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Core Chem 1A - Term 1 > Spectroscopy > Flashcards

Spectroscopy Flashcards

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1
Q

de Broglie relationship

A

λ = h / mv

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2
Q

c =

A

λv

v = frequency

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3
Q

ṽ =

A

1 / λ

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4
Q

E =

A

hv = hc / λ = hcṽ

= kBT = RT

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5
Q

State definition

A

The term used to summarise and define the particular set of energies that a molecule has

A molecule in a certain state has a well defined energy and is represented by the position on the y-axis on an energy level diagram

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6
Q

Transition definition

A

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

A transition is shown as a vertical arrow between two states showing the direction of change

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7
Q

`Selection rules tell us…

A

which transition states are allowed

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8
Q

Boltzman distribution:

A

N1 / N0 = exp(-(E(N1) - E(N0)) / kBT) = exp(-ΔE / kBT)

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9
Q

Approximate wavelength and type of spectroscopy used for radio waves

A

100m - 102m

Nuclear spin (NMR)

Electron spin (ESR) for shortest wavelengths

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10
Q

Approximate wavelength and type of spectroscopy used for microwaves

A

10-1m - 10-3m

Electron spin (ESR)
or Molecular rotation
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11
Q

Approximate wavelength and type of spectroscopy used for infrared waves

A

10-3m - 10-6m

Molecular vibration

Molecular rotation for longest wavelengths

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12
Q

Approximate wavelength and type of spectroscopy used for visible light rays

A

10-6m - 10-6.5m

Molecular vibration for longer wavelengths

Electronic energies for shorter wavelengths

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13
Q

Approximate wavelength and type of spectroscopy used for ultra-violet rays

A

10-6.5m - 10-8m

Electronic energies

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14
Q

Approximate wavelength and type of spectroscopy used for X-rays

A

10-8m - 10-12m

Electronic energies for longest 3/4 of wavelengths

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15
Q

Approximate wavelength and type of spectroscopy used for γ-rays

A

10-12m - 10-16m

Nuclear energies

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16
Q

A =

A

A = -log10(It / I0) = εcl

c = concentration
I = intensity of light
l = length of box
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17
Q

Symbol and units for the molar absorption coefficient

A

ε

dm3 mol-1 cm-1

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18
Q

ε can be determined by…

A

A plot of A vs. c

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19
Q

%T =

A

(It / I0) x 100

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20
Q

Complementary colour combinations

A

Voilet & Yellow

Blue & Orange

Red & Green

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21
Q

Origins of the Beer-Lambert Law

A

Light absorbed is proportional to the number of light-absorbing molecules:

dI = -α c I dx

(dx is change in box length)

Integrates to:

I = I0 exp (-αcl)

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22
Q

What does the wavelength of absorption tell us?

A

Gives us information on the energy gap, ΔE, between ground and excited states

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23
Q

The molar extinction coefficient is…

A

A constant for a particular molecule at a certian wavelength

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24
Q

Conjugation…

A

Shifts λmax to longer wavelength (lower energy) and increases ε

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25
Q

What effect does a larger box have on ε?

A

Larger box = Larger charge separation = Stronger interaction with the electric field = Larger ε

(Separation between energy levels decreases as size of box increases)

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26
Q

Electronic spectroscopy involves…

A

Moving electrons between molecular orbitals (changing the electronic structure)

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27
Q

What is the chromophore?

A

The part of the molecule that absrobs light

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28
Q

What orbitals are involved in electronic spectroscopy for organic molecules?

A

Mostly involves n, π or π* orbitals

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29
Q

What orbitals are involved in electronic spectroscopy for inorganic molecules?

A

Often involves transfer of electrons between d-orbitals

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30
Q

Electronic spectroscopy is very useful for…

A
  • Measuring the concentration of a known chromophore
  • Following the course of a reaction
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31
Q

Hooke’s Law

A

F = -kx

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32
Q

For a simple harmonic oscillator, the mass oscillates with frequency…

A

v = 1 / 2π sqrt( k / m)

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33
Q

Energy in a simple harmonic oscillator

A

The total energy is constant, but interconverts between potential and kinetic energy

34
Q

What does a reduced mass do?

A

Allows two connected masses to be replaced by a single effective mass

35
Q

Equations for 2 masses connected by a spring

A

ve = 1 / 2π sqrt( k / μ)

1/μ = 1/m1 + 1/m2

k = force constant

μ = reduced mass

36
Q

Reduced mass: Potential energy =

A

V = 1/2 kx2

37
Q

Values of energy allowed, E(v) =

A

hνe(v + 1/2)

νe = vibrational frequency

v = vibrational quantum number (0,1,2,3…)

38
Q

Quantum oscillator: How to work out ΔE

A

eg v = 0 –> v = 1 transition (the fundamental)

E(v) = hνe(v + 1/2)

ΔE = E(v = 1) - E(v = 0)

ΔE = hνe

39
Q

Quantum oscillator: one selection rule says…

A

Δv = ± 1

40
Q

A vibrational mode is only IR active if…

A

the molecular dipole moment changes during vibration

41
Q

Dipole moment, μ =

A

qd

as molecule vibrates d changes, so does μ

42
Q

ṽe =

A

(1 / 2πc) sqrt(k / μ)

43
Q

Give general pattern of force constants of bond types

A

triple bonds > double bonds > single bonds

stronger bonds have higher force constants

44
Q

If there are n atoms in a molecule, how many vibrational nodes are there?

A

3n - 6

except for linear molecules, where there are 3n - 5

45
Q

Fingerprint region includes frequencies…

A

< 1200 cm-1

46
Q

Describe the O-H peak on an IR spectrum

A

Around 3500 cm-1

Broad, smooth

47
Q

Describe the C-H peak on an IR spectrum

A

Around 3000cm-1

Usually broad, spiky

48
Q

Describe the C≡N peak on an IR spectrum

A

Around 2250 cm-1

Very strong

49
Q

Describe the C≡C peak on an IR spectrum

A

2050 - 2250 cm-1

Weak

50
Q

Describe the C=O peak on an IR spectrum

A

1700 - 1800 cm-1

Very strong

51
Q

Describe the N-H peak on an IR spectrum

A

3300 - 3450 cm-1

Medium

52
Q

Describe the C=C peak on an IR spectrum

A

1630 - 1680 cm-1

Weak to medium

53
Q

Anharmonicity causes…

A

Energy levels to get closer together as v increases

54
Q

Magnetic moment, μ =

A

γ sqrt(I(I + 1)ћ)

ћ = h / 2π

γ = magnetogyric ratio of the nucleus

55
Q

What values can the second quantum number, mI take?

A

from -I to I

56
Q

NMR: E =

A

-γmIћB

B = magnetic field

57
Q

NMR: Selection rules

A

ΔmI = ±1

58
Q

NMR is strongly…

A

Isotope - specific

(I = 1/2 gives best for NMR, eg H1, C13…)

59
Q

NMR: What effects the magnetic field at the nuclues?

A

Electron density around the nucleus

60
Q

Beff =

A

Bapp (1-σ)

σ = shielding constant

61
Q

What effects NMR chemical shifts?

A

Electron density
- The more electrons around a nucleus, the more it is shielded from the applied magnetic field. Electronegative atoms shift the chemical shift to much higher

Aromatic and other π-bonding
- Shifts are normally at a higher frequency

62
Q

Approximate chemical shift for an aldehyde

A

9

63
Q

Approximate chemical shift for a carboxylic acid

A

11

64
Q

Approximate chemical shift for C=CH

A

5-7

65
Q

Approximate chemical shift for an aromatic compound

A

7-8

66
Q

Approximate chemical shift for -CH2-O-, -CH2-X, -CH2CO-R

A

2-5

67
Q

Approximate chemical shift for alkanes

A

1-2

68
Q

NMR: Difference invenergy between ±1/2, ΔE =

A

γћB

69
Q

νNMR =

A

γB / 2π

70
Q

Chemical shift, 𝛿 is

A
  • A ratio of the peak position in Hz to the operating frequency in MHz, so it is:
  • Independent of magnetic field strength
  • Reported in Hz
71
Q

How to calculate coupling constant, J

A

Difference between Δν for 2 peaks

72
Q

How to calculate Δν for a peak

A

𝛿 x νopper / 106 ppm

73
Q

For a molecule CxHyNzOn, rings plus double bonds =

A

x - 1/2 y + 1/2 z + 1

74
Q

Describe the simplest mass spectrometer

A
  • Electron impact ionisation
  • Molecule goes through ionisation chamber
    M + e- –> M+ + 2e-
  • Goes through magnetic field
    Lightest ions are deflected most
  • Goes through detector
75
Q

Basic scheme of mass spectrometry

A
  1. Vaporise and ionise molecule
  2. Accelerate ions
  3. Separate ions by mass (various techniques)
  4. Detect ions
76
Q

Describe Matrix assisted laser desorption and ionisation

A
  • Prepare the sample in a matrix that absorbs light, and so can be vaporised by a pulsed laser beam
  • Include a salt so that ions (eg Na+, Ag+) attach to molecules
  • Ions go through an extraction grid, which is negatively charged, to the mass spectrometer
  • Good for polymers and biological macromolecules
77
Q

Describe Electrospray ionisation

A
  • A high voltage is applied to the liquid molecule to produce an aerosol
  • A fine spray of charge droplets are dispersed
  • Solvent evaporation then takes place
  • Ions are then ejected from the droplets
  • Often method of choice for biomolecules, but produces multiply charged ions
78
Q

Explain an old method of separating oins by m/z

A
  • Using a magnetic field
  • Ion beam accelerated by Vacc
  • eVacc = 1/2 mv2
  • Ions follow circular orbit in field
79
Q

Describe a modern method of separating ion by m/z

A
  • Quadrupoles
  • Consists of 4 cylindrical rods, parallele to one another
  • Ions are separated based on the stability of their trajectories in the oscillating electric fields that are applied to the rods
80
Q

Describe a method of finding m/z

A
  • Time of flight
  • Generate ions at a discrete time (t = 0)
  • Accelerate ions with an electric field (the ions aquire kinetic energy, Ekin = 1/2 mv2 = zeVacc
  • 1/2 m (d/t)2 = zeVacc
  • m/z ∝ t2
  • By recording time between ion generated and arrival time at detector can deduce m/z
81
Q

What is the nitrogen rule?

A

Molecular weight is always even from molecules made up of common elements unless they contain in an odd number of nitrogen atoms

Core Chem 1A - Term 1 (6 decks)

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