7r-Electron delocalizations

The resulting crystal proved amenable to a conclusive XRD analysis. As shown in Eq. (26), the C—C ring bond is lengthened over what it is in the structurally similar cyclopropene (1.304 A) and the B—C bonds shortened relative to the electronically analogous bond in trivinylborane (1.558 A). Thus, one can safely conclude that there is extensive 7r-electron delocalization and Hiickel aromatic character in the borirene ring. 

Of enormous interest to our present discussion would be XRD data on monomeric boroles in order to determine the structural consequences of 7r-electron delocalization in this four-electron, formally antiaromatic system.30 However, many attempts to grow suitable crystals of 94 have failed, and prolonged storage of solutions has produced only yellow dimers, presumably similar in structure to 100. Hence, in estimating the relative importance of structures 94a-c, spectral data must be our guide. 

The next cyclic alkadiene, 1,3-cyclopentadiene, has been experimentally studied by MW, GED and XR methods. The carbon skeleton is planar (C2v symmetry), and the small C=C—C angles compared to those in 1,3-butadiene (124.3°) or d.s-1-butene (126.40)58 do not seem to influence noticeably the lengths of the CC bonds, although other effects, such as 7r-electron delocalization, might have an opposite effect. The apparently normal structure parameters observed for 1,3-cyclopentadiene might therefore be a result of different forces having opposite effects on the structure parameters. 

Only for a special class of compound with appropriate planar symmetry is it possible to distinguish between (a) electrons, associated with atomic cores and (7r) electrons delocalized over the molecular surface. The Hiickel approximation is allowed for this limited class only. Since a — 7r separation is nowhere perfect and always somewhat artificial, there is the temptation to extend the Hiickel method also to situations where more pronounced a — ix interaction is expected. It is immediately obvious that a different partitioning would be required for such an extension. The standard HMO partitioning that operates on symmetry grounds, treats only the 7r-electrons quantum mechanically and all a-electrons as part of the classical molecular frame. The alternative is an arbitrary distinction between valence electrons and atomic cores. Schemes have been devised [98, 99] to handle situations where the molecular valence shell consists of either a + n or only a electrons. In either case, the partitioning introduces extra complications. The mathematics of the situation [100] dictates that any abstraction produce disjoint sectors, of which no more than one may be non-classical. In view if the BO approximation already invoked, only the valence sector could be quantum mechanical9. In this case the classical remainder is a set of atomic cores in some unspecified excited state, called the valence state. One complication that arises is that wave functions of the valence electrons depend parametrically on the valence state. 

Aromatic cyclic 7r-electron delocalization does indeed stabilize the planar structure with bond equalization (84ZOR897)—the problem is that, in addition to that effect, there may exist some others that may eventually overshadow it. Thus, the foregoing warrants the conclusion that the preference of a planar or nonplanar geometry of heterocycle depends on a number of factors including aromaticity (antiaromaticity), which may not even be the most important. In any case, this factor should not be disregarded if one wishes to obtain a correct overall energy balance. For example, aromaticity is reflected in the values of inversion barriers. Thus, for antiaromatic 2-azirine the nitrogen inversion barrier is, as was mentioned earlier, 37.7 kcal/mol, whereas in the case of its saturated... 

Multiple Heck reactions have also been applied in a number of ways to prepare polymers (Scheme 40). One-dimensionally 7r-conjugated polymers are attractive materials because of their optical and electrical properties resulting from 7r-electron delocalization along their main chains. Among these, poly(p-phenylenevinylene) (PPV) shows high electrical conductivity, large non-linear optical responses, and electroluminescent activity. One approach... 

It was shown in Section 21-3 that benzene is 36-38 kcal more stable than the hypothetical molecule 1,3,5-cyclohexatriene on the basis of the differences between experimental heats of combustion, or hydrogenation, and heats calculated from bond energies. We call this energy difference the stabilization energy (SE) of benzene. We have associated most of this energy difference with 7r-electron delocalization, which is the delocalization energy (DE). The difference between SE and DE will be small only if our bond-energy tables are reliable and steric and strain effects are small. 

Despite the fact that optical applications require thin films of poly(3-alkylthiophene)s, the photochemistry of these materials has been characterized in solution but only scarcely in the solid state. The UV/Vis spectra of these films of poly(3-butylthiophene) show an absorption band in the visible range corresponding to a n—n transition whose energy depends on 7r-electron delocalization. 

Representative examples of ring proton and carbon chemical shifts of all known l,4-(oxa/thia)-2-azoles were reported in CHEC-II(1996). A special notice should be given for H and 13C nuclear magnetic resonance (NMR) spectra of both dithiazolium 5 (X = Y = S) and oxathiazolium salts 6 (X = 0 Y = S) and 7 (X = S Y = 0) <1996CHEC-II(4)489>. A S downfield shift for both 3-H and 5-H as well as C-2 and C-5 is correlated with a potential 7r-electron delocalization and thus the aromaticity of these ring systems <1996CHEC-II(4)498>. 

The higher rate of HC1 elimination of 4-chloro-l-phenyl-1-butanone with respect to 5-chloro-2-pentanone by a factor of 9.9 confirmed the participation of the C=0 group. The 7r-electron delocalization of the benzene ring increases the basicity and nucleophilicity of the carbonylic oxygen and led to greater stabilization of the transition state through assistance to the C—Cl bond cleavage in the transition state (equation 80). 

The aromatic character of 1,3,4-thiadiazole can be demonstrated with the aid of micro-wave spectroscopy. Using the differences between the measured bond lengths and covalent radii, aromaticity, as shown by 7r-electron delocalization, diminishes in the order 1,2,5-thiadiazoles > thiophene > 1,3,4-thiadiazole > 1,2,5-oxadiazole (66JSP(19)283). The micro-wave spectrum was further refined by later workers (7lJST(9)l63). 

More precise numerical data, such as heats of combustion, are not available but theoretical methods (see Section 4.38.2.1) suggest that l,6,6aA4-trithiapentalenes exhibit a naphthalene-like 7r-electron delocalization. It should be noted, however, that the resonance... 

The very concept of resonance originated in VB theory and was extensively exploited in qualitative explanations of stability and other properties of tt-electron systems [78], surviving vitriolic attacks by some Soviet philosophers of the time. Recent re-examination of these ideas [61,65,79-82], based on new semiempirical and ab initio results, shows that the concept of resonance or 7r-electron delocalization is very subtle and prone to misinterpretation when improperly isolated. 

Calculations of the geometry of 106 have been reported by a number of groups l The most accurate data are those from the calculations of Kraka and Cremer based on MP2/DZ+P and CCSD(T)/DZ+P calculations The C(l)-C(6) distance in the latter work was found to be 1.572 A, which is clearly longer than the value found for cyclopropane. The C(l)—C(6) bond order is significantly smaller than 1, indicating a partial bond between these atoms. 7r-Electron delocalization, as reflected by the ratio of calculat-ed bond lengths [C(l)—C(2) 1.461,C(2)—C(3) 1.357, C(3)—C(4) 1.446 A], appeared to be... 

Zahradnik and Koutecky made a series of molecular orbital calculations by the HMO method, using the Longuet-Higgins model for the sulfur atom, on thiazole and the isomeric thiadiazoles. They found the largest 7r-electron stabilization for the 1,2,4 isomer, but very little dilFerence between the 1,2,5 and 1,3,4 isomers. The calculated bond orders, however, show a larger 7r-electron delocalization in the 1,2,5 than in the 1,3,4 isomer, in agreement with the results of Bak et The formal double bonds have a lower bond order in the 1,2,5 than in the 1,3,4 isomer, whereas the reverse is true for the formal single bonds. These relations hold for three different sets of parameters for the carbon-sulfur bond. 

If pyran-4-one is a 67r-electron aromatic system, it must adopt the betaine structure (212 X = O), which would exhibit a large dipole moment. The observed moment for (18) is 3.7 D. A dipole moment of 1.9 D is obtained by vector summation of bond and group moments. Clearly, if mesomeric effects were absent, the difference between the calculated and observed moments would be very small. The magnitude of the observed dipole moment suggests there is considerable 7r-electron delocalization in pyran-4-one. However, the calculated value for the betaine structure (212 X = 0) is 21 D (73MI22203) and thus it appears that the extent of delocalization is, after all, quite small. 

The TT-bonding leads to 7r-electron delocalization along the polymer chains and, thereby, to the possibility of charge carrier mobility, which is extended into three-dimensional transport by the interchain electron transfer interactions. In principle, broad Ti-electron bandwidths (often several eV) [10,11] can lead to relatively high carrier mobilities. 

A significant feature of this compoimd is the property of the C7-N2-N1-C2-C3 moiety with its planar, extended chain of atoms and its bond lengths and angles typical of sp hybridization. The resonance structures that could describe the 7r-electron delocalization are ... 

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