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nmr > Chemical Shift > Flashcards

Chemical Shift Flashcards

1
Q

what energy level transition is NMR concerned with

A

nuclear spin energy levels

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2
Q

what is the nuclear spin quantum number

A

I

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3
Q

what is the formula for calculating I

A

I = h/2pi + root(I(I+1))

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4
Q

what values can I take

A

0, 1/2, 1, 3/2, 2, 5/2 etc.

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5
Q

what values of I are NMR active

A

nuclei in which I does not = 0 are NMR active

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6
Q

how is the value of I calculated

A 
protons / neutrons / I
even      / even       / 0
odd       / odd         / 1
even     / odd      / 1/2, 3/2 etc
odd      / even / 1/2, 3/2 etc
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7
Q

give examples of 1=1/2 nuclei

A

1H 13C 15N 19F 29S

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8
Q

what is a nucleus with a I value greater than 1/2 known as

A

quadrupolar

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9
Q

give examples of nuclei with I=1

A

2H 14N

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10
Q

give examples of I=3/2 nuclei

A

7Li 23Na

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11
Q

give examples of I=5/2 nuclei

A

27Al

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12
Q

describe the physical properties of spin 1/2 nuclei

A

spherical charge distribution

electric energy is independent of orientation (acts as point charge)

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13
Q

how many projections does a nucleus have on the z axis

A

(2I+1) projections of Iz

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14
Q

what is the formula for Iz

A

Iz = (h/2pi) x m

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15
Q

what is the formula for the magnetic quantum number and what values can it take

A

m = 2m + 1 and can take values of 1 - -1

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16
Q

what is the magnetic moment of a nucleus proportional to, and what is the constant of proportionality

A

mu is proportional to I

the constant of proportionality is the gyromagnetic ratio (gamma)

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17
Q

what are the units of the gyromagnetic ratio

A

radT-1s-1

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18
Q

what is the energy of a magentic moment (mu) in electric field (B)

A

E = -mu x B

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19
Q

how can the energy of a magnetic moment in I also be written

A

E = -m(h/2pi) x nu B

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20
Q

how is the energy of a transition calculated

A

E(m=1/2) - E(m=-1/2)

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21
Q

what is the formula for angular frequency wrt. Bo

A

W = gamma x B0

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22
Q

what is the formula for angular frequency wrt nu

A

w = 2pi nu

23
Q

what is formula for the Larmor frequency

A

nu = -gammaBo/2pi

24
Q

what is the typical field strength of an NMR machine

A

9.4 T = Bo

25
Q

what is the formula for the population of energy levels

A

n(upper)/n(lower) = e^deltaE/kT

difference is always very small

26
Q

other than the energy levels population what else does nmr intensity depend on

A

abundance of the NMR active isotope

27
Q

what is the formula for the frequency of an NMR nucleus

A

nu = gammaBo/2pi

28
Q

what is the strength of the induced field proportional to

A
  • sigma Bo
29
Q

what does sigma represent

A

the extent of chemical sheilding

30
Q

why is the induced field inversely proportional to the applied field

A

the induced field acts in the opposite direction to the applied field

31
Q

what is the formula for localised magnetic field B(loc)

A

B(loc) = Bo + Binduced

=Bo(1-sigma)

32
Q

what is the formula for the NMR frequency of B(loc)

A

nu = gammaBo/2pi (1-sigma)

33
Q

what is the formula for chemical shift

A

shift (delta) = 10^6 (nu-nuref)/nuref

34
Q

what is the formula for chemical shift involving sigma

A

shift (delta) = 10^6 (sigmaref - sigma)/(1-sigma ref)

35
Q

what are the 2 main components of chemical shielding

A 
sigma p (paramagnet shielding)
sigma d (diamagnetic shielding)
36
Q

what effect does paramagnetic shielding have

A

augments Bo - deshields

37
Q

what effect does diamagnetic shielding have

A

opposes Bo - shields

38
Q

what is the size of sigma p in different nuclei

A

in spherical nuclei sigma p =0

in heavy nuclei sigma p is large

39
Q

what is the effect of sigma d and sigma p in 1H nuclei

A

minimal

40
Q

what are the 4 effects that make up total sigma

A

sigma = sigma dloc + sigma ploc + sigma ring current + sigma other

41
Q

what does local diamagnetic shielding depend on

A

number of electrons in an atom

higher e- density = greater shielding = smaller chemical shift

42
Q

what does local paramagnetic shielding depend on

A

mixing of ground states and excited states

extent of mixing depends on energy gap between homo and lumo and distance between nucleus and electrons

43
Q

how does the interaction of Bo and magnetic flux effect shielding

A

when Bo is in the same direction as flux, the proton in the flux will be deshielded and vice versa

44
Q

describe flux about an alkyne bond

A

deshielding occurs around the bond

- effects all pi systems

45
Q

describe flux around an aromatic ring

A

shielding occurs in the centre of the ring

desheilding around the around the outside of the ring

46
Q

what effect does H-bonding have on shielding

A

large deshielding effect as e- density is removed from protons

47
Q

give an example where h bonding increases chemical shift

A

with increasing ethanol conc the position of the OH peak increases frequency

48
Q

how does a positive charge effect shielding

A

deshields

49
Q

how does a negative charge effect shielding

A

increases shielding

50
Q

what is the effect of a paramagnetic compound (unpaired e-)

A

substantial shielding

51
Q

why do unpaired electrons give rise to such large magnetic field

A

gyromagnetic ratio for an electron is 660 times greater than that of a proton

52
Q

what is the role of a shift reagent

A

speads out peaks of similar frequency to increase resolution

53
Q

how do solvents effect the NMR

A

different solvents have a different affinities for different molecules

54
Q

what effect does temperature effect an NMR spectrum

A

can be used to view thermodynamic parameters

nmr (5 decks)

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