Section B: spectroscopy

Question 1

describe the action of light as a wave

Answer 1

• electromagnetic (E) and magnetic (H) fields travel together in space
• they fluctuate in phase, both in time and along direction of travel

Question 2

how can ultraviolet be detected?

Answer 2

by the blackening of Ag salts;

insects can ‘see’ UV light

Question 3

how can infrared be detected?

Answer 3

using a thermometer

Question 4

describe the theory of UV-visible spectroscopy

Answer 4

• 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

Question 5

what are some characteristics of chromophores?

Answer 5

• polyunsaturated systems
• polyaromatic systems
• systems containing C=O, C=S, C-P, etc.
• molecules containing transition metal ions

Question 6

which electron transitions lie in the UV-vis spectra?

Answer 6

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

Question 7

how does the speed of light relate to the wavelength and frequency of light?

Answer 7

c = λ x v

Question 8

how can the energy of a light particle (photon) be calculated?

Answer 8

E = h x v = (h x c) / λ

Question 9

what is the speed of light?

Answer 9

2.998 x 10^8 m/s

Question 10

which wavelengths does chlorophyll absorb at?

Answer 10

400-450 nm (blue-violet) and 600-700 nm (red) to reflect a green-yellow colour

Question 11

what is Hooke’s Law?

Answer 11

F = - kx (restorative force)

where k is the force constant- related to bond strength

Question 12

what units does k, the force constant, have?

Answer 12

N/m or kg/s^2

Question 13

what is the relationship between the magnitude of the force constant and the strength of the bond?

Answer 13

the greater the force constant, the stronger the bond

Question 14

which equation relates the force constant, the reduced mass and the frequency of absorption?

Answer 14

v(0) = 1/2π SQRT(k/μ)

Question 15

explain the theory behind infrared spectroscopy

Answer 15

• 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

Question 16

how is the reduced mass, μ, calculated?

Answer 16

μ= m1m2 / (m1 + m2) (x 1.67 x 10^-27 kg)

Question 17

what is the mass of a proton?

Answer 17

1.67 x 10^-27 kg

Question 18

what is the isotope substitution formula?

Answer 18

v0heavy / v0light = SQRT(μlight / μheavy)

Question 19

describe the lack of absorption spectrum for homonuclear diatomic molecules

Answer 19

• no dipole moment
• μ0 does not chance during the vibration
• no absorption spectrum

Question 20

can there be an absorption spectrum for molecules with no permanent dipole moment?

Answer 20

• 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

Question 21

what is the relationship between the magnitude of the dipole change and the IR absorption intensity?

Answer 21

if the dipole moment change is large, the IR absorption intensity is strong

Question 22

which vibrations are IR-active for CO2?

Answer 22

asymmetric stretching ✓
bending ✓
symmetric stretching x

Question 23

what is FTIR spectroscopy?

Answer 23

Fourier transform IR spectroscopy - simultaneous collection of data over a wide spectral range to obtain an infrared spectrum

Question 24

what is the nature of CH2 and CH3 absorption peaks?

Answer 24

sharp peaks

Question 25

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

Answer 25

'fingerprint' region - used in quantitative/qualitative analysis - applications in forensic science

Question 26

what is unexpected about the absorption spectrum of pure ethanol?

Answer 26

it has free O-H instead of hydrogen bonding

Question 27

explain the nuclear spin

Answer 27

- 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'

Question 28

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

Answer 28

ΔE = γB0h / 2π

Question 29

explain the theory behind NMR

Answer 29

- 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

Question 30

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

Answer 30

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

Question 31

what is Beff?

Answer 31

the magnetic field felt by the nucleus | Beff = B0 - σB0 = B0(1-σ)

Question 32

what is the frequency taking into account shielding?

Answer 32

v = γB0(1-σ) / 2π

Question 33

explain the concept of shielding

Answer 33

- 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

Question 34

how can shielding be used to identify different nuclei?

Answer 34

- 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

Question 35

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

Answer 35

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

Question 36

what does it mean if two species are magnetically equivalent?

Answer 36

they have the same chemical shift value

Question 37

explain how the chemical shift can be negative

Answer 37

- 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

Question 38

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

Answer 38

TMS = tetramethylsilane (CH3)4Si δ=0 ppm - unreactive liquid - mixes with common solvents - single NMR peak - peak occurs lower than that for most protons

Question 39

how is the chemical shift calculated?

Answer 39

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

Question 40

what is the reason for splitting patterns in NMR?

Answer 40

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

Question 41

what can NMR be used for?

Answer 41

- 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