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CHEM1603 Dina > Section B: spectroscopy > Flashcards

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
Q

what is the region 1600-400 cm^-1 used for?

A

‘fingerprint’ region

  • used in quantitative/qualitative analysis
  • applications in forensic science
26
Q

what is unexpected about the absorption spectrum of pure ethanol?

A

it has free O-H instead of hydrogen bonding

27
Q

explain the nuclear spin

A
  • 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
Q

how is the energy of the spin states of an NMR-active nucleus calculated?

A

ΔE = γB0h / 2π

29
Q

explain the theory behind NMR

A
  • 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
Q

how does the Larmor frequency, vL, relate to the gyromagnetic ratio, γ, and to B0?

A

since ΔE = γB0h / 2π
and ΔE = h x vL ,
vL = γB0 / 2π

31
Q

what is Beff?

A

the magnetic field felt by the nucleus

Beff = B0 - σB0 = B0(1-σ)

32
Q

what is the frequency taking into account shielding?

A

v = γB0(1-σ) / 2π

33
Q

explain the concept of shielding

A
  • 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
Q

how can shielding be used to identify different nuclei?

A
  • 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
Q

what is the effect of an electron-donating species on the NMR frequency of an adjacent nucleus?

A
  • if an adjacent species is electron-donating, the shielding electron density is increased ie. σ increases
  • therefore the NMR frequency decreases
36
Q

what does it mean if two species are magnetically equivalent?

A

they have the same chemical shift value

37
Q

explain how the chemical shift can be negative

A
  • 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
Q

what chemical is used as a standard for NMR and why?

A

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
Q

how is the chemical shift calculated?

A

[ v(sample) - v(reference) ] / v(reference x 10^6

40
Q

what is the reason for splitting patterns in NMR?

A

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
Q

what can NMR be used for?

A
  • 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

CHEM1603 Dina (9 decks)

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