Peripheral bonding

For designating the position of fusion, the peripheral bonds of the base component, starting with the 1,2-bond, are consecutively assigned italic letters in alphabetical order, following the direction of numbering thus in quinoxaline (35) the 1,2-bond is assigned... 

Aromaticity. The aromaticity indices (based on deviations in peripheral bond orders) for quinoxaline and related azaheterocycles have been calculated and they show good correlation with independently calculated resonance... 

The bond lengths for the dianions of I, III, IV and VII shown in Fig. 5 indicate that these dianions all have a fully-symmetrical nuclear arrangement and that the bond lengths of the peripheral bonds of these anions are fairly equivalent. This means, in agreement with the results obtained using the static theory, that addition of two more electrons to molecules I, III and VII to form their dianions results in a complete disappearance of the bond alternations existing in these molecules. Dianions of ss) III , and iv have been prepared and are known to be stable species. 

Open M4 units are also quite rare. In the selenobromides M4Se16Br2 (M=Nb, Ta) there are tetranuclear units M4(p-Se2)r,, which result from condensation of three M2( r-Se2)2 clusters. The M-M distances within them are about 3.1 A.52 Single crystal structure determination of V5S8 revealed the presence of rhombic vanadium clusters in the structure with hinge V-V bond of 2.88 A, peripheral bonds of 3.04 A and strong intercluster bonding (V-V 2.92 A).53... 

Cluster Configuration Peripheral Bonds Delocalized (r-honds Delocalized ir-bonds Antiaromaticity... 

Aromaticity indices based on a statistical evaluation of peripheral bond orders have been derived for five-ring heterocycles a value of 50 for 1,3,4-oxadiazole compares with values of 43 and 66 for furan and thiophene respectively <85T1409>. Somewhat in contrast, O—C, C—N, and N—N bond orders of 1.3124, 1.9062, and 1.3348 (MMX, EXE calculation) for 1,3,4-oxadiazole led to the conclusion that the molecule was not aromatic <91H(32)2023>. 

An index of aromatic character based upon statistical evaluation of the deviations in peripheral bond orders has been devised and applied to five-membered ring heterocycles and their mesoionic derivatives including 1,2,3,4-thiatriazole <85TI409> and 5-phenyl-l,2,3,4-thiatriazol-3-oxide <93T8441>. See also <93QSAR146>. 

The carbon-hydrogen bonds are the prototype of peripheral bonds. In the perspective of the CNE effect, the extraction of an atom that was partially charged in the host... 

This echoes Sanderson s claim [244,245] that all contributing CH bond energies are equal. The behavior observed here for the CH bonds is typical for peripheral bonds. Indeed, it holds tme also for CX bonds (X = Cl,Br,l), where scx + CNE constant [44]. 

The thermal rearrangement of spiropentanes to methylenecyclobutanes proceeds via two successive bond cleavages. First, a peripheral bond breaks to give a 1,3-diradical 28, and then a radical bond breaks to give a 1,4-diradical 29, until ring closure yields the product(s). 

Second, the ordinary Lewis structure describing the [l.l.l]propellane molecule is less adequate than is usual for hydrocarbons. This is demonstrated by the fairly large interaction matrix elements between the occupied localized orbital of the central bond and the vacant localized peripheral antibonding orbitals, and between the unoccupied localized central antibond orbital and the occupied localized peripheral bond orbitals. Bonds in [1.1.1 [propellane are quite a bit more delocalized than is usual in saturated hydrocarbons, and this is perhaps the reason why the empirical correlations for NMR coupling constants fail. 

Thermolysis (200°) of bicyclo[1.1.0]butane (3) severs two bonds to produce 1,3-butadiene, but in the presence of 1, 3 undergoes cleavage of the central allyl carbenoid intermediate, which reacts with an electron-deficient alkene to give an allylcyclopropane (equation II).3... 

One contra-intuitive feature of the Wiener index is that more distant atom pairs make a larger contribution to W than adjacent atom pairs (Randic and Zupan, 2001). A typical characteristic of W is that the central C-C bonds make a greater contribution than the peripheral bonds (Figure 5.5). For many bond additive physicochemical properties, such as boiling point, the opposite may be true, (i.e., the terminal bonds are considered more important to determine the magnitude of the property). 

An appropriately substituted pyridine substrate may be cyclized to a 1, 7-naphthyridine by completion of any one of the 10 peripheral bonds in the product. 

Aromaticity. An aromaticity index, based on deviation of peripheral bond orders,1691 has been applied to pyrazine (89% of that for benzene) and some derivatives.257,376,379,383 The aromaticity of l,s4-dihydropyrazines has been studied.565,1734... 

As structural criteria of aromaticity, indexes N= + b (where R are bond lengths, and a and b are parameters characteristic for a given pair of atoms) and Julg s parameter J = 1 — 225/ S,s(l —d Jdf (where n is the number of peripheral bonds rs, are their lengths, and d is their mean length) have been applied. The less the value SaA and closer to 1 the value J, the greater is the aromaticity of a heterocycle. Polarizability exaltations F = M — M , where M and M arc the mean dipole polarizability and the mean atomic or group polarizability, respectively. 

Bicyclo[1.1.0]butanes (1 equation 1), the smallest bicyclic hydrocarbons, are highly strained but stable at ambient temperature. Under the influence of a catalytic amount of a nickel(0) complex, however, they undergo cleavage of the central bond and one of the four peripheral bonds and enter into cycloaddition reactions with electron deficient alkenes to afford allylcyclopropane derivatives. Reaction of... 

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