Volume 157, number 4 CHEMICAL PHYSICS LETTERS 12 May 1989 ON THE INTER-RING SEPARATION OF THE LOWEST EXCITED AND IONIZED STATES OF [2.2]PARACYCLOPHANE Sylvio CANUTO ’ and Michael C. ZERNER Quantum Theory Project, University of Florida. Gainesville, FI. 3261 I, USA Received 20 January 1989; in linal form 2 February 1989 The geometries of the ground state, the four lowest excited states and the lowest triplet state of [2,2]paracyclopbane are ob- tained from SCF (STO-3G) calculations. The calculated structure for the ground state, ‘A,, is in very good agreement with exper- iment. The inter-ring separations in the lowest triplet (B,,) and singlet (Bzg) states are calculated to be 0.20 and 0.31 A shorter than the same distance in the ground state, respectively. This observation is used to explain the large red-shift observed in the phosphorescence and fluorescence spectra. The inter-ring separation in the ground state radical cation has also been calculated and it is used in the comparison of the absorption spectrum of the cation with the photoelectron spectrum of neutral [2.2]paracyclophane. 1. Introduction In [ m.m lparacyclophanes two benzene rings are held together in a sandwich structure by two bridges of methylene ( CH2) ,,1in the para positions [ 1 ] As different inter-ring distances are associated with dif- ferent values of m, the [mm] paracyclophanes pro- vide a regular series of molecules in which the in- teraction of the two benzene systems can be systematically examined. In a previous paper [2], we have studied the ultraviolet absorption spectra of the zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA [ m. m ] paracyclophanes. For m B 3 the spectra can be easily understood as a doubling of the para-xylene bands, as it might be expected for weakly interacting dimers. In [ 3.3lparacyclophane the inter-ring sep- aration has been determined to be 3.30 A, nearly the value of the benzene distance in solution. The spec- trum differs from those of higher-order paracyclo- phanes with the lowest bands exhibiting the signs of strongly interacting dimers. [ 2.2lparacyclophane is even more interesting. The separation of the aro- matic rings is = 3.1 A, thus shorter than those of ben- zene solution and the X-ray analysis [ 31 has estab- lished that the structure of [ 2.2lparacyclophane is ’ Permanent address: Departamento de Fisica, Universidade Federal de Pernambuco, 50000 Recife PE, Brazil. considerably distorted into two boat-like confor- mations (see fig. 1) . The ultraviolet absorption spec- trum is more complex and a theoretical interpreta- tion of this spectrum that conforms with all known experimental features has been given in the previous paper [2]. Several of the lowest excited states in- volve transitions from orbitals which are antibond- ing into orbitals which are bonding with respect to the inter-ring interaction. These excited states should thus have inter-ring separations which are even smaller than those of the ground state. The charac- terization of the molecular structure in the excited states of the [ 2.21 paracyclophane thus become of great importance for understanding the nature of the molecular forces involved. The emission spectrum acquires particular significance for aspects related to Fig. 1. The schematic structure of [2.2]paraqclophane. 0 009-2614/89/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland Physics Publishing Division ) 353