Resonance arrows

Mechanisms, like resonance structures, utilize curved arrows. (Resonance structures are ways of illustrating the Vcirious resonance forms that contribute to the resonance hybrid. If you need more review, refer to Organic Chemistry I For Dummies. ) Many of the same rules apply to both however, there are some important differences ... 

Fig. 2.5 An ion kinetic energy distribution of field desorbed He ions taken with a pulsed-laser time-of-flight atom-probe. In pulsed-laser stimulated field desorption of field adsorbed atoms, atoms are thermally desorbed from the surface by pulsed-laser heating. When they pass through the field ionization zone, they are field ionized. Therefore the ion energy distribution is in every respect the same as those in ordinary field ionization. Beside the sharp onset, there are also secondary peaks due to a resonance tunneling effect as discussed in the text. The onset flight time is indicated by to, and resonance peak positions are indicated by arrows. Resonance peaks are pronounced only if ions are collected from a flat area of the...

Note the structures above are resonance structures and are indicated by resonance arrows (- - ). Resonance structures do not exist independently they only approximate what the actual species looks like. 

Figure B2.5.13. Schematic representation of the four different mechanisms of multiphoton excitation (i) direct, (ii) Goeppert-Mayer (iii) quasi-resonant stepwise and (iv) incoherent stepwise. Full lines (right) represent the coupling path between the energy levels and broken arrows the photon energies with angular frequency to (Aco is the frequency width of the excitation light in the case of incoherent excitation), see also [111].

Figure Cl.5.8. Spectral jumping of a single molecule of terrylene in polyethylene at 1.5 K. The upper trace displays fluorescence excitation spectra of tire same single molecule taken over two different 20 s time intervals, showing tire same molecule absorbing at two distinctly different frequencies. The lower panel plots tire peak frequency in tire fluorescence excitation spectmm as a function of time over a 40 min trajectory. The molecule undergoes discrete jumps among four (briefly five) different resonant frequencies during tliis time period. Arrows represent scans during which tire molecule had jumped entirely outside tire 10 GHz scan window. Adapted from...

To deal wifh circumslances such as fhe bonding m ozone fhe nolion of resonance befween Lewis sfrucfures was developed According fo fhe resonance concepf when more fhan one Lewis sfrucfure may be written for a molecule a single sfrucfure is msuf ficienl fo describe if Ralher fhe frue sfrucfure has an eleclron dislribulion fhaf is a hybrid of all fhe possible Lewis sfrucfures fhaf can be written for fhe molecule In fhe case of ozone fwo equivalenf Lewis sfrucfures may be wriffen We use a double headed arrow fo represenf resonance befween fhese fwo Lewis sfrucfures... 

If IS imporfanf fo remember fhaf fhe double headed resonance arrow does not mdi cate a process m which fhe fwo Lewis sfrucfures mferconvert Ozone for example has... 

Electron delocalization can be important in ions as well as in neutral molecules Using curved arrows show how an equally stable resonance structure can be generated for each of the following anions... 

In Section 1 9 we introduced curved arrows as a tool to systematically generate resonance structures by moving electrons The mam use of curved arrows however is to show the bonding changes that take place in chemical reactions The acid-base reactions to be discussed in Sections 1 12-1 17 furnish numer ous examples of this and deserve some preliminary comment... 

Adenine is a weak base Which one of the three nitrogens designated by arrows in the struc tural formula shown is protonated in acidic solution" A resonance evaluation of the three protonated forms will tell you which one is the most stable... 

FIGURE 13.13 The magnetic moments (blue arrows) of the two possible spin states of the methine proton affect the chemical shift of the methyl protons in 1,1-dichloroethane. When the magnetic moment is parallel to the external field if.o (green arrow), it adds to the external field and a smaller 3 0 is needed for resonance. When it is antiparallel to the external field, it subtracts from it and shields the methyl protons. 

Fig. 2. The proton magnetic resonance spectrum of 5-nitrobenzofuroxan, in acetone at — Sl C. The bands marked by arrows arise from the 5-nitro tautomer.

The two individual line-bond structures for acetate are called resonance forms, and their special resonance relationship is indicated by the doubleheaded arrow between them. The only difference between resonance forms is the placement of the r and nonbonding valence electrons. The atoms themselves occupy exactly the same place in both resonance forms, the connections between atoms are the same, and the three-dimensional shapes of the resonance forms are the same. 

Resonance forms differ only in the placement of their tt or nonbonding electrons. Neither the position nor the hybridization of any atom changes from one resonance form to another. In the acetate ion, for example, the carbon atom is sp2-hybridized and the oxygen atoms remain in exactly the same place in both resonance forms. Only the positions of the r electrons in the C=0 bond and the lone-pair electrons on oxygen differ from one form to another. This movement of electrons from one resonance structure to another can be indicated by using curved arrows. A curved arrow always indicates the movement of electrons, not the movement of atoms. An arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow. 

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

Figure 11.12 Resonance forms of the allyl and benzyl carbocations. Electrostatic potential maps show that the positive charge (blue) is delocalized over the ir system in both. Electron-poor atoms are indicated by blue arrows.

The double-headed arrow is used to separate resonance structures. 

Because all three bonds are identical, a better model of the nitrate ion is a blend of all three Lewis structures with each bond intermediate in properties between a single and a double bond. This blending of structures, which is called resonance, is depicted in (9) by double-headed arrows. The blended structure is a resonance hybrid of the contributing Lewis structures. A molecule does not flicker between different structures a resonance hybrid is a blend of structures, just as a mule is a blend of a horse and a donkey, not a creature that flickers between the two. 

The curved arrows show how one resonance structure relates to another. Notice that the formal negative charge is located on the ortho and para positions, exactly where reaction takes place most quickly. Other ortho- and para-directing groups include —NH2, —Cl, and —Br. All have an atom with a lone pair of electrons next to the ring, and all accelerate reaction. 

Fig. 16. The UV-visible spectra of Ag,jj /Kr mixtures (Ag/Kr = l/10 )at 10-12K (A) After a 30-min irradiation centered at the atomic resonance absorption lines. (B ) The outcome of a 10-min, 423-nm Agj irradiation, showing major decay of the bands associated with Ag, (indicated by arrows) and the appearance of two new bands near 450 nm. (C) The result of a 5-min, 25K bulk thermal annealing period, showing regeneration of the original Ag3 spectrum eind loss of the new band near 445 nm USD.

The resonance interaction of chlorine with the benzene ring can be represented as shown in 13 or 14, and both of these representations have been used in the literature to save space. However, we shall not use the curved-arrow method of 13 since arrows will be used in this book to express the actual movement of electrons in reactions. We will use representations like 14 or else write out the canonical forms. The convention used in dashed-line formulas like 14 is that bonds that are present in all canonical forms are drawn as solid lines, while bonds that are not present in all forms are drawn as dashed lines. In most resonance, a bonds are not involved, and only the n or unshared electrons are put in, in different ways. This means that if we write one canonical form for a molecule, we can then write the others by merely moving n and unshared electrons. 

The double-headed arrow is intended to imply the existence of a resonance hybrid a stmcture with an electronic distribution intermediate between the two shown. Every instmctor knows the hazards of this portrayal. Firstly, the doubleheaded arrow is misinterpreted by some students to mean either (i) that there is an equilibrium condition involving the two different species, or (ii) that flipping occurs between the two species. A second problem is demonstrated by those students who ask Are these not the same If we rotate one of the molecules by 60°, we see that they are identical . We can hypothesise that the latter problem may be exacerbated by the tendency of textbooks (and probably teachers) to talk about these two different resonance stmctures as though we are referring to two different molecules - when, in fact, we are talking about different electron distributions in just one molecule. It seems so important for instractors to refer to just one set of six carbon atoms joined by ct bonds, and then to discuss alternative distributions of the six TT electrons within that system. 

The compound above has two important resonance structures. Notice that we separate resonance structures with a straight, two-headed arrow, and we place brackets around the structures. The arrow and brackets indicate that they are resonance structures of one molecule. The molecule is not flipping back and forth between the different resonance structures. 

CURVED ARROWS THE TOOLS FOR DRAWING RESONANCE STRUCTURES ... 

In the beginning of the course, you might encounter problems like this here is a drawing now draw the other resonance structures. But later on in the course, it will be assumed and expected that you can draw all of the resonance structures of a compound. If you cannot actually do this, you will be in big trouble later on in the course. So how do you draw all of the resonance structures of a compound To do this, you need to leam the tools that help you curved arrows. 

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