Common Examples of Resonance

Problem 16.4 Draw a second resonance structure for each carbocation. Then draw the hybrid. 

Problem 16.6 Use resonance theory and the Hammond postulate to explain why 3-chloro-1-propene (CH2= CHCH2CI) is more reactive than 1 -chloropropane (CH3CH2CH2CI) in SnI reactions. 

When are resonance structnres drawn for a molecule or reactive intermediate Because resonance involves delocalizing n bonds and nonbonded electrons, one or both of these structural features must be present to draw additional resonance forms. There are four conunon bonding patterns for which more than one Lewis structure can be drawn. 

The asterisk [ ] corresponds to a charge, a radical, ora lone pair. 

This is called allyl type resonance because it can be drawn for allylic carbocations, allylic carb-anions, and allylic radicals. 

and Z may all be carbon atoms, as in the case of an allylic carbocation (resonance structures A and B), or they may be heteroatoms, as in the case of the acetate anion (resonance structures C and D). The atom Z bonded to the multiple bond can be charged (a net positive or negative charge) or neutral (having zero, one, or two nonbonded electrons). The two resonance structures differ in the location of the double bond, and either the charge, the radical, or the lone pair, generalized by [ ]. 

Cyclic, completely conjugated rings like benzene have two resonance structures, drawn by moving the electrons in a cyclic manner around the ring. Three resonance structures can be drawn for other conjugated dienes, two of which involve charge separation. 

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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 ... 

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... 

Resonances are common and unique features of elastic and inelastic collisions, photodissociation, unimolecular decay, autoionization problems, and related topics. Their general behavior and formal description are rather universal and identical for nuclear, electronic, atomic, or molecular scattering. Truhlar (1984) contains many examples of resonances in various fields of atomic and molecular physics. Resonances are particularly interesting if more than one degree of freedom is involved they reflect the quasi-bound states of the Hamiltonian and reveal a great deal of information about the multi-dimensional PES, the internal energy transfer, and the decay mechanism. A quantitative analysis based on time-dependent perturbation theory follows in the next section. 

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. 

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... 

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]. 

Other magnetic measurements of catalysts include electron paramagnetic resonance and magnetic susceptibility. Although those are not as common as NMR, they can be used to look at the properties of paramagnetic and ferromagnetic samples. Examples of these applications can be found in the literature [87. 

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... 

In this chapter, we first present a brief overview of the experimental techniques that we and others have used to study torsional motion in S, and D0 (Section II). These are resonant two-photon ionization (R2PI) for S,-S0 spectroscopy and pulsed-field ionization (commonly known as ZEKE-PFI) for D0-S, spectroscopy. In Section HI, we summarize what is known about sixfold methyl rotor barriers in S0, S, and D0, including a brief description of how the absolute conformational preference can be inferred from spectral intensities. Section IV describes the threefold example of o-cholorotoluene in some detail and summarizes what is known about threefold barriers more generally. The sequence of molecules o-fluorotoluene, o-chlorotoluene, and 2-fluoro-6-chlorotoluene shows the effects of ort/io-fluoro and ortho-chloro substituents on the rotor potential. These are approximately additive in S0, S, and D0. Finally, in Section V, we present our ideas about the underlying causes of these diverse barrier heights and conformational preferences, based on analysis of the optimized geometries and electronic wavefunctions from ab initio calculations. 

As is now common practice, H NMR spectra (NMR - nuclear magnetic resonance) have been recorded on most examples of the fused heterocyclic rings mentioned in later sections in this chapter, and as such the primary literature is replete with the data for these and should be consulted directly. 

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