Spectroscopy Overview

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HL Chemistry - Option A Modern Analytical
Chemistry
Principles of Spectroscopy
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Definition

• Spectroscopy - The study of the interaction of
  electromagnetic radiation with matter

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Introduction

• Spectroscopy is an analytical technique which
  helps determine structure.
• It destroys little or no sample.
• The amount of radiation absorbed by the sample is
  measured as wavelength is varied.

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Major Types of Spectroscopy

• Infrared (IR) spectroscopy measures the bond
  vibration frequencies in a molecule and is used
  to determine the functional group.
• Mass spectrometry (MS) fragments the molecule and
  measures the masses.
• Nuclear magnetic resonance (NMR) spectroscopy
  detects signals from hydrogen atoms and can be
  used to distinguish isomers.
• Ultraviolet (UV) spectroscopy uses electron
  transitions to determine bonding patterns.

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Electromagnetic Spectrum
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Electromagnetic Spectrum
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Properties ofElectromagnetic Radiation

• Electromagnetic Radiation
• energy radiated in the form of a WAVE caused by
  an electric field interacting with a magnetic
  field
• result of the acceleration of a charged particle
• does not require a material medium and can travel
  through a vacuum

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Transmission of Radiation

• Transmission
• rate at which radiation passes through a
  transparent material is less than through a
  vacuum
• depends upon the kinds and concentrations of
  atoms, ions, and molecules in the medium
• radiation must interact with material
• interaction must not undergo permanent energy
  transfer

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Reflection of Radiation

• Reflection always occurs when radiation passes
  from one medium to another
• Reflection is greatest when two materials have
  large differences in their refractive indecies

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Scattering of Radiation

• Types of Scattering
• Scattering by Large Molecules
• can be measured
• a function of the size and shape of molecule
• Raman Scattering
• part of the radiation undergoes quantized
  frequency changes
• scattering involving molecules which are
  considerably smaller than the wavelength of
  radiation
• blue sky results from greater scattering of
  shorter wavelength visible light

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Summary of the Type of EM Interactions

• Absorption - EM energy transferred to absorbing
  molecule (transition from low energy to high
  energy state)
• Emission - EM energy transferred from emitting
  molecule to space (transition from high energy to
  low energy state)
• Scattering - redirection of light with no energy
  transfer

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Atomic Absorption

• Absorption occurs with only a few well-defined
  frequencies
• Electronic excitation
• Several branches of spectroscopy are based on
  this concept, a few of which are discussed here

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Absorption vs. Emission
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En
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Eo
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Absorption
Emission
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Absorption
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Absorption of Radiation

• The energy of an exciting photon must equal
  the energy difference between the ground state
  and one excited state for absorption to occur
  Quantum Leap

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The Spectrum and Molecular Effects
gt
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Fluorescence (Emission)
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Emission of Radiation

• Radiation results from the relaxation of
  electrons from higher (excited) states to lower
  energy states
• Emission Spectroscopy and Fluorescence
  Spectroscopy are based on this concept

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Equation Definitions

• E energy (Joules, ergs)
• c speed of light (constant)
• l wavelength
• h Plancks constant
• n nu frequency (Hz)
• nm 10-9 m
• Å angstrom 10-10 m

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Key Formulas

• E hn
• h 6.626 x 10-34 J-s
• n frequency in Hz, E energy
• l c/n
• c 3.0 x 108 m/s
• l wavelength, n frequency in Hz

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Visible Light
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Visible Light
Red Orange Yellow Green Blue Indigo Violet
R O Y G B I V
700 nm 650 nm 600 nm 550 nm 500 nm 450 nm 400 nm
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Complementary Colors
Absorbed Observed
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Major Types of Light Spectroscopy

• Absorption spectroscopy
• Measures amount of light absorbed
• Most common, non-destructive
• Concentration, pH measures, purity, ID
• Atomic emission spectroscopy
• Measures light emitted from burned sample
• Elemental analysis

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Types of Light Spectroscopy(continued)

• Fluorescence spectroscopy
• Samples fluoresce when they emit at higher l than
  what they absorb
• Measures solvent interactions, distances,
  molecular shape, and motion
• Circular Dichroism spectroscopy
• Absorption of circular polarized light
• Chiral compound identification
• Transmission spect. (colorimetry)

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UV vs. IR vs. NMR

• UV has broad peaks relative to IR NMR
• UV has less information than IR NMR
• UV spectra are easier to collect
• UV spectra are faster to collect
• UV spectrometers are cheaper
• UV spectra require only nanograms of material or
  chemicals

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Single Beam Spectrophotometer
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Dual Beam Spectrophotometer
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Cuvettes (sample holder)

• Polystyrene
• 340-800 nm
• Methacrylate
• 280-800 nm
• Glass
• 350-1000 nm
• Suprasil Quartz
• 160-2500 nm

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Definitions

• Io intensity of light through blank
• IT intensity of light through sample
• Absorption Io - IT
• Transmittance IT/Io
• Absorbance log(Io/IT)

Io
IT
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Absorbance Beers Law
Increasing absorbance
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Beers Law
Io
IT
Io
IT
pathlength b
pathlength b
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Beers Law
Absorbance ebC
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Absorption Methods

• Readout for an inexpensive photometer.

transmittance scale is linear absorbance scale
is exponential.. thus, one usually reads
transmittance, then calculates absorbance
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UV Activity

• Needs chromophores
• CC, CO, NN, NO2,
• p -gt p and n -gt p transitions
• napthoquinones, anthocyanins
• Non-absorbers
• long chain aliphatics
• alcohols, ethers, non-conjugated mols.

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UV Activity
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p
p
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EM Radiation
E
M
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Circular Polarization
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Circular Dichroism
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CD can distinguish chirality
L-isomer D-isomer
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pKa Measurement with UV
n
i
Titration of Phenylephrine
Ai - A
pKa pH log
A - An