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Chemistry 2 > Chemistry Y1: Spectroscocpy > Flashcards

Chemistry Y1: Spectroscocpy Flashcards

1
Q

What are the advantages of spectroscopic methods

A
  • Fast
  • measure small sample sizes
  • Non-destructive
  • Can analyse mixtures
  • Sample doesn’t need to be pure
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2
Q

What are the spectroscopy techniques

A

*Identify compounds by determining absorption/ emission spectra
*Quantify substances by measuring appropriate wavelength
*determine molecular structure
*Follow reactions

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3
Q

Energy of radiation equation?

A

E = hc/λ
h= planks constant (J/s)
C= speed of light (ms-1)
Λ= lambda wavelength (m)

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4
Q

How is EM radiation absorbed by atoms/molecules?

A
  • Atoms and molecules exists in quantised energy levels
  • Bonds may bend or rotate only
    *e can only move between specific orbitals
  • these energy differences quantify absorbtion
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5
Q

Order Rotational levels, Electronic levels and vibrational levels in terms of size of energy gaps

A

Rotational > Vibrational > electronic

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6
Q

What is the Energy range of UV?

A

1nm-400nm

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7
Q

What wavelength have electronic energy levels?

A

UV/ Visible

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8
Q

What wavelength have molecular vibrations as energy levels?

A

IR

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9
Q

What wavelength have Nuclear spin energy levels?

A

Radio (nmr)

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10
Q

What is the Energy range of Visible light?

A

400nm-750nm

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11
Q

How can you determine the concentration of a sample?

A

by measuring how much light has been absorbed

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12
Q

What factors control Amax?

A

*Concentration of a species
*Path length of the cuvette (wavelength)

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13
Q

What is beer lamberts law, provide an equation

A

A linear relationship between A and c

(Value of ε and A must be at the same wavelength)

A=εcl

ε=Molar extinction coefficient (L mol-1 cm-1)
C= concentration
L= path length (cm)

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14
Q

What does a higher molar coefficient (ε) mean

A

The species is more effective at absorbing light at a given wavelength

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15
Q

When is molar coefficient (ε) highest?

A

At Amax

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16
Q

What are negative and positive deviation

A

Negative= conc lower than expected for beer lambert

Positive= conc higher than expected for beer lambert law

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17
Q

what 2 orbitals are formed from the overlap of 2 atomic orbitals

A

2 molecular orbitals:
1. Bonding molecular orbital
2. Anti-bonding molecular orbital

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18
Q

Which bond requires more energy: Lp -Pi or Pi-Pi

A

Pi-Pi = 190nm MORE ENERGY
lP-pi = 270nm LESS ENERGY

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19
Q

What causes the peak in alkenes?

A

The energy gap between Pi-Pi*
Pi= Highest occupied (molecular orbital)
Pi* = Lowest occupied (molecular orbital)

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20
Q

Do Larger or smaller alkenes have the bigger wavelength?
What does this mean?

A

Larger alkenes have the bigger wavelength, SO the energy gap (HOMO) is getting smaller

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21
Q

Do increased number of double bonds increase or decrease Lander max

A

more double bonds = Higher lander max

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22
Q

Conjugation of double bonds:
What happens when conjugation increases?

A

The Pi and Pi* orbitals get closer together = longer wavelength

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23
Q

What does conjugation do?

A

It allows free movement of e within a molecule
More conjugation, the smaller the HOMO-LUMO energy gap

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24
Q

What are the 2 vibrational modes?

A
  1. Bending vibrations
  2. Stretching vibrations
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25
Q

Convert 10um into wavenumbers (sm-1)

A

10um = 10 x 10-6
=10 x 10-4
=1.0 x 10-3

so, 10um = (1.0 x 10-3)/ (1.0 x 10-3) = 1000cm-1
1cm -cm-1

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26
Q

What phases can IR spectroscopy samples be in?

A

(g), (aq), (s)

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27
Q

What is the reduced mass equation/

A

μ=m1m2/ m1+m2

μ= Mass of atoms (amu)

*Convert to Kg

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28
Q

What is Hooke’s law?

A

⋁=1/2πc √k/μ

V= frequency of vibrations (cmS⁻¹)
K= bond strength (KgS⁻¹)
μ = mass of atoms (Kg)
c= Speed of light (cmS⁻¹)

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29
Q

What must there be for IR absorption to occur

A

A dipole change during vibration

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30
Q

Dipole changes: What does X-X mean

A

No dipole
No change on stretch
IR inactive

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31
Q

Dipole changes: WHat does X-Y mean?

A

Has dipole
Increases on stretch
IR active

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32
Q

What happens to IR absorption with decreased bond polarity?

A

iR absorption becomes less intense

33
Q

Stronger bonds = Bends
Weaker bonds = Stretches
TRUE OR FALSE?

A

FALSE
Stronger bonds = Stretches
Weaker bonds = Bends

34
Q

What is below 1500cm⁻¹ known as?

A

The fingerprint region
-Allows us to distinguish between 2 molecules with the same functional group

35
Q

What is above 1500cm⁻¹ known as on spectroscopy?

A

The functional group region

36
Q

What peak would we expect from C=O

A

A sharp/strong peak at 1700cm⁻¹
*The stretch frequency can be affected groups bonded to the carbonyl
*A sharp peak means neighbouring group is quite polar

37
Q

What does conjugation with N (amide) do to Carbonyl?

A

*The carbonyl gets weakened
*The conjugating group (N), lowers the IR stretching frequency by ~30-40cm⁻¹ usually
* THe C(=O)N bond creates a dipole where N gets a slightly +ve charge

38
Q

What happens when Carbonyl is bonded to an electronegative species?

A

IR stretching frequency increases
The bond is strengthened

39
Q

What does a C-H peak tend to look like

A

*Can be quite a strong peak despite not being v electronegative
*Their peak depends on what other molecules are bonded to e.g. C-O
*The peaks are the cumulation of all the C-H bonds in the molecule
-Making it look quite strong

40
Q

Why do we need NMR spectroscopy?

A

Important for understanding structure of unknown molecules

41
Q

What are the 2 classes of atomic nuclei?

A

With spin= ODD mass no. and or ODD atomic no.

without spin= everything else

42
Q

What 2 things are needed for a magnetic moment?

A
  1. Object is spinning
  2. Object has a charge
43
Q

What are the 2 possible spins of H1 and C12?

A

+1/2 and -1/2

44
Q

Magnetic moments with NO field are?

A

*Randomly oriented
*Equal in energy

45
Q

Magnetic moments WITH field are?

A

*Aligned with externally applied magnetic field
EITHER - Spin parallel/aligned with +1/2
OR - Spin anti-parallel/ aligned against -1/2

46
Q

What is magnitude of ΔE determined by?

A

*Identity of the element
*Size of the externally applied field

47
Q

How are nuclei ‘flipped’ from ‘Spin aligned’ to ‘Spin opposed’

A

Using radiowave EM radiation equal to ΔE

48
Q

What do you use to represent a nuclear magnetic moment

A

A vector arrow

49
Q

What is the chemical shift equation?

A

δ=Vsample-Vstandard/(Operating Frequency ) x10⁶

V’s relate to wavelength of RF emr absorbed

50
Q

What is the standard compound of chemical shift?

A

Tetramethylsilane (CH3)4Si

51
Q

What are the 4 factors affecting Proton chemical shifts?

A

1.Inductive effects by electronegative groups
2. Magnetic anisotropy
3.H-bonding
4. Magnetic fields of neighbouring protons

52
Q

What causes nucleus ‘shielding’?

A

Circulating electrons generate a local magnetic field (Be), this shields the nucleus from the applied magnetic field (Bo)
*i.e the magnetic field felt by the nucleus is reduced (Beff)

53
Q

What is the calculation for Beff

A

Beff = Bo-Be

54
Q

Order: CHBr₃, CH₃Br, CH₄ and CH₂Br₂ in terms of chemical shift

A

CH₄< CH₃Br < CH₂Br₂ < CHBr₃

  • Because number of e-ve groups increases
    =decreased Be
    =increased Beff
    = increased δ
55
Q

What fields are protons in Aromatics, Alkenes, Alkynes … etc with II bonds under?

A

-They interact with Bo (applied field)
1.Bo
2.Be (shielding field of valence e)
3.Field due to II system

-They all have either a ‘shielding’ or ‘deshielding’ effect

56
Q

Why does H bonding affect H NMR

A

*The δ range is large + hard to predict
*Solvation
*Acidity
*Concentration
*Temperature

57
Q

What are equivalent protons?

A

They’re chemically/ electronically/ Symmetrically similar.
They have the same δ on NMR spectrum

e.g. CH3CHClCH3

The H on each end methyl group are all equivalent

58
Q

In Hnmr What determines the amount of peaks

A

The number of different H groups

59
Q

How do we work out how many H are in each envorinment

A

Integration

60
Q

What is the 3 bond rule?

A

Only non-identical protons on adjacent carbons will induce coupling

61
Q

What is the n+1 rule?

A

*Do the hydrogen(s) of interest have an H’s on adjacent groups

NO- Singlet peak
YES - How many = n

62
Q

Why to we get splitting into single, doublet, triplet… etc.

A

Spin-Spin splittng/coupling

63
Q

What are the exceptions to coupling rules

A

NH and OH protons DON’T couple to adjacent Hs = ‘Exchangeable’ protons
*This can be exploited by adding D2O to sample - OH and NH disapear

64
Q

Why is proton noise decoupling needed?

A

chance of finding C13-C13 is low, but C13-H1 is high
This stops H1 coupling
So, each C is a singlet

65
Q

How do you interpret 13C nmr?

A

No. peaks= no. different C environments
δ is dependent of group C is bonded to
scale usually =1-230ppm
0ppm = δ of (CH3)4Si

66
Q

What doest symmetry do to the number of peaks in 13C nmr

A

Reduced the number of peaks

67
Q

What is distortionless enhancement by polarisation transfer? Why is it useful?

A

It’s a method for determining the presence of 1⁰, 2⁰, 3⁰ and 4⁰ C atoms
It differentiates between CH, CH2, CH3, CH4

Shows CH and CH3 pointing up (or down)
Shows CH2 pointing down (or up)
Shows no peak for 4⁰

68
Q

What is the Half equation of electron impact?

A

M ——> M⁺ + e⁻

M⁺= moleculare ion/ parent ion

69
Q

What does fragmentation of the Molecular ion/Parent ion create

A

A⁺ + B⁺ daughter ions

70
Q

For Cl2 what 2 peaks would you expect to find- ISOTOPIC EFFECTS OF HYDROGEN

A

1 peak 35Cl : 1 peak 37Cl
3 : 1

71
Q

In Experimental analysis (Empiricle/molecular formula) if the elements don’t add up to 100% what do we assume?

A

The rest is made up by Oxygen

72
Q

How to do elemental analysis?

A
  1. Find %s
  2. Divide %s by the Mr of that element
  3. Divide them all by the smallest ratio
73
Q

What is Empirical formula?

A

The simplest ratio of the elements using integers

74
Q

What is molecular formula?

A

The actual ratio of the element using the total Mr and empirical formula

75
Q

What is the equation for double bond equivalence?

A

DBE = (2c +2 -H) / 2

*Measuring the degree of unsaturation in a molecule

e.g. DBE of 2 would suggest 1 double bond or saturated ring
DBE of 3: triple bond/ double bond/ saturated ring anf double

Oxygen has no affect

76
Q

Where would you expect a C=O

A

1745cm⁻¹

77
Q

Where would you expect a C-(triple bond)-N

A

2250cm⁻¹

78
Q

Where would you expect a O-H

A

3340cm⁻¹

Chemistry 2 (7 decks)

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