NMR

Question 1

What happens in an applied magnetic field for nucleus

Answer 1

It spins on its axis. In mag field, the axis of rotation will precess around the mag field.

Question 2

What is the frequency of precession called

Answer 2

Resonance or Larmor frequency

Question 3

Nuclear magnetic moment equation

Answer 3

μ = Magnetogyric ratio (γ) * Spin quantum number (I)

Question 4

What is magnetogyric ratio. Meaning if negative or positive

Answer 4

A number given for every nucleus. Signs determines anti/clockwise precession. Sort of how fast its going

Question 5

What does nuclear spin quantum number I result in

Answer 5

The orientation of nuclear spin

Question 6

Equation which quantises orientation of magnetic moment in z azis

Answer 6

μz prop to m*(h/2π) where m = +I .. -I and h is Plancks

Question 7

How many allowed nuclear spin states of μz are there

Answer 7

2I + 1 where I is the nuclear spin

Question 8

What happens to μz orientations when magnetic field is absent

Answer 8

All have same energy ie degenerate

Question 9

Energy of nuclei in applied field equation

Answer 9

E = γBm*h/2π where m is +I .. -I

Question 10

How to work out Larmor frequency using difference in energy of spin states

Answer 10

ΔE = hν = γBh/2π so frequency v = γB/2π

Question 11

What is the importance of the Larmor frequency of a nucleus in NMR

Answer 11

Irradiation causes transitions between nuclear spin energy levels

Question 12

What is the Zeemen effect

Answer 12

The splitting of energy levels/spin states in a magnetic field

Question 13

Effective magnetic field equation

Answer 13

Beff = B0(1 - σ) where σ is the shielding constant

Question 14

What causes the shielding constant

Answer 14

Electrons opposing the induced B due to their motion

Question 15

Where can e- reinforce applied B (deshielding)

Answer 15

Benzene ring

Question 16

Nomenclature for higher frequency NMR

Answer 16

Desheilded and downfield!

Question 17

Why does F cause more desheilding than I in 1H NMR?

Answer 17

Fs electronegativity pulls e- density away from carbon and H bond

Question 18

What is the coupling constant

Answer 18

Written like 1JAX where 1 is how many bonds and AX are the 2 nuclei. Measured in Hz. Same if looking at X or A spectrum.

Question 19

Amount of NMR splitting peaks

Answer 19

2nI + 1 where n is how many neighbours the nucleus sees. I = spin

Question 20

What can affect coupling constant magnitude?

Answer 20

Intervening bonds, delocalised orbitals, heavy nuclei

Question 21

Find coupling constant

Answer 21

Difference in ppm betw 2 peaks x resonance frequency in MHz gives constant in Hz

Question 22

AMX system for X. Explain its spectrum if I=1/2

Answer 22

X sees M first -> doublet. Then X sees A so causes original lines to split into doublets again. In total 4 lines where coupling constant is smaller for second couplet. Called doublet of doubles.

Question 23

In H-P-F, would HP or PF have the bigger coupling constant

Answer 23

PF because coupling occurs through intervening electrons. F has more than H.

Question 24

Splitting for AMX2 systems

Answer 24

There will be either a triplet of doublets or doublet of triplets depending on what the chosen molecule sees first. If A sees M first then it will be a doublet split into triplets aka a doublet of triplets.

Question 25

What is the purpose of the spectrometer frequency?

Answer 25

To encourage nuclei to become excited

Question 26

What happens during decoupling in NMR

Answer 26

Coupling nucleus irradiated at its resonance frequency -> rapid transition between all spin states -> other nuclei sees it as 0 spin (average) -> decoupled nucleus does not cause peaks to split!

Question 27

Notation for a decoupled 11B NMR spectra with BH4-

Answer 27

11B{1H} NMR

Question 28

What happens to spectrum where only 10% nuclei are spin active?

Answer 28

One singlet of 90% in middle and 2 smaller SATELLITE PEAKS of 5% on either side.

Question 29

If nucleus has 2 isotopes that are active, which one gives the largest coupling constant of the satellite peaks?

Answer 29

Heavier nuclei TEND to give bigger coupling constants but depends on magnetogyric ratio.

Question 30

Which coupling constant is bigger in B NMR if A-B-C and A-B is covalent and B-C is dative

Answer 30

A-B will be the main split and bigger coupling constant

Question 31

Does BX3 or BX4 have a higher ppm

Answer 31

BX3 is more deshielded to higher ppm

Question 32

Why does BF3 not follow the halide sequency of the most shielded

Answer 32

F has similar sized full pz orbital as B’s empty pz orbital -> returns some e- density in sigma bonding -> greater expected shielding

Question 33

What happens if 2 nuclei of I=3/2 get coupled

Answer 33

1:2:3:4:3:2:1 as each 1:1:1:1 gets split into 4 other ones

Question 34

What is quadrupole moment + its symbol

Answer 34

Measure of distortion from spherical charge distribution, eQ

Question 35

What is Heisenberg Uncertainty Principle

Answer 35

ΔE*Δt >= h/4π where ΔE is uncertainty in energy of state and Δt is uncert in time specified

Question 36

Why does I=1/2 give well defined spectra but others may not

Answer 36

I=1/2 has a long lived ground and excited state. Therefore Δt is large so ΔE is small so well defined energies -> sharp line in spectrum. For quadrupolar, it transitions through spin states rapidly so Δt is very small so it gives a larger ΔE and broader and shorted peaks in spectrum since the area stays the same

Question 37

When can quadrupolar nuclei still give sharp peaks

Answer 37

A symmetrical molecule so there is no field gradient (dipole) at the nucleus

Question 38

When will quadrupolar nuclei not be seen in spectra

Answer 38

Large eQ + unsymmetrical environ. Also if measuring I=1/2 spectra and has a quadrupolar neighbour

Question 39

What does it mean if a molecule is fluxional

Answer 39

Equatorial and axial atoms are seen as the same as they exchange rapidly giving a different spectra because NMR only sees an average.

Question 40

What happens to the spectra as you raise the temperature for a fluxional molecule

Answer 40

1. Loss of fine structure. 2. Coalescence: it collapses to a single broad peak. 3. Sharpening to a single narrow peak

Question 41

Why does fluxionality affect NMR

Answer 41

It is a slow technique so it only sees averaged out set of atoms