JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 6, 163-170 (1993) zyxwv [ 41 METACYCLOPHANE: STO-3G MOLECULAR STRUCTURE, STRAIN ENERGY AND CNDO/S-CI ELECTRONIC TRANSITION ENERGIES LEONARDUS W. JENNESKENS* AND JAAP N. LOUWENT zyxw Debye Institute, Department zyxwvuts of Physical Organic Chemistry, Utrecht University, Padualaan 8, 3584 zyx CH Utrecht, The Netherlands The molecular structure of [4]metacyclophane is optimized at the ab inirio STO-3G SCF level and zyx a genuine minimum is located. In the STO-3G structure the benzene ring is less bent and geometrically distorted than in the previously reported MNDO structure. Although some bond alternation is calculated for the carbon-carbon bond lengths of the bent benzene ring, their values still fall in the range of highly delocalized compounds. Despite the fact that the STO-3G strain energy [SE(tot.)] is larger than the corresponding MNDO strain energy, the distribution of SE(tot.) over the bent benzene ring [SE(bb.)] and the oligomethylene bridge [SE(br.)] is similar. To facilitate the hitherto unsuccesful identification of [ 4 ] metacyclophane, its electronic transition energies were calculated with CNDO/S-CI using the STO-3G geometry as input geometry. The reliability of the CNDO/S-CI results was assessed by a comparison of calculated and experimental electronic transition energies for p- and, m-xylene, [ 5 ]para- and [ 5 ] metacyclophane and [ 4 ] paracyclophane following the same approach. INTRODUCTION Small [nlcyclophanes are of interest for the investi- gation of the occurrence of electron delocalization in bent benzenes. Consequently, their synthesis and char- acterization have received considerable attention in recent years. In the [nlparacyclophane series both [5] -4 and [4]para~yclophane~.~ have been identified spectroscopically and characterised in solution and matrix, respectively. In contrast, in the [n] meta- cyclophane series, [S]metacyclophane is still the smallest unequivocally characterized representative. ' Although semi-empirical MNDO' calculations indicated that the next lower homologue [4] meta- cyclophane is less strained than [4] paracyclophane9-10 it has hitherto escaped spectroscopic identification. Nevertheless, compelhg evidence for its formation as a fleeting intermediate was deduced from the thermo- lysis of its Dewar benzene valence isomer tri- cycle [6.1.1.0(3,9)] de%a-2,8( 10)-diene in either sealed ampoules at 150-200 C or by flash vacuum thermolysis at 400-500 "C. Obviously, the severe thermolysis con- ditions used for its generation hamper the isolation and identification of the elusive [4] metacyclophane. .- Similar observations have been reported previously in attempted syntheses of [5] zyxw -I3 and zyx [4] paracycl~phanel~ by thermal isomerization of the corresponding 1,4- bridged Dewar benzene isomers. The available exper- imental data suggest that the identification of [4]metacyclophane will have to wait for new synthetic approaches. Note that also irradiation at -50°C of tricyclo [6.1.1.0(3,9)] deca-2,8(10)-diene gave the cyclo- propane edge-bridged prismane derivative which rearranged to a fulvene; no evidence for the formation of [4] metacyclophane was found. Is With respect to the theoretical investigation of small [n]cyclophanes, it has recently been shown that the minimal ab initio STO-3G l6 basis set gives an improved description of the geometries of these strained mol- ecules in comparison with semi-empirical MND0911' and molecular mechanics results; especially the extent of distortion of the benzene ring is better described. Although STO-3G results have been pub- lished for [n] paracyclophanes with n = 8, 7, 6, 5 and and for [S]metacyclophane, 25 to our knowl- 4 10.22 - 24 edge no data are available for [4]metacyclophane. In this paper we report its STO-3G molecular structure, strain energy [SE(tot.)] and the distribution of SE(tot.) * Author for correspondence. t Present address: Akzo Research Laboratories Arnhem, Corporate Research, P.O. Box 9300, 6800 SB Arnhem, The Netherlands. 0894-3230/93/030163-08$09.00 zyxwvut 0 1993 by John Wiley & Sons, Ltd. Received June 1992 Revised 28 September 1992