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Chemistry 2Y > Spectroscopy > Flashcards

Spectroscopy Flashcards

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1
Q

What is the order of energy levels for vibrational, electronic and rotational?

A

Electronic > Vibrational > Rotational

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2
Q

What types of light on the EM spectrum are electronic?

A

UV/visible

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3
Q

What types of light on the EM spectrum are vibrational?

A

Infrared

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4
Q

What types of light on the EM spectrum are rotational?

A

Radio

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5
Q

What can UV/visible spectroscopy be used to investigate?

A

Presence of double bonds
Transition metals
Concentration

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6
Q

What can infrared spectroscopy be used to investigate?

A

Functional groups

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7
Q

What can radio (NMR) spectroscopy be used to investigate?

A

Molecular structure

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8
Q

How can you determine energy of radiation, using Plancks constant (formula + units)?

A

Energy of radiation = Plancks constant (J) x Speed of light (m/s) x Lamnda wavelength (m)

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9
Q

What is the visible spectrum in order of decreasing wavelength?

A

Red Orange Yellow Green Blue Indigo Violet
ROY G BIV

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10
Q

What is the Beer-Lambert Law (formula)?

A

Absorption = molar extinction coefficient x concentration x path length

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11
Q

What can you deduce from plotting absorption against concentration on a graph?

A

If there is a linear relationship (you get a straight line), it adheres to the Beer-Lambert law within that range of concentrations

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12
Q

What are electrons in bonding orbitals found?

A

In bonding orbitals, it is probable that the electrons will be found in between the two nuclei

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12
Q

What does a high molar absorption coefficient tell you?

A

That the compound is very effective at absorbing light (of a certain wavelength)§

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12
Q

What is the molar absorption coefficient?

A

A measurement of how strongly a chemical species absorbs light at a given wavelength

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12
Q

What are electrons in anti-bonding orbitals found?

A

In anti-bonding orbitals, it is probable that electrons will be found on opposite sides of the nuclei

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13
Q

Which orbitals are higher energy out of bonding and anti-bonding?

A

Anti-bonding orbitals are higher energy

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14
Q

On a UV spectroscopy diagram, which electron transition will be the higher peak?

A

The transition from anti-bonding to bonding orbitals, as there is a greater energy gap

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15
Q

On a UV spectroscopy diagram, what does the smaller peak represent?

A

The non-bonding to anti-bonding electron transition; this requires less energy, compared to bonding to anti-bonding

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16
Q

What does HOMO stand for?

A

Highest Occupied Molecular Orbital

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17
Q

What does LUMO stand for?

A

Lowest Unoccupied Molecular Orbital

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18
Q

When do electrons enter anti-bonding orbitals?

A

When they are excited

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19
Q

What is a conjugated molecule?

A

A molecule made up of alternating double and single bonds

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20
Q

What is the relationship between conjugation, and HOMO & LUMO?

A

With increasing conjugation, HOMO and LUMO get closer together

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21
Q

What does increasing conjugation mean wrt wavelength?

A

More conjugation = lower energy gap = longer wavelength

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22
Q

At what wavelengths will the substance appear colourless/white?

A

Less than 400nm or greater than 750nm

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23
Q

What are the two types of vibration in molecules?

A

Bending and stretching vibrations

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24
Q

What is a bending vibration?

A

Where the bond length doesn’t change

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25
Q

What is a stretching vibration?

A

Where the bond length is changing

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26
Q

What is the Y-axis on IR spectroscopy graphs?

A

% of either absorption or transmittance

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27
Q

What is the X-axis on IR spectroscopy graphs?

A

Wavenumber

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28
Q

What is the unit of wavenumber?

A

cm^-1

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29
Q

How do you convert from um to cm^-1?

A

Convert to cm, then take the reciprical
Example:
1um = 0.01 cm
1 / 0.01 = 100 cm-1

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30
Q

What is wavenumber determined by?

A
  • Bond strength, quantified by force constant (k)
  • Mass of atoms, quantified by reduced mass (u)
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31
Q

How can you deduce reduced mass (u)?

A

Multiply the atomic masses, then divide by the sum of the masses:
Reduced mass (u) = m1m2 / m1 + m2

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32
Q

What can you use to determine a predicted wave number?

A

Hooke’s law (equation given in exam)

33
Q

What determines whether a bond is IR active or inactive?

A

For a bond to be IR active, the bond must polar, causing a vibrational stretch

34
Q

What is the relationship between polarity of bond and intensity of absorption?

A

The more polar the bond, the more intense the IR absorption

35
Q

What is the broadening of a peak on an IR spectroscopy diagram indicative of?

A

Hydrogen bonding, so often an O-H bond

36
Q

Where is the functional group region on an IR spectroscopy diagram?

A

4000cm-1 to 1500cm-1

37
Q

Where is the fingerprint region on an IR spectroscopy diagram?

A

1500cm-1 to 625cm-1

38
Q

What does a strong sharp peak at around 1700cm-1 indicate?

A

A double bond, commonly C=O or C=C

39
Q

How does bond strength affect wavenumber?

A

The stronger the bond, the higher the wavenumber (and the higher IR stretching frequency)

40
Q

How does mass of atoms affect wavenumber?

A

The greater the mass, the lower the wavenumber (and the lower the IR stretching frequency)

41
Q

At what frequency would a triple bond appear?

A

Around 2100cm-1

42
Q

What bonds are found above 2800cm-1?

A

X-H bonds, like C-H, N-H and O-H

43
Q

What affect does conjugation have on wavenumber?

A

Conjugated molecules have a lower wavenumber (typically lower than all others)

44
Q

Why can a peak look strong despite the bond not being strong?

A

The accumulation of many bonds (such as many C-H) in a molecule can make the peak look strong, even though individually they are weak

45
Q

What are the two types of nuclei?

A

With spin
Without spin

46
Q

What does it mean for a nucleus to be with spin?

A

It has an odd mass number and/or atomic number (i.e is an isotope)

47
Q

What are some examples of nuclei with spin?

A

1-H (proton) and Carbon-13

48
Q

What are the two possible spins of isotopes?

A

+1/2 and -1/2

49
Q

What are nuclear magnetic moments generated by?

A

They are generated by the charged spinning nucleus

50
Q

What is the orientation and energy of nuclear magnetic moments in the presence of no magnetic field?

A

The magnetic moments are randomly oriented, and are equal in energy

51
Q

What is the orientation of nuclear magnetic moments in the presence of an externally applied magnetic field?

A

They will either spin parallel/aligned with the magnetic field, or antiparallel/aligned against the magnetic field

52
Q

What are the respective energies of magnetic moments in the presence of a magnetic field?

A

Magnetic moments that are antiparallel/aligned against the magnetic field will be of higher energy, compared to those aligned with the field

53
Q

What is delta E wrt nuclear magnetic moments?

A

The energy required to flip the nuclei from spin aligned to spin opposed

54
Q

What two main factors determine delta E?

A
  • Identity of the element
  • The size of the externally applied field
55
Q

What three other factors can affect delta E?

A
  • Inductive effects by electronegative groups
  • Magnetic anisotrophy
  • Hydrogen bonding
56
Q

What is the shielding magnetic field,

A

It is generated by electrons surrounding the nucleus when an external magnetic field is applied, and opposes it

57
Q

What is the relationship between electron density and delta E?

A

The greater the electron density around the nucleus of the H, the greater the shielding, and so the lesser the delta value

58
Q

What happens if hydrogen is bonded to a very electronegative element (e.g. O-H)?

A

There will be a lesser electron density around the nucleus of the H, so less shielding, meaning a greater overall field and delta value

59
Q

What is magnetic anisotrophy?

A

Compounds with pi bonds can either be shielding (decrease delta) or deshielding (increase delta)

60
Q

What is magnetic anisotrophy dependent on?

A

The positioning of the proton relative to the pi system

61
Q

How does hydrogen bonding affect delta value in NMR?

A

Compounds containing NH and OH and undergo hydrogen bonding are difficult to predict where the peak will be on the delta scale

62
Q

What is the unit of delta value in NMR?

A

ppm

63
Q

What is typically the delta value of 1-H in aromatic compounds (Ar-H)?

A

6-7ppm

64
Q

What does a low delta value for 1-H indicate?

A

Alkyl groups

65
Q

What does a large range of delta values indicate?

A

Hydrogen bonding (compounds containing N-H/O-H)

66
Q

Spin spin splitting

A

Do practice questions

67
Q

If a NMR peak disappears after being shaken with D2O, what is likely to be present in the compound?

A

An NH or OH group

68
Q

What is the natural abundance of carbon-13, and what are the implications of this for coupling?

A

1.1%, so it is unlikely that you will find 13-C next to a 13-C, so very low chance of seeing this coupling

69
Q

Why may there be splitting on 13-C NMR?

A

Because 1-H can commonly be found next to a 13-C; proton noise decoupling can be used to remove these peaks

70
Q

What 3 features will there be in DEPT spectrums?

A
  • CH and CH3 groups will be pointing up
  • CH2 peaks will be pointing down
  • Quaternary C will have no peak
71
Q

Summarise briefly how mass spectrometers work:

A
  1. Sample vaporised
  2. Electron beam ionises sample
  3. Particles accelerated into magnetic field
  4. Magnetic field separates particles based on mass/charge ratio
  5. Particles detected
72
Q

What is fragmentation is mass spec?

A

The parent/molecular ion can fragment into daughter ions; the most intense peak on mass spec indicates the most stable fragment

73
Q

What are the two main isotopes of Cl, and what are their relative abundances?

A

35-Cl and 37-Cl, with a natural abundance of 75:25 respectively

74
Q

What are the two main isotopes of Br, and what are their relative abundances?

A

79-Br and 81-Br, with a 50:50 natural abundance

75
Q

What does a m+1 small peak indicate?

A

Most commonly 13-C

76
Q

What does a m+2 small peak indicate?

A

18-O

77
Q

How do you work out the number of double bond equivilance?

A

(2n + 2) - no. hydrogens / 2
This will give you the number of DBE present

78
Q

What is the effect of nitrogen on DBE?

A

For each nitrogen, add 1 H. So the equation is:
(2n + 2 + no. N) - no. hydrogens /2

79
Q

What is the effect of oxygen on DBE?

A

Oxygen has no effect on DBE equation, so use normal (2n + 2) - no. hydrogens /2

80
Q

What is the effect of halogens on DBE?

A

Each halogen will replace 1 H. So the equation is:
(2n + 2 - no. halogens) - no. hydrogens / 2

81
Q

What does one DBE indicate?

A

One double bond (e.g. C=O, C=C, C=N) or a saturated ring structure

82
Q

What does two DBE indicate?

A

C triple bond N
or
Saturated ring + double bond
or
Two double bonds

83
Q

What does four DBE indicate?

A

Benzene ring

Chemistry 2Y (7 decks)

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