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					Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald


 Splitting Tree Diagram for a single value of Jax
Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald

Splitting Tree Diagram for two different values of Jax
   Chem 546 Fall 2010     Introduction to Spectroscopy   Prof. Rob Ronald
• Increased field strength can simplify the analysis of 1H NMR Spectra.
  At higher field it is clearly apparent that these two signals two
  separate patterns and are not coupled to each other.
   Chem 546 Fall 2010     Introduction to Spectroscopy   Prof. Rob Ronald
• When two nuclei (protons) are coupled the splittings that are
  observed are equal for both nuclei.
• Actually, the signals are not split, but the presence of near
  neighbors with different spin states results in different populations
  depending on the number of spin states involved – a result of the
  nearly equal populations of I = +½ and I = -½ spin states in the
  Boltzmann distribution.
• The picture shows the situation where two protons are coupled
  into two doublets with the same JAX value.
   Chem 546 Fall 2010    Introduction to Spectroscopy   Prof. Rob Ronald
• For an A2X system the single proton is coupled into a 1:2:1 triplet
  because there are now four spin states in the neighboring 2-protons –
  two of the states are degenerate.




• You should work out on your own that an A3X system will have eight
  spin states for the X proton and that there will be two states
  containing three degenerate states.
• Coupled systems can be analyzed by splitting tree diagrams.
 Chem 546 Fall 2010    Introduction to Spectroscopy   Prof. Rob Ronald

• When the coupling patterns are more complicated
  than those predicted by the N+1 rule they are
  referred to as 2nd order couplings
• The distinction between1st and 2nd order coupling is
  determined by the ratio of the J value to the
  difference in chemical shift of the interacting nuclei
  (Δν/J).
   – When Δν/J ≥10 the coupling patterns follow the N+1 rule
     (1st order coupling observed).
   – When Δν/J ≤10, then the patterns become more complex
     (2nd order coupling) – this type of coupling is often observed
     with geminally coupled systems, and also in alkenes.
• Usually, when Δv/J ≥ ~4 the splitting pattern has
  enough 1st order character for the 1st order N+1
  couplings to be recognized even though the pattern
  may be distorted.
  Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald

• The picture shows the
  predicted band shapes for a
  two proton coupling (AX)
  with varying Δν/J values –
  note that upon casual
  inspection when Δν/J = ~2
  then the pattern looks very
  much like a A3X quartet.
• One of the advantages of
  using higher field NMR
  spectrometers is that the
  increased chemical shift
  dispersion simplifies the 3J
  coupling patterns – more of
  them become 1st order and
  predicted by the N + 1 Rule.
Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald
Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald




                          • The transition from first order
                            coupling of an AX2 system (top)
                            to an AB2 system (bottom). For
                            this system J = 10Hz and v
                            varies from 10 to 160 Hz.
   Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald

• The 3JHH in sp3 systems is angle dependent; this is shown by
  the Karplus equation (Karplus and Conroy):
              3J = A + B cosθ + C cos2θ

  Where θ is the dihedral angle, and A, B, and C are empirical
  constants: 7, -1 and 5, respectively, the theoretical constants
  are: 4.22, -0.5, and 4.5. See pp 223-228 in Pavia
  Lampmann,and Kris.
• Typical values of 3JHH for “alkanes” is around 7.5 Hz.
• For 1H spectra 4JHH couplings are small 0-3 Hz and are usually
  only observed when the conformation between the protons has
  a “W” shape – the so-called W-coupling.


• Small 4JHH couplings are seen often in aromatic systems, which
  are planar and offer good W orientations.
• Small 4JHH couplings are also seen in allylic systems: the allylic
  protons that can have a W orientation with respect to a vinyl
  proton will show small couplings.
   Chem 546 Fall 2010    Introduction to Spectroscopy   Prof. Rob Ronald




• The Karplus curve for the dependence of the vicinal 3JHH
  coupling on the dihedral angle,Φ : the heavy line is the
  theoretical curve; the shaded area is the range of experimental
  results. Note that the prediction that 3J180° > 3J0° is confirmed.
  Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald

• Actually, the magnitude of vicinal (3J) coupling
  constants depends upon four factors:
   – 1. The dihedral angle, Φ, between the C-H (this is the
     prediction of Karplus)
   – 2. The C-C bond length, Rμv
   – 3. The H-C-C-H valence angles, θ and θ’
   – 4. The electronegativity of the substituents on the H-C-C-H
Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald


• Some examples of coupling constants with
  different dihedral angles in various systems.
Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald
Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald
  Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald

• Some examples of bond length
  dependence
 Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald


• Some examples of bond angle dependence



        148°                119°




         138°               117°




         125°                116°
  Chem 546 Fall 2010   Introduction to Spectroscopy      Prof. Rob Ronald

• Some examples of substituent effects
  – For ethanes: 3J = 9.41 – 0.80ΔE
  – For ethylenes: 3Jtrans = 19.0 – 3.3ΔE
                      cis = 11.7 – 4.7ΔE
                   3J

                                                      ΔE = E(X) – E(H)
  Chem 546 Fall 2010     Introduction to Spectroscopy   Prof. Rob Ronald

                       Spin Decoupling
• It is possible to do a spin-decoupling experiment to
  show which protons are J-coupled to which.
• This experiment has largely been replaced by COSY –
  a 2D experiment that shows all of the J-couplings.
• It is worthwhile to look at the decoupling experiment
  as it will make the broadband decoupling employed in
  routine 13C spectra more understandable.
• If the spectrum is acquired while one set of protons
  suspected of being coupled to another set is irradiated
  – the irradiation “pumps” those protons back and forth
  between the spin states.
• During the acquisition of the FID the vicinal protons
  see both states equally and their coupling averages
  out so the set coupled to the irradiated set collapses to
  a single line at the midpoint frequency of the pattern.
Chem 546 Fall 2010   Introduction to Spectroscopy   Prof. Rob Ronald
  Chem 546 Fall 2010     Introduction to Spectroscopy   Prof. Rob Ronald

                          Summary
• Integration
  – The integral spectrum gives the relative number of protons giving
    rise to the signals
  – The accuracy of the integration is about ±10%
• Spin-Spin Splitting – Coupling
  – Coupling Constants, J, are field independent and are given in Hz
  – One bond coupling is mostly important in 13C-1H situations, and
    is usually suppressed to simply 13C spectra. The DEPT sequence
    recovers the 13C-1H coupling information.
  – Two bond coupling is also referred to as Geminal coupling.
  – Geminal coupling is mostly observed when there are chiral
    centers present, or if there is restricted rotation. The protons are
    then referred to as diastereotopic.
  – 1H-C-1H Geminal couplings are typically in the range of 12-15Hz
    for CH2 groups and 0-3 for =CH2 groups in alkenes.
  – Geminal couplings often give rise to so-called A-B quartets.
Chem 546 Fall 2010     Introduction to Spectroscopy   Prof. Rob Ronald
                     Summary (Cont)
– Three bond coupling is referred to as Vicinal Coupling: H-C-C-H
– Vicinal coupling is observed in most 1H spectra and provides
  important structural information.
– The multiplicities induced by vicinal couplings are predicted by the
  N+1 Rule where N is the number of three bond neighbors
– Typical values for vicinal couplings are on the order of 6-9 Hz.
– The magnitude of the vicinal coupling is angle dependent and can
  be estimated using the Karplus equation, and this can be used to
  assign stereochemistry.
– The magnitude of the vicinal coupling constant in alkenes can be
  used to assign cis/trans stereochemistry of 1,2-disubstituted
  alkenes: cis ~10Hz; trans ~16Hz.
– Four bond coupling H-C-C-C-H is very small (0-3Hz) and not
  often observed.
– Four bond coupling is most favorable when the five atom system
  is planar and has a “W” configuration.
– Four bond couplings are observed in many aromatic systems
  Chem 546 Fall 2010    Introduction to Spectroscopy   Prof. Rob Ronald

                       Summary (Cont)
• Spin Decoupling
  – Decoupling is a technique that is used to remove the effects of
    coupling in the spectrum.
  – Decoupling can be used to determine which signals in a spectrum
    are J coupled to each other by selectively irradiating signals that
    are suspected to have coupling partners.
  – COSY (Correlation Spectroscopy) is a pulse sequence that gives
    a 2D map of all of the J couplings in a molecule and is often used
    in place of decoupling.
• You should consult the appendices at the end
  of the text for lists of various spectral data.

				
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posted:9/23/2012
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