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Physical y3 term 1 > PHYS BRITTON > Flashcards

PHYS BRITTON Flashcards

1
Q

if a question asks for non active nmr nuclei, what are we looking for?

A

even numbers because to be spin active we need odd numbers of electrons. HOWEVER 2h is an exception as this can be NMR active

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2
Q

magnetic moment of spinning nucleus

A

u = yP

mag moment = magnetogyric ratio x angular momentum

nb p = I

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3
Q

formula for orientations of spin states in a mag field

A

2I +1

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4
Q

Energy associated with each state =

A
  • (mhyB0) / (2pi)

where m = state eg 1/2 or -1/2

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5
Q

Energy ( that we need to apply to cause NMR signal) =

A

delta E = hv = hbar x y x Bo = hyBo/2pi

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6
Q

order of energy levels in nmr dependent on … (2)

A

m and y

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7
Q

larmor freq is defined as ….

A

frequency at which a nucleus precesses

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8
Q

larmor frequency dependent on (2)

A

strength of magnetic field Bo
and magnetogyric ratio y

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9
Q

larmor freq (rad s-1) =

A

w = -y Bo

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10
Q

larmor freq (Hz)

A

v = -yBo/ 2pi

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11
Q

why does H nmr have higher signal and senistivity than c nmr?

A

because magnetogyric ratio of H (yH) > yC

also signal is effected by natural abundance and 1H = 100% 13C=1%

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12
Q

why is NMR a sensitive technique compared to other soectroscopies?

A

because the gap between the energy levels is small

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13
Q

spin up nuclei have m =

A

1/2

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14
Q

spin down nuclei have m =

A

-1/2

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15
Q

what has higher/ lower energy, spin up or spin down nuclei?

A

spin up lower (alpha)
spin down higher (beta)

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16
Q

eqn for boltzmann distribution of spin up ans spin down nuclei =

A

N1/2
——– = e ^(deltaE/KT)
N -1/2

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17
Q

eqn for contribution to nmr =

A

dif in population of spin up spin down / total polulation

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18
Q

how do stronger magents affect population difference and thus intensity of signal?

A

Stronger magnets , bigger population difference, more signal, more intense signal

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19
Q

how do stronger magnets effect resolution?

A

higher field strength gives better resolution

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20
Q

eigenvalues for hamiltonian operator for ψ-1/2?

A

opposite sign because hamiltonian operator is negative

eigenvalue = 1/2yHbarBo

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21
Q

eigenvalues for hamiltonian operator for ψ1/2?

A

opposite sign because hamiltonian operator is negative

eigenvalue = -1/2yHbarBo

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22
Q

how can population of energy levels be changed?

A

by applying larmor frequency

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23
Q

what effect does a 90 degree pulse have on spin populations?

A

there is no difference in spin populations after a 90 pulse - populations of spin up and spin down are equal

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24
Q

what effect does a 180 pulse have on spin populations?

A

spin populations are inverted , so there are now more spin down than spin up

Mo now points down -z axis

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25
Q

Angle of an Rf pulse determined by (2)

A

its power (B1)
its duration (tp)

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26
Q

equation for angle of rf pulse =

A

theta (angle) = (360y/2pi) x B1Tp

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27
Q

what effect does doubling the duration of a 90 degree pulse have?

A

you get a 180 degree pulse

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28
Q

where does magnetisation point after a 90 y pulse

A

along x axis (Mx)

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29
Q

where does magnetisation point after a 90 x pulse

A

down the y axis (My)

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30
Q

what is the reference point for nmr ?

A

TMS usually at 0

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31
Q

what is free induction decay

A

the oscillating voltage induced by the magnetisation vector the coil

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32
Q

what effect does amount of magnetisation in transverse axis (x) have on intensity of nmr signal

A

they are linked. the more magnetisation in the transverse axis(x) , the stronger the nmr signal

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33
Q

maxiumum intensity is when the magnetisation vector is where?

A

on the x axis

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34
Q

The chemical shift, , for a peak in a 1H NMR spectrum is 3.5 ppm. What is the
difference in frequency between this peak and TMS (at 0 ppm) on a 500.4 MHz
spectrometer?

A

omega = v-vo (Hz)
delta = (v-vo)/vo x 10^6

delta = omega/vo x 10^6
delta x vo = omega x 10^6

(delta x vo) / 10^6 = omega

3.5 x 500.4 / 10^6 = omega mhz

we need to consider changing 500.4MHz to Hz

(3.5 x 500.4 x 10^6 )/ 10^6 = omega Hz = 1751.4 Hz

A: 1751.4 Hz

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35
Q

The chemical shift, , for a peak in a 1H NMR spectrum is 8.1 ppm. If the transmitter
frequency is set to  0 = 400.4 MHz, what is the offset frequency, , for this peak?

A

omega = v-vo (Hz)
delta = (v-vo)/vo x 10^6

delta = omega/vo x 10^6
delta x vo = omega x 10^6

(delta x vo) / 10^6 = omega

8.1 x 400.4 / 10^6 = omega mhz

we need to consider changing 400.4MHz to Hz

(8.1 x 400.4 x 10^6 )/ 10^6 = omega Hz = 3243.2 Hz

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36
Q

treu or false

The Larmor frequency for a nucleus is proportional to the strength of the static
magnetic field B0.

A

true

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37
Q

true or flase

NMR signal intensity is dependent on population diff between spin states

A

true

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37
Q

true or false
j cpupling is dependent on field strength

A

false

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38
Q

true or false
line width of a peak in a nmr spectrum is dependent on the t2 relaxation time of the protons accociated with that peak

A

true

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39
Q

true or false
spin spin coupling is repsonsible for peak splitting in nmr spectrum

A

true

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40
Q

photons from which region of EM spectrum will most liekly induce a rottional transition ?

A

microwave

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41
Q

List, in order of energy (highest to lowest), the energy levels for the 17O nucleus in a
static magnetic field.
Y (
17O) = −36.26  106
rad s−1 T−1
I = 5/2

A

amount of levels = 2i+1 = 6

usually we have positive values of gamma and lowest energy is positive eg 1/2m lowest then -1/2 m for I=1/2

however, here we have a rare case where gamma is positive so this trend is flipped

therefore lowest energy = -5/2 for I=5/2

therefore highest to lowest energy =======
m5/2, m3/2, m1/2, m–1/2, m–3/2 m–5/2

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42
Q

true or false
The Larmor frequency is in the MHz range for superconducting NMR
magnets.

A

true

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43
Q

true or flase
the nmr signal is stored in KHZ

A

true

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44
Q

true or false
multiple aquisitions are not required for quadrature detection

A

true

45
Q

the line widths of hnmr peaks are dependent on transverse relaxation time of photons?

true or false

A

true

46
Q

true or false

NMR spectroscopy measures the absorption and emission of radio frequency
radiation.

A

false - it measures the precession of a magnetisation vector

47
Q

what factors will effect t1 relactaion time in water ?

A: Adding 1 mM GdCl3.
B: Decreasing the temperature.
C: Measuring the T1 relaxation time on a 600 MHz instrument.
D: Shimming the magnet.
E: Removing dissolved gaseous oxygen from the water.

A

all apart from D (shimming the magnet) will effect t1.

48
Q

true or false

In pulsed gradient spin echo NMR spectroscopy, the normalised signal, (S(G)/S(0)), is dependent on the strength of the PGSE gradients.

A

true

49
Q

true or flalse

In pulsed gradient spin echo NMR spectroscopy, the normalised signal, (S(G)/S(0)), is dependent on the temperature of the
sample.

A

true

50
Q

true or flalse

In pulsed gradient spin echo NMR spectroscopy, the normalised signal, (S(G)/S(0)), is dependent on the magnetic field strength of the spectrometer

A

false

51
Q

true or flalse

In pulsed gradient spin echo NMR spectroscopy, the normalised signal, (S(G)/S(0)), is dependent on the separation between the PGSE gradients

A

true

52
Q

true or flalse

In pulsed gradient spin echo NMR spectroscopy, the diffusion co-efficient of a molecule can be measured.

A

true

53
Q

Which of the following cannot affect the line width of a peak in a NMR spectrum?
A: the temperature of the sample
B: chemical exchange
C: the homogeneity of B0
D: the tip angle of the radio frequency pulse
E: the presence of a paramagnetic salt.

A

D the tip angle of the radiofrequency pulse

54
Q

Which of the following will affect the line width of a peak in a NMR spectrum?
A: the temperature of the sample
B: chemical exchange
C: the homogeneity of B0
D: the tip angle of the radio frequency pulse
E: the presence of a paramagnetic salt.

A

all but D

A: the temperature of the sample
B: chemical exchange
C: the homogeneity of B0
NOT D: the tip angle of the radio frequency pulse
E: the presence of a paramagnetic salt.

55
Q

will this improve the resolution of the sample in nmr?
increasing temp of sample

A

yes

56
Q

will this improve the resolution of the sample in nmr?
aquiring the NMR spextrum on a spectrometer with higher frequency proton resonance frequency

eg 600 MHZ instead of 400 MHZ

A

yes

57
Q

will this improve the resolution of the sample in nmr?

changing the solvent to one with a lower viscocity

A

yes

58
Q

will this improve the resolution of the sample in nmr?
removing dissolved gaseous 02 from sample

A

yes

59
Q

will this improve the resolution of the sample in nmr?

increasing the number of signal averages

A

no

60
Q

true or false
the NMR signal arises from the decay of maganetisation as the system returns to thermal equilibrium

A

false

61
Q

true or flase
the line widths of 1H NMR are dependent on dipolar coupling

A

true

62
Q

A 1 H NMR spectrum is collected on a 400 MHz spectrometer, using four signal
averages, a spectral width of 10 kHz and a repetition time of 1 s. The T1 relaxation
time for protons in the sample is 1.45 s. What pulse angle will result in maximum
signal for the experimental parameters used?

A

cos(theta) = exp^(-TR/T1)
cos(theta) = exp^(-1/1.45)
cos(theta) = 0.50175
cos(0.50175) = 60

therefore 60 degrees

63
Q

will this increase S/N ratio of peaks in NMR?
increase signal averages

A

yes

64
Q

will this increase S/N ratio of peaks in NMR?
add 10mM of Gd3+ to the sample

A

yes
dipolar coupling effected by paramagnetic ions in the sample
will increase s/n ratio

note: will reduce t1 relaxation times of molecules around it

65
Q

will this increase S/N ratio of peaks in NMR?
adding paramagentic substance to sample?

A

yes

66
Q

will this increase S/N ratio of peaks in NMR?
increase repeptition time

A

yes

67
Q

will this increase S/N ratio of peaks in NMR?
collect data from a spectrometer with higher MHZ

A

yes

68
Q

Which of the following cannot affect the integral of a peak in a NMR spectrum?
A: the number of signal averages
B: the repetition time
C: the homogeneity of B0
D: the tip angle of the radio frequency pulse
E: the concentration of the sample

A

C the homogeneity of B0

69
Q

can the integral of a peak in NMR be affected by :
number of signal averages

A

yes

70
Q

can the integral of a peak in NMR be affected by : the repetition time

A

yes

71
Q

can the integral of a peak in NMR be affected by : the homogeneity of BO

A

no

72
Q

can the integral of a peak in NMR be affected by : the tip angle of the radiofrequency pulse

A

yes - maximum peak height / intensity is at a specific tip angle

cos(theta)= exp^(-tr/t1)

73
Q

can the integral of a peak in NMR be affected by : the cocnetration of the sample

A

yes

74
Q

Radiation of which wavelength is most likely to induce a purely rotational transition
in a molecule?
A: 1 m
B: 1 cm
C: 1 mm
D: 1 microm
E: 1 nm

A

B 1cm
microwaves between 1mm and 30cm

75
Q

what properties of an NMR spectrum does relaxation effect ?

A

width and intensity of peaks

76
Q

does chemical exchnage affect the appearence of an NMR spectrum?

A

yes

77
Q

nuclei near electronegative species will experience a stronger or weaker mag field than those that are near e donating species?

A

stronger

78
Q

Nmr spectra has ppm on the axis , but what are we actually plotting?

A

frequency

79
Q

true or false t1 and t2 relaxtation times originate from dipole dipole relaxation

A

true
less neighbouring spins nearby will mean it relaxes more slowly.tr

80
Q

true or flase
t1 and t2 relaxation times are influenced by paramagnetic species.

A

true
dipolar interactions between the spins on neighbouting nuclei influence t1 and t2 relaxation . these interactions are influenced by paramagnetic species.

they will reduce the t1 relaxation times of moleules around them

81
Q

t1 is the ……. relaxation time

A

spin lattice relaxation

82
Q

treue or false
in liquids there is no splitting

A

true - due to the rapid motion of molecules, the distnace and angle between molecules averages so there is no splitting.

83
Q

what is tc

A

rotational correlation time

84
Q

tc is dependent on what (3)

A

temp size of molecule and viscocity of solvent

eg small molecule will have lower tc as its faster

85
Q

true or false
spin lattice relaxtaion time is depdndent on rotational correlation time

A

true

86
Q

true or false
spin spin relaxation only occurs in the transverse plane

A

true

87
Q

what effect does t2 have on lifetime of a spin state?

A

reduces lifetime

88
Q

what effect does t2 relaxation have on shape of peak?

A

causes line broadening

89
Q

which statements are correct ??

spin spin relaxation (t2) occurs through
1) inter and intra molecular relaxation
2) magnetic filed inhomogeneity

A

both are correct

90
Q

a broad peak shows what

A

fast decay (short T2)

91
Q

a narrow peak shows what

A

slow decay (long t2)

92
Q

true or false

t2 can be longer than t1

A

false . it can never be longer than t1

93
Q

true or false
at fast motion t1 and t2 have the same value

A

true

94
Q

true or false
multiple aquisitions can affect signal intensity

A

true

95
Q

in an inversion recovery experiment for T1 relaxation time , what relationship is present between the intenisties of the peaks in the spectrum and Mz(tao)?

A

proportional

96
Q

Tnull=

A

T1ln2

97
Q

true or false, in CPMG experiment echoes are repeatedly refocused , this minimised the effct of the inhomogenous field

A

true

98
Q

NMR signal in measured in…..

A

MHz

99
Q

NMR signal stored in

A

KHz

100
Q

Nyquist theorem suggests data should be sampled how many times to correctly assign a frequency?

A

twice per wavlength

101
Q

sample rate (delta t) =

A

1/SW

1/ maximum frequency

102
Q

quadrature detenction decreases signal to noise
true or false

A

false - it INCREASES signal to noise

103
Q

signal averaging / multiple aqusitions allows signal to noise to be improved, true or false

A

true

104
Q

true or false
signal adds coherently

A

true

105
Q

true or false
noise adds coherently

A

false, noise adds more slowly than signal (which adds coherntly) if n is the number of averages, noise adds as rootN akaN^1/2

106
Q

organise these 2d NMR techniques into through bond and through space interactions
COSY NOSEY HMQC

A

COSY HMQC through bond interactions

NOESY throigh space interactions

107
Q

what 2d nmr technique is used for analysing molecular dynamics / chemical exchange ?

A

EXSY

108
Q

4 steps for a 2d pulse sequence

A

preparation
eveolution
mixing
acquisition

109
Q

true or false
magnetisation transfer through scalar coupling can be meausreed using a two pulse sequence and will result in diagonal cross peaks

A

false - will result in off diagonal cross peaks becsuse the larmor freq of the first and second pulse will be different

110
Q

REVISE FROM NUCLEAR OVERHAUSER EFFECT IN TOPIC 3

A

Physical y3 term 1 (3 decks)

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