Chemistry 332 - Winter 1997

Chapter 13 - Structure Determination

Determination of Chemical Structure

• chemical methods
  based on known reactivity patterns
• qualitative tests
  e.g., double bonds decolorize Br2
• spectroscopic methods
  based on interactions with light

Properties of Light

• as electromagnetic waves:
  c = l v
  c = speed of light (3 x 108 m/sec)
  l = wavelength
  v = frequency (per sec)
  
  
• as particles (photons):
  E = h v
  h = Planck's constant

The Electromagnetic Spectrum
(see Figure 13.1, page 379, in McMurry)

• x-rays ~ 0.1 - 10 nm
• ultraviolet ~ 10 - 400 nm
• visible ~400 - 800 nm
• infrared ~ 1 - 100 µm
• microwaves ~ 0.01 - 10 cm
• radiowaves ~ 0.1 - 100 m
  (0.1 - 100 MHz - compare AM/FM frequencies)

Absorption Spectroscopy

• spectrum: plot of absorbance as a function of wavelength (or frequency)
• y-axis: absorbance or transmittance
• x-axis: wavelength range
• peaks of absorbance (or valleys of transmission)
  represent specific molecular interactions with the light

Molecular Energy Levels

• energy levels are quantized
• absorption occurs when the energy of the light (per photon) exactly matches the energy difference between an occupied and an unoccupied energy level
• E = E2 - E1 = h v
• absorption spectroscopy reveals the spacings between energy levels in a molecule

Types of Molecular Energy

• nuclear > 500 kcal/mol - gamma rays
• electronic ~ 100 kcal/mol - ultraviolet/visible
• vibrational ~ 1 kcal/mol - infrared
• rotational ~ 1 cal/mol - microwaves
• magnetic* ~ 0.1 cal/mol - radiowaves
  * induced by a strong external magnetic field
  NMR for nuclear magnetic effects
  EPR for electron magnetic effects

Infrared Spectroscopy

• probes different molecular vibrations
• absorption occurs when the frequency of the IR wave
  matches a vibrational frequency of the molecule
• molecules typically have many possible vibrations
• bond stretching vibrations are most characteristic
  (detect different kinds of A-B bonds)
• bond bending involves several atoms at once
  more complex vibrations (rock, scissor, wag, twist)
• one exception: a completely symmetrical bond vibration does not absorb IR
  e.g., the C=C double bond stretch in ethylene is IR-silent

IR Spectra

• x-axis:
  wavelength range typically 2 - 15 µm
  wavenumber also used (second x-axis)
  wavenumber (v-bar) = 1/l in cm-1
• y-axis:
  usually transmittance (0 - 100%)
  (valleys correspond to absorptions)
  "peak" position noted by wavenumber

Regions of an IR Spectrum

• Bonds to H (4000 - 2500 cm-1)
• Triple Bonds (2500 - 2000 cm-1)
• Double Bonds (2000 - 1500 cm-1)
• "Fingerprint" (1500 - 600 cm-1)
• Table 13.1 will be provided for quizzes and exams

Interpreting an Unknown IR Spectrum

• work from left to right (decreasing wavenumber)
• look for characteristic bond stretches
• try to confirm functional groups with additional absorptions
• if possible, compare to a known spectrum

IR Examples


Ultraviolet/Visible Spectroscopy

• x-axis:
  typically 200 - 800 nm
• y-axis:
  typically absorbance in % or log units
  (peaks correspond to absorptions)
  peak positions noted by lmax

Electronic Absorption Spectra

• uv/vis absorptions correspond to electron transitions
  from one molecular orbital to another
• generally pi bonds are clearly seen in the uv region
• absorptions are called pi->pi* transitions
  (from bonding to antibonding orbitals)
  
  
• one pi bond (ethylene) 171 nm (lmax)
• conjugated dienes and polyenes absorb at longer wavelengths
  (lower energies, or more closely spaced orbitals)
  • 1,3-butadiene 217 nm
  • benzene 254 nm
  • beta-carotene 483 nm (absorbs violet - looks yellow)
• Table 13.2 will be provided for quizzes and exams

Photochemistry

• what do molecules do once an electron is promoted to a higher energy orbital?
• many possibilities:
  • emit light (fluorescence)
  • lose energy as heat
  • rearrange
  • decompose
  • transfer an electron to (or from) another molecule

NMR Spectra

• the most powerful and versatile technique for structure determination
• based on nuclear spin of certain atoms:
  H-1, C-13, N-15, F-19, P-31, etc.
• an external magnetic field is applied,
  the nuclear spins either align with or against the applied field,
  which creates two distinct energy levels
• absorption occurs when the energy of the radiowave is the same as the difference between the two magnetic energy levels (called the resonance condition)

NMR Instrumentation

• larger magnetic fields require higher frequency radiowaves
  (30, 60, 90, 200, 400, 500 MHz instruments are typical)
• for 21,000 Gauss,
  H-1 nuclei resonate at about 90 MHz
  C-13 nuclei resonate at about 22.6 MHz
• Note - although C-13 is only about 1.1% of all carbon in nature, this is sufficient for most samples to show good spectra

MRI - Magnetic Resonance Imaging

• special technique for spatial visualization of H nuclei in a whole body
• distinct forms of H2O and organic H are visualized

Interpreting Proton (H-1) NMR Spectra

• 1) number of different absorptions
  * how many different kinds of H are present
• 2) location of the absorptions
  * environments of the H's
• 3) intensities of the absorptions
  * how many of each type of H
• 4) splitting patterns
  * interactions with neighboring H's

Equivalent Hydrogens

• equivalent hydrogens absorb together
• e.g., only one absorption for methane, ethane, ethylene, benzene
• but two for propane, butane, isobutane

Chemical Shifts

• different environments for individual hydrogens
  cause them to absorb at slightly different resonance positions
• shielded/upfield:
  higher electron density requires a stronger field for resonance
• deshielded/downfield:
  lower electron density requires a weaker field for resonance
  (using a constant radiofrequency)

TMS as a Reference

• the protons (and carbons) of (CH3)4Si are highly shielded
• TMS is set as the reference (zero on the chemical shift delta scale)
• delta (ppm units) = shift from TMS in Hz / radiofrequency in MHz
  
• e.g., Cl-CH2-O-CH3 shows 2 absorptions for H NMR and two for C NMR
• on a 90 MHz NMR, H resonances are at 90,000,300 Hz and at 90,000,480 Hz
  (the instrument is tuned so that TMS absorbs at exactly 90,000,000 Hz)
  • delta = 300 Hz / 90 MHz = 3.3 ppm
  • delta = 480 Hz / 90 MHz = 5.3 ppm
    
• for C NMR, the absorptions are at 22,601,290 Hz and 22,601,910 Hz
  (where TMS is at 22,600,000 Hz)
  • delta = 1290 Hz / 22.6 MHz = 57.1 ppm
  • delta = 1910 Hz / 22.6 MHz = 84.5 ppm
    
• Note that on a higher-field instrument (e.g., 500 MHz),
  the chemical shifts would be the same
  although the absolute differences (in Hz) would be larger
  e.g., the proton resonances would be at
  500,001,650 Hz (3.3 ppm) and
  500,002,650 Hz (5.3 ppm)
• higher fields afford higher resolution:
  • better separation of absorptions
  • resonances typically look sharper at higher fields

Integrated Peak Areas

• indicate relative abundance of each type of hydrogen (or carbon)
• experimentally usually shown as stair-steps
  with the relative heights indicating relative area
• e.g., methyl tert-butyl ether shows two H NMR absorptions,
  with integrated areas in the ratio 1:3,
  representing the 3 methyl Hs and the 9 tert-butyl Hs
• the C NMR shows 3 peaks in relative areas 1:1:3
  (however, instrumental methodology often makes integration of C-13 peaks less reliable than H integrations)
• problem - C4H7Br3 gives only two H NMR peaks, in area ratio 4:3

Chemical Shift Correlations

• different functional groups lead to distinctive chemical shifts
• generally, electron withdrawal leads to deshielding (higher shifts)
• Table 13.3 will be provided for quizzes and exams
  • simple alkyl C-Hs 0.5 - 2.0 ppm
  • next to a C=C (allylic) 1.5 - 2.0
  • next to a C=O (ketone) 2.0 - 3.0
  • next to aromatic ring (benzylic) 2.5 - 3.0
  • next to an electronegative element (O, halogen) 2.5 - 4.0
  • on a double bond C=C-H (vinylic) 5.0 - 6.5
  • on an aromatic ring 6.5 - 8.0
  • on an aldehyde H-C=O 9.5 - 10
  • on a carboxylic acid O=C-O-H 11 - 12

Interpretation of NMR Spectra

• like IR, work from left to right
• identify the total number of resonances
• locate characteristic chemical shifts
  e.g., carboxylic acid, aldehyde, aromatic, vinylic, electronegative element
• identify C8H10 ,
  which gives only two absorptions at 7 ppm and 2.7 ppm
• predict the number of resonances in the C NMR spectrum of p-xylene

C-13 Chemical Shifts

• sp3 carbons 0 - 100 ppm
• sp2 carbons 100 - 200 ppm
• C=O carbons 170 - 210 ppm
• Figure 13.17 will be provided for quizzes and exams

Spin-Spin Splitting Patterns

• recognize common splitting patterns
• singlet - a single sharp peak
• doublet - two equal peaks
• triplet - three peaks in the ratio 1:2:1
• quartet - four peaks in the ratio 1:3:3:1

The (n+1) Rule

• in general, when there are n neighboring Hs, the absorption will be split to (n+1) lines
• e.g., CH3-CHCl2
  two H NMR absorptions at 2.5 ppm and 5.9 ppm (area ratio 3:1)
• the peak at 2.5 ppm due to the methyl group is a doublet
  (the chemical shift of 2.5 ppm is measured at the middle of the doublet)
• the methyl Hs either feel a shielding or a deshielding effect from their neighbor (C-H),
  depending on whether its spin is aligned with or against the applied magnetic field
• the neighboring C-H is half the time aligned with the magnetic field
  and half the time aligned against the magnetic field,
  so the methyl peak is split to two equal peaks
• the CH peak is split to a quartet by its 3 neighboring methyl Hs
• statistically, the 3 Hs can be aligned in four different ways
  (all with, all against, 2 with + 1 against, or 2 against + 1 with),
  with the latter two possibilities more likely
• see the splitting trees in your text (page 398) for a quartet and a triplet

Coupling Constant, J (Hz)

• the separation of the lines of a multiplet is J, the coupling constant, measured in Hz
• all peaks in a multiplet are separated by the same amount, J
• all peaks in the spin-coupled multiplet are separated by the same amount, J
• coupling is strong for nearest neighbors H-C-C-H,
  and small for more distant neighbors
• equivalent Hs don't split one another
  (i.e., ethane is a singlet, not 2 quartets)
• the value of J (e.g., about 7 Hz for simple alkyl groups)
  is the same regardless of the field strength of the instrument
• low-field instruments often give overlapping multiplets,
  which is less of a problem in high-field instruments