7r-bond order

Finally, the CN and CC 7r-bond orders can be determined by use of a relation between bond orders and bond length (122) and the experimentally measured bond length (159). 

In Table 1-9 we have collected only the 7r-bond orders calculated by allvalence-electrons methods and compared their values with those deduced from experimental bond lengths. Both data are indicative of an aromatic molecule with a large dienic character. The 2-3 and 4-5 bonds especially present a large double-bond character, whereas both C-S bonds are relatively simple. 

Using a similar approach, Evans and co-workers obtained values of hsi = -1.50, kcsi = 0.55, 8 = 0.15 for a series of trimethylsilyl-substituted naphthalene radical anions based on a Q value of -26.1 49). For phenyltrimethylsilylacetylene radical anion, the values of the heteroatom parameters that gave the best fit of the Huckel calculated spin densities with experimental values, using a Q value of 28, were hs, = -1.3, and kCSi = 0.65 when a 8 value of 0.1 was assumed. A resulting C—Si 7r-bond order of about 0.3 is obtained 43). 

The work of Taddei et al.230 on imidazol2,1 -6]thiazole 337 and derivatives has interesting implications on the structure of azapen-talenes, and an important aspect of this study is that the molecular geometry used for calculations on 6-phenylimidazo[2,l-6]thiazole 417 was obtained from X-ray structure determinations130b (Section V,A). The reactivity of this system (Scheme 18, Section IV,C,4,b) is better correlated with Tr-electron densities than with total charges, and 7r-bond orders (by the PPP method) show that the thiazole part of the molecule is more localized than the imidazole part (Section VII). Proton chemical shifts, except that of the H2 proton a to sulfur (Section V,G,2), vary linearly with the total charge carried by the ring carbon atoms. 

The differences between the experimental bond distances of sulfur bases (Figs. 18 and 19), and those of the parent molecules.7 6 The differences between calculated 7r-bond orders of uracil and of its thio analogs (numbers in parentheses). 

The IR spectra of coumarin and isocoumarin show carbonyl frequencies characteristic of lactones. There is no evidence to suggest that contributions from betaines (207) and (208) are significant. Moreover, EHT MO calculations predict high ir-bond orders for the carbonyl groups in these molecules. IR data and C=0 7r-bond orders are presented in Table 10. 

An MO study of all twenty pyranopyrandiones has been reported (73MI22200). Use was made of HMO calculations together with the co technique. Calculations were also performed on some methyl-substituted pyranopyrandiones and afforded values for the delocalization energy, 7r- bond orders and tt- charge densities. Empirical resonance energies were obtained... 

Due to the higher reactivity of the C-C bonds at the poles of the C70 molecule that have the highest 7r-bond orders, they are exclusively involved in the above mentioned dimerizations-cycloadditions. Therefore, five [2+2] addition isomers of (C70)2 and two of the cross-dimers (C60)(C70) were formed that were finally resolved in the pure forms with the aid of recycling high-pressure liquid chromatography (HPLC). In all these all-carbon dimers (Scheme 3), the [2+2] connecting bonds can be easily cleaved under electroreductive conditions, suggesting that such kinds of dimeric materials can be used as prototypes in molecular switches based on fullerene architectures. 

Typical coupling constants for isothiazoles are given in Table 5. The electronegative nitrogen atom reduces 3/3>4 and 4/3>5 from the values of 3.50 Hz and 0.27 Hz, respectively, in thiophene. The 4J values correlate quite well with 7r-bond orders calculated by MO methods (74CJC833). 

Similarly, the vicinal coupling constants, 3-/H( >h( +1)> also take alternating values related to the 7r-bond order by a linear relationship86. 

Bond distances (in A) NBO s-characters (in %), Lowdin s 7r-bond orders and Lowdin atomic charges of deliberately distorted benzene and perfluorobenzene as obtained by the MP2(fc)/6-31G model. 

Molecule Bond Distance s-character 7r-bond order A... 

Figure 7. The least-square fit quadratic curves relating the CC bond distances in the D3/, distorted benzene to the average s-character (a) and the 7r-bond order (b).

Figure 10. Schematic illustration of a tendency of each benzene fragment in naphthalene to retain its aromaticity by producing cis 1,3-butadiene partial localization in its twin-ring as described by the resonance structures (7a) and (7b) yielding the resulting predominant canonical structure (7c). This intuitive argument is supported by the (HF/6-31G ) bond distances and the corresponding 7r-bond orders given within parentheses.

Geometry of cyclopropene and its deliberately deformed structure, s-characters (in %) of hybrid orbitals and Lowdin 7r-bond orders. 

Perusal of CC bond distances and the accompanying 7r-bond orders presented in Table 7 reveals that an appreciable MN effect occurs in systems 20-22. This is evidenced by alternation of their IBDs values and differences (benzene) relative to free benzene gauge value. Bond fixation is reflected in the corresponding bond localization indices L(d). They read 0.064(0.068), 0.099(0.167), 0.155(0.242) and 0.153(0.238) for 12, 20, 21, and 22, respectively, where the first number refers to the MP2 results whereas the Hartree-Fock model calculations are given within parentheses. It follows that oxa-heteroanalog 21 exhibits the largest MN effect. It is also apparent that the HF model exaggerates the bond fixation in the aromatic moiety, but its qualitative predictions are correct in most cases. 

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