Isomers resonance structures

New problems on structure of cisplatin from number of known isomers, resonance structures of acetylacetonate and glycinate, structures of glycinato complexes, complexes of the ambidentate thiocyanato complexes, structure of a trien complex, and names of the Figure 3.18 complexes... 

It can be seen from resonance structures (2) to (4) that a — I — M-substituent deactivates the 3- and 5-position most strongly in electrophilic substitution. If this deactivation of the 5-position is strong enough to overcome the activating effects of the sulfur in the 5-position, substitution will be directed to the 4-position to an increasing extent. Tirouflet and Fournari studied the nitration of 2-substituted thiophenes of this type. The analysis was carried out polarographically, and the percentage of 4-isomer was as follows ... 

Consequently, structures 85b and 85c must be considered resonance structures rather than valence isomers. Hyperfine coupling constants were computed for a series of dithiazolyl radicals and related compounds [96MRC913]. An absolute mean deviation of 0.12 mT with respect to experimental data is reported for 10 sulfur hyperfine coupling constants obtained from UB3-LYP/TZVP calculations. 

Azulene, an isomer of naphthalene, lias a remarkably large dipole moment for a hydrocarbon (/i = 1.0 D). Explain, using resonance structures. 

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 ... 

Suppose that a molecule is composed of two atoms of phosphorus and one of oxygen. Draw structures for two possible isomers of the molecule. For the more stable structure, draw the resonance structures. Which structure is least important ... 

A variety of acyclic and cyclic S-N compounds decompose at moderate temperatures (100-150 °C) with the formal loss of a symmetrical NSN fragment, but this molecule has never been detected. The lowest energy isomer, linear NNS, is generated by flash vacuum pyrolysis of 5-phenyl-l,2,3,4-thia-triazole.40 Ab initio molecular orbital calculations indicate that the resonance structure N = N+-S is dominant.41... 

Table 3 presents the experimental enthalpies of formation of polynitrobenzenes and Table 4 presents the calculated additivity values and DSEs for these same compounds. Enthalpy-of-formation values have been determined experimentally for all three dinitrobenzene isomers in the gaseous state. The enthalpy-of-formation difference between the meta and para isomers is indistinguishable from 0. Conventional wisdom suggests that the para isomer should be destabilized relative to the meta because of adjacent positive charges in key ionic or polar resonance structures. Thus it seems that electronic effects due to meta/para dinitro substituent position are small. This small enthalpy-of-formation difference is similar to that for the meta and para dicyano, difluoro and dichloro benzenes, but does not mimic the ca 22 kJ mol 1 difference for the phthalic acids with which the... 

Figure 13.4 C(7)-C(8) isomer resulting from monoaddition of benzyne to Cjg and Kekul resonance structure of C70, illustrating the benzenoid character of the equatorial hexagons.

This isomer represents the first example of a [5,6]-adduct in which the fusion bond remains intact. This addition mode was explained by the relatively high double bond character of the [5,6]-bond C(7)-C(8), which underlines the importance of the Kekule structure of C7Q that involves the benzenoid hexagons at the equatorial belt. No such resonance structure is required to describe the proper bonding situation within C50 (see also Chapters 1 and 14). 

Problem 2.44 What is the difference between isomers and contributing resonance structures ... 

A perusal of the / values in Table 1 also shows that/aa is higher in the [3,2-6]-annelated systems than in the corresponding [2,3-6] isomers. For example / , in thieno[3,2-6]thiophene (3) and N-benzylthieno[3,2-6]pyrrole (18) is 1.55 and 1.3 Hz compared to 1.17 and 1.0 Hz for the corresponding [2,3-6 ]- annelated systems (7) and (19). This relatively high value for / - in the [3,2-6]-fused systems is a consequence of the transmission of the spin interactions between the protons through the v- framework rather than the cr-framework. In qualitative terms this means that several resonance structures can be written for such systems wherein electronic perturbations at C-2 are effectively transmitted to C-5 rather than C-6 (Scheme 4). 

A double-headed arrow is often used to indicate that two structures drawn are resonance structures rather than tautomers or other separable isomers. 

Compounds with a cyclopentadienyl-silicon cr-bond adopt a variety of bonding arrangements, which can be classified on the basis of the hybridization of the corresponding cyclopentadienyl carbon atom, as displayed in Scheme 1. In species of type 1, the silicon atom is bound to an sp3 -hybridized carbon atom, i.e. to an ally lie carbon within a cyclopentadiene unit. The silicon atom is bound to an sp2-hybridized carbon atom in species of type 2-6, including silylcyclopentadienyl radicals (2), ionic silylcyclopentadi-enide species (3), and silylcyclopentadienyl fragments bound in a /j5 -fashion to a metal centre (4). Species 5 represents the ionic resonance structure of a silafulvene, and structure 6 stands for the vinylic isomers of a cr-cyclopentadienylsilane (type 1), but these two types of compound are not examined in detail in this article. 

The phenolic enone 169 was recovered unchanged from treatment with sodium methoxide however, treatment of furanone 70 under the same basic conditions gave smoothly the enone isomer 169. Therefore, the lack of closure of 169 is the result of an unfavorable equilibrium. The process 170 -169 can be looked at as a 5-Exo-Trigonal process due to the resonance structure 171B. 

Some compounds exhibit resonance. That is, there is more than one acceptable Lewis structure for a compound. Another way to say this is that no single Lewis structure fully describes the actual structure. Resonance structures differ only in how the electrons are distributed among the atoms. If the atom connectivity is different, the two structures are isomers, which can be isolated and observed separately. 

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