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NMR Flashcards

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1
Q

what major benefit does NMR analysis possess?

A

NMR can measure all states of matter
- liquid samples
- high pressure NMR can be done using specialist probe and sapphire sample tube
- gels can be analysed using hybrid probe that spins sample at MAGIC ANGLE
- solids packed into small rotor (1.3mm to 4mm) & inserted into probe, spinning v. fast at MAGIC ANGLE to stimulate molecular tumbling

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2
Q

why does the type of rotor matter for NMR analysis?

A

because smaller rotors spin at faster rates

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3
Q

what sample prep is needed for liquid samples?

A
  • deuterated solvents used to dissolve solid sample
  • internal standards (TMS) often used to calibrate chemical shift
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4
Q

what sample prep is needed for solid samples?

A
  • solids are packed into tiny rotor
  • an external standard is used to calibrate spectrum - adamantane. No d-solvents needed
  • choice of external standard depends on nuclei you wish to observe
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5
Q

how does NMR work, simply?

A

NMR signal of over 20 nuclei can be acquired with appropriate probe. NMR signal is detected in presence of a strong external magnetic field

This is achieved by submerging a magnetic coil in liquid He, within N2, allowing it to super conduct

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6
Q

what does nuclear spin arise from?

A

it arises from the unpaired proton and neutron spins in the nucleus

all isotopes with odd atomic number/atomic mass will have a nuclear spin

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7
Q

what is the principal nuclear spin quantum number given as?

A

L

where there are 2l + 1 m(l) levels associated with a nuclear spin l.

m(l) takes values l-1 to -l
(m(l) can be thought of as denoting orientation w/ or against the magnetic field)

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8
Q

what is the gyromagnetic factor?

A

it is defined as the ratio of its angular momentum to its spin angular momentum, hence relating the magnetic moment of a nuclei to its spin

the GMR is a constant for the particle or specific nuclear isotope youre studying.

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9
Q

what happens when a “magnetic nucleus” is placed in a magnetic field?

A

the nuclei will align with the magnetic field (B(0)) - low energy - or against the magnetic field - high energy.

Hence, 𝜇 = 𝛾𝐼
(arrows on diagram should be flipped)

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10
Q

what does ΔE depend on?

A

ΔE depends on:
- size of magnetic moment
- strength of external magnetic field

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11
Q

How can ΔE be measured\?

A

By applying electromagnetic radiation of particular frequency in the radio region of EM spectrum causing the nuclei to flip provided the resonance condition (E=hv) is satisfied

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12
Q

what is the Lamor frequency?

A

The Lamor frequency (resonance frequency) for a given field strength (B(0)) is determined by the nuclide being observed
- every nuclide has a characteristic magnetic moment

To be precise, we can describe the magnetic moment (μ) as being proportional to the spin angular momentum (l) with the GMR for each nuclei

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13
Q

How are different environments determined in NMR?

A

Because the resonance frequency also depends on the chemical environment of a nuclei in a molecule, causing a local magnetic field, B(local), from interaction with the electron cloud.

This effect is known as chemical shift

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14
Q

what does a greater GMR mean?

A

greater GMR means a greater energy gap between high and low energy conformations.

the larger ΔE is, the easier/more sensitive it is to measure - why 13C NMR is harder to measure than 1H NMR)

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15
Q

what equation gives the Lamor frequency of a nucleus? what does this mean?

A

ω = γB(0)
ω = Lamor frequency
γ = gyromagnetic ratio
B(0) = external magnetic field

It means to observe a particular nuclei, we must tune the probe to that particular frequency.

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16
Q

what equation gives Tesla from Hz and GMR?

A

Tesla = Hz / GMR

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17
Q

whatre the units of the GMR?

A

Typically given as; MHz / T

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18
Q

What is the magnetic field strength of the spectrometer in CTL - 8 at 300 MHz? 1H GMR is 42.58 MHz/T

Give in terms of Tesla

A

300 MHz / 42.58 MHz/T = ~7T

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19
Q

Calculate the Lamor (resonance) frequency of a 1H nucleus in a 5.00 T field.

A

ω = 42.58 MHz/T x 5 T = 212.9 MHz

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20
Q

how do you convert chemical shift values (Hz) into ppm?

A
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21
Q

ΔE is relatively small, meaning nuclei can interchange between states easily. Whats the significance of this?

A

At 298K, the distribution between states is approx. equal, meaning very few nuclei are undergoing resonance at any one time

As the signal depends on these few resonating frequencies, NMR is very sensitive to detect these

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22
Q

what can be done to increase signal intensity?

A

Increase the external magnetic field (B(0)), making ΔE greater and therefore increasing the population difference between m = 1/2 and m = -1/2

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23
Q

what equation describes the distribution of nuclei between the 2 states?

A

It is described by the Boltzmann distribution:

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24
Q

does increasing or decreasing temperature affect sensitivity?

A

Yes, decreasing temperature INCREASES sensitivity, according to the Boltzmann equation, as it increases the population of the ground state

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25
what is receptivity?
Receptivity (R(x)) is a measure of how easy it is to acquire an NMR signal of a particular nuclide compared to another one. The smaller the Rx value, the more difficult the nuclide is to detect - and hence make a spectrum from
26
how can receptivity, Rx, be calculated?
It is defined as the product of natural abundance (%) and NMR sensitivity |γ^3|I(I+1) where γ is GMR, and l is nuclear spin
27
whats the purpose of the probe?
To transmit and receive resonance frequency energies between the spectrometer and the sample
28
How is the NMR machine tuned and matched for observing different nuclei?
A weak signal is transmitted to the probe - dummy scan The transmitted frequency is then centred by adjusting the matching and tuning at the base of the probe Matching involves adjusting the probe so that the minimum of the wobble curve is at the base of the display - y-axis Tuning involves ensuring that this occurs at the frequency of the transmission - centre of the screen's x-axis
29
Why must the magnetic field be locked?
Because the magnetic field is not perfectly stable and drifts - particularly during longer experiments - vibrations in environment - moving metallic objects
30
How is the magnetic field locked?
A lock substance, usually deuterated solvent, is added to the sample because: - spin 1 nuclei - so has relatively short relaxation time - reduced proton signal from solvent - commercially accessible
31
whats the need for shims?
Shims are used to adjust the homogeneity of a magnetic field, correcting any inhomogeneities in the applied magnetic field during an NMR experiment
32
how do shims work?
Shims are located at specific places around the sample in the magnet By applying different electrical currents across these coils, it is possible to compensate for deficiencies in the field homogeneity - affecting different axis of the field component.
33
how is shimming done?
To shim the magnet, an indicator of field homogeneity is needed. This can be provided by the lock signal, whose intensity is proportional to the field's homogeneity. To shim, you make changes to the shims and observe the lock intensity. Continue making changes until a maximum intensity is reached.
34
How is the lock signal optimised?
a dedicated lock channel pulses on the lock nucleus, observing signals while the lock is active a narrow lock signal is desired, acquired by adjusting various shim currents, aiming for an optimal lock signal
35
what does fourier transform NMR allow for?
FT-NMR allows us to apply a strong radio frequency pulse at the centre of a range of frequencies in order to excite an entire region - causing all signals in the region to give a simultaneous response
36
After a radio frequency pulse is applied, what happens next?
each nucleus oscillates with its resonance frequency before decaying away - Free Induction Decay (or FID) A fourier transform is then done which converts the signal as a function of time into the equivalent function of frequency - it separates out the component signals to give a spectrum.
37
whats the use of receivers?
to detect signals, storing digitally but generated as analogue data
38
how do modern receivers differ to older versions?
sometimes, the placement of the receiver can leave artefacts on the spectrum. modern receivers circumvent this by acquiring a significantly larger bandwidth and thus do not need to be centred - instead, a digital filter is used to reduce the spectrum to only the desired region
39
what converts digital data to analogue in modern receivers?
an Analogue to Digital Converter (ADC)
40
what happens if the signal received is too large to fit within the ADC bandwidth?
ADC overflow - leaving artefacts on the system (Dummy scans would show this)
41
what happens if the signal received is too small to fit within the ADC bandwidth?
Poor digitalisation of noise - poor signal to noise ratio
42
why are different pulse powers and lengths used for different nuclei?
Because different nuclei have characteristic relaxation times and depend on the radio frequency energy it has received during excitation
43
what factors affect the relaxation time?
Temperature, physical state and nuclei
44
what acquisition parameters should be optimised when recording NMR?
- relaxation delay - is delay between pulses long enough? - RF pulses - is power and length correct? - spectral window - are all chemical shifts included in the spectrum? 10% additional empty space either side of spectrum is recommended. - digital resolution - is distance between data points big enough? - signal to noise - at least 250:1 for <1% error - excitation bandwidth - are all excitation of resonances uniform?
45
what is digital resolution defined as?
digital resolution is the distance between data points - it is defined by the spectral width / half the acquired points. TD = inverse of acquisition time (1/AQ)
46
why can uniform excitation of 13C resonances be problematic?
because 13C has a large chemical shift range, making it difficult to uniformly excite the entire spectral region with a single pulse
47
what 2 types of relaxation can nuclei undergo after excitation?
spin-lattice relaxation spin-spin relaxation
48
why is it important to know the relaxation time of the nuclei youre observing?
because we cannot send another pulse for excitation until all spins have relaxed back to the ground state this requires optimisation of D1 to be at least 5 times the slowest relaxation signal (up to 30s for 1H, up to 60s for 13C)
49
what processing parameters should be adjusted for a good spectrum?
- Zerofilling - Line broadening - Accurate phasing
50
what is zerofilling?
Zerofilling corresponds to the number of points that are used in the spectrum - SI This should at minimum be equal to TD (time-domain)
51
why is it important to optimise line broadening?
because line broadening increases S:N ratio but decreases resolution - so ideal for signals with low S:N.
52
why is accurate phasing important?
peaks must be symmetrical, otherwise integral values will be inaccurate
53
why is baseline correction important?
a flat baseline is important for accurate integration values
54
how much larger should integration regions be relative to the peak width?
should be at least 20 times the peak width in order to cover 99% of the peak area although, consistency is all that matters - if one region is +/- 50 Hz for one peak, all other peaks should be integrated this way also
55
why do carbon satallites sometime appear in spectra?
carbon satallites are due to coupling to the 1% of 13C in the sample - if you include these for one peak, they must be included for all
56
what does 13C's low abundance mean for recording NMR spectra?
it means 13C-13C coupling is rarely seen, meaning direct carbon-carbon connectivities cannot be assigned
57
why is 13C NMR usually {1H decoupled}? how does this relate to the NOE?
because each carbon may be coupled to several protons, resulting in complex spectra and it further reduces signal intensity by spreading the resonance decoupling produces saturation of proton resonances, generating a nuclear Overhauser enhancement of the carbon signal - further increasing 13 C signal intensity
58
why is it important to obtain 13C NMR in addition to 1H NMR?
because 13C resonances are indicative of the chemical environment, showing functional groups that arent seen in 1H NMR - carbonyls
59
is it possible to distinguish between protonated and unprotonated carbon environments in 13C NMR?
Yes It is often possible to distinguish non-protonated 13C environments due to their low intensity relative to protonated carbons - this is principally due to long carbon relaxation times which result from a lack of a directly bonded proton
60
how does proton decoupling work?
- sample is irradiated with 2 different frequencies - Larmor frequency for 13C and 1H - this excites all 13C nuclei and causes all protons to undergo rapid transitions between nuclear spin states - on 13C timescale, each proton is in an average / effectively constant nuclear spin - this results in 1H-13C spin-spin interactions not being observed
61
what does DEPT stand for?
Distortionless Enhancement by Polarisation Transfer
62
why are DEPT experiments used?
theyre used to enhance the sensitivity of carbon observations & for editing 13C spectra
63
how do DEPT experiments enhance the 13C signal?
the gain in sensitivity is due to starting the experiment with proton excitation and then subsequently transferring the magnetisation onto carbon (polarisation transfer) this gains stems from the larger population differences associated with protons, which are 4 times that of carbon
64
how does the editing feature in DEPT experiments aid structure elucidation?
the editing feature allows us to alter the amplitude and sign (+/-) of carbon resonances according to the number of directly attached protons - allowing identification of carbon multiplicities
65
what is done in DEPT experiments to allow various carbon resonances to be seen?
a final proton pulse is done at different angles (45, 90, 135) resulting in different signs (+/-) for different carbon environments
66
why do quaternary carbons not produce responses in standard DEPT experiments?
because they do not posses a directly bonded proton.
67
can quaternary be seen in other 13C experiments?
Yes, albeit more weakly in APT and J-Mod variants. (DEPT-Q) In these experiments, quaternary carbons can be observed and inverted
68
what are COSY experiments used for?
COSY NMR (2D NMR) is used to identify nuclei that share a scalar (J) coupling The presence of off-diagonal peaks (cross-peaks) in the spectrum correlates to coupled partners
69
why are COSY experiments used?
because they are a very efficient way of establishing connectivities when a large number of coupling networks need to be identified - as it maps all correlations within a single experiment
70
when does COSY find use?
it finds use when homonuclear decoupling is unsuitable, where selective decoupling is not possible due to resonance overlap
71
whatre the applications of NMR?
NMR can tell us about: - type of atoms present in sample - relative amounts of atoms in sample - specific environments of atoms in molecule - purity & composition of sample - as well as structural information, incl. constitutional & conformational isomerism
72
what are the 2 types of quantitative NMR?
Relative concentration determination Absolute concentration determination
73
what is relative concentration determination and how is it done?
- compares integrals of interest to one another - allows you to measure accurate ratios of different species
74
whatre the typical applications of relative concentration determination?
purity evaluation & isomer ratio determination
75
what is absolute concentration determination and how is it done?
- integrals of interest are compared to a known standard (calibration compound or calibrant) - the calibrant can also be used for chemical shift referencing
76
how is relative concentration determined?
the molar ratio of Mx/My between 2 compounds x and y is determined using the formula shown: where I is the integral and N is the number of nuclei giving rise to the signal - how many protons in the environment?
77
how is absolute concentration determined?
- calibrant signals must be integrated & normalised according to number of protons - calibrant signals are then compared to the integrals of interest - the concentration of compound x can then be determined using the formula given: Where I, N, and C are the integral area, number of nuclei, and concentration of the compound of interest (x) and the calibrant (cal), respectively.
78
how is the purity of a compound, x, determined?
The purity of a compound x, can be found using the formula given. Where; I = integrated area N = number of nuclei M = molecular mass W = gravimetric weight P = purity of the compound of interest(x) and the calibration compound (cal), respectively.

CHEM286 (6 decks)

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