IGLO STUDY OF BENZENE AND SOME OF ITS ISOMERS AND RELATED MOLECULES -SEARCH FOR EVIDENCE OF THE RING CURRENT MODEL

IGLO calculations of the magnetic susceptibility tensors as well as of the C-13 and H-1 shielding tensors are performed and analyzed for ben zene (1), the hypothetical cyclohexatriene (2) with alternating single and double bonds, some benzene isomers like fulvene (3), and other re lated molecules, including ethylene (8), the two 1,3-butadienes (9,10) , and eight hexatrienes (11-18). The susceptibilities of the noncyclic polyenes as well as the benzene isomers 4 and 5 are rather well descr ibed by an increment system both for the in-plane and out-of-plane com ponents, that of benzene only for the in-plane component, while for th e out-of-plane component there is an additional shielding of 42 ppm cg s, which is reduced to 33 ppm cgs in cyclohexatriene. The other two mo lecules of this study with a somewhat enhanced out-of-plane susceptibi lity are fulvene (13 ppm cgs) and cyclopentadiene (17 ppm cgs). Eviden ce is accumulated that the increased susceptibility in benzene comes f rom the pi-electron system and is an indication of a nearly-free circu lar current in the pi-system of benzene. This becomes particularly man ifest if one uses the center of the molecule as a gauge origin for the zz-component of the shielding due to the occupied pi-MOs. For this ga uge the shielding is almost entirely diamagnetic, as required for a fr ee current. In cyclohexatriene this current is somewhat attenuated, as seen from larger paramagnetic contributions. The increased susceptibi lities in fulvene and cyclopentadiene are also due to the pi-system, t hough no genuine ring current effects can be detected. In the case of the H-1 shielding a different mechanism is responsible for the deshiel ding in ethylene (and polyenes) with respect to methane (or paraffins) and for the extra deshielding in benzene relative to ethylene. The la tter is due to the occupied pi-MOs and related to the ring current. It is calculated to be similar to 5 ppm in the out-of-plane component, b ut it is reduced to less than 2 ppm in the isotropic shielding. Ring c urrent effects are not detectable in any other molecules of this study , but in complementary investigations of other Huckel or anti-Huckel a nnulenes. The C-13 shift tensors show a very complicated pattern with large variations already between the isomeric hexatrienes, which are h ard to interpret within an increment system. While the shift tensors i n fulvene don't differ too much from what one expects for carbon atoms in a similar bonding situation, bis-(methylene)cyclobutene and tris(m ethylene)cyclopropane are affected by strong ring strain effects. Benz ene differs from polyenes mainly in a shielding of the out-of-plane co mponent that is unusually strong for a CH carbon in an unsaturated sys tem. Conclusions on a ring current effect would be premature.