Benzene diamagnetic ring current

Figure 5 The diamagnetic ring current effect in benzene.

This lack of a major diamagnetism may be attributed to two factors. First even in the completely delocalized cyclopropenium cation the diamagnetic ring current effects are small and in the range of only 25% of those in benzene as concluded from NMR data146. ... 

Diprotonation of (97) with H2SO4 gives a species which is protonated at both carbonyl groups, not the delocalized annulene dication, presumably for steric reasons.140 Oxidation dications of perylenes, e.g. (98), are [4n + 2] species with diamagnetic ring currents, whereas the dibcnzo[ /. // ] derivative [two more benzene rings at the top and... 

We start with some biographical notes on Erich Huckel, in the context of which we also mention the merits of Otto Schmidt, the inventor of the free-electron model. The basic assumptions behind the HMO (Huckel Molecular Orbital) model are discussed, and those aspects of this model are reviewed that make it still a powerful tool in Theoretical Chemistry. We ask whether HMO should be regarded as semiempirical or parameter-free. We present closed solutions for special classes of molecules, review the important concept of alternant hydrocarbons and point out how useful perturbation theory within the HMO model is. We then come to bond alternation and the question whether the pi or the sigma bonds are responsible for bond delocalization in benzene and related molecules. Mobius hydrocarbons and diamagnetic ring currents are other topics. We come to optimistic conclusions as to the further role of the HMO model, not as an approximation for the solution of the Schrodinger equation, but as a way towards the understanding of some aspects of the Chemical Bond. 

The role of ionic structures is crucial, and it can be appreciated based on a simple symmetry analysis. Indeed, as shown recently (24), the mixing of all ionic structure types into the spin-alternant determinant in benzene mediates the circular electronic motion (between the two determinants) and induce the diamagnetic ring current in a magnetic field. By contrast, the exclusion of the diagonal ionics in CBD excludes this motion. Exercise 5.4 demonstrates qualitatively this dichotomy of ring currents. 

It is experimentally known that benzene exhibits diamagnetic ring currents, while cyclobutadiene (CBD) does not. In this exercise, we will consider the wave functions of benzene and CBD as primarily made of their spin-alternant determinants HqC and I1qC, and we will model ring currents by the passage from one spin-alternant determinant to the other (HqC Hqc), which is associated with a collective circular flow of electrons. 

This ring current in benzene is termed diamagnetic and is characteristic of aromatic compounds in general. The presence of a diamagnetic ring current provides a useful experimental criterion for the presence of aromaticity in a compound. Other examples of the use of this method are provided in Section 16.10. 

Comparison of the n —To-values for 342 and 343 reveals that the annelation of one benzene ring suppresses more strongly the diamagnetic ring current in the 14-membered ring than the annelation of one naphthalene nucleus. An appreciable intensification of the diatropicity was observed in the benzonaphtho derivative 344. However, the tj —To-value for 344 was found to be smaller than that for the fully symmetrical dinaphtho derivative 340. 

The aromatic nature of the [18]porphyrin-(2.1.0.1) derivative 3.135, as well as analogs 3.136, 3.137, and 3.144 (systems we will hereafter refer to as corrphycenes), was confirmed by the observation of sustained diamagnetic ring current effects in its H NMR spectrum. The UV-vis absorption spectrum (in benzene) is also consistent with the proposed aromaticity and with an 18-7t-electron formulation. Specifically, a... 

Aromaticity The property of having (4 +2)7t electrons in a closed circuit of molecular orbitals. The commonest example is benzene, where n equals one, around a six-carbon ring. A more precise definition is the ability to sustain an induced diamagnetic ring current, i.e. one that is diatropic. 

The acetylacetonate chelates 8°1 27 form an extensively studied class of complexes for which the suggestion that cyclic conjugation should lead to aromatic stability was applied historically for the first time 81.82), However, Musso etal. 83.84) showed by analysis of vibration spectra that the ti bonds in the chelated ligands are completely delocalized and the use of a mesityl substituent in position 3 as an indicator for diamagnetic ring currents showed no diamagnetic anisotropy comparable to that in benzene. They therefore discarded the concept of cyclic delocalization and aromatic character in these compounds. 

Most striking is the difference between the low temperature spectr. of (4n)annulenes and (4n+2)annulenes, as previou.sly discussed in Chapter I. The latter give evidence of a diamagnetic ring current with the outer protons deshielded, as in benzene, and the protons within the ring shielded, whereas the former have the outer protons shielded and the inner protons deshielded. This is illustrated by the chemical shifts listed in the following table ... 

Benzene is the prototypical aromatic molecule with 6 n electrons, perfect Z)6h symmetry, aromatic stabilization energy of about 36 kcal/mol, a NICS value of roughly -9 ppm and an appreciable amount of diamagnetic ring current. Aromaticity of an arbitrary molecule is some times judged through its resemblance with benzene via parameters like Polansky index, molecular similarity, Clar s sextet etc. 

Cemusak et al have used the Coupled HF method to study ring currents in six cyclic isomers of (CH)2B2N2. Correlated calculations show that all six have planar structures and Tt-electron diamagnetic ring currents. The nuclear shielding produced by the ring current is compared with that of benzene. 

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