NMR 3 Flashcards
Metal hydrides have a highly __________ chemical shift
NEGATIVE
Chemical shift increases as electron density at nucleus _________
Decreases
What causes shifts to higher frequency?
Electronegative substituents
Positive charge
Increase in oxidation state
Why is deuterated solvent necessary for H and C NMR?
To avoid swamping the spectrum with signal from solvent.
Provides stability as can lock on deuterium resonance and compensate for any drift in magnetic field
What is an isotopologue?
A molecule that differs only in its isotopic composition.
What is homonuclear coupling?
Coupling between atoms of the same isotope
All couplings visible in same spectrum
Heteronuclear coupling is:
Coupling between atoms of different isotopes.
Coupling evident in separate spectra of different nuclei
What is broadband decoupling?
As NMR runs, B nuclei are made to undergo rapid reversal of their spin orientation which causes coupling to A to disappear.
BB decoupling is when all of the frequencies of a particular nucleus are decoupled.
Also possible to selectively decouple only certain frequencies
How does selective decoupling work?
Spectra can be simplified by selective irradiation at specific frequencies.
Rapidly interchanges spin states, making nucleus spin inactive.
Non first order spectra:
Complicated and don’t show expected pattern.
Observed when difference in chemical shift between two atoms of same nucleus is similar in magnitude to the coupling constant
What is an isotopomer?
Isomer with same number of each isotope but differing in their position.
Paramagnetism:
Cause wide shift range as unpaired electrons have own magnetic field that affect chemical shift.
Broadened signals diminishes quality of NMR spectrum to extent coupling is rarely resolved.
Paramagnetism leads to shorter relaxation times which means peaks are broader and rate of spectral acquisition can be high.
What is dynamic NMR?
Reversible molecular processes which result in detectable changed in an NMR property
INTRAmolecular processes involve:
Rotation about bonds
Ligand reorganisation
All called FLUXIONAL processes
INTERmolecular processes involve:
Exchange of free and bound ligands.
For seperate ligands to be observed rate of exchange between sites < 2πΔv
NMR is sensitive to only quite slow motions
Slow exchange leads to:
Separate resonances for each site, degree of broadening dependant on how much time nuclei spend in each site
As exchange rate increases lines become broader
Fast exchange leads to:
Distinction between different sites completely lost
System behaves as one single site
Key points of variable temperature NMR of a dynamic system:
High temperature: FAST EXCHANGE
Lines sharpen as fast exchange limit reached. Single peak.
Intermediate temperature: lines broaden, coalesce into single peak
Low temperature: process slowed, likely to see slow exchange
How to determine energy of activation parameters from dynamic NMR experiments:
Measure NMR at various temperatures. Obtain values for k. Use in Eyring equation to estimate ΔH and ΔS: Plot ln(k/T) vs 1/T Slope is -ΔH/R Intercept is ΔS/R + 23.76
