NMR 3

Question 1

Metal hydrides have a highly __________ chemical shift

Answer 1

NEGATIVE

Question 2

Chemical shift increases as electron density at nucleus _________

Answer 2

Decreases

Question 3

What causes shifts to higher frequency?

Answer 3

Electronegative substituents
Positive charge
Increase in oxidation state

Question 4

Why is deuterated solvent necessary for H and C NMR?

Answer 4

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

Question 5

What is an isotopologue?

Answer 5

A molecule that differs only in its isotopic composition.

Question 6

What is homonuclear coupling?

Answer 6

Coupling between atoms of the same isotope

All couplings visible in same spectrum

Question 7

Heteronuclear coupling is:

Answer 7

Coupling between atoms of different isotopes.

Coupling evident in separate spectra of different nuclei

Question 8

What is broadband decoupling?

Answer 8

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

Question 9

How does selective decoupling work?

Answer 9

Spectra can be simplified by selective irradiation at specific frequencies.
Rapidly interchanges spin states, making nucleus spin inactive.

Question 10

Non first order spectra:

Answer 10

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

Question 11

What is an isotopomer?

Answer 11

Isomer with same number of each isotope but differing in their position.

Question 12

Paramagnetism:

Answer 12

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.

Question 13

What is dynamic NMR?

Answer 13

Reversible molecular processes which result in detectable changed in an NMR property

Question 14

INTRAmolecular processes involve:

Answer 14

Rotation about bonds
Ligand reorganisation
All called FLUXIONAL processes

Question 15

INTERmolecular processes involve:

Answer 15

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

Question 16

Slow exchange leads to:

Answer 16

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

Question 17

Fast exchange leads to:

Answer 17

Distinction between different sites completely lost

System behaves as one single site

Question 18

Key points of variable temperature NMR of a dynamic system:

Answer 18

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

Question 19

How to determine energy of activation parameters from dynamic NMR experiments:

Answer 19

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