Which  -clamped conjugated monocycles exhibit ring currents? Remco W. A. Havenith, a Leonardus W. Jenneskens,* b Patrick W. Fowler* c and Alessandro Soncini c a Department of Chemistry, University of Warwick, Coventry, UK CV4 7AL b Debye Institute, Department of Physical Organic Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands. E-mail: jennesk@chem.uu.nl; Fax: +31-30-2534533; Tel: +31-30-2533128 c Department of Chemistry, University of Exeter, Stocker Road, Exeter, UK EX4 4QD. E-mail: P.W.Fowler@exeter.ac.uk; Fax: +44-1392-263434; Tel: +44-1392-263466 Received 26th January 2004, Accepted 10th March 2004 First published as an Advance Article on the web 1st April 2004 Quenching/survival of ring currents in π-clamped conjugated monocycles is controlled by the match or mismatch in parity between the frontier orbitals of the central π-conjugated 4n+2/4n monocycle and those of the clamps. Changes in ring current are not primarily caused by bond alternation or ‘Mills–Nixon’ effects; current and geometry changes on clamping are both consequences of electronic structure. Introduction The correspondence between 4n+2/4n π-electron counts and diatropic/paratropic ring currents in planar Hückel monocycles stems from their characteristic frontier-orbital structure. 1 Using the ipsocentric partition of total (σ + π) current density into orbital contributions, 2 it can be shown that the induced currents in the monocycles are dominated by HOMO–LUMO virtual excitations, and hence can be ascribed to just four of the 4n+2 π-electrons in the diatropic case, and to two of the 4n π-electro- ns in the paratropic case. 1 As part of the exploration of the concept of aromaticity, synthetic chemists have sought for many years to subvert the Hückel rules and to make unconventional monocycles by altering their natural geom- etries. Benzene rings with strong bond alternation, e.g. 1 3 and 2, 4 and planarised cyclooctatetraene (COT), e.g. 6, 5 have been produced using a “clamping” strategy, where annelation is used to impose the desired geometric constraints (Scheme 1). In this way, systems with positive and negative ‘Mills–Nixon effects’, 6 i.e., positive and negative bond length differences, ΔR = R 1 - R 2 have been devised (see also Scheme 1). As explicit mapping of current density shows, ring currents survive in some of these novel clamped systems but not in others; in 2 the characteristic diatropic benzene ring current remains strong whereas it is quenched in 1 and in the tris(cyclo- butadieno)-clamped model 4; 7 in 6 and 7 the paratropic ring current expected on the basis of the Hückel 4n rule is indeed found, but is then predicted to disappear in the COT analogue of 4, e.g. exo-8. 8 This variety of behaviour has a ready explan- ation in the orbital model based on the ipsocentric approach: the survival/quenching of currents in functionalised 7–9 and con- strained 10 systems such as 1–4 and 6–8, is rationalised by con- sidering the extent to which the HOMO and LUMO of the central ring remain intact, preserving the character of the dom- inant virtual excitation(s). Annelation with saturated moieties, as in 2, 6, and the related model systems containing cyclo- butano clamps (3 7,9 and 7 8 ) leaves the π HOMO and π* LUMO of the monocycle essentially undisturbed and, hence, preserves the diatropic/paratropic current. On the other hand, annelation with strongly interacting unsaturated moieties inserts new π and π* levels into the gap, changes the set of active virtual excit- ations and, hence, destroys the current (Fig. 1). This quenching effect has been demonstrated computationally for 1, 4 and its exo-COT analogue 8. 7–9 Our subject here is another clamping group that frequently features in discussions of aromaticity: the unsaturated 3,4-di- methylenecyclobuteno unit. 6,11,12 Although this group is π-conjugated, the predicted bond alternation for tris(3,4-di- methylenecyclobuteno)benzene (5, Scheme 1) is comparable to that in tris(cyclobutano)benzene (3) where the clamping group is saturated, and the computed nucleus-independent chemical shift (NICS) value (ca. -10 ppm. 12 ) in 5 indicates little change in the magnetic character of the central ring, compared to benzene itself. This appears to pose a problem for our rational- isation of currents in terms of a saturated/unsaturated dichot- omy. 7–9 How does this clamping group fit with the scheme presented in Fig. 1? The present paper reports calculations of current maps for tris(3,4-dimethylenecyclobuteno)benzene (5) and the COT analogues endo-9 and exo-10 (Scheme 1), which lead to a gener- alisation of the survival/quenching criteria. It is shown that pictorial molecular orbital theory explains these ab initio results and gives a concise predictive rationalisation of the effects of the different π and σ clamping groups on magnetic properties. Computational details Geometries Geometries were optimised at the RHF/6-31G** level of theory using GAMESS-UK. 13 Under the constraint of D 3h symmetry, tris(3,4-dimethylenecyclobuteno)benzene (5) was found to be a true minimum, as shown by a Hessian calculation (Table 1). In the case of tetrakis(3,4-dimethylenecyclobuteno) COT, there are two conceivable valence isomers denoted endo (9) or exo Fig. 1 The effect of clamping on the electronic structure of benzene and 1,3,5,7-cyclooctatetraene (COT). In benzene and COT itself the ring current arises from a translationally (T ) allowed HOMO–LUMO vertical excitation, and a rotationally (R) allowed HOMO–LUMO vertical excitation, respectively. 2 When saturated groups are attached (left), this transition is undisturbed, as the extra (usually σ) orbitals from the clamping group lie outside the active π space, but when unsaturated groups are attached (right), other magnetically active orbitals often intrude into the frontier region. The present paper discusses a case where no π intruder orbitals are in fact present. DOI: 10.1039/ b401125b 1281 This journal is © The Royal Society of Chemistry 2004 Org. Biomol. Chem. , 2004, 2, 1281–1286