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Chemistry > Basics of Structural Determination. > Flashcards

Basics of Structural Determination. Flashcards

1
Q

What is the meaning of the following abbreviation?

R

A

Any alkyl group.

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2
Q

What is the meaning of the following abbreviation?

Ph

A

Phenyl

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3
Q

What is the meaning of the following abbreviation?

Ar

A

Aryl (any aromatic group).

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4
Q

What is the meaning of the following abbreviation?

Ac

A

Acetyl

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5
Q

What is the meaning of the following abbreviation?

tBu

A

tert-Butyl (Other groups attached to a carbon that has 3 methyl groups attached to it).

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6
Q

What is the meaning of the following abbreviation?

nBu

A

Normal Butyl

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7
Q

What is the meaning of the following abbreviation?

Me

A

Methyl

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8
Q

What is the meaning of the following abbreviation?

Et

A

Ethyl

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9
Q

What is the meaning of the following abbreviation?

nPr

A

Normal propyl

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10
Q

What is the meaning of the following abbreviation?

iPr

A

Isopropyl (group attached to central carbon in propyl group)

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11
Q

What assumption is made when applying formal charges?

A

Assumes that electron density is shared equally between atoms.

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12
Q

Trivial name of propanone.

A

Acetone

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13
Q

Trivial name of ethanoic acid.

A

Acetic acid

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14
Q

Trivial name of diethyl ether.

A

Ether

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15
Q

Trivial name of methyl benzene.

A

Toluene

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16
Q

Trivial name of ethanoic anhydride.

A

Acetic anhydride

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17
Q

Trivial name of trichloromethane.

A

Chloroform

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18
Q

In X-ray crystallography, what are atomic displacement parameters used to show?

A

Uncertainty in the position of the atoms.

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19
Q

Describe briefly the process of X-ray crystallography?

A

Single crystal of sample mounted on needle and cooled by a jet of nitrogen. X-rays are focused onto the sample through a tube in which there is a vacuum.

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20
Q

Why is the sample cooled in X-ray crystallography?

A

To minimise vibration so decrease uncertainty.

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21
Q

What is the X-ray source?

A

Different elements, such as Molybdenum.

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22
Q

How are X-rays generated?

A

A core electron is knocked out of an atom, and others drop down to lower energy levels, releasing X-rays.

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23
Q

What two things do X-ray diffraction patterns show?

A

Bond lengths and how molecules pack together.

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24
Q

What is the name of a diagram that shows all parts of the structure at least once?

A

A unit cell.

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25
Q

What do contours on a map of electron density join up?

A

Areas of equal electron density.

26
Q

What does it mean if an area has more contours on an electron density map?

A

Higher electron density.

27
Q

Why do hydrogen atoms often not appear on electron density maps?

A

They have very low electron density.

28
Q

How can the position of hydrogen atoms be determined in X-ray crystallography?

A

Prior knowledge of bond lengths and angles.

29
Q

Give the advantages of X-ray crystallography.

A

Best method for structural determination because it gives the location of all of the atoms.

30
Q

Give the disadvantages of X-ray crystallography.

A

Good quality crystals can be difficult, or impossible, to obtain.
It can sometimes be difficult to locate hydrogen atoms, but this is not usually an issue.

31
Q

Name the steps in mass spectrometry.

A

Vaporisation, Ionisation, Acceleration, Ion Drift, Detection (Victor Is An IDiot Duck)

32
Q

Describe the electron bombardment method of ionisation.

A

High energy electrons fired at the sample, knocking electrons from sample particles, forming cations. It usually leads to fragmentation.

33
Q

Describe the electrospray method of ionisation.

A

Sample dissolved in volatile solvent and forced through fine hollow needle that is connected to the positive terminal of an energy supply. This leads to the formation of positively charged droplets that have gained a proton from the solvent. The solvent evaporates until the droplets may contain only one particle of the sample.

34
Q

Describe the acceleration stage in mass spectrometry .

A

Positively charged sample particles accelerate towards a negatively charged plate. Lighter particles reach higher velocities.

35
Q

Describe ion drift.

A

Sample particles are focused into a beam by a hole in the negatively charged plate, then travel down the flight tube.

36
Q

Describe detection in mass spectrometry.

A

Positively charged sample particles collide with the negatively charged detector and gain an electron from it, producing a current. The larger the current, the higher the abundance of an isotope.

37
Q

What is MS/MS?

A

A process where certain ions in the sample are isolated and dubbed a parent ion, then fragmented to produce daughter ions. This allows you to work backwards from daughter ions in the future to work out the identity of the original compound.

38
Q

Give an example of where MS/MS might be useful.

A

Analysing residues at the site of an explosion. Daughter ions can be analysed to determine the composition of an explosive.

39
Q

What are the advantages of mass spectrometry?

A

Gives the molecular formula.
Excellent for analysis of mixtures.
Tiny samples (down to a few million molecules) can be analysed.

40
Q

What are the disadvantages of mass spectrometry?

A

It is only really useful for identifying known compounds, as it can be difficult to interpret otherwise.

41
Q

What does the term ‘quantised’ mean with respect to the energy levels of electrons?

A

Electrons can only possess certain energies.

42
Q

What are the implications of the energy levels of electrons being quantised?

A

Moving an electron from one energy level to another requires a fixed energy change (i.e. the input or release of a fixed amount of energy).

43
Q

As well as the energy levels of electrons, what other types of energy are quantised?

A

Translational, rotational and vibrational energies.

44
Q

What is the equation for the change in energy required to promote an electron from one energy level to another?

A

ΔE=hν

where h = Planck’s Constant (6.62607015×10−34 J/s ) and ν= frequency of light.

45
Q

What happens when electrons are promoted to a higher energy level?

A

Light of the frequency corresponding to the energy change is absorbed.

46
Q

What happens when electrons drop to a lower energy level?

A

Light of the frequency corresponding to the energy change is released.

47
Q

How does Nuclear Magnetic Resonance (NMR) work?

A

Some nuclei possess spin. In a strong magnetic field, spin interacts with the applied field, producing a set of nuclear spin energy levels. Radiowaves with the frequency corresponding to ΔE are absorbed, giving a peak.

48
Q

What two components of an atom can possess spin?

A

Electrons.

The nucleus.

49
Q

What number specifies nuclear spin?

A

The nuclear spin quantum number, I.

50
Q

What values can the nuclear spin quantum number take?

A

Integral and half-integral values.

51
Q

A nucleus with spin ‘I’ gives rise to how many energy levels when placed in a magnetic field?

A

2I + I

52
Q

Different isotopes of the same element have different values of I. True or false?

A

True.

53
Q

What values does I take in nuclei with odd masses?

A

Half-integral values.

54
Q

What values does I take in nuclei with odd numbers of protons and neutrons?

A

Integral values.

55
Q

What values does I take in nuclei with even numbers of protons and neutrons?

A

No spin.

56
Q

What two factors determine the energy difference between spin states?

A

The nucleus itself and the strength of the local magnetic field.

57
Q

Explain why local magnetic field decreases with higher electron density, and hence why the energy difference between spin states is lowered.

A

Electrons set up their own magnetic field that opposes the applied field, so the higher the electron density, the weaker the magnetic field experienced by the nucleus.

58
Q

What is meant if something ‘deshields’ the nucleus?

A

Something withdraws electron density from the nucleus, so it experiences a greater local magnetic field.

59
Q

Why is a chemical shift scale used instead of frequency in NMR?

A

The exact frequency at which a nucleus resonates is dependent on the magnetic field, so the values would differ dependent on magnetic field and create confusion. It makes more sense to quote resonance frequency from an agreed reference compound, to normalise it.

60
Q

Chemical shift (in ppm) = ?

A

10^6 x (frequency of resonance - frequency of reference) ÷ frequency of reference.

61
Q

Why is magnetic field strength not included in the equation for chemical shift?

A

It affects all of the compounds equally so cancels out (it would be equivalent to multiplying all of the frequencies in the equation by k).

62
Q

Give an example of a reference sample used in NMR.

A

Tetramethylsilane.

Chemistry (9 decks)

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