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Structural chemistry > The fundamentals L1 > Flashcards

The fundamentals L1 Flashcards

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1
Q

What do protons, neutrons and electrons all possess

A

Protons neutrons and electrons have intrinsic spin, angular momentum of 1/2 and orbital angular momentum

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2
Q

How does the nucleus behave

A

The nucleus behaves as a single particle with spin angular momentum

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3
Q

Define total angular momentum of the nucleus

A

Total angular momentum of nucleus is the sum of spin and orbital angular momenta of all protons and neutrons in the nucleus

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4
Q

How if at all are spin and orbital angular momenta quantised

A

Spin and orbital angular momenta are quantised in both magnitude and direction

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5
Q

How do we calculate magnitude

A

Magnitude = square root of [I(I+1)ħ]
Where I is nuclear spin
h is (h/2pi) and h is planks constant

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6
Q

How do we calculate direction

A

Iz = mIħ
Iz = Z component of I (with respect to an arbitrary Z axis
Mi - magnetic quantum number

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7
Q

For a given nucleus how many possible spin states are there

A

For a given nucleus with spin I there are 2I+1

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8
Q

How do we know the value of I for a particular nucleus

A

Spins of protons or neutrons can pair up and spins cancel
Protons do not pair up with neutrons. It is not possible to predict the value of I only whether it is zero, half integer or integer

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9
Q

Describe nuclei with odd number of protons and or neutrons

A

Nuclei with odd number of protons and neutrons have a total angular momentum that is not zero therefore they have spin
They are therefore nmr active

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10
Q

Describe nuclei with even number of protons and or neutrons

A

Nuclei with even number of protons and neutrons do not possess spin and are nmr inactive therefore cannot be detected

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11
Q

Define isotope specific

A

Different isotopes of the same elements have different spins

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12
Q

What does spin of a nucleus produce

A

Spin of nucleus creates movement of electrical charge which produces a magnetic field

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13
Q

What does a nucleus with non zero spin have

A

A nucleus with non zero spin has a magnetic moment (mu)
Magnetic moment is a vector quantity quantised in the same way as I

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14
Q

How do you calculate magnetic moment (mu)

A

Magnetic moment = magnetogyric ratio * I

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15
Q

What is the magnetogyric ratio

A

Proportionality constant specific to each nucleus

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16
Q

Describe nuclear spins in absence of an applied field

A

In absence of an applied magnetic field, nuclear spins are randomised
2I+1 spin states are degenerate

17
Q

Describe nuclear spins in presence of an external magnetic field

A

In the presence of an external magnetic field the 2I+1 spin states have different energies. The spins either align or oppose withe the magnetic field

18
Q

What is B0

A

B0 is the applied external magnetic field

19
Q

What are quantum restrictions

A

Quantum restrictions prevent spins aligning exactly with B0

20
Q

What do energy difference between spin states depend on

A

External magnetic field and magnetogyric ratio

21
Q

How do we calculate the energy difference between spin states

A

E=hv=(h/2pi)magnetogyric ratioB0

22
Q

Define resonant frequency

A

Resonant frequency is the frequency corresponding to energy of transition

23
Q

How do you calculate resonant frequency

A

(Magnetogyric ratio*B0)/2pi

24
Q

What are the selection rules in terms of energy levels

A

For a given B0 the spacing of E-levels are equal.
Change in magnetic quantum number must be plus or minus 1

25
Q

What happens to the nucleus in an applied magnetic field

A

In an applied magnetic field the nucleus processes about it own axis of spin, the frequency of precession is equal to the resonant frequency which is known as the lamor frequency.

26
Q

What happens upon application of radio frequency radiation of the same frequency to the nucleus

A

Causes resonance in the nucleus and energy can be absorbed and transitions between the spin states can occur

27
Q

What leads to population of higher energy states

A

Thermal motion

28
Q

What does the Boltzmann distributions tell us

A

Population difference between energy levels

29
Q

What is the equations for Boltzmann distribution

A

N upper/ N lower =e^deltaE/kt
Where k is the Boltzmann constant

30
Q

What induces transitions between spin states

A

Irradiation of sample in external magnetic field at larmor frequency induces transitions between spin states

31
Q

Describe the probability of excitation

A

The probability of excitation (lower to upper) is the same as the probability of emission (upper to lower)

32
Q

What does detection of transitions rely on

A

Detection of transitions relies on very small population difference between energy levels.
Ie the intensity of NMR signals depends on difference between the number of absorptions and emissions

33
Q

What does sensitivity for NMR depend on

A

Sensitivity greatest for nuclei with large magnetogyric ration
Sensitivity increases with increasing B0 and increases with decreasing temp

34
Q

What is relaxation in terms of NMR

A

After excitation spins relax to re establish Boltzmann distributions. In absence of relaxation populations of spin states equal so no NMR signals detected so spins become saturated

35
Q

What are the two distinct relaxation processes

A

Spin lattice relaxation and spin spin relaxation

36
Q

Outline the spin lattice relaxation process

A

Loss of energy to surrounding eg solvent molecules, wall of nmr tube via tumbling and vibrations
Tumbling at rate similar to larmor frequency creates oscillating field with enables relaxation

37
Q

Outline spin spin relaxation

A

Energy exchange between nuclear spins leads to randomisation of spins but no net loss of energy

38
Q

Describe the effect of relaxation on linewidths

A

Relaxation of nuclear spins are generally slow so linewidths are narrow, but in some cases relaxation of nuclear spins are very fast and peaks so broad they are difficult to observe

39
Q

How do we measure linewidths

A

Measurement of linewidths. Distance between half height on peak

Structural chemistry (11 decks)

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