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Chemistry unit 4 > 4.12 Structure Determination > Flashcards

4.12 Structure Determination Flashcards

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1
Q

What does the mass spectrometer do?

A

The mass spectrometer analyses gaseous samples and compounds, providing information about relative molecular mass

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2
Q

What form of mass spec can be used to determine a molecular formula?

A

High-resolution mass spectroscopy

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3
Q

What are the four stages of mass spec?

A

Ionisation, acceleration, deflection, and detection

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4
Q

How is the positive ion formed in mass spec?

A

A high-energy electron gun knocks off an electron to produce the cation

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5
Q

How is the cation accelerated?

A

Using an electric field

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6
Q

How is the cation deflected?

A

Using a variable magnetic field

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7
Q

How is the cation detected?

A

Using electric plates

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8
Q

What is the base peak on the mass spectra?

A

The base peak is the tallest peak, the one with the greatest abundance

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9
Q

What is the molecular ion peak?

A

The molecular ion peak is on the far right, it is caused by the produced cation

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10
Q

Why is the cation a free radical?

A

Because it has a lone pair of electrons

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11
Q

What is the equation for mass spec?

A

M(g) + e- ==> M+(g) + 2e-

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12
Q

What is the equation for the splitting cation?

A

M+* ==> X+ + Y*

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13
Q

What is infrared spectroscopy used for?

A

It is used to provide information about the types of bonds present in a molecule

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14
Q

How is IR spectroscopy possible?

A

different bonds absorb different frequencies of IR radiation

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15
Q

What is the fingerprint region?

A

It is the part of the spectrum between 1500cm and 400cm, it’s pattern is unique to each compound

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16
Q

what does n.m.r. spectroscopy provide?

A

It provides information about the position of certain atoms in a molecule

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17
Q

What does n.m.r. spec rely on?

A

It relies on a property of atomic nuclei called nuclear spin

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18
Q

What property must an atom have to have nuclear spin?

A

The nuclei of the atom must have an odd mass number

19
Q

What can be said about a nucleus in the spin state opposed to the magnetic field?

A

A nucleus in the spin state opposed to the field is at a higher energy than a nucleus aligned with it

20
Q

What state must a sample be to produce an n.m.r. spectrum?

A

The sample must be a liquid or solvent

21
Q

What properties must the solvent be for the sample to be dissolved in it?

A

It must be proton free so that it does not affect the spectrum, EG CCl4

22
Q

What is the first stage of n.m.r. spectroscopy?

A

The sample is put in a cylindrical tube and lowered into the spectrometer

23
Q

What happens after the sample is in the spectrometer?

A

It is spun on its axis between the poles of a very powerful electromagnet. The applied field is varied

24
Q

Why is the n.m.r. spectrum produced?

A

Each atom in the sample absorbs a different frequency of electromagnetic radiation

25
Q

Why is a reference sample added to the sample?

A

It is difficult to measure the precise resonance frequency of H or C so the difference between the resonance frequencies of the sample and the reference are measured

26
Q

What is the reference sample used?

A

TMS - Si(CH4)3

27
Q

Why is chemical shift calculated?

A

Chemical shift is calculated to allow results from different spectrometers to be compared

28
Q

What does chemical shift values between molecules depend on?

A

The chemical environment

29
Q

What is the definition of a chemical environment?

A

It involves any atoms covalently bonded to it, and neighbouring atoms or groups of atoms

30
Q

What are equivalent atoms?

A

Atoms in the same chemical environment

31
Q

What are non-equivalent atoms?

A

Atoms in different chemical environments

32
Q

What is the resonance frequency of an atom affected by?

A

It is affected by the local electron density

33
Q

Why does electron density affect chemical shift?

A

Electrons oppose the magnetic field, reducing its effect on the resonating nucleus and causing it to be shielded

34
Q

What is the effect of electronegative atoms?

A

Chemical shift increases in atoms that are covalently bonded to electronegative atoms such as oxygen as the become deshieded

35
Q

What does the area under each peak represent in an H n.m.r. spectra? This is the same as the height of peaks in an integrated spectra

A

The area under each peak is proportional to the number of atoms in the same chemical environment

36
Q

What can be worked out from the height of integrated spectra peaks?

A

The relative number of atoms in each chemical environment

37
Q

What are 3 reasons why TMS is chosen as the reference standard?

A
  • The H and C nuclei are highly shielded due to the electronegativity of silicon being low, meaning the peaks will be upfield
  • All hydrogen and carbon atoms are equivalent, producing sharp peaks
  • TMS is cheap, non-toxic and does not react with the sample
38
Q

What does high-resolution proton n.m.r. give?

A

Gives fine detail about the relative position of each hydrogen atom in a compound, due to spin-spin coupling

39
Q

When are protons said to be coupled?

A

When they are in different neighbouring chemical environments

40
Q

What is the effect of spin-spin coupling?

A

A single peak is split into a cluster of peaks in high res n.m.r. spectra

41
Q

What rule is used to work out the splitting pattern for a peak corresponding to a chemical environment?

A

n + 1 rule

42
Q

What splitting pattern is produced by CH?

A

Doublet

43
Q

What splitting pattern is produced by CH2?

A

Triplet

44
Q

What splitting pattern is produced by CH3?

A

Quartet

Chemistry unit 4 (12 decks)

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