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Light and spectroscopy
Light interacting with molecules gives rise to spectroscopy
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- an electric (E) and magnetic (H) field oscillate constantly, up and down, at right angles
to each other
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- the light is travelling at a certain speed which is a constant in a vacuum, equal to 3E8
m/s
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C
- the ratio between the speed in a vacuum and the speed in a
medium, is called the refractive index of that material ’n’, which
indicates how much the light has been slowed down
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- cones contain different types of photorhodopsin
molecules, each one sensitive to a different wavelength,
and convert this to an electrical signal

- the prism can separate white light into its different wavelengths as it has a refractive
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index and different wavelengths travel at different speeds
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Invisible light (UV & IR):
- UV-visible spectroscopy interacts with electrons
- IR spectroscopy interacts with vibrations of molecules
- Radio waves - NMR spectroscopy interacts with the magnetic part of light
Light as a particle:
- a unit of light is a photon
- its properties are mass, velocity and momentum
- particle wave duality explains how something can go from particle properties to wave
properties
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This is expressed as λ = h / mv
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626E-34 J s given)
the energy (E) associated with a light particle is…
High energy = high frequency & short wavelength
E =hν or = hc/λ
Low energy = low frequency & long wavelength

Electronic spectroscopy :
- electrons can change their orbit by jumping from
one energy level to another by absorbing or emitting radiation/light
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- light absorbing species are called chromophores
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- the position and intensity of the band give info about the bonding and atoms involved
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- transitions in unsaturated organic compounds can occur in the visible range but other
transitions occur in the UV range, invisible to the human eye, so we must look at
ranges between UV AND visible light
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, and transition
metal ion containing molecules
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for single compounds it indicates the kind of electronic transition involved

Electronic structure
- electrons orbit around the nucleus and their energy is quantised i
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related to the
radius of the orbit by giving it a principal quantum number ’n’
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- the jump is related to frequency and wavelength of the light absorbed by ΔE = h ν =
hc / λ
- if there is light with a frequency to correspond to the energy change between the
inner orbit and outer orbit, that light will be absorbed
- three p orbitals for each principal quantum number : px, py, pz
- molecular orbitals form by combining atomic orbitals together

- the angular momentum dictates the orbital shape
- sp3 orbitals = s+px+py+pz (tetrahedral shape)
-sp2 = s+px+py pz is
unhybridised and
forms π bonds by
overlapping sideways
(trigonal planar)

- sp = s + px while pz and py form π bonds, which gives a triple bond
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- λmax = 163 nm in ethene

- orbitals can interact along the polyunsaturated chain
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30nm per additional
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C=C bond
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the transition will move to smaller energy

Benzene
- sp2 hybridised planar molecule
- λmax = 203 nm (between this and 295 if there are substituents)

Chlorophyll - polycyclic aromatics
- absorbs blue-violet at 400-450 nm and red at 600-700 nm and so appears yellow/green
- this is because there are 2 different π to π* transitions in the structure
Beer-Lambert Law
- once absorbance is measured, we
calculate concentration in moles dm-3

Absorbance A = (log10 Io/It)
= ε
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L

Infrared Spectroscopy
- atoms vibrate around their equilibrium in patterns called vibrational modes
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- there is a restoring force F = -k x (x is the
F = -k x
change in r and k (N/m) is the force constant
related to bond strength)
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7E-27
or νo
- to calculate νo (s-1) in wave numbers (cm-1) divide νo by speed
of light (c) 3E10
-K can be calculated from IR
frequencies to deduce bond
νo (heavy) =
μ (light)
strength
νo (light) √ μ (heavy)
-we can use different values to compare
different bonds
-different isotopes give different reduced

(cm-1) = 130

k=μ

νo
130

k
√μ

2

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masses and vibrational
frequency changes but k stays
the same
the isotope substitution
formula is used to identify
which vibrations involve a
high frequency
particular atom based on a
frequency shift

1/2 frequency of symmetric

highest frequency

H2O
- different vibrational modes with a characteristic frequency
Transmission to Absorbance:

- absorbance values used to apply Beer-Lambert
law

Absorbance = -log10 T%
100

How IR works
- when atoms vibrate, they change the electric
dipole momentum (way the charge is distributed across a molecule), causing
fluctuation in the electric field that interacts with the infrared light
- homonuclear diatomic molecules have no dipole moment
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- if dipole moment change is large, IR absorption is strong
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- % transmission is measured
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We can identify many IR peaks as due to specific vibrations of different functional
groups
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Saturated, aliphatic
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-CH3 symmetrical stretch

- unsaturated e
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alkenes, aromatics,
alkynes are above 3000 cm-1
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- hydrogen bonding lowers the O-H
stretch frequency and broadens the
absorption band
N-H stretching is sharper and lower than O-H
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g NMR
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The position determines the bond strength
1670-1570 cm-1 coupling indicates a peptide link
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g
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C-O-H bond stretch occurs where C=O usually is in ethanol
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spinning around the nuclear axis gives a magnetic field
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against has the highest energy and is unstable
with has the lowest energy and is stable

Larmor Frequency
- the nuclear spin axis is tilted towards the external field direction and it precesses
around it
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- it is determined by γ, the gyromagnetic ratio
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increasing the strength of the magnetic field will increase the Larmor frequency
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- total nuclear spin quantum number (I) is determined by the number and proportion of
protons and neutrons
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- when there is no external field, all spin directions have the same energy
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the difference between the two energies is hνL in an external magnetic field
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- the sample is placed inside an NMR tube
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- a radio transmitter sends the electromagnetic signal to the sample to cause the spins to
flip when the radiation matches the energy difference between the spin up and spin
down, and the radiation is therefore absorbed, giving a peak
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it is tuned to the specific Larmor frequency
that is being observed
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Shielding and Chemical Shift
- nuclei in molecules are surrounded by electron clouds which cause a magnetic field
that opposes the external field
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This is shielding
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effective field felt by the nucleus is Beff

Beff = Bo (1-σ)

- NMR frequency is therefore lowered:
- effect of shielding causes a chemical shift and nuclei in
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ν = γBeff = γBo (1-σ)
2π
2π

different bonding environments experience different
effective magnetic fields and have different NMR frequencies
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g
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Relative Substituent Effects
- CH3 has a greater tendency to push electron density in
than H
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Standards
- used to compare 2 sets of data
- we measure chemical shift values relative to the NMR standard with a reference NMR
frequency νref
- the chemical shifts are then recorded as δ (ppm)
δ = νsample - νref x 106
- the standard is unreactive, liquid which mixes with
νref
common solvents, has a single NMR peak at lower
frequency than most protons due to high electron density
from the highly donating Si atom, giving greater shielding
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- splitting is due to magnetic interactions between protons at different sites in the
molecule
- multiplets are due to magnetic coupling of spins of protons that are adjacent to each

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other and in magnetically non-equivalent environments
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Triplet

Quartet

- if there are n equivalent neighbours, the main NMR peak will be split into (n+1) components to
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form a multiplet

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