IGLO calculations of the magnetic susceptibility tensors as well as of the 13C and 'H shielding tensors areperformed and analyzed for benzene (l), the hypothetical cyclohexatriene (2) with alternating single and double bonds,some benzene isomers like fulvene (3), and other related molecules, including ethylene (8), the two 1,3-butadienes (9,lo), and eight hexatrienes (11-18). The susceptibilities of the noncyclic polyenes as well as the benzene isomers 4 and5 are rather well described by an increment system both for the in-plane and out-of-plane components, that of benzeneonly for the in-plane component, while for the out-of-plane component there is an additional shielding of 42 ppm cgs,which is reduced to 33 ppm cgs in cyclohexatriene. The other two molecules of this study with a somewhat enhancedout-of-plane susceptibility are fulvene (13 ppm cgs) and cyclopentadiene (17 ppm cgs). Evidence is accumulated thatthe increased susceptibility in benzene comes from the n-electron system and is an indication of a nearly-free circularcurrent in the n-system of benzene. This becomes particularly manifest if one uses the center of the molecule as a gaugeorigin for the zz-component of the shielding due to the occupied PMOS. For this gauge the shielding is almost entirelydiamagnetic, as required for a free current. In cyclohexatriene this current is somewhat attenuated, as seen from largerparamagnetic contributions. The increased susceptibilities in fulvene and cyclopentadiene are also due to the x-system,though no genuine ring current effects can be detected. In the case of the 1H shielding a different mechanism isresponsible for the deshielding in ethylene (and polyenes) with respect to methane (or paraffins) and for the extradeshielding in benzene relative to ethylene. The latter is due to the occupied n-MOs and related to the ring current.It is calculated to be - 5 ppm in the out-of-plane component, but it is reduced to less than 2 ppm in the isotropic shielding.Ring current effects are not detectable in any other molecules of this study, but in complementary investigations ofother Huckel or anti-Huckel annulenes. The 13C shift tensors show a very complicated pattern with large variationsalready between the isomeric hexatrienes, which are hard to interpret within an increment system. While the shifttensors in fulvene don't differ too much from what one expects for carbon atoms in a similar bonding situation, bis-(methy1ene)cyclobutene and tris(methy1ene)cyclopropane are affected by strong ring strain effects. Benzene differsfrom polyenes mainly in a shielding of the out-of-plane component that is unusually strong for a CH carbon in anunsaturated system. Conclusions on a ring current effect would be premature.
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|Anno di pubblicazione:||1994|
|Titolo:||IGLO study of benzene and some of its isomers and related molecules. Search for evidence of the ring-current model|
|Autori:||U. FLEISCHER; W. KUTZELNIGG; P. LAZZERETTI; V. MUEHLENKAMP|
|Appare nelle tipologie:||Articolo su rivista|
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