Inorganic NMR

Question 1

What is I?
(uppercase i)

Answer 1

Spin angular momentum number
Commonly just spin

Question 2

What must I be for NMR activity?

Answer 2

Must be equal to or larger than 1/2

Question 3

What occurs to a state at an I value when B is applied?

Answer 3

State splits into 2I+1 non-degen energy levels

Question 4

What is the equation of energy including I?
(uppercase i)

Answer 4

E = - γℏm_IB

Question 5

What is required for an element to be quadrapolar?

Answer 5

I >= 1
(upppercase i)

Question 6

How is % abundance seen when multiple peaks?

Answer 6

Shared between them

i.e. doublet is both 1/2 abundance

Question 7

What is wanted for NMR in terms of a nucleus?

Answer 7

Abundant
High gyromagnetic ratio
Highest possible applied field strength

Question 8

What is the ΔE in NMR and its size wrt kT?

Answer 8

ΔE is between ground and excited state caused during NMR

ΔE &laquo_space;kT

Question 9

What does energy difference between states mean for NMR sensitivity?

Answer 9

ΔE &laquo_space;kT
Small population difference so weak net absorption and intensity in NMR
This means that the small splitting

Question 10

How does a secondary magnetic field develop?

Answer 10

Denoted B_i
Produced by e- precession

Question 11

How does the overall B₀ relate to an effective field?

Answer 11

B = B₀(1-σ)

σ = shielding constant

Question 12

What is the shielding (ramsey) formula?

Answer 12

σ = σ_d + σ_p

σ_d = diamagnetic shielding (opposing B₀)

σ_p = paramagnetic shielding (augments B₀)

Question 13

What causes diamagnetic shielding?

Answer 13

Unperturbed spherical motion of e-
Induced by applied mag field

Question 14

What causes paramagnetic shielding?

Answer 14

Hinder rotation of e- cloud about nucleus
Such as other nuclei or non-spherical charged distribution

Question 15

How does paramagnetic shielding relate to difference in energy?

Answer 15

σ_p α -1/ΔE

Depends on extent of magnetic field mixing excited electronic states into ground state wavefunction

Question 16

How does ppm relate to ΔE for octahedral complexes?

Answer 16

As ppm increases, 1/ΔE increases

Question 16

How does ppm relate to ΔE for octahedral complexes?

Answer 16

As ppm increases, 1/ΔE

Question 17

How does ppm relate to σ?

Answer 17

δ = 10⁶(σ_ref-σ)

Question 18

What is the shift of transition metal hydrides?

Answer 18

Negative proton shift values
As excited states low in E so mixed with ground state wavefunction
Causes deshielding at M and shielding at H

Question 19

How does the shift of TM hydrides change depending on de- counts?

Answer 19

The shift is more negative when there are more d-electrons

Question 20

Define scalar coupling

Answer 20

Indirect nuclear-nuclear coupling mediated by e- in a bond

Question 21

What is the j-coupling for an I=1/2 nuclei?

Answer 21

Coupling to n indentical spin 1/2 nuclei gives n+1 multiplet with binomial intensities

Coupling between mag equivalent nuclei doesnt give splitting

Question 22

What is the binomial tree for spin 1/2 coupling?

Answer 22

Question 23

What does the strength of 1-bond ¹J(¹³C,¹H) coupling constant depend on?

Answer 23

Depends on extent of s-orbital involvement in bonding
More s-character has larger J

Question 24

What is 1-bond ¹J(¹³C,¹H) in a complex relative to not?

Answer 24

Decreased when in a complex relative to free
Due to partial rehydrbidization to sp³

Question 25

How can geminal (2-bond) coupling give stereochem info?

Answer 25

e.g. square planar complexes
cis and trans coupling differ strongly in magnitude
²J trans&raquo_space; ²J cis

Question 26

What is vicinal (3-bond coupling) dependent on?

Answer 26

Depends on dihedral angle (φ) between planes contaning coupling nuclei

³J(X,Y) = Acos2φ + Bcosφ + C

Question 27

How do you decouple a spin from other spins?

Answer 27

Rf irradiation @ Larmor frequency of a spin

Rapidly swaps between α & β spin states so average observed to other nuclei - no splitting

Question 28

How does frequency and coupling relate in magnitude in weak coupling? (standard J coupling)

Answer 28

|Δv|&raquo_space; |J|
This is standard J-coupling

Question 29

What is the magnitude of frequency and coupling constant in strong coupling (2nd order patterns)?

Answer 29

|Δv|≈ |J|
Distortion in intensities
Midpoint no longer shows true δ

Question 30

How does 2nd order strong coupling look compared to weak coupling?

Answer 30

Δδ similar to J in an AB system
This is seen as roofing

Question 31

What is the requirement for magnetic equivalent nuclei?

Answer 31

Identical chemical shifts
Same coupling constants to other nuclei in moleucle

Question 32

How are quadrapolar nuclei present on NMR spectrum?

Answer 32

Splits to 2I+1 m_I states
Equal prob of all states so equal intensities
Actual ppm in the middle

Question 33

What is the NMR of PF₅?

Answer 33

One peak
F exchange by Berry mechanism so indistinguishable

Question 34

What occurs to protons on methylene ligand (M=CH₂) in NMR?

Answer 34

At high T:
geminal protons exchange faster than NMR timescale so couple as 1 environemnt
At low T:
protons don’t exchange as insufficient energy, separate environments and can see some geminal coupling

Question 35

What is the coalescence temperature, T_c?

Answer 35

Temp at which spectrum changes from separate peaks to single, flat-topped peak
Related to when groups can exchange or rotate

Question 36

How does an EWG change rate of exchange in =CH₂?

Answer 36

EWG (e.g. Ph) takes e- density away from =CH₂
Easier to break π-alkene bond and rotate protons