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Core Chem 1A - Term 1 > Spectroscopy > Flashcards

Spectroscopy Flashcards

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
What effect does a larger box have on ε?
Larger box = Larger charge separation = Stronger interaction with the electric field = Larger ε (Separation between energy levels decreases as size of box increases)
26
Electronic spectroscopy involves...
Moving electrons between molecular orbitals (changing the electronic structure)
27
What is the chromophore?
The part of the molecule that absrobs light
28
What orbitals are involved in electronic spectroscopy for organic molecules?
Mostly involves n, π or π\* orbitals
29
What orbitals are involved in electronic spectroscopy for inorganic molecules?
Often involves transfer of electrons between d-orbitals
30
Electronic spectroscopy is very useful for...
- Measuring the concentration of a known chromophore - Following the course of a reaction
31
Hooke's Law
F = -kx
32
For a simple harmonic oscillator, the mass oscillates with frequency...
*v* = 1 / 2π sqrt( k / m)
33
Energy in a simple harmonic oscillator
The total energy is constant, but interconverts between potential and kinetic energy
34
What does a reduced mass do?
Allows two connected masses to be replaced by a single effective mass
35
Equations for 2 masses connected by a spring
*v*e = 1 / 2π sqrt( k / μ) 1/μ = 1/m1 + 1/m2 k = force constant μ = reduced mass
36
Reduced mass: Potential energy =
V = 1/2 kx2
37
Values of energy allowed, E(*v*) =
hνe(*v* + 1/2) νe = vibrational frequency *v* = vibrational quantum number (0,1,2,3...)
38
Quantum oscillator: How to work out ΔE
eg *v* = 0 --\> *v* = 1 transition (the fundamental) E(*v*) = hνe(*v* + 1/2) ΔE = E(*v* = 1) - E(*v* = 0) ΔE = hνe
39
Quantum oscillator: one selection rule says...
Δ*v* = ± 1
40
A vibrational mode is only IR active if...
the molecular dipole moment changes during vibration
41
Dipole moment, μ =
qd as molecule vibrates d changes, so does μ
42
ṽe =
(1 / 2π*c*) sqrt(k / μ)
43
Give general pattern of force constants of bond types
triple bonds \> double bonds \> single bonds stronger bonds have higher force constants
44
If there are n atoms in a molecule, how many vibrational nodes are there?
3n - 6 except for linear molecules, where there are 3n - 5
45
Fingerprint region includes frequencies...
\< 1200 cm-1
46
Describe the O-H peak on an IR spectrum
Around 3500 cm-1 Broad, smooth
47
Describe the C-H peak on an IR spectrum
Around 3000cm-1 Usually broad, spiky
48
Describe the C≡N peak on an IR spectrum
Around 2250 cm-1 Very strong
49
Describe the C≡C peak on an IR spectrum
2050 - 2250 cm-1 Weak
50
Describe the C=O peak on an IR spectrum
1700 - 1800 cm-1 Very strong
51
Describe the N-H peak on an IR spectrum
3300 - 3450 cm-1 Medium
52
Describe the C=C peak on an IR spectrum
1630 - 1680 cm-1 Weak to medium
53
Anharmonicity causes...
Energy levels to get closer together as *v* increases
54
Magnetic moment, μ =
γ sqrt(I(I + 1)ћ) ћ = h / 2π γ = magnetogyric ratio of the nucleus
55
What values can the second quantum number, mI take?
from -I to I
56
NMR: E =
-γmIћB B = magnetic field
57
NMR: Selection rules
ΔmI = ±1
58
NMR is strongly...
Isotope - specific (I = 1/2 gives best for NMR, eg H1, C13...)
59
NMR: What effects the magnetic field at the nuclues?
Electron density around the nucleus
60
Beff =
Bapp (1-σ) σ = shielding constant
61
What effects NMR chemical shifts?
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
Approximate chemical shift for an aldehyde
9
63
Approximate chemical shift for a carboxylic acid
11
64
Approximate chemical shift for C=CH
5-7
65
Approximate chemical shift for an aromatic compound
7-8
66
Approximate chemical shift for -CH2-O-, -CH2-X, -CH2CO-R
2-5
67
Approximate chemical shift for alkanes
1-2
68
NMR: Difference invenergy between ±1/2, ΔE =
γћB
69
νNMR =
γB / 2π
70
Chemical shift, 𝛿 is
- 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
How to calculate coupling constant, J
Difference between Δν for 2 peaks
72
How to calculate Δν for a peak
𝛿 x νopper / 106 ppm
73
For a molecule CxHyNzOn, rings plus double bonds =
x - 1/2 y + 1/2 z + 1
74
Describe the simplest mass spectrometer
- 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
Basic scheme of mass spectrometry
1. Vaporise and ionise molecule 2. Accelerate ions 3. Separate ions by mass (various techniques) 4. Detect ions
76
Describe Matrix assisted laser desorption and ionisation
- 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
Describe Electrospray ionisation
- 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
Explain an old method of separating oins by m/z
- Using a magnetic field - Ion beam accelerated by Vacc - eVacc = 1/2 mv2 - Ions follow circular orbit in field
79
Describe a modern method of separating ion by m/z
- 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
Describe a method of finding m/z
- 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
What is the nitrogen rule?
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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