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CHEM431 > NMR Notes > Flashcards

NMR Notes Flashcards

1
Q

What is NMR concerned with?

A

The observation of transitions between energy levels associated with nuclear spin in a magnetic field

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2
Q

Uses of NMR in inorganic chemistry

A
  1. Elucidating molecular structure
  2. Configuration and mode of bonding of ligands
  3. Molecular fluxionality (molecular dynamics, variable temp. NMR)
  4. Ligand-exchange reactions
  5. Following course of reaction
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3
Q

What is required for NMR activity?

A

Nuclear spin quantum number (I) must be greater than 1
(nuclei passes I, normally NMR associated with diamagnetic complexes)

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4
Q

mi

A

Nuclear spin magnetic quantum number, can have values of:
I, I-1, …., -I

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5
Q

Suitable NMR solvents

A

Solvents with deuterium
CDCl3, CD3COD3, etc.
(H2O is a common impurity)

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6
Q

How is solution NMR with D labelled solvents done?

A

Spectrometer is locked on the D frequency, everything is observed in the same field value which minimizes field drift

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7
Q

NMR parameters

A

Chemical shift (gamma, ppm)
Coupling constant (J, Hz)

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8
Q

Chemical shift

A

Change in position of a resonance line from that of a reference or standard

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9
Q

What is chemical shift proportional (or inversely proportional) to?

A

Inversely proportional to electron density and shielding

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10
Q

What is chemical shift influenced by?

A

Bo (the field)
Electron density
Neighbouring nuclei (coupling)

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11
Q

Coupling constant (J)

A

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)

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12
Q

Formula for number of NMR lines

A

Coupling to ‘n’ equivalent nuclei with nuclear spin I (not equal to 0) gives:
2nI + 1 lines

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13
Q

Basic difference between NMR and EPR

A

In NMR, our observing nucleus instead of an observing electron

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14
Q

Why is there commonly more overlap in NMR than EPR?

A

Chemical shift values are more similar in NMR

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15
Q

What effects coupling constant (J)?

A
  1. Identity of coupled nuclei
  2. Nature/number of bonds connected to coupled nuclei
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16
Q

Is coupling to nuclei with I > 1/2 often observed?

A

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

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17
Q

Why does I > 1/2 often result in non-observed coupling?

A

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

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18
Q

What is the effect of coupling to transition metals with I = 1/2 and natural abundance < 100%?

A

Multiplet with high degree of symmetry

19
Q

Decoupling

A

Technique that is applied to simplify spectra by removing coupling to the observing nucleus

20
Q

Broad band decoupling

A

An additional band of frequencies is applied at the same time as the exciting frequency for the observing nucleus

21
Q

What is a consequence of decoupling?

A

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
22
Q

Hydride/hydrido complexes

A

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
23
Q

Chemical shift trend (hydride)

A

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)
24
Q

What does more positive (larger) chemical shift mean?

A

Larger shift = deshielded
Lower (more -) shift = shielded

25
Q

Hydride chemical shift range

A

0 to -30 ppm (high field)

26
Q

Hydride chemical shift trend

A

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

27
Q

Characteristic coupling of hydrides

A

(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

28
Q

Downside of 31P NMR

A

P has low receptivity and 100% relative abundance, must pulse for a very long time

29
Q

Why are phosphines and phosphates good ligands?

A

Due to the lone pair on the P atom

30
Q

Are chemical shifts or coupling constants of more value in assigning configuration?

A

Coupling constant

31
Q

What does coupling to transition metal with less than 100% relative abundance result in?

A

Satellite peaks

32
Q

What is the downside of 31P NMR?

A

Low receptivity and 100% relative abundance of 31P (must pulse for a long time)

33
Q

Why are 31P{1H} spectra obtained?

A

So that all coupling reflects 31P-31P or 31P-M coupling or coupling to other I = 1/2 nuclei

34
Q

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?

A

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

35
Q

What is the J value of (P-M) a measure of?

A

Electron density in the bond (therefore bond strength) and therefore a function of the pi-acceptor strength of the ligand trans to P

36
Q

13C NMR accumulation times?

A

Long, especially for low solubility and high molecular mass

37
Q

Why is 13C-13C coupling not seen?

A

Extremely low relative abundance of 13C make it extremely unlike to have two side by side

38
Q

Where are the chemical shifts for 13CO transition metal complexes located?

A

Significantly downfield shifted (150-250 ppm)

39
Q

What is the 3d, 4d, 5d trend for CO resonances?

A

CO resonances shifted to higher field (lower chemical shift, greater shielding) for:
3d -> 4d -> 5d transition metal

40
Q

What is the chemical shift trend for bridging vs terminal carbonyls

A

Bridging = lower wavenumber (IR), higher chemical shift

41
Q

What is the chemical shift trend in substituted carbonyl complexes

A

Increased back donation by transition metal increases the chemical shift of 13CO (pi-acceptor ability of other ligands)

42
Q

Dynamic NMR

A

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

43
Q

Associative exchange process
Dissociative exchange process

A

Dissociative (breaking bonds) - coupling is lost between exchangeable and adjacent nuclei

Associative (retaining bonds) - coupling remains

CHEM431 (2 decks)

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