Pyrrole bond lengths

Figure S.S Structure of pyrrole (bond lengths in pm, bond angles in degrees).

The sydnones may be represented by structures (123a-d), of which the zwitterionic structure (123a) most clearly implies an aromatic sextet. The diamagnetic susceptibility exaltation for Af-phenylsydnone of ll.Ox 10 cm moP is comparable with the corresponding value for pyrrole (10.2x10 ). 3-p-Bromophenylsydnone (123 R = H, R = p-bromophenyl) is essentially planar however, the O—N bond and 0(1)—C(5) bond lengths are not very different from normal single bond distances. 

However, it was pointed out that two other observations are out of line with the iron(I) formulation and more consistent with an iron(II)-porphyrin radical anion [290] (1) the low-intensity red-shifted Soret band in the UV-VIS spectrum with broad maxima in the a,(3-region compared to, for instance, Fe(TPP) in THF, is typical of a porphyrin radical, and (2) the bond lengths of the porphyrin core indicate population of the (antibonding) LUMO of the ligand (i.e., the presence of an extra electron in the re-system). The presence of porphyrin radical character in the electronic ground state was also inferred from the paramagnetic NMR-shifts of the pyrrole protons at the meso and p-carbon atoms [291]. 

The lack of reactivity of (45) and CEP-pyrrole towards alcohols is attributed, at least partly, to the double bond character of the phosphorus-nitrogen bond as evidenced by the crystallographically determined abnormally short P-N bond lengths. 

The difference between the Fe-S bond lengths in the high spin and low spin states is about 0.15 A, which is also in line with the Fe-S bond lengths for the low spin tris(l-pyrrole-dithiocarbamato)iron(III) hemikis(dichloro-... 

A polymeric chain stracture is observed in Iit2ln(pyr) I, (Hpyr = pyrrole n = 2 in the vapour) the distorted trigonal bipyramidal geometry at each indium features equatorial coordination of the ethyl (In C bond lengths are 2.141(5) and 2.143(5) A) and amido... 

Figure 1 Bond lengths for pyrrole, indole and carbazole as estimated by HMO theory...

Schomaker and Pauling s electron diffraction studies27 gave approximate values to the bond angles and bond lengths of pyrrole. The microwave rotational spectrum gave more accurate dimensions and also confirmed the planarity of the molecule. The data indicated... 

The aromaticity obtained by means of the DE values and their differences in the single pseudoazulene systems are also the result of calculations of the bond orders (Fig. 1). The calculated bond alternations46 57-77-79-82-117118-129 are in line with the heteroaromaticity of the systems. Similar relationships were calculated for azulene.219 The extent of the bond alternation, however, differs from one system to the other (see Fig. 1). In indeno[2.1-f>]-l-benzo-pyran (44a) the bond lengths of the central cyclo[fo]pyran system indicate pronounced bond alternation, and in the opinion of the authors, the systems have little aromatic character.129 Calculations with 1 //-pyrindines (26) and 2//-pyrindines (29) show that 26 is more stable than 29, but both are less aromatic than indole.63 A comparison of SCF calculations for pseudoazulenes 26 and 29 with their aza analogs 68 and 69-73, which contain a pyrrole-type nitrogen, shows only minor perturbations of carbon-carbon bond lengths on replacing a =CH— moiety by =N—.66... 

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