Diamagnetic susceptibility exaltation in hydrocarbons

Abstract

Diamagnetic susceptibility exaltation, defined as the difference between the measured molar susceptibility and the susceptibility estimated neglecting the contribution of ring current, is found to be a property solely of aromatic compounds by calculating the exaltations of a wide variety of aromatic and nonaromatic hydrocarbons. Nonbenzenoid aromatic compounds exhibit exaltation, pseudoaromatic compounds do not. Exaltation is proportional to the theoretically calculated "London diamagnetism" of aromatic compounds. espite more than 30 years of strong interest in the

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

“…Aromatic compounds sustain diatropic ring currents with bond length equalization, while antiaromatic compounds sustain paratropic ring currents in spite of localized structures [31][32][33][34]. As a result, diatropic ring currents cause exaltations of diamagnetic susceptibility from the sum of that of acyclic reference compounds [35,36]. Also in NMR chemical shifts, shielding and deshielding effects are observed inside and outside aromatic rings, respectively, while antiaromatic paratropicity results in the opposite effects [32,34].…”

Efficient mapping of ring currents in fullerenes and other curved carbon networks

“…A widely accepted criterion of aromaticity for a cyclic system is its ability to sustain a diatropic ring current induced by the presence of a perpendicular external magnetic field [10][11][12][13][14][15][16]. Such currents can be inferred indirectly from their effects on observable magnetic properties (exaltation of magnetisability [17][18][19], downfield chemical shifts of external hydrogen nuclei [20]) and properties that are sub-observables, in Hirschfelder's sense [21], such as the characteristically negative NICS value at the ring centre or some other chosen point [22]. Inferences of this type all have difficulties associated with masking of ring-current effects by other contributions to the property [23][24][25][26][27], but ring-current aromaticity can in fact be deduced directly from calculation of the induced current itself, by solution of the Schrödinger equation for the molecule immersed in the magnetic field, using an appropriate distributed-origin method [28][29][30][31].…”

“…This idea was taken up in the 'pentagonproximity' model for NICS(0) values in faces of leapfrog fullerenes [42]. Currents may also extend over larger circuits: calculations on the functionalised fullerene derivative C 60 F 15 X 3 (X = [CBr(CO 2 Et) 2 ]), for example, show a global (diatropic) current delocalised over a [18] trannulene π-subsystem of the cage, which has the typical two-orbital, four-electron signature of the (4n + 2) monocycle [38]. From the existing results it is clear that there are patterns here that can often be rationalised using simple mechanical or orbital arguments.…”

Diamagnetic susceptibilities and susceptibility exaltations of some 7-substituted cycloheptatrienes

“…It is interesting that the introduction of a second substituent on C,, e.g. 10 with two C, methyl groups, lowers the susceptibility exaltation back to a value slightly less than that of the unsubstituted system 1.…”

“…It is difficult on the basis of chemical shift arguments to establish the presence of a diamagnetic ring current in a molecule, particularly as the whole question of the origin of the typically low-field resonances of aromatic protons has been called into question (9 (10). calculated assuming a C2*-C, (0) bond increment = 4.6 a series of 7-substituted cycloheptatrienes and have found the magnitude of the exaltations to be very strongly dependent on the substituent used.…”