Spectroscopy

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Spectroscopy

• Organic Chemistry
• Fall 2008

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Anasazi Multinuclear NMR
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Paragon 1000 FT-IR
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Infrared Spectroscopy
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History

• IR radiation discovered by Sir William Herschel
  in 1800
• Coblentz recorded extensive data on the
  interaction of infrared radiation with organic
  and inorganic compounds showing that each
  compound had a unique spectrum
• First double beam instruments (Perkin Elmer and
  Beckman) came in the 1940s
• First FT-IR instruments came out in 1980s from
  perkin Elmer

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Basic Theory

• Molecules with covalent bonds may absorb IR
  radiation
• The absorption is quantized so only certain
  frequencies of IR radiation
• IR radiation will cause molecules to move to
  higher rotational levels and higher vibrational
  levels
• Because each vibrational excited state has
  rotational sublevels the peaks are broad bands
• The molecule must have a change in dipole moment
  during the vibration to be IR active

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Types of Vibrational Transitions

• There are two modes of vibrations
• Stretching
• Involve changes in bond length resulting in
  changes of interatomic distance
• Two modes symmetrical and asymmetrical
• Bending
• Involves changes in bond angle or change in the
  position of a group of atoms
• Scissoring, rocking, wagging, and twisting

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Requirements for Absorption

• The natural frequency of vibration of the
  molecule must equal the frequency of the incident
  radiation
• The frequency must be equal to the ?E between the
  vibrational state
• The vibration must cause a change in µ
• The amount of radiation absorbed is proportional
  to the square of the rate of change in the µ
• The ?E is modified by coupling to rotational
  energy levels

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FT-IR Instruments
centerburst
FFT
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Stretching Vibrations
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Stretching and Bending Modes
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Basic Equation

• The stretching vibrational modes can be modeled
  by using Hookes Law for springs
• Frequency 1 / 2p v / µ
• force constant of the bond
• µ reduced mass
• µ M1M2 / M1 M2

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Typical Spectrum - Polystyrene
Functional Group Region - Stretching
Fingerprint Region
T
Aromatic
Aromatic
Cm-1
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Sampling Techniques

• Broad categories
• Transmittance and Reflectance
• Solids
• Nujol, KBr mull
• Evaporative films
• Diamond anvil cell
• Liquids
• Sealed cell Demountable cell (Beta Cell)
• IR Cards (PE and Teflon films)
• Gas
• Gas cell (path length up to one meter)

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Nuclear Magnetic Resonance Spectroscopy
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History of the Method

• First Commercial Instrument Varian HR-30 in 1952
• Permanent magnet
• Only for hydrogen spectra
• First Fourier Instrument Varian FT-80 in 1978
• Electromagnet (water cooled)
• Used 1,000,000 mainframe computer
• Latest design Varian 900 mHz Superconducting
  FT-NMR (2.5 Million)
• Related technique Magnetic Resonance Imaging
  (the dont spin the sample !!)

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Basic Principles

• Paramagnetic nuclei (odd number of protons or
  neutrons) will behave as magnets when placed in
  magnetic field
• EM radiation in the radiofrequency range will
  cause the nuclear magnets to absorb energy and be
  promoted to higher energy arrangement with their
  magnetic fields aligned against the applied field
• When the excited nuclei fall back to lower energy
  they release a resonant frequency to that
  absorbed

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Nobel Laureates in NMR

• E. Bloch and F. Purcell (1952) for demonstrating
  the NMR effect in 1946
• R. Ernst and W. Anderson (1991) for discovering
  and developing pulsed FT-NMR methods and 2-D
  methods
• Fenn, Tanaka, and Wuthrich (2002) for discovering
  methods for analysis of 3D structure of proteins
  by NMR and MS

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Basic Design of the Instrument
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Paramagnetic Nuclei in Magnetic Field
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Energy Difference is Field Dependent
v ? Bo / 2p ? magnetogyric ratio Bo
magnetic field
Larmor Frequency (v)
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The NMR Spectrum
FFT
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Chemical Shifts

• From the basic equation it would appear that all
  nuclei of hydrogen or carbon would resonate at
  the same combination of Bo and frequency (same
  Larmor frequency)
• But each set of nuclei see a different effective
  field (Beff Bo sBo)
• s is the screening constant or the diamagnetic
  shielding constant
• For most compounds the value of sBo is different
  for each unique nucleus present in the molecule
• Beff and the separation seen are both dependent
  upon the applied magnetic field, thus higher
  fields yield better resolution of closely
  appearing peaks

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Measurement of Chemical Shifts

• The absolute value of the frequency cannot be
  accurately measured (varies by ppm of Hz),
  therefore an internal standard is used (TMS) and
  the relative chemical shift from TMS is measured
  and divided by the field strength of the
  instrument so that it is independent of field
  strength
• Chemical shift ?S ?R / ?NMR

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The Spectrum
Deshielded region
TMS
Shielded region
Higher ?
Lower ?
Chemical Shift (ppm)
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Sample Preparation Techniques

• For carbon samples with the EFT-60 instrument you
  need a neat sample or at least a 40 mg/mL
  deuterated solvent sample (0.75 to 1.0 mL)
• For proton samples with the EFT-60 you can obtain
  a good spectrum with a single scan with a 5
  solution of the solid or liquid sample in a
  deuterated solvent
• Common deuterated solvents chloroform, benzene,
  acetone, acetonitrile)
• Sample MUST be filtered through a glass fiber
  filter to remove any metallic particles and dust

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Pentane, proton spectrum
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Pentane, carbon spectrum
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Pentane, IR spectrum
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Steps to Work Spectral Problems

• Calculate the index of hydrogen deficiency
• Look at the FT-IR and determine the functional
  groups present (CO, O-H)
• Look at the H-NMR to determine the presence of
  aromatic/aliphatic hydrogens
• Count number of carbons from the C-NMR
• Using the chemical formula (if given) draw
  possible structures
• See which structure fits the H-NMR splitting
  pattern

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Different Spectral Data

• UV-Vis a peak indicates multiple double bonds or
  aromatic ring(s)
• FT-IR indicates functional groups
• H-NMR indicates number of unique hydrogens,
  splitting indicates neighboring hydrogens,
  position indicates type of H
• C-NMR indicates number of unique carbons,
  position indicates type of carbon

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Proton NMR Spectra

• Number of signals number of unique kinds of
  hydrogen
• Intensity of signals (area) number of hydrogens
  giving each signal
• Position of signals type of hydrogens
• Splitting of signals number of adjacent
  hydrogens (n1 rule)
• Spacing of splitting coupling constant

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Common IR Peaks

• C-H stretch 2960-2850 cm-1
• C-H unsaturated stretch 3100-3120
• CC 1680-1620
• CC 2260-2100
• CC-H 3350-3300
• Ar-H 3030-3000
• Benzene, CC 1600, 1500
• O-H stretch 3650-3400
• C-O stretch 1150-1050
• CO stretch 1780-1640
• N-H stretch 3500-3300
• C-N stretch 1230, 1030
• CN stretch 2260-2210
• RNO2 1540

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Common H-NMR Peaks

• CH3 0.7 - 1.3 ppm
• CH2 1.2 - 1.4 ppm
• CH 1.4 - 1.7 ppm
• C-H 4.5 - 6.5 ppm
• OC-CH3 2.1 - 2.6 ppm
• Ar-CH3 2.2 - 2.7 ppm
• C-H 2.5 - 3.1 ppm
• C-H 9.5 10 ppm
• C-O-H 1.0 6.0 ppm
• X-C-H 2.0 4.0 ppm

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Triplet
CDCl3
Quartet
Multiplet
1750 cm-1
C6H12O2
1150 cm-1
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Spectroscopy Problems
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IR Overtones 1600-2000 cm-1
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1-Hexene, proton spectrum
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1-Hexene, carbon spectrum
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1-Hexene, FT-IR spectrum
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Limonene, proton spectrum
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Limonene, carbon spectrum
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Limonene, FT-IR spectrum
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1-Bromopentane, proton spectrum
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1-Bromopentane, carbon spectrum
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1-Bromopentane, FT IR spectrum
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1-Pentanol, proton spectrum
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1-Pentanol, carbon spectrum
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1-Pentanol, FT-IR spectrum
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Sec-Butyl methyl ether, proton
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Sec-Butyl methyl ether, carbon
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Unknown, proton spectrum
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Unknown, carbon spectrum
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Unknown, FT-IR spectrum
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sec-Butylamine, proton
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Sec-Butylamine, carbon
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Sec-Butylamine, FT-IR
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2-Butanone, proton
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2-Butanone, carbon
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2-Butanone, FT-IR
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Unknown 2, proton
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Unknown 2, carbon
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Unknown 2, FT-IR
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