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Chapter 12

IR and NMR - Lecture and Textbook Notes Chapter 12 and 13

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Cecilia Kutas

CHM247H Jasmyn Lee Chapter 12: Structure Determination: Mass Spectrometry and Infrared Spectroscopy IR Spectroscopy Infrared Spectroscopy  Irradiation of organic molecules with light of wavelength (λ) 2.5 μm to 25 μm causes bond vibtations to occur  This is within the infrared portion of the electromagnetic spectrum  This region is commonly expressed as a wavenumber range (1/λ) Wave Numbers  2.5μm to 25μm represents 2.5 x 10 to 2.5 x 10 cm  Wavenumbers often quoted as reciprocal of wavelength in cm -1  Wavenumber ranger in which infrared energy absorption is observed is 400 to 4000 cm Origin of Bond Vibrations  Covalent bonds continuously vibrate – like two atoms joined by a spring  Energy of these vibrations is quantized – only specific vibrational energy levels allowed in a molecule  Energy of a particular vibration given by o E = hν o ν = bond vibrational frequency  When frequency of applied infrared radiation matches that of bond vibrational frequency, an absorption is observed  This is noted by a “dip” in infrared spectrum  This makes “spring” expand and contract more than normal  Bond vibrations made up of stretching and bending motions  Different bonds vibrate (stretch and bend) at different frequencies  o c = speed of light o N = Avogadro’s Number o μ = reduced mass (m1m2/m 1 m )2 m 1nd m e2ual masses of each atom in the bond o K = force constant (measure of bond strength)  Equation shows position of absorption for a bond vibration depends on o Bond strength o Masses of atoms connected by bond  The stronger the bond and the lighter the atoms connected by the bond, the higher the wavenumber of absorption Infrared Absorption Chart  T = transmittance of incident light, i.e. 100% transmittance means no light absorbed by compound Information Obtained  Possible to determine what type of bonds present in a molecule from its infrared spectrum 1 CHM247H Jasmyn Lee  This leads to the identification of functional groups  The type of functional groups (eg/ C=C, C≡C, C=O) present in a molecule can be deduced Regions Of The Spectrum  The infrared spectrum can broadly be divided into four regions – 1. Fingerprint Region – unique for each organic compound -1  400 to 1500 cm 2. Double Bond Region – where C=C (alkenes), C=O (carbonyls) and C-N (imines) exhibit stretching vibrations -1 1500 to 2000 cm 3. Triple Bond Region – where C≡C (alkynes) and C≡N (nitriles) exhibit stretching vibrations  2000 to 2500 cm -1 4. Single Bond to Hydrogen Region – where C-H, O-H (alcohols and carboxylic acids) and N-H (amines) exhibit stretching vibrations  2500 to 4000 cm -1 Some Functional Groups -1  Alkanes - C-H stretch seen at 2900 cm  Alkenes- C=C stretch seen at 1640 – 1680 cm -1  Alkynes - C≡C stretch seen at 2100 cm -1 -1  Alcohols - O-H stretch seen at 3300 – 3600 cm (amines in similar region)  Carbonyl - C=O stretch seen at 1700 cm -1 Identification of Organic Compounds – NMR and IR Spectroscopy Electromagnetic Radiation  As a particle, one unit is a photon o A photon contains a discrete amount of energy, called a quantum  As a wave – characteristic length (λ) and frequency (ν) reported in cycles/sec (s ), also called hertz (Hz) 2 CHM247H Jasmyn Lee  E = hν = hc (1/λ) 8 o C = 3.0 x 10 m/s o H = Planck’s constant, 1.58 x 104cal.s -10 X ray: 10 m UV: 10 m Vis: 10 m E = h IR: 10 m (IR spectroscopy) Microwave: 10 m -2 Radiowave: 10 m (NMR) Molecular Vibrations  Stretching energies > bending energies o Roughly 10 vibrations/sec -1  ν1(A1) Symmetric Stretch – 908 cm  ν2(E) Symmetric Bend – 434 cm -1  ν (F ) Asymmetric Stretch – 1283 cm -1 3 2  ν4(F2) Asymmetric Bend – 631 cm -1  Different functional groups vibrate at different energies, allowing identification of functional groups in a molecule Four Distinct Regions of An IR Spectrum 3 CHM247H Jasmyn Lee Carbonyl Groups O O R H R R Aldehyde Ketone O O O R OH R OR R NHR Acid Ester Amide 4 CHM247H Jasmyn Lee Examples and Practice Problems In Slides 5 CHM247H Jasmyn Lee Chapter 13: Structure Determination: Nuclear Magnetic Resonance Spectroscopy 1H NMR Spectroscopy 13.1 Nuclear Magnetic Resonance Spectroscopy 1 13  The proton, H, and the C nucleus have spins o Positively charged nuclei o Spin on an axis; act like a magnet o Interact with external magnetic field 1 H NMR + +  Spinning protons generate tiny magnetic field a) Randomly oriented nuclear magnetic fields – absence of magnetic field; spins are oriented randomly b) Oriented nuclear magnetic fields with external field – place nuclei between poles of a strong magnet  A spinning H or C nucleus can orient so that its own tiny magnetic field is aligned parallel or antiparallel to the external field – not same energy or likelihood  Parallel slightly lower in energy than Antiparallel  favoured  Nuclei are irradiated with electromagnetic radiation of proper frequency – energy absorption occurs o Lower energy state “spin flips” to the higher energy state  magnetic nuclie are said to be in resonance with the applied resonance  Frequency necessary for resonance depends on 1. The strength of the external magnetic field  If strong magnetic field is applied – energy difference between the two spin states is larger and higher-frequency (higher energy) radiation is required for a spin flip 2. On the identity of the nuclei  Applied external magnetic field ed Radiofrequency absorb 13.2 The Nature of NMR Absorptions  Provides a map of the C-H framework of the molecule  External magnetic field applied to a molecule causes the electrons surrounding the nuclei to set up tiny local magnetic fields of their own which act in opposition to the applied field – the effective field actually felt by the nucleus is weaker than the applied field 6 CHM247H Jasmyn Lee o Nuclei are shielded from the full effect of the applied field by the surrounding electrons o Each chemically distinct nucleus in a molecule is in a slightly different electronic environment, each nucleus is shielded to a slightly different extent – the effective magnetic field felt by each is different  Differences can be detected - distinct NMR signal for each distinct C or H nucleus in a molecule  Overview of process: o Dissolve compound in suitable solvent (usually CDC3 ) and place in thin glass tube o Put compound in a magnetic field o Irradiate with radio waves to bring into resonance o Get information about Hydrogen’s in the molecule  Magnetic Resonance Imaging (MRI) – uses the principles of nuclear magnetic resonance to image tissue o MRI normally uses the magnetic resonance of protons on water and very sophisticated computer methods to obtain images o Other nuclei within the tissue can also be used ( P) or an imaging (contrast) agent can be administered  Horizontal Axis – effective field strength felt by the nuclei  Vertical Axis – intensity of absorption of rf energy  Difference between NMR and IR o Timescale – NMR is much slower o IR takes an “instant” picture and “freezes the image” o NMR takes a slow “time averaged picture” 13.3 Chemical Shift  Nuclei that absorb on the downfield side of the chart require lower field strength for resonance – they have less shielding o Downfield = more shielding o Upfield = less shielding  TMS – tetramethylsilane, (C3 4 Si – reference absorption peak; is added to sample for reference point  Chemical Shift – the position on the chart at which a nucleus absorbs o Chemical shift of TMS = 0 δ  1 δ = 1ppm of the spectrometer operating frequency o Eg/ Instrument operating at 200 Hz, 1δ = 1 millionth of 200 000 000 Hz (200 Hz) o Can use formula: o Allows measurement of spectra using different machines – the chemical shift of an NMR absorption in δ units is constant, regardless of the operating frequency of the spectrometer
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