Option: MolecSpec Flashcards
What are the two forms of shielding?
Diamagnetic - opposes applied mag field
Paramagnetic - augments applied mag field
What is the dependence of σp?
σp α (-1/ΔE)(1/r3)
depends on effects of mag field mixing excited elec states into gs wavefn
What effect does isotope change NMR signal?
Shifts the signal most when light elements changed
As alters vib freq, elec population, etc.
What is a quadrupolar nuclei?
Nucleus with spin I > 1/2 posses an elec quadrupole moment
This interacts with local elec field gradients, which gives fast relaxation and v broad signals
What occurs to quadrupolar nuclei in high symm?
Sharper resonances than expected
This is because the elec field gradient has cancelled out
What is the coupling between quadrupolar and non-quadrupolar nuclei?
Coupling to quadrupolar nuc not resolved due to fast relaxation
(except in high symm when there is 2I+1 with equal intensities)
What is the binomial splitting when I=1 or I=3/2
When I=1 then add three numbers (2 when I=1/2)
When I=3/2 then add fours numbers
What is the NOE experiement?
One or more nuclei irradiated selectively
Intensity changes are seen as a result of dipolar relaxation (dipole-dipole coupling with r-6)
Nearby nuclei then also seen in spectrum
How can NMR distinguish hydrogen complexes?
1J is 25-34 Hz when metal coords to a H2 bond in η2 manner
2J is 5 Hz when there are H-M-H (two separate ligands)
Why is there faster relaxation in molecular hydrogen (η2) complexes?
H v close in proximity and so dipole-dipole relaxation occurs
What are the stages in which you collect a 2D spectra?
What are the pulses in the COSY (correlated spec) experiment?
What happens to a boron cluster when the spectra is H-decoupled?
No coupling between Bs which have a H bridge between them
What occurs due to magnetization in NOESY and EXSY?
NOESY - magnetization transferred between neighborough spins via NOE (dipolar coupling)
EXSY - magnetization transferred between sites by chemical exchange
What are the pulses required for NOESY and EXSY?
Magnetization transferred during τm
Why does solid state generally give broad lines?
Dipolar coupling
Long T1 relax times
Chemical shift anisotropy
How is dipolar coupling different in solid state?
Not averaged by molec tumbling like in solution
Broadens signal
How does long T1 relaxation times effect solid-state NMR?
Means pulse repetition rates slower, giving longer accumulation times and poor signal to noise
Also less scans possible
How does chemical shift anisotropy effect solid state NMR?
Chemical shielding depends on orientation, not averaged in solid state
This leads to line broadening
How can you get sharp peaks in solid state NMR?
Prob nucleus w/ low abundance so little homonuclear dipolar coupling (heteronuc removed with decoupling techniques)
Magic angle spinning (MAS) techniques
What is magic angle spinning technique (MAS)?
Solid state sample spun at 54.44° which averages broadening with (3cos2θ -1) dependence so sharper lines
A few with this dependence as dipolar coupling & shift broadening reduced
How does rate of MAS change peaks?
Slower - resonance with side bands separated by spinning freq with intensity reflecting profile of anisotropy
Faster - single isotropic average peak
What must you use when you have spin-dilute solid state NMR?
Cross polarisation
Gives better signal to noise ratio
What is the process of cross polarization when solid-state spin dilute nuclei?
Magnetization transferred from abundant spin I with high gyromag to a dilute low gyromag nucleus
Enhancement of the dilute nucleus occurs
Faster pulse repetition rates then possible based on the T1 of the abundant nuc
What is observed when MAS used with cross polarization and high power decoupling?
Spinning side band artifact resonances are observed (from MAS)
Only observed when other techniques are also employed as weak
Why can you not do X-ray crystallography of Zeolites?
Al and Si have similar e- densities so cannot determine how much is one or the other
What is Lowenstein’s rule for zeolites?
No Al-O-Al linkages can occur in a zeolite
How does doping zeolites with N effect the shift?
As amount of N increases then peak observed which is more +ve
Also better resolution as improved crystallinity
What occurs to sub at zeolite for high T?
More sub at high temp
How long does a NMR experiment take?
102 - 10-9 s
Threfore compared to most analytical is v slow
What is the berry pseudorotation mechanism?
What is the cramer rotation of alkenes?
Coord alkenes will rotate at a certain temp
Means at low T the protons have separate peaks
What is the rate of exchange at the coalescence point?
kex = πδv/Sqrt[2]
where δv is the change in freq between the nuclei in different sites
What can you use kex in NMR?
Analysis using arrgenius plot
Analysis in terms of Eyring transition-crowd theory (ΔG‡)
Why is there broadening with/without exchange?
Linewidths larger when there is exchange present
Short lifetime means energy of different forms less well defined so broadening
linewidth α 1/k, so as exchange faster then more defined
How can you find volume of activation (ΔV‡) from NMR?
Variable pressure NMR
If as P increases then rate decreases then a +ve ΔV‡
If as P decreases then rate increases then a -ve ΔV‡
How does extent of line-broadening in T-dependent NMR show the mechanism?
All resonances broaden @ same rate as T increases if in same environment
As equally as likely
i.e. if some protons exchangable and others not then the one which are not exchange slower so are less broad
What is a magnetization transfer experiment and when is it used?
Nuclei at an exchanging site selectively excited, which is transferred to another site by chemical exchange (e.g. NOESY)
Used when there is a system in the slow exchange regime
What is seen in NMR spectra for mag transfer experiments?
When transfer then mag transfer occurs, so only radiated peak and the other site is irradiated
e.g. if H2 & H3 transfer, then mag transfer occurs so signals are seen at the 2 and 3
What is the structure of a 2D exchange spectroscopy?
π/2 pulses are 90 degree
τm is the magnetization time, and this is when different things occur
What is the process of NOESY and EXSY?
NOESY - during τm mag transferred between neighbouring spins via NOE
EXSY - during τm mag transferred between sites by chem exchange
How can EXSY be used to find rates of exchange?
If strong cross-peaks in EXSY then fast exchange
When are crosspeaks in EXSY seen if multiple atoms different?
Only crosspeaks when only 1atom different (halogen)
e.g. BBr3, BClBr2, BCl2Br, BCl3
How can you react a non-reversible reaction?
Know NMR signals of intermediates
Observe peaks at different times during the reaction
How does isotopic sub change NMR spectra?
Small changes in chemical shifts as sub alters bond vib frequencies and so small changes in elec distributions and extent of shielding
Larger effects with lighter elements (H/D have largest % change)
How can you work out the Sn splitting in NMR of SnH3-nDn?
SnH3 - quartet
SnH3 - 7 peaks in modified pascals triangle
SnH2D - triplet from H and then D causes each line to split into 3
SnHD2 - doublet from H and then 2xD causes quintet (I=1)
How can you find the intensities of different peaks in (XeF6)4?
Use prob of active 129Xe to find intensity of peaks
Numbers add up to 4 due to 4xXe
How do oxolate ligands compare to thiolate equivalents in terms of shift?
Thiolate equivalents have lower chemical shifts
σp α (-1/ΔE) <1/r3>
This is because larger distance between nucleus and surrounding e- (more covalency and larger orbital size)
How can ring whizzing happen in Cp ligands?
1,2 shift - C2/4/5 change environment but 3 stays in same. C2/5 broadened more than C3/4
1,3 shift - C5 stays in same environment after one shift but C2/3/4 all change environment. C3/4 broadened more than C2/5
