Resonance common examples

The common characteristics of the above mentioned heterocycles are electron withdrawing and a site of unsaturation that can stabilize the negative charge developed by the displacement reaction through resonance. For example, the thiazole activated halo displacement is similar to that of a conventional activating group as shown in Scheme 1. The activation is derived from the electron affinity and the stabilization of the negative... 

Sometimes a given set of atoms can covalently bond with each other in multiple ways to form a compound. This situation leads to something called resonance. Each of the possible bonded structures is called a resonance structure. The actual structure of the compound is a resonance hybrid, a sort of weighted average of all the resonance structures. For example, if two atoms are connected by a single bond in one resonance structure and the same two atoms are connected by a double bond in a second resonance structure, then in the resonance hybrid, those atoms are connected by a bond that is worth 1.5 bonds. A common example of resonance is found in ozone, 0, shown in Figure 5-7. 

ESR include electron magnetic resonance (EMR) and electron paramagnetic resonance (EPR). ESR is more limited than NMR, since it can only be used to study species with one or more unpaired electron spins. Common examples of such species are the following ... 

Resonance Lamp.—Such lamps (sometimes called low pressure lamps) are often used as line sources in photochemical studies. These usually contain a small amount of a metal vapor (e.g., mercury, cadmium, zinc, etc.) and several mm pressure of a rare gas. They operate at relatively low current (ca. 100 ma.) and high voltages (several thousand volts). This is in contrast to a typical medium pressure lamp which may operate off a 110-220 v. power supply delivering ca. 3-5 amp. The most common example in photochemistry is the mercury resonance lamp which has strong emission of the unreversed resonance lines at 2537 A. and 1849 A. (ca. 90% or more of the total) along with other, much weaker lines ( resonance lines are those which appear both in absorption and emission). There is little continuum. Sources of this type are widely used for photosensitized reactions. 

Sometimes more than one satisfactory Lewis structure can be written and there is no reason to select one over another. In such cases a single structural formula is inadequate for a correct representation, and we say that the true structure is a resonance hybrid of the several structures. Common examples of species requiring resonance structures are ozone, 03, carbonate ion, CO " and benzene, C6H6. These... 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

There are a few molecules in which an atom will have less than eight valence electrons. The most common examples of these contain H, Be, B, and Al. For example, boron trifluoride, BF3, has a central boron atom surrounded by three fluorine atoms. After filling the octets around the fluorine atoms, there are two possible solutions. One is to leave boron with only six valence electrons, while the second is to draw resonance structures for the molecule. 

Formally, one can think of the Raman transition probability being proportional to the elements of the polarizability tensor of a bound electron as the exciting frequency approaches the resonance frequency, these elements are enhanced in a Lorentz model of the bound electron. A common example of this mechanism is furnished by the ring-breathing (in-plane expansion) modes of porphyrins. Another mechanism, called vibronic enhancement, involves vibrations which couple two electronic excited states. In both mechanisms, the enhancement factors are nearly proportional to the intensities in the absorption spectrum of the adsorbate. 

An exception to the straightforward correspondence between C shifts in zeolites (or other catalysts) and solution values occurs when the structure of the compound is significantly perturbed on the catalyst. The most common example is protonation equilibria on acidic catalysts. Indeed, there have been a number of reports of the use of protonation shifts of amines 151,521, phosphines (151, and phosphine oxides (531 as probes of catalyst acidity. Similar effects are (x ca-sionally encountered in in situ experiments when a basic molecule is formed as an intermediate or product. An interesting case is the conversion of acetone to hydrocarbons on zeolites, which may involve the intermediacy of diacetone alcohol, mesityl oxide, phorone, and isophorone—all ketones. The chemical shifts of the carbonyl carbons of all these species in acidic zeolites were found to be up to 10 ppm downfield of the corresponding values in reference compilations. Furthermore, although the chemical shifts of the olefinic carbons a to the carbonyl were in reasonable agreement with values for CDCI solutions, the resonances of the olefinic carbons p to the carbonyl were very broad and shifted 20-30 ppm downfield 54. ... 

When heteroatoms are attached to the positive carbon, the cation gains added stability via donation of electrons (back donation) to the electron deficient center, which leads to greater stability, often by resonance delocalization of the charge on the heteroatom. Two common examples of this are oxo-stabilized cations such as 106 and sulfur-stabilized cations such as 107. In both cases, back donation from the heteroatom leads to resonance stabilized cations. Both 106 and 107 are more stable than a simple tertiary cation bearing three... 

Resonance Fluorescence. Resonance fluorescence occurs when the atoms absorb and reemit radiation at the same wavelength. The most common examples correspond to transitions originating in the ground state (resonance transitions). For example, resonance fluorescence is observed for zinc at 213.86 nm, for nickel at... 

The most common example of the resonance theory is the description of the benzene structure. The experimentally precisely determined and accurately known carbon-carbon bond length is consistent with the model as average of the resonance structures. When Pauling s resonance description of the benzene structure was criticized, the physicist Edward Teller and his colleagues provided spectroscopic evidence to support it [40]. The Nobel laureate physicist Philip Anderson was oblivious of Teller s and his co-workers paper (Private communication from Philip Anderson to the author by e-mail in 2009), and 68 years after Teller s contribution, in 2008, Anderson communicated another supportive paper for Pauling s model [41]. 

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