Section B: spectroscopy Flashcards

1
Q

describe the action of light as a wave

A
  • electromagnetic (E) and magnetic (H) fields travel together in space
  • they fluctuate in phase, both in time and along direction of travel
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2
Q

how can ultraviolet be detected?

A

by the blackening of Ag salts;

insects can ‘see’ UV light

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3
Q

how can infrared be detected?

A

using a thermometer

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4
Q

describe the theory of UV-visible spectroscopy

A
  • absorption bands due to transitions between electronic energy levels
  • transitions involving tightly-bonded electrons occur in the UV range (not visible to the human eye)
  • transitions among valence electrons in unsaturated organic compounds can occur in the visible range, often in chromophores
  • if an atom absorbs UV-visible light, the electron can be promoted to a higher energy orbit
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5
Q

what are some characteristics of chromophores?

A
  • polyunsaturated systems
  • polyaromatic systems
  • systems containing C=O, C=S, C-P, etc.
  • molecules containing transition metal ions
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6
Q

which electron transitions lie in the UV-vis spectra?

A

π-π* transitions lie in the UV range

π-σ* transitions usually lie at too high an energy to be observed in UV-vis spectra

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7
Q

how does the speed of light relate to the wavelength and frequency of light?

A

c = λ x v

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8
Q

how can the energy of a light particle (photon) be calculated?

A

E = h x v = (h x c) / λ

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9
Q

what is the speed of light?

A

2.998 x 10^8 m/s

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10
Q

which wavelengths does chlorophyll absorb at?

A

400-450 nm (blue-violet) and 600-700 nm (red) to reflect a green-yellow colour

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11
Q

what is Hooke’s Law?

A

F = - kx (restorative force)

where k is the force constant- related to bond strength

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12
Q

what units does k, the force constant, have?

A

N/m or kg/s^2

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13
Q

what is the relationship between the magnitude of the force constant and the strength of the bond?

A

the greater the force constant, the stronger the bond

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14
Q

which equation relates the force constant, the reduced mass and the frequency of absorption?

A

v(0) = 1/2π SQRT(k/μ)

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15
Q

explain the theory behind infrared spectroscopy

A
  • atoms vibrate about their equilibrium positions in well-defined patterns
  • the covalent bond can be modelled as a spring
  • use of Hooke’s Law
  • if the vibration causes a change in the electric dipole moment (μ0), this results in a fluctuating electric field that can interact with the infrared light
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16
Q

how is the reduced mass, μ, calculated?

A

μ= m1m2 / (m1 + m2) (x 1.67 x 10^-27 kg)

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17
Q

what is the mass of a proton?

A

1.67 x 10^-27 kg

18
Q

what is the isotope substitution formula?

A

v0heavy / v0light = SQRT(μlight / μheavy)

19
Q

describe the lack of absorption spectrum for homonuclear diatomic molecules

A
  • no dipole moment
  • μ0 does not chance during the vibration
  • no absorption spectrum
20
Q

can there be an absorption spectrum for molecules with no permanent dipole moment?

A
  • yes, if they are more complication than homonuclear diatomic molecules
  • some vibrations can cause a change in μ0
  • these vibrations are IR-active
    eg. CO2
21
Q

what is the relationship between the magnitude of the dipole change and the IR absorption intensity?

A

if the dipole moment change is large, the IR absorption intensity is strong

22
Q

which vibrations are IR-active for CO2?

A

asymmetric stretching ✓
bending ✓
symmetric stretching x

23
Q

what is FTIR spectroscopy?

A

Fourier transform IR spectroscopy - simultaneous collection of data over a wide spectral range to obtain an infrared spectrum

24
Q

what is the nature of CH2 and CH3 absorption peaks?

A

sharp peaks

25
what is the region 1600-400 cm^-1 used for?
'fingerprint' region - used in quantitative/qualitative analysis - applications in forensic science
26
what is unexpected about the absorption spectrum of pure ethanol?
it has free O-H instead of hydrogen bonding
27
explain the nuclear spin
- nuclear spin is a quantised property - each nucleus contains a mixture of protons and neutrons that contribute to the spin quantum number - usually, only nuclei with an odd mass number are NMR-active - 2H and 14C are exceptions - we are mostly interested in 1H and 13C, with total spins I = 1/2 - a spin 1/2 nucleus has two possible spin states: 'spin up' and 'spin down'
28
how is the energy of the spin states of an NMR-active nucleus calculated?
ΔE = γB0h / 2π
29
explain the theory behind NMR
- atomic nuclei have a property called 'spin' - they are charged particles and so spinning around the nucleus results in a local magnetic field (they behave like magnets) - usually the N-S axis is random - in the presence of an external magnetic field, B0, they become aligned with or against the external field direction - there is a small energy difference between the 'up' and 'down' spins - both are present at room temperature but a slightly higher proportion is aligned with the field, giving a net magnetic moment - the nuclear field is tilted with respect to the external field direction and it precesses around B0 - the frequency at which this occurs is the Larmor frequency, vL - vL is determined by γ, the gyromagnetic ratio, and B0 - γ is a constant for each nucleus, but different for each - if we send light into the system, we can see an NMR resonance observed as an absorption of energy at the characteristic Larmor frequency for each nucleus
30
how does the Larmor frequency, vL, relate to the gyromagnetic ratio, γ, and to B0?
since ΔE = γB0h / 2π and ΔE = h x vL , vL = γB0 / 2π
31
what is Beff?
the magnetic field felt by the nucleus | Beff = B0 - σB0 = B0(1-σ)
32
what is the frequency taking into account shielding?
v = γB0(1-σ) / 2π
33
explain the concept of shielding
- nuclei are surrounded by electron clouds - the circulating electrons cause a small magnetic field that opposes B0 - therefore, the actual magnetic field felt by the nucleus is slightly lower than expected - this is shielding - amount of shielding = σB0, where σ is the shielding constant
34
how can shielding be used to identify different nuclei?
- shielding causes a chemical shift of the NMR frequency - nuclei in different bonding environments experience slightly different magnetic fields, Beff - Beff is usually decreased from B0
35
what is the effect of an electron-donating species on the NMR frequency of an adjacent nucleus?
- if an adjacent species is electron-donating, the shielding electron density is increased ie. σ increases - therefore the NMR frequency decreases
36
what does it mean if two species are magnetically equivalent?
they have the same chemical shift value
37
explain how the chemical shift can be negative
- in some large polycyclic highly conjugated or aromatic molecules - there may be protons in the inside of the ring - the ring current now opposes B0 - shifted to negative δ values
38
what chemical is used as a standard for NMR and why?
TMS = tetramethylsilane (CH3)4Si δ=0 ppm - unreactive liquid - mixes with common solvents - single NMR peak - peak occurs lower than that for most protons
39
how is the chemical shift calculated?
[ v(sample) - v(reference) ] / v(reference x 10^6
40
what is the reason for splitting patterns in NMR?
splitting patterns are due to the magnetic coupling between protons at different sites: - protons (a) and (b) are on adjacent carbons - they are in different shielding environments ie. they have different chemical shifts - when proton (a) undergoes its resonance and flips its spin, the other nucleus could be in either the 'up' or 'down' state - these will cause the magnetic field felt by proton (a) to be slightly different - two closely-spaced NMR lines are observed
41
what can NMR be used for?
- to identify the proton environments present and hence to work out the molecular structure - to test possible structural models made based on chemical composition and other spectroscopy results - to identify unknown substances by comparing their fingerprint regions to those of known compounds - for quantitative analysis of absolute or relative concentrations of known compounds present