Dipolar resonance structures

The BE-matrices of hydrogen cyanide and hydrogen isocyanide (notice that since the neutral "carbenic" resonance structure has not the octets fully completed, the dipolar resonance structure is taken instead) are ... 

A-5. Azulene, shown in the following structure, is highly polar. Draw a dipolar resonance structure to explain this fact. 

The carbonyl groups of amides absorb at particularly low IR frequencies about 1640 to 1680 cm-1 (Figure 12-13). The dipolar resonance structure (shown next) places part of the pi bond between carbon and nitrogen, leaving less than a full C=0 double bond. 

Simple is clearly a subjective concept. Recall that we earlier mentioned considerations of resonance stabilization for enamines. More precisely, in the classical arrow-pushing description of organic molecules, much of the reaction chemistry and the thermochemistry of enamines is describable in terms of contributions from the covalent and dipolar resonance structures... 

If one visualizes the dipolar resonance structure 29b as being somewhat similar to the isomeric open-ring structure of the parent thiazirene 2, then the above results, together with those of the previously summarized computational studies, could suggest the isolability of thermodynamically stable thiazirenes. 

Part of the folklore of nonbenzenoid hydrocarbons suggests fulvenes are on the nonaro-matic/aromatic border. It is thus not obvious whether these species really belong in this chapter. Yet, because their aromaticity is so much less than that found for their isomeric benzenoid derivatives we feel confident to proceed. Other than the parent hydrocarbon species 103 [i.e. 127 wherein (R, R ) = (H, H) most of the other thermochemically char-acterized fulvenes have substimtion on the exomethylene carbon cf (R R ) = (H, Me), (Me, Me) and (Ph, Ph) for reference, the suggested enthalpies of formation of the (H, H), (H, Me), (Me, Me) and (Ph, Ph) species are 224, 185, 144 and 402 kJmor respectively. Were all differences in steric interactions and contributions from the dipolar resonance structures of the generic type 128 negligible, then AH (127, R R ) and A//f(CH2=CR R ) would be linearly related. We find that a nearly perfect straight line... 

The conjugate addition of a nucleophile to an a, -unsaturated ketone-or aldehyde is due to the same electronic factors that are responsible for direct addition. We ve seen that carbonyl groups are polarized so that the carbonyl carbon is positive, and we can even draw a dipolar resonance structure to underscore the point ... 

The resonance stabilization in these systems is thought to arise from the conjugation between the C-C and C-X double bonds, whereby the dipolar resonance structures contribute significantly to the resonance hybrid. 

The most obvious characteristic of fluoroorganic compounds is the extreme stability of the carbon-fluorine bond. The stability increases with the number of fluorine substituents bound to the same carbon atom. This increase of stability is reflected in the lengths of the C-F bonds in the series CHjF (140 pm) > CHjFj (137 pm) > CHF3 (135 pm) > CF4 (133 pm) (calculation at the MP2/6-31+G level of theory) [14]. The main reason for this stabilization is the nearly optimum overlap between the fluorine 2s and 2p orbitals and the corresponding orbitals of carbon this enables the occurrence of dipolar resonance structures for multiply fluorine-substituted carbon (Figure 1.9, see p. 14). The consequences on chemical reactivity of this self-stabilization of multiple fluorine substituents on the same carbon atom are discussed in more detail in Section 2.1.3. 

Wiberg and Schreiber have examined the rotational barriers in acrolein and vinylamine. If resonance structure 13 participates in the description of acrolein, then the population of C3 should be greater in the rotated conformer relative to the planar form. Similarly, if resonance structure 14 participates in the description of vinylamine, then the population on C2 should be smaller in the rotated conformer relative to the ground state planar conformer. The calculated topological charges (Table 28) indicate an electronic distribution exactly opposite to that expected. The authors conclude that dipolar resonance structures are unimportant in these molecules. 

Cyclopropenes and cyclopentadienes with exocyclic double bonds provide the possibility of dipolar resonance structures that suggest aromatic character in the cyclic structure. 

For methylenecyclopropene, a microwave structure determination has established bond lengths that show the strong alternation anticipated for a localized structure. The molecule does have a significant (1.90 D) dipole moment, implying a contribution from the dipolar resonance structure. The net stabilization calculated at the MP/6-31G level is small and comparable to the stabilization of 1,3-butadiene. The molecular geometry... 

The fulvalene systems are not predicted to be aromatic by any of the theoretical estimates of stability. Even simple resonance considerations would suggest polyene behavior, since only dipolar resonance structures can be drawn in addition to the single nonpolar structure. 

MO calculations (HF/STO-3G and HF/3-21G) indicate a rotational barrier that is substantially reduced relative to the corresponding barrier in ethene. The TS for the rotation is calculated to have a charge separation of the type suggested by the dipolar resonance structure. ... 

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