Tautomers are isomers of organic compounds that readily interconvert by a chemical reaction called tautomerization.
This reaction commonly results in the formal migration of a hydrogen atom or proton, accompanied by a switch of a single bond and adjacent double bond. The concept of tautomerizations is calledtautomerism. Because of the rapid interconversion, tautomers are generally considered to be the same chemical compound. Tautomerism is a special case of structural isomerism and can play an important role in non-canonical base pairing in DNA and especially RNA molecules.
In solutions in which tautomerization is possible, a chemical equilibrium of the tautomers will be reached. The exact ratio of the tautomers depends on several factors, including temperature, solvent, and pH.
Common tautomeric pairs are:
- ketone - enol, e.g., for acetone (see: keto-enol tautomerism)
- ketene - ynol, e.g., for Ethenone
- amide - imidic acid, e.g., during nitrile hydrolysis reactions
- lactam - lactim, an amide - imidic acid tautomerism in heterocyclic rings, e.g., in the nucleobases guanine, thymine, and cytosine
- enamine - imine
- enamine - enamine, e.g., during pyridoxalphosphate-catalyzed enzymatic reactions.
- anomers of reducing sugars in solution interconvert through an intermediate open chain form.
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies (or resonance frequencies). At these frequencies, even small periodic driving forces can produce large amplitude oscillations, because the system stores vibrational energy.
Resonance occurs when a system is able to store and easily transfer energy between two or more different storage modes (such as kinetic energy and potential energy in the case of a pendulum). However, there are some losses from cycle to cycle, called damping. When damping is small, the resonant frequency is approximately equal to the natural frequency of the system, which is a frequency of unforced vibrations. Some systems have multiple, distinct, resonant frequencies.
Resonance phenomena occur with all types of vibrations or waves: there is mechanical resonance, acoustic resonance, electromagneticresonance, nuclear magnetic resonance (NMR), electron spin resonance (ESR) and resonance of quantum wave functions. Resonant systems can be used to generate vibrations at a specific frequency (e.g. musical instruments), or pick out specific frequencies from a complex vibration containing many frequencies (e.g. filters).