CHEM 400 Organic Spectral Analysis

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CHEM 400 Organic Spectral Analysis
Fall 2009
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Dispersive IR Spectrometers
• All spectrometers consist of four basic parts
  that are coupled with all four parts of the
  spectroscopic process - irradiation,
  absorption-excitation, re-emission-relaxation and
  detection.

excited state
Absorption-Excitation Spectrometer needs to
contain the sample
Detection-reemission Spectrometer needs to
detect the photons emitted by the sample and
ascertain their energy
hn
Relaxation
Energy
rest state
rest state
Irradiation Spectrometer needs to generate
photons
hn
hn
hn
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Dispersive IR Spectrometers
• Those four parts are
• Source/Monochromator
• Sample cell
• Detector/Amplifier
• Output
• Dispersive IR spectrometers were the first IR
  instruments, however their simplicity and
  longevity allows them to continue in service
  for most routine organic analyses their speed and
  resolution is adequate
• For the most part, their design is austere and
  relies on simple mechanics and optics to generate
  a spectrum, very similar to simply rotating a
  glass prism to see different bands of visible
  light

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Dispersive IR Spectrometers
• Here is a general schematic

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Dispersive IR Spectrometers
• Source is a heated nichrome wire which produces a
  broad band continuum of IR light (as heat)
• The beam is directed through both the sample and
  a reference cell
• A rapidly rotating sector (beam chopper)
  continuously switches between directing the two
  beams to a diffraction grating

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Dispersive IR Spectrometers
• The diffraction grating slowly rotates, such that
  only one narrow frequency band of IR light is at
  the proper angle to reach the detector
• A simple circuit compares the light from the
  sample and reference and sends the difference to
  a chart recorder

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Dispersive IR Spectrometers
• On the older instruments the motor in the chart
  recorder was synchronized ( calibrated) to the
  motor on the diffraction grating
• Because each spectrum is the result of the
  tabulation of the spectroscopic process at each
  frequency individually, it is said to record the
  spectrum in the frequency domain

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Dispersive IR Spectrometers
• Advantages simple, easy to maintain last the
  life of the source and moving parts
• Disadvantages to cover the entire IR band of
  interest to chemists it is necessary to use two
  diffraction gratings
• At high q, the component frequencies are more
  spread out, so the resulting spectra appear to
  have various regions expanded or compressed
• The limit to resolution is 2-4 cm-1

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Fourier Transform IR Spectrometers
• FT-IR is the modern state of the art for IR
  spectroscopy
• The system is based on the Michelson
  interferometer
• Laser source IR light is separated by a beam
  splitter, one component going to a fixed mirror,
  the other to a moving one and are reflected back
  to the beam splitter
• The beam splitter recombines the two to a pattern
  of constructive and destructive interferences
  known as an interferogram a complex signal, but
  contains all of the frequencies that make up the
  IR spectrum

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Fourier Transform IR Spectrometers
• The resulting signal is essentially a plot of
  intensity vs. time
• Such information if plotted would look like the
  following
• This is meaningless to a chemist we need this
  to be in the frequency domain rather than time.

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Fourier Transform IR Spectrometers
• By applying a mathematical transform on the
  signal a Fourier transform the resulting
  frequency domain spectrum can be observed
• FT-IRs give three theoretical advantages
• Fellgetts advantage every point in the
  interferogram is information all wavelenghts
  are represented
• Jacquinots advantage the entire energy of the
  source is used increasing signal-to-noise
• Connes advantage frequency precision
  Dispersive instruments can have errors in the
  ability to move slits and gratings reproducibly
  FTIR is internally referenced from its own beam

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Dispersive and Fourier
  Transform
• Fourier Transform IR Spectrometers
• Justiks advantage does it give me what I need
• Single-beam instrument collect a background
  (air has IR active molecules!)
• Fast all frequencies are scanned simultaneously
• No referencing!
• Computer based scaling and editing of the
  spectrum to squeeze out the most data spectra
  are proportional (no stretching or squeezing of
  regions), comparison with spectral libraries
• Disadvantages expenisve relative to dispersive
  instruments, and the components take more
  expertise and service calls to replace

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Sample size typically the size of the beam
  mms ? mgs
• Non-destructive sample can be recovered with
  varying degrees of difficulty
• Liquid samples the easiest IR spectra are those
  of neat liquid samples
• Solid samples are too dense for good IR spectra
  inter-molecular coupling of vibrational states
  occurs and peaks are greatly broadened
• In the liquid state full 3-D motion is available,
  and these effects are averaged out and diminished
• The thickness of a sample can be decreased to
  reduce these effects further
• ? Thin film liquid samples are best!

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Liquid samples
• Sample cell cannot possess covalent bonds (SiO2,
  or glass is out)
• The most common cell is a pair of large
  transparent windows of inorganic salts
• Most common
• NaCl cheap, transparent from 650 4000 cm-1,
  but fragile
• Less common AgCl, KBr, etc. if you need
  transparency below 650, limit is practically 400

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Solution samples
• One way solids can be handled is as a solution
• Key is that the solvent picked will cover the
  least amount of the spectrum as possible, as it
  will also be present
• Common solvents typically are symmetrical, or
  have many halogenated bonds low cm-1 CCl4,
  CHCl3, CH2Cl2, etc.
• The cell in this case is two NaCl (or other)
  windows with a spacer, the sample is loaded via a
  syringe into the cell

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Solution samples
• A newer method involves the use of a polyethylene
  matrix, that will hold allow a solution sample to
  evaporate, leaving small portions of the sample
  embedded in the matrix
• The samples are liquid-like
• The only interference is that of hydrocarbon

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Solid Samples
• The most common treatment for solid samples is to
  mull them with thick mineral oil (high MW
  hydrocarbon) - Nujol
• Just like with the polyethylene cards, the
  molecules of the sample are held in suspension
  within the oil matrix
• Again, the interference is that of hydrocarbon

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Solid Samples
• The connoisseurs method (with no organic
  interference) is to press the solid with KBr into
  a pellet
• Under high pressure the KBr liquefies and entraps
  individual molecules of the sample in the matrix
• These spectra are the only spectra of solids that
  are as interference free as liquids

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• FT-ATR Attenuated Total Reflection
• Solves problem of achieving high resolution of
  solid samples with little or no sample
  preparation
• Sample wide variety of types
• Solids of limited solubility
• Films
• Threads
• Pastes
• Adhesives
• Powders

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• FT-ATR Attenuated Total Reflection
• Measures the changes that occur in a totally
  internally reflected IR beam when it comes into
  contact with a sample
• IR beam is directed onto an optically dense
  crystal with a high refractive index at a certain
  angle
• Internal reflectance creates an evanescent wave
• Wave protrudes only a few microns (0.5 µ - 5 µ lt
  l) beyond the crystal surface and into the sample
• Evanescent wave acts as a bubble of infrared
  that sits on the surface of the crystal.

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• FT-ATR Attenuated Total Reflection
• Must be good contact between the sample and the
  crystal surface
• Pressure is applied (20 40 psi)
• Surfaces that are deformable give best spectra
• Special anvils allow the pressure to be spread
  over the surface of a powder and allow good
  spectra to be obtained
• In regions of the infrared spectrum where the
  sample absorbs energy, the evanescent wave will
  be attenuated or altered
• The attenuated energy from each evanescent wave
  is passed back to the IR beam, which then exits
  the opposite end of the crystal and is passed to
  the detector in the IR spectrometer.

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• FT-ATR Attenuated Total Reflection
• Need a high refractive index ATR crystal (2.38
  and 4.01 at 2000 cm-1)
• Best crystals high resolution instruments
  POOR durability
• Thallium bromide
• Thallium iodide
• Germanium and ZeSe plate
• Poorest crystal low resolution instruments
  HIGH durability
• Diamond (single bounce fine for organic!)
• ATR crystal can be dipped into the liquid or oils

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• FT-ATR Attenuated Total Reflection
• Applications
• Materials science one can observe layers of a
  polymer surface, surface oxidation and aging,
  etc.
• Routine organic spectra rapid sample
  preparation lt 5 minutes for dry samples
• Biology study of surfaces

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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following four IR spectra of p-cresol

Neat Sample
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following four IR spectra of p-cresol

KBr Pellet
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following four IR spectra of p-cresol

CCl4 Solution
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following four IR spectra of p-cresol

FT-ATR
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following three IR spectra of
  m-nitroanisole

Nujol Mull
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following three IR spectra of
  m-nitroanisole

KBr Pellet
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following three IR spectra of
  m-nitroanisole

CCl4 Solution
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following three IR spectra of
  p-nitrophenol

Nujol Mull
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following three IR spectra of
  p-nitrophenol

KBr Disc
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• IR Spectroscopy
• Instrumentation and Experimental Aspects
• The IR Spectrometer Experimental aspects
• Differences in Spectral Appearance
• Compare the following three IR spectra of
  p-nitrophenol

FT-ATR