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.

• Peptide Bonding between Amino Acids

  • Higher-ordered structures such as peptide chains and proteins are formed when amino acids bond to each other.
  • The amide group has three resonance forms, which confer important properties.
  • First, the stabilization afforded from the resonance structures effectively stabilizes it by 80kj/mol, making it less reactive than similar groups.
  • The peptide bond is uncharged at normal pH values, but the double bonded character from the resonance structure creates a dipole, which can line up in secondary structures.

• 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.

• 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.

• NMR and MRIs

  • Magnetic resonance imaging is a medical imaging technique used in radiology to visualize internal structures of the body in detail.
  • Magnetic resonance imaging (MRI), also called nuclear magnetic resonance imaging (NMRI) or magnetic resonance tomography (MRT), is a medical imaging technique used in radiology to visualize internal structures of the body in detail.
  • MRI utilized the property of nuclear magnetic resonance (NMR) to image the nuclei of atoms inside the body.
  • This electromagnetic field has just the right frequency (known as the resonance frequency) to become absorbed and then reverse the rotation of the hydrogen protons in the magnetic field.

• 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

• 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.

• Forced Vibrations and Resonance

  • The phenomenon of driving a system with a frequency equal to its natural frequency is called resonance.
  • The phenomenon of driving a system with a frequency equal to its natural frequency is called resonance.
  • It is interesting that the widths of the resonance curves shown in depend on damping: the less the damping, the narrower the resonance.
  • Heavy cross winds drove the bridge into oscillations at its resonant frequency.
  • The damping decreased when support cables broke loose and started to slip over the towers, allowing increasingly greater amplitudes until the structure failed.

• 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.