s the electron is on and therefiore wich molecular orbital the electron is in
The NO logand can exist as NO. keeping the unpaired electron, NO+ where the electron is donated or NO- where the electron is accepted.
NO. - is diamagnetic so should not show peaks in EPR
NO+ - should show coupling to Fe but not N
NO- - should show coupling to N but not Fe
WHat are EXAFs?
Etended X-ray Absorption Fine Structure
A synchrotron-based technique
Incoming X-rays eject a core electron and the electron is observed as a spherical wave which can be backscattered
What happens in an EXAFS experiment?
An electron is ejected
The electron sees the electron density of neighbouring atoms and is back scattered
Back scattering can inpinge on teh original atom in phase or out of phase
This affects the absorption characteristics of the atom being observed
What is the difference between in phase and out of phase?
What is scattering dependent on?
Atomic number of scatterng nuclei
Their distance from the absorbing atom
The extent of bond vibration
Scattering generates an absorption spectrum with peaks showing constructive interference and trophs showing destructive interference
The line eventually dies away
What are the experimental considerations of EXAFS?
High beam intensities
access elements with approx Z ≥ 15 (P)
do not need single crystals
use on liquids
use on biological sample
used widelu in the study of heterogenous catalysts
How do you process EXAFS spcetra?
Correct for the slope
Change X axis from energy to k using k α √E-E0 where E is the observed energy and E0 is the energy at absorption edge
Remove background data amd normalise
WHat is a limitation of EXAFS data?
DAta looks identical for cis and trans
No radical information
