resonance

Examples of resonance in the following topics:

• Resonance

  • Resonance structures depict possible electronic configurations; the actual configuration is a combination of the possible variations.
  • This intermediate has an overall lower energy than each of the possible configurations and is referred to as a resonance hybrid.
  • Therefore, three valid resonance structures can be drawn.
  • Sometimes, resonance structures involve the placement of positive and negative charges on specific atoms.
  • Because atoms with electric charges are not as stable as atoms without electric charges, these resonance structures will contribute less to the overall resonance structure than a structure with no charges.

• Acidity of Phenols

  • A similar set of resonance structures for the phenolate anion conjugate base appears below the phenol structures.
  • The resonance stabilization in these two cases is very different.
  • An important principle of resonance is that charge separation diminishes the importance of canonical contributors to the resonance hybrid and reduces the overall stabilization.
  • An energy diagram showing the effect of resonance on cyclohexanol and phenol acidities is shown below.
  • The additional resonance stabilization provided by ortho and para nitro substituents will be displayed in the third diagram above.

• Resonance

  • Likewise, the structure of nitric acid is best described as a resonance hybrid of two structures, the double headed arrow being the unique symbol for resonance.
  • The above examples represent one extreme in the application of resonance.
  • The application of resonance to this case requires a weighted averaging of these canonical structures.
  • The basic principles of the resonance method may now be summarized.
  • The stability of a resonance hybrid is always greater than the stability of any canonical contributor.

• Reactions of Fused Benzene Rings

  • Naphthalene is stabilized by resonance.
  • Three canonical resonance contributors may be drawn, and are displayed in the following diagram.
  • From heats of hydrogenation or combustion, the resonance energy of naphthalene is calculated to be 61 kcal/mole, 11 kcal/mole less than that of two benzene rings (2 * 36).
  • As expected from an average of the three resonance contributors, the carbon-carbon bonds in naphthalene show variation in length, suggesting some localization of the double bonds.

• The Chemical Shift

  • Although the eleven resonance signals are distinct and well separated, an unambiguous numerical locator cannot be directly assigned to each.
  • Also, it should give a single sharp nmr signal that does not interfere with the resonances normally observed for organic compounds.
  • Note that νref is the resonant frequency of the reference signal and νsamp is the frequency of the sample signal.
  • The first feature assures that each compound gives a single sharp resonance signal.
  • From the previous discussion and examples we may deduce that one factor contributing to chemical shift differences in proton resonance is the inductive effect.

• Structural Determination

  • Structural determination using isotopes is often performed using nuclear magnetic resonance spectroscopy and mass spectrometry.
  • Structural determination utilizing isotopes is often performed using two analytical techniques: nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS).
  • Mass spectrometry and nuclear magnetic resonance detect the difference in an isotope's mass, while infrared spectroscopy detects the difference in the isotope's vibrational modes.
  • Nuclear magnetic resonance and mass spectrometry are used to investigate the mechanisms of chemical reactions.

• Detection and Observation of Radicals

  • The same unpaired or odd electron that renders most radical intermediates unstable and highly reactive may be induced to leave a characteristic "calling card" by a magnetic resonance phenomenon called "electron spin resonance" (esr) or "electron paramagnetic resonance" (epr).
  • This complexity is the result of hyperfine splitting of the resonance signal by protons and other nuclear spins, an interaction similar to spin-spin splitting in nmr spectroscopy.

• Signal Strength

  • We can take advantage of rapid OH exchange with the deuterium of heavy water to assign hydroxyl proton resonance signals .
  • As shown in the following equation, this removes the hydroxyl proton from the sample and its resonance signal in the nmr spectrum disappears.
  • Hydrogen bonding shifts the resonance signal of a proton to lower field ( higher frequency ).
  • iv) Intramolecular hydrogen bonds, especially those defining a six-membered ring, generally display a very low-field proton resonance.
  • For most of the above resonance signals and solvents the changes are minor, being on the order of ±0.1 ppm.

• Proton NMR Spectroscopy

  • This important and well-established application of nuclear magnetic resonance will serve to illustrate some of the novel aspects of this method.
  • If the magnetic field is smoothly increased to 2.3488 T, the hydrogen nuclei of the water molecules will at some point absorb rf energy and a resonance signal will appear.
  • Since protons all have the same magnetic moment, we might expect all hydrogen atoms to give resonance signals at the same field / frequency values.
  • This secondary field shields the nucleus from the applied field, so Bo must be increased in order to achieve resonance (absorption of rf energy).
  • In the upper diagram, those compounds that give resonance signals at the higher field side of the diagram (CH4, HCl, HBr and HI) have proton nuclei that are more shielded than those on the lower field (left) side of the diagram.

• Bonding in Coordination Compounds: Valence Bond Theory

  • Finally, Linus Pauling integrated Lewis' proposal and the Heitler-London theory to give rise to two additional key concepts in valence bond theory: resonance and orbital hybridization.
  • It is in this aspect of valence bond theory that we see the concept of resonance.