resonance structure

Examples of resonance structure in the following topics:

• Resonance

  • Resonance structures depict possible electronic configurations; the actual configuration is a combination of the possible variations.
  • Each contributing resonance structure can be visualized by drawing a Lewis structure; however, it is important to note that each of these structures cannot actually be observed in nature.
  • 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.

• Resonance

  • This averaging of electron distribution over two or more hypothetical contributing structures (canonical forms) to produce a hybrid electronic structure is called 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 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.
  • On the other hand, if two or more canonical forms have identical low energy structures, the resonance hybrid will have exceptional stabilization and unique properties.

• Formal Charge and Lewis Structure

  • In particular, chemists use Lewis structures (also known as Lewis dot diagrams, electron dot diagrams, or electron structures) to represent covalent compounds.
  • Non-valence electrons are not represented when drawing the Lewis structures.
  • Lewis structures can also be drawn for ions.
  • These equivalent structures are known as resonance structures and involve the shifting of electrons and not of actual atoms.
  • Two of the contributing structures of nitrogen dioxide (NO2).

• 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.
  • The contributing structures to the phenol hybrid all suffer charge separation, resulting in very modest stabilization of this compound.
  • An energy diagram showing the effect of resonance on cyclohexanol and phenol acidities is shown below.

• 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).
  • Nuclear magnetic resonance and mass spectrometry are used to investigate the mechanisms of chemical reactions.
  • NMR and MS detect isotopic differences; detecting these differences allows information about the position atoms in a product's structure to be determined.
  • Identify the uses of isotopic labeling in structural determination and the primary techniques used to study isotopically-labeled molecules

• 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.
  • Valence bond structures are similar to Lewis structures, except where a single Lewis structure is insufficient, several valence bond structures can be used.
  • It is in this aspect of valence bond theory that we see the concept of resonance.

• Reactions of Fused Benzene Rings

  • Naphthalene is stabilized by resonance.
  • Three canonical resonance contributors may be drawn, and are displayed in the following diagram.
  • The two structures on the left have one discrete benzene ring each, but may also be viewed as 10-pi-electron annulenes having a bridging single bond.
  • The structure on the right has two benzene rings which share a common double bond.
  • This contrasts with the structure of benzene, in which all the C–C bonds have a common length, 1.39 Å.

• 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 hydrogen atoms in a given molecule are all structurally equivalent, averaged for fast conformational equilibria.
  • The first feature assures that each compound gives a single sharp resonance signal.

• Signal Strength

  • From one of the spectrum signals (colored red) or on hydrogen atom(s) in the structural formulas the spectrum, a diagram follows showing an enlarged spectrum and the relationship will be colored blue.
  • Hydrogen bonding shifts the resonance signal of a proton to lower field ( higher frequency ).
  • Two structurally equivalent structures may be drawn for the enol tautomer (in magenta brackets).
  • For most of the above resonance signals and solvents the changes are minor, being on the order of ±0.1 ppm.
  • However, two cases result in more extreme changes and these have provided useful applications in structure determination.

• 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).
  • Most organic compounds exhibit proton resonances that fall within a 12 ppm range (the shaded area), and it is therefore necessary to use very sensitive and precise spectrometers to resolve structurally distinct sets of hydrogen atoms within this narrow range.