Distance in rings

The frequently observed equalisation of experimental interatomic distances in ring structures such as (3.126), however, often does not enable a distinction to be made between ordinary primary covalent bonds and dative bonds. Consequently arrowed bonds are often omitted from the structural formulae of these types of compound). 

Fig. 29. Variation in periodic distances in ring-banded spherulites in PCL/SAN blends with (T .-Tg) for different compositions data taken from [107]...

How do the bond distances of 1 2 epoxypropane change on protonation of the nng oxygen" Assum ing that the longer C—O bond is the weaker of the two do the bond distances in the protonated form correlate with the regioselectivity of acid catalyzed ring opening ... 

In resonance terms the shorter carbon-oxygen bond distance in phenol is attrib uted to the partial double bond character that results from conjugation of the unshared electron pair of oxygen with the aromatic ring... 

The CC bond distances in cyclopentadienyl anion, C5H5, are all equal, because the anion is aromatic (see Chapter 12, Problem 10). Electrophiles that interact electrostaticaUy with the anion, such as Na", interact equally with all five carbons, and do not disturb the anion s aromatic character. On the other hand, electrophiles that make covalent bonds, such as H", might interact more strongly with one particular carbon and destroy the aromaticity of the ring. 

Energies are in a.u., charges iq) and BOPs in electrons, distances in A and angles in degrees. There are two kinds of oxygen, since the ring is puckered. 

It is seen that the calculation of a radial distribution curve is closely similar to the calculation of a simplified theoretical intensity curve for the usual visual method, the summation being over the rings seen on the photograph instead of over the interatomic distances in the assumed model but whereas in the usual treatment the calculation may need to be repeated for many models, a single curve only is required for the new method. 

Bromine and Chlorine.—Accurate values of the internuclear distances in the molecules Bra and Cl2 are known from band spectral studies, namely, Br-Br = 2.281 A. and Cl-Cl = 1.988 A. The visual method led to results (2.289 A. and 2.009 A., respectively) in satisfactory agreement with these.8 Radial distribution curves for these substances are shown in Fig. 3, the data used being given in Tables II and III. For bromine, with seven rings, three different estimates of intensities lead to the same Br-Br distance, 2.270 A., less... 

There is little doubt that the C-C distance in the benzene ring retains in its methyl derivatives the value 1.39 A. found for benzene itself.19 The main question regarding these substances is whether the distance from methyl carbon to ring carbon is determined by the usual single-bond radius, having thus the value 1.54 A., or whether... 

The photographs obtained for pyridine and py-razine are so closely similar to those of benzene as to leave no doubt that the three molecules have nearly identical structures. The radial distribution curve for pyridine (Fig. 1) calculated from the data given in Table II has well-defined peaks at 1.38, 2.39 (= V3 X 1.38), and 2.76 (= 2 X 1.38) A. The sharpness of the 2.39 peak indicates that the six meta distances in the ring are nearly equal. The calculated intensity curve for the model with C-C = 1.39 A., C-N = 1.33 A., C-H = 1.08 A., the angle C-N-C = 119° and the angles C-C-C = 121°, and having nearly equal meta and para distances, is shown in Fig. 3. The comparison of s0 and s values for this model (Table II) leads to the values C-H =... 

The initial results on the [NaA ] clusters have been extended to the heterocyclic ring species [XAls] (X = Si, Ge, Sn, Pb) bearing a single charge and as a result are not prone to dissociation of an electron like the [Al4] or [Ga4H2] anions [69]. A planar four-membered ring structure is in all cases the most stable structural isomer with Al—Al distances in the range 2.57-2.61 A. 

We can therefore conclude that in 4n systems antiaromaticity will be at a maximum where a molecule is constrained to be planar (as in 59 or the dianion of 83) but, where possible, the molecule will distort itself from planarity and avoid equal bond distances in order to reduce antiaromaticity. In some cases, such as cyclooctatraene, the distortion and bond alternation are great enough for antiaromaticity to be completely avoided. In other cases (e.g., 96 or 103), it is apparently not possible for the molecules to avoid at least some p-orbital overlap. Such molecules show paramagnetic ring currents and other evidence of antiaromaticity, although the degree of is not as great as in molecules such as 59 or the dianion of 83. 

A crystallographic scale of acidity has been developed. Measuring the mean C—H O distances in crystal structures correlated well with conventional P a(DMSO) values. An ab initio study was able to correlate ring strain in strained hydrocarbons with hydrogen-bond acidity. ... 

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