NMR Notes Flashcards
What is NMR concerned with?
The observation of transitions between energy levels associated with nuclear spin in a magnetic field
Uses of NMR in inorganic chemistry
- Elucidating molecular structure
- Configuration and mode of bonding of ligands
- Molecular fluxionality (molecular dynamics, variable temp. NMR)
- Ligand-exchange reactions
- Following course of reaction
What is required for NMR activity?
Nuclear spin quantum number (I) must be greater than 1
(nuclei passes I, normally NMR associated with diamagnetic complexes)
mi
Nuclear spin magnetic quantum number, can have values of:
I, I-1, …., -I
Suitable NMR solvents
Solvents with deuterium
CDCl3, CD3COD3, etc.
(H2O is a common impurity)
How is solution NMR with D labelled solvents done?
Spectrometer is locked on the D frequency, everything is observed in the same field value which minimizes field drift
NMR parameters
Chemical shift (gamma, ppm)
Coupling constant (J, Hz)
Chemical shift
Change in position of a resonance line from that of a reference or standard
What is chemical shift proportional (or inversely proportional) to?
Inversely proportional to electron density and shielding
What is chemical shift influenced by?
Bo (the field)
Electron density
Neighbouring nuclei (coupling)
Coupling constant (J)
Yield independent, primarily based on structure not magnetic strength
Nuclear resonance is affected by the presence of neighbouring magnetic nuclei (spin orientations of neighbouring nucleus)
Formula for number of NMR lines
Coupling to ‘n’ equivalent nuclei with nuclear spin I (not equal to 0) gives:
2nI + 1 lines
Basic difference between NMR and EPR
In NMR, our observing nucleus instead of an observing electron
Why is there commonly more overlap in NMR than EPR?
Chemical shift values are more similar in NMR
What effects coupling constant (J)?
- Identity of coupled nuclei
- Nature/number of bonds connected to coupled nuclei
Is coupling to nuclei with I > 1/2 often observed?
No, often not observed and frequently a broadened line results for the observing nucleus
I > 1/2 nuclei can be ignored from the point of view of coupling but not from the point of view of electron density
Why does I > 1/2 often result in non-observed coupling?
Nuclei with I > 1/2 possess quadrupole moments
- positive charge is not evenly distributed over a sphere but concentrated more along one direction
- as molecule tumbles in solution, quadrupolar nucleus tumbles with it, spin-spin coupling is averaged to zero
What is the effect of coupling to transition metals with I = 1/2 and natural abundance < 100%?
Multiplet with high degree of symmetry
Decoupling
Technique that is applied to simplify spectra by removing coupling to the observing nucleus
Broad band decoupling
An additional band of frequencies is applied at the same time as the exciting frequency for the observing nucleus
What is a consequence of decoupling?
Nuclear overhauser effect
- rapid relation of irradiated nuclei can change the energy distribution of the observing nuclei
- signal intensity, charges, and integrations are no longer reliable (cannot integrate if there is decoupling)
- NOE gives a useful gain in signal/noise ratio
Hydride/hydrido complexes
Complexes containing hydrogen atoms directly bonded to transition metals
- important as useful intermediates in many catalytic reactions as a result of a key step in the metal hydrides with a variety of unsaturated organic ligands
Chemical shift trend (hydride)
For square planar and octahedral complexes containing (Pt2+, Rh+, Rh2+, etc.), the hydride chemical shift increases as the ligand field strength of the ligand trans to the hydride increases
- pi acceptor ligand pulls electron density away from metal, thus away from H causing increased chemical shift on H)
What does more positive (larger) chemical shift mean?
Larger shift = deshielded
Lower (more -) shift = shielded
Hydride chemical shift range
0 to -30 ppm (high field)
Hydride chemical shift trend
For square planar and octahedral complexes containing (Pt 2+, Rh +, Rh 2+, etc.), hydride chemical shift increases as the ligand field strength (pi accepting ability) of the ligand trans to the hydride increases
(pi acceptor ability)
Cl- < Br- < I- < CN-
(good pi acceptors_
CO, CN-, PX3, PR3, P(OR)3
Characteristic coupling of hydrides
(1) Another hydride
(2) Spin 1/2 nuclei of another non-equivalent hydride
(3) Spin 1/2 transition metals
(4) Spin > 1/2 transition metals in highly symmetric complexes
Downside of 31P NMR
P has low receptivity and 100% relative abundance, must pulse for a very long time
Why are phosphines and phosphates good ligands?
Due to the lone pair on the P atom
Are chemical shifts or coupling constants of more value in assigning configuration?
Coupling constant
What does coupling to transition metal with less than 100% relative abundance result in?
Satellite peaks
What is the downside of 31P NMR?
Low receptivity and 100% relative abundance of 31P (must pulse for a long time)
Why are 31P{1H} spectra obtained?
So that all coupling reflects 31P-31P or 31P-M coupling or coupling to other I = 1/2 nuclei
What causes the higher chemical shift for P(OR)3 compared to PR3?
What does the J value of P(OR)3 tell us relative to PR3?
The O atom pulls electron density away from the P atom (deshielding)
J for P(OR)3 is larger, suggesting it possible has more s character than normal sp3 hybridized nuclei
What is the J value of (P-M) a measure of?
Electron density in the bond (therefore bond strength) and therefore a function of the pi-acceptor strength of the ligand trans to P
13C NMR accumulation times?
Long, especially for low solubility and high molecular mass
Why is 13C-13C coupling not seen?
Extremely low relative abundance of 13C make it extremely unlike to have two side by side
Where are the chemical shifts for 13CO transition metal complexes located?
Significantly downfield shifted (150-250 ppm)
What is the 3d, 4d, 5d trend for CO resonances?
CO resonances shifted to higher field (lower chemical shift, greater shielding) for:
3d -> 4d -> 5d transition metal
What is the chemical shift trend for bridging vs terminal carbonyls
Bridging = lower wavenumber (IR), higher chemical shift
What is the chemical shift trend in substituted carbonyl complexes
Increased back donation by transition metal increases the chemical shift of 13CO (pi-acceptor ability of other ligands)
Dynamic NMR
Spectrum can be changed by chemical exchange/process by which 2+ atoms otherwise expected to be non-equivalent become equivalent by changing sites
- can be observed through dynamic, temperature controlled NMR
Associative exchange process
Dissociative exchange process
Dissociative (breaking bonds) - coupling is lost between exchangeable and adjacent nuclei
Associative (retaining bonds) - coupling remains
