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Chemistry Alevel OCR YR 13 > Spectroscopy > Flashcards

Spectroscopy Flashcards

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1
Q

what is NMR spectroscopy?

A

nuclear magnetic resonance. Technique uses a combination of a very strong magnetic field and radio frequency radiation. The nuclei of some atoms can absorb this radiation. Energy for absorption can be measured and recorded as an NMR spectrum.

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2
Q

NMR only works with what kind of nuclei?

A

-nuclei with an odd number of protons or odd number of neutrons
-for organic compounds, NMR is relevant to 1H(proton NMR) and 13C

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3
Q

How are nuclear spins significant in NMR spectroscopy?

A

-the nucleus has two different spin states with different energies.
-with the right combo of a strong magnetic field and radio frequency radiation, the nucleus can absorb energy and rapidly flips between two spin states. This is called resonance.

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4
Q

How does radio frequency radiation compare to infrared radiation used in IR spectroscopy ?

A

radio frequency has less energy. The frequency required for resonance is proportional to the magnetic field strength and it is only in strong and uniform magnetic fields that this small quantity of energy can be detected.

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5
Q

where are NMR spectrometers found?

A

MRI body scanners

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6
Q

what is a chemical shift?

A

the frequency shift in electrons needed for NMR to take place

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7
Q

what is the standard reference chemical which all chemical shifts are measured against?

A

Tetramethysilane (TMS) given at chemical shift value of 0ppm

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8
Q

what is the amount of chemical shift determined by?

A

chemical environment, especially the presence of nearby electronegative atoms

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9
Q

NMR requires what?

A

different energy and frequency producing absorption peaks at chemical shifts. This means carbon and hydrogen arrangement in a molecule can be mapped out.

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10
Q

How do you use the NMR spectrometer?

A

-the sample is dissolved in a solvent and placed in a narrow NMR sample tube, together with a small amount of TMS inside an NMR spectrometer.
-it is spun to even out imperfections in the magnetic field within the sample
-spectrometer is zeroed against TMS standard and the sample is given a pulse of radiation containing radio frequencies, whilst maintaining a constant magnetic field
-absorptions from resonance are detected and displayed on computer
-sample can be recovered by evaporation of solvent.

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11
Q

What solvent is used in NMR spectroscopy?

A

Deuterated solvent is used where 1H atoms are replaced by 2H atoms, this womb produce an NMR signal.

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12
Q

What specific solvent is commonly used in NMR spectroscopy?

A

Deuterated trichloromethane (CDCl3), but this will still produce a peak in a carbon-13 NMR spectrum. The computer usually filters out this peak before displaying the spectrum.

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13
Q

what information does carbon-13 NMR give us about a molecule?

A

-the number of different carbon environments (from the number of peaks)
-the types of carbon environments present (from chemical shift)

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14
Q

How is the chemical environment of a carbon atom determined by the position of the atom within the molecule?

A

-carbon atoms that are bonded to different atoms have different environments and will absorb different chemical shifts
-if two carbon atoms are positioned symmetrically, they have the same chemical environment and contribute the same peak

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15
Q

what information does a proton NMR spectrum provide?

A

-number of different proton environments (from the number of peaks)
-types of proton environments present (from the chemical shift)
-relative numbers of each type of proton (from integration traces or ration numbers of the relevant peak areas)
-the number of non-equivalent protons adjacent to a given proton (from the spin-spin splitting pattern)

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16
Q

what principles apply to proton NMR?

A

-If two or more protons are equivalent, they will absorb at the same chemical shift, increasing the size of the peak
-protons of different types have different chemical environments and are non-equivalent- they absorb at different chemical shifts

17
Q

In proton NMR, the ratio of the relative areas under each peak gives the ratio of what?

A

the ratio of the number of protons responsible for each peak

18
Q

what is an integration trace?

A

the relative areas under various peaks in a spectrum

19
Q

In proton NMR, what are splitting patterns caused by?

A

caused by the protons spin interacting with the spins of nearby protons that are in different environments

20
Q

what is the n+1 rule?

A

for a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern = n+1

21
Q

relative peak areas with splitting patterns follow what pattern?

A

pascals triangle

22
Q

why is assigning OH and NH protons difficult?

A

the broadening of peaks due to hydrogen bonding means OH and NH peaks are not usually involved in splitting patterns.

23
Q

what is the proton exchange technique for identifying -OH and -NH protons?

A

-a proton NMR spectrum is run as normal
-small volume of deuterium oxide, D2O is added, mixture is shaken and second spectrum is run
deuterium exchanges and replaces the OH and NH protons in the sample with deuterium atoms
-the second spectrum is run on CH3OD

24
Q

what is the typical sequence for identification in a combined technique?

A

-elemental analysis
-mass spectra
-infrared spectra
-NMR spectra

25
Q

what is elemental analysis used for?

A

use of percentage composition by mass to determine the empirical formula of a compound

26
Q

what is mass spectra used for?

A

use of molecular ion peak to determine the molecular mass and fragment ions to determine parts of the molecule. The molecular formula can then be determined from the empirical formula and molecular mass

26
Q

what is mass spectra used for?

A

use of molecular ion peak to determine the molecular mass and fragment ions to determine parts of the molecule. The molecular formula can then be determined from the empirical formula and molecular mass

27
Q

what is the infrared spectra used for?

A

use of absorption peaks to identify bonds present and functional groups

28
Q

what is NMR spectra used for?

A

to determine the number and different types of carbon and hydrogen atoms from the chemical shifts of peaks and the order of atoms within molecules from splitting patterns.

29
Q

when an external magnetic field is applied to the magnetic field of the nuclear spin, what two directions do they align in?

A

nuclei spin in either the direction of the external magnetic field or against it. Those that spin in the direction of the magnetic field have a lower energy.

30
Q

How does NMR work?

A

-NMR fires out radio waves. At a specific frequency the nuclei that are aligned with the magnetic field absorb the energy and flip to the higher level
-those with higher energy can drop to lower energy and emit radio waves
-overall more energy is absorbed than emitted as there are more nuclei aligned with the magnetic field. NMR measures the amount of energy absorbed.

31
Q

In NMR, why is the energy absorbed by nuclei varied?

A

-It is dependent in the environment it is in
-a nucleus can be shielded from an external magnetic field from electrons surrounding the nucleus
-atoms and groups of atoms adjacent to the nucleus affect the level of electron shielding.

32
Q

In NMR, the environment is determined by?

A

the groups of atoms that exist near to the nuclei being examined. We look at the full chain, not just the atoms immediately bonded to the atom

33
Q

In NMR, what is the chemical standard used and why?

A

-chemical standard is TMS (tetramethylsilane)
-standard is needed as nuclei absorb different amounts of energy
-TMS is used as it will produces one large peak away from sample peaks. It is also inert, non-toxic and volatile

34
Q

what is the chemical shift?

A

-the difference between the TMS peak and peaks produced by the substance under test
-TMS can be used to calibrate NMR machines

35
Q

a higher chemical shift means…

A

the electron shielding is lower

36
Q

what non-hydrogen based solvents are substances dissolved in to avoid confusing the spectra?

A

non-hydrogen based solvents must be used or they will be shown as peaks on the spectrum
-solvents like deuterium (D2O) do not have hydrogen and has an even number of nucleons so will not be detected by the NMR machine
-CCl4 can also be used

37
Q

when D2O is added…

A

proton exchange occurs with -OH and NH2 groups. The D atoms replace H atoms. As D has an even number of nucleons, its nucleus does not possess a spin property, so it does not signal H NMR

Chemistry Alevel OCR YR 13 (13 decks)

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