Resonance structures single

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

A single resonance structure is shown below for each of several molecules. Consider other resonance structures. Comment on those that would be expected to make a major stabilizing contribution to the molecule in question. 

In addition to intermolecular interactions in the solid state, the relative importance of the two resonance structures A and B is an important issue in the structural determinations of selena- and tellura-diazoles. ° The Se-N bond lengths fall within the range 1.78-1.81 A and the Te-N bond lengths are 2.00-2.05 A compared to single bond values of 1.86 and 2.05 A, respectively. It can be concluded that resonance structure A is more important than B for the Se and, especially, the Te... 

The predicted bond order for a given bond is listed at the intersection of the two atoms of interest in the bond orders table. The illustration at the left shows the predicted bond orders for this molecule (where 1.0 is a traditional single bond, 2.0 is a double bond, and so on). The C-H bonds all have predicted bond orders of about. 9, while the C-C bonds have predicted bond orders of about 1.4. The latter arc consistent with the known resonance structure for allyl cation. ... 

Molecules with alternating single and double bonds ( conjugated polyenes ) often exhibit unusual physical and chemical properties. Chemists have postulated the involvement of zwitterionic resonance structures to account for these properties. 

The biradical resonance structure for ozone requires two singly occupied MOs, and it is clear that an RHF type wave function, which requires all orbitals to be doubly occupied, cannot describe this. A UHF type wave function, however, allows the a and /3 orbitals to be spatially different, and can to a certain extent incorporate both resonance structures. Systems with biradical character will often have a (singlet) UHF wave function different from an RHF. 

Resonance hybrid (Section 2.4) A molecule, such as benzene, that can t be represented adequately by a single Kekule structure but must instead be considered as an average of two or more resonance structures. The resonance structures themselves differ only in the positions of their electrons, not their nuclei. 

In para-amino benzoic acid, there is another resonance structure right next to the six-sided ring. It is a carboxyl group, shown with a single bond between carbons, and a double bond between the carbon and the oxygen. This is also a place where the electron can bounce around between the three nuclei. 

Structural isomers are molecules that have the same formula but in which the atoms are connected in a different order. Two isomers of disulfur difluoride, S2F2, are known. In each the two S atoms are bonded to each other. In one isomer each of the S atoms is bonded to an F atom. In the other isomer, both F atoms are attached to one of the S atoms, (a) In each isomer the S—S bond length is approximately 190 pm. Are the S—S bonds in these isomers single bonds or do they have some double bond character (b) Draw two resonance structures for each isomer, (c) Determine for each isomer which structure is favored by formal charge considerations. Are your conclusions consistent with the S—S bond lengths in the compounds ... 

When there is electronic resonance, the bond lengths, bond angles, and nuclear positions are intermediate between those corresponding to the individual resonance structures, as found in the crystal structure of the tin dimer.32 This type of bond, that is, a single bond plus a resonating unshared electron pair, was subsequently found to occur on the 100 surfaces of silicon.33... 

Never break a single bond when drawing resonance structures. By definition, resonance stractures must have aU the same atoms connected in the same order. 

Notice that we have two resonance structures, each of which has charge separation. Even though the molecule has no net charge, nevertheless, we cannot draw a single resonance structure that is free of charge. If we try to do so, we will end up with a structure that violates the second commandment ... 

The previous chapter was devoted solely to drawing resonance structures. If you have not yet completed that chapter, do so before you begin this section. We said in the previous chapter that resonance would find its way into every single topic in organic chemistry. And here it is in acid-base chemistry. 

When we learned how to draw resonance structures, we saw two conunand-ments that we must not violate (1) never break a single bond, and (2) never exceed an octet for second-row elements. When drawing mechanisms, we are trying to understand where the electrons actually moved to break and form bonds. Therefore, it is OK to break single bonds. In fact, it happens in almost every reaction. So when drawing mechanisms there is only one commandment to follow never exceed an octet for second-row elements. 

Which of these options is the best Lewis structure Actually, no single Lewis structure by itself is an accurate representation of NO3. Any single structure of the anion shows nitrate with one NDO double bond and two N— O single bonds. In Section 9 1, we show that single and double bonds between the same types of atoms have different lengths and different energies. In contrast, experiments show that the three nitrate N—O bonds are identical. To show that the nitrate N—O bonds are all alike, we use a composite of the three equivalent Lewis structures. These are traditionally called resonance structures. Resonance stmctures are connected by double-headed arrows to emphasize that a complete depiction requires all of them. 

Experimental studies support our conclusion about the resonance structures of NNO. As we describe in Section 9-1. double bonds are shorter than single bonds. The length of the nitrogen-oxygen bond in NNO is less than typical N—O single bonds but greater than typical NDO double bonds. 

A clue to the nature of the third itt MO can be found in the placement of electrons in the two resonance structures for ozone, which are shown with color highlights in Figure 10-36a. Notice that in one resonance structure, the left outer atom has three lone pairs and a single bond, while the right outer atom has two lone pairs and a double bond. In the other resonance structure, the third lone pair is on the right outer atom, with the double bond to the left outer atom. The double bond appears in different positions in the two stmctures, and one of the lone pairs also appears in different positions. These variations signal delocalized orbitals. 

Resonance Molecules with two or more valid structures are said to be resonant. The actual structure is neither of the alternatives but a lower-energy molecule with delocalized valence electrons. Benzene with its alternating double and single bonds is an example of a resonant structure. Benzene actually has no single... 

In accord with the resonance structure drawn, there is little B=B bonding in this diborane(4) derivative and the B—B distance is found to be 1.859 A,68 which is considerably longer than even the normal expected value of 1.7 A for a boron-boron single bond.67 This is consistent with the normal repulsion of negative charges on adjacent atoms not stabilized by 7r-bonding. 

So each bond has a bond order of 1.5, which is consistent with the observed bond length. These two resonance structures are often called Kekule structures because they were first proposed in 1865 by Kekule, who imagined that the molecule converted very rapidly from one form to the other. This, however, is not the case the molecule never has either of the Kekule structures but only a single structure, which is intermediate between these two hypothetical structures and is approximately represented as follows ... 

There are many other molecules in which some of the electrons are less localized than is implied by a single Lewis structure and can therefore be represented by two or more resonance structures. For example, the three bonds in the carbonate ion all have the same length of 131 pm, which is intermediate between that of the C—O single bond in methanol (143 pm) and that of the C=0 double bond in methanal (acetaldehyde) (121 pm). So the carbonate ion can be conveniently represented by the following three resonance structures ... 

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