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Core Chemistry 1 > Spectroscopy and proof of structure > Flashcards

Spectroscopy and proof of structure Flashcards

1
Q

Use of Mass Spectrometry

A
  • molecular ion gives molecular weight (MW)
  • exact MW and isotope patterns may give molecular formula
  • fragmentation pattern may suggest molecular fragments
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2
Q

Use of Infrared Spectroscopy

A
  • look for peaks characteristic of functional groups
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3
Q

Use of HNMR

A
  • Integrals give number of equivalent protons
  • Chemical shifts can be correlated with functional groups
  • J couplings give connectivity between groups of protons
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4
Q

Strengths of HNMR spectroscopy

A
  • quick to determine type of H sites and their relative numbers
  • chemical shifts correlate well with chemical functionality
  • coupling patterns are useful in determining connectivity between H types
  • different isomers give very different spectra
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5
Q

Limitations of HNMR spectroscopy

A
  • no info about overall molecular weight
  • no direct info about atoms other than H
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6
Q

Strengths of Mass Spectrometry

A
  • quick to determine molecular mass
  • isotope patterns may reveal presence of e.g. halogens, approx number of C atoms, odd numbers of N, etc
  • some chemical groups (e.g. benzyl) give identifiable fragment ions
  • fragments can also confirm the presence of some functional groups (being lost)
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7
Q

Limitation of mass spectrometry

A
  • fragmentation patterns may be complex
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8
Q

Strengths of IR Spectroscopy

A
  • easy to check presence/ absence of certain chemical groups
  • the precise vibrational frequencies of some groups can distinguish between other similar groupings, e.g. carbonyl, carboxylic acid and amide.
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9
Q

Limitation of IR Spectroscopy

A
  • molecular vibration can be complex - do not over-interpret
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10
Q

Photon and energy relationship

A

photon energy = molecular energy gap

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11
Q

Degenerate States

A

Two different states (defined by different quantum numbers) can have the same energy

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12
Q

Units for wavelength, λ

A

metres, m

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13
Q

Units for Frequency, v

A

s -1 , Hz

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14
Q

Units for wavenumber, ṽ

A

cm -1

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15
Q

A “state” definition

A

defines an atom or molecule by its quantum numbers

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16
Q

A “transition” definition

A

the process by which one state turns into another state, usually by the exchange of energy with the surroundings by electromagnetic radiation

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17
Q

What do the selection rules do?

A

They tell us which transitions are allowed and which are not

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18
Q

How is data recorded in spectroscopy?

A

In the frequency domain - scanning the frequency of radiation we pass through the sample and seeing what is absorbed

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19
Q

What causes a shadow?

A

When samples containing the atoms are placed between the lamp and a screen due to the atoms absorbing the specific atomic line spectrum

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20
Q

Define the functional group region

A

4000-1500 cm -1

In this region, there is localised vibrational motion of atoms with functional groups which is helpful in indicating the general molecular structure.

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21
Q

Define the fingerprint region

A

1200-100 cm -1

In this region, there is global vibrational movement involving extended parts of the molecule. These are unique to a particular structure and act as a ‘fingerprint’. Isomers with the same functional groups will show different fingerprint spectra.
(many atoms move at the same time)

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22
Q

Factors that determine vibrational frequencies

A
  • bond strength
  • types of bonds
  • stretching v bending
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23
Q

How does bond strength impact force constant and vibrational wavenumber

A

As bond strength becomes weaker, force constant decreases and vibrational wavenumber decreases

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24
Q

How does type of bonding effect vibrational wavenumber and frequency?

A

Vibrational wavenumber:
Bonds to hydrogen > Bonds between first row atoms > bonds to heavier atoms

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25
Q

How does vibrational frequency vary from stretching and bending?

A

symmetric stretch = not IR active

asymmetric stretch > degenerate bends

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26
Q

What is the impact of isotopic substitution?

A

It changes the reduced mass and shifts vibrational frequencies.
Force constants remain the same.

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27
Q

What is the result of anharmonicity?

A

energy levels get closer together as v increases

(v = vibrational quantum number)

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28
Q

NMR outline on nuclei acting as magnets

A
  • nuclei can interact with an applied magnetic field giving rise to sets of energy levels and can induce transitions
  • magnetic moments on different nuclei within a molecule can interact with each other
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29
Q

What do energy levels refer to?

A

they describe the energy of the entire molecule in that state

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30
Q

What does Fourier Transforms (FT) do?

A

Allows a time-domain spectrum to be calculated into a frequency domain spectrum

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31
Q

Measuring wavelength of absorption

A
  • tend to be broad
  • gives information on the energy gap between ground and excited states
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32
Q

Measuring intensity of absorption

A
  • not all transitions have the same probability and therefore strength (sometimes referred to as a cross- section, σ)
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33
Q

What are chromophores?

A

they are molecules in a given material that absorb particular wavelengths of visible light

34
Q

What length molecule is favourable for absorption?

A

the longer the molecule, the greater the wavelength therefore greater the absorption

(longer the molecule, closer the energy levels)

35
Q

Why are transition metal ions coloured?

A

transitions between d-orbitals

36
Q

Isosbestic points

A

absorbence doesn’t change

37
Q

What does electronic spectroscopy involve?

A

electrons moving between molecular orbitals (changing the electronic structure)

38
Q

Why is electronic spectroscopy useful?

A

to measure the concentration of a known chromophore or to follow the course of a reaction.

39
Q

What is the harmonic oscillator in terms of energy?

A

a potential energy curve showing potential energy as a function of displacement

  • total energy is constant, but interconverts between potential and kinetic energy
40
Q

Selection rules for IR spectroscopy

A
  1. ΔV = ± 1
  2. a vibrational mode is only IR active if the molecular dipole moment changes during the vibration
    (IR spectra are only observed for heteronuclear diatomics, not homonuclear ones)
41
Q

What sort of vibrational frequencies is greater: stretching or bending

A

stretching frequencies > bending frequencies

*can have symmetric or antisymmetric stretches

42
Q

If a molecule has n atoms, how many vibrations (degrees of vibrational freedom), for linear molecules?

A

3n - 5

43
Q

If a molecule has n atoms, how many vibrations (degrees of vibrational freedom), for non-linear molecules?

A

3n - 6

44
Q

Functional group vs molecule vibrations

A
  • functional group vibrations are found in the functional group region
  • whole molecule vibrations are found in the fingerprint region
45
Q

How does isotopic substitution affect vibrational frequencies?

A

changes the reduced mass and shifts vibrations, but force constants remain the same

46
Q

What does anharmonicity mean for when v (vibrational quantum number) increases?

A

it causes the energy levels to get closer together

47
Q

What does nuclear spin mean for the nucleus?

A

it gets a magnetic moment

48
Q

In an applied magnetic field, in how many ways can the magnetic moment align with the field?

A

2I +1

49
Q

What do you need for NMR?

A
  • big magnet (very high field strength, superconducting magnet)
  • sample placed in magnet
  • radiofrequency (RF) transmitter to obtain spectra
50
Q

Basic interpretation of NMR spectra

A

expect to see a separate peak for each magnetic nucleus in a chemically distinct environment

51
Q

Why is the magnetic field experienced at each nucleus in a molecule not identical?

A

it is affected by electron density around the nucleus which shields it from the applied magnetic field

52
Q

How to calculate effective magnetic field?

A

B eff = app (1-σ)

where σ is the shielding constant

53
Q

Why does electron density impact effective magnetic field?

A
  • when electron orbital is put into a magnetic field, it induces motion of the electrons which generates an induced field
  • greater the electron density around the proton, the greater the shielding and the lower the B eff
54
Q

Factors affecting chemical shift?

A
  • electron density
  • electronegativity of neighbouring groups
  • magnetic anisotropy
  • aromatic and other pi-bonding
55
Q

What do correlation charts show?

A

typical ppm ranges for different functional groups

56
Q

What essentially is the cause of spin-spin coupling?

A

when the magnetic moments of two protons interact

57
Q

What is the chemical shift?

A

a ratio of the peak position in Hz to the operating frequency in MHz

  • it’s independent of the magnetic field strength and it’s reported in ppm
58
Q

What is the coupling constant?

A
  • it is a measure of the interaction between a pair of protons
  • it is independent of the magnetic field strength
  • reported in Hz
59
Q

What is coupling like to equivalent protons?

A

if the nuclei (spin 1/2) are equivalent, there are characteristic ‘multiplets’ of n+1 lines:

1H: 1:1 doublet
2H: 1:2:1 triplet
3H: 1:3:3:1 quartet
etc

60
Q

Why is C13 NMR much less sensitive than H1 NMR?

A

C13 is in such low natural abundance (about 1.1%) thus gives very weak signals.

(& lower gyromagnetic ratio)

61
Q

Why does 13C NMR have higher chemical shift?

A
62
Q

Why does 13C NMR only show single line per site in molecule?

A

13C - 13C coupling is negligible due to 13C being in such low abundance

63
Q

What is the basic scheme of mass spectrometry?

A
  1. Vaporise and ionise molecule
  2. Accelerate ions
  3. Separate ions by mass (mass filters, time-of-flight)
  4. Detect ions
64
Q

Hard vs soft ionisation techniques

A

Hard: electron impact - suitable for small/ volatile molecules

Soft: electrospray - for delicate molecules

65
Q

Electron impact ionisation

A
  • leads to extensive fragmentation
  • sample must be thermodynamically stable since it undergoes harsh conditions

parent peak (M+) corresponds to molecular mass of compound

66
Q

Matrix assisted laser desorption and ionisation (MALDI)

A
  • good for polymers and biological macromolecules
  • prepare sample in a matrix that absorbs light (soft technique)
  • minimal fragmentation
  • ions can be +ve or -ve and may include a metal ion attached (e.g. Na+)
67
Q

Time-of-flight

A
  • ions are accelerated with equal kinetic energy
  • m/z is proportional to V acc t 2
  • recording time between ion generation and arrival time at the deterctor, m/z can be deduced
68
Q

What can be lost in fragmentation?

A

both radicals and neutrals but only ions are detected

69
Q

What does fragmentation do?

A

it follows a pathway that leads to the most stable ions thus giving these as strong peaks in the mass spectrum

70
Q

Molecular ion peak (M+)

A

M+ peak is the highest possible mass from a molecule, M (molecular mass)

*M+ peak may be weak or absent if molecular ion is very unstable

71
Q

Nitrogen rule

A

odd number of nitrogen atoms = odd molecular mass

  • molecular weight is always even from molecules made up on the common elements
72
Q

For the ring + double bond rule, what does RDB = 4 may mean?

A

it is aromatic

73
Q

What are the key isotopes of C, Br and Cl?

A

C13 and C12
Br79 and Br81
Cl35 and Cl37

74
Q

Peaks for bromine in mass spec

A

51% Br79 and 49% Br81 so approximately equal intensity

75
Q

Peaks for carbon in mass spec

A

98.9% C12 and 1.1% C13 so clusters of peaks for all large organic molecules

76
Q

What is a quick way to estimate the number of C’s in mass spec?

A

Divide M+1:M ratio by 1.11%

77
Q

Peaks for chlorine in mass spec

A

75% Cl35 and 25% Cl37 so a 3:1 ratio

78
Q

Peaks for boron in mass spec

A

20% B10 and 80% B11

79
Q

Stable fragments: molecules containing benzyl

A

gives strong stabilised fragment ion at 91 units, which can rearrange to stable “tropylium ion” by resonance

80
Q

Stable fragments: Carbonyl containing compounds

A

they often give the stable ‘acylium ion’ fragment

81
Q
A

Core Chemistry 1 (9 decks)

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