Aromatic exaltation

Pascal increment system for diamagnetic susceptibility aromatic exaltation M... 

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. 

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic. 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

All the same, the quantitative determination of the aromaticity and antiaromaticity from the ring current model may be complicated by at least two problems. First, experimentally observable values of magnetic susceptibilities and their exaltations and anisotropies as well as the H-NMR chemical shifts are not necessarily determined exclusively by ring currents hence, all other effects have to be identified and removed. Naturally, for this model to work, the contribution by the ring current must be predominant. Another problem is that the calculated results on ring current intensities for molecules from the diatropic-paratropic border area may vary qualitatively depending on the method of calculation (80PAC1541). 

Another quantitative characteristic of the magnetic manifestation of aromaticity is represented by the exaltation of the total magnetic susceptibility A (68JA811 75MI2). For conjugated compounds, this parameter is given by the difference between Xm and Xm standing, respectively, for the experimentally measured molar susceptibility and the molar susceptibility... 

The pK values of phenols in singlet and triplet states are valuable guide to substituent effect in the excited states, specially for the aromatic hydrocarbons. In general, the conjugation between substituents and -electron clouds is very significantly enhanced by electronic excitation without change in the direction of conjugative substituent effect. The excited state acidities frequently follow the Hammett equation fairly well if exalted substituent constants a are used. 

Magnetic criteria have received wide application mainly as a qualitative test for aromaticity and antiaromaticity. The values of the exaltation of diamagnetic susceptibility (in 10-6A cm-3 mol-1), and therefore aromaticity, decrease in the sequence thiazole (17.0) > pyrazole (15.5) > sydnone (14.1). The relative aromaticity of heterocycles with a similar type of heteroatom can be judged from values of the chemical shifts of ring protons. The latter reveals paramagnetic shifts when Tr-electron delocalization is weakened. For example, in the series of isomeric naphthoimidazoles aromaticity decreases in the sequence naphthof 1,2-djimidazole (8 = 7.7-8.7 ppm) > naphtho[2,3- perimidine (8 = 6.1-7.2 ppm). This sequence agrees with other estimates, in particular with energetic criteria. 

Magnetic rotation (Faraday) effects have been little used for assessing aromaticity. Furan has only a very weak effect, and when allowance is made for steric strain in the ring the exaltation, Ep> which can be considered to be a measure of aromaticity, is —0 compared with 45 for pyrrole, 52 for thiophene and 182 p,r for benzene (69Mi3iooi). 

The usefulness of magnetic susceptibility and diamagnetic susceptibility exaltation measurements as a means of assessing aromaticity has been outlined elsewhere (68JA811, 74AHC(17)255, B-75MI22206). 

---