THEORETICAL-STUDY OF [4]PARACYCLOPHANE AND ITS DEWAR BENZENE AND PRISMANE VALENCE ISOMERS

Ab initio calculations were carried out for [4]paracyclophane (1) and its Dewar benzene (2) and prismane (3) valence isomers. The compounds were studied using AO basis sets ranging from 6-31G type to such of triple-zeta type including d and f functions on the carbon atoms and p functions at the hydrogen atoms. Complete geometry optimizations were performed with basis sets up to double-zeta quality. The critical geometrical distortion angle of the benzene ring of 1 is calculated to be 29.7-degrees which is 5-degrees larger than for the [5]paracyclophane molecule. The calculated strain energies of the three isomers (91.3, 1.6, and 4 kcal/mol for compounds 1-3) show that only the relative energy of the benzene isomer is affected by the short (CH2)4 chain. Furthermore it is found that the SCF approximation is not suitable for the calculation of the energy difference between the benzene and the corresponding Dewar benzene form due to a partial diradicaloid character of 1. The employment of MP2, MP4SDTQ, CAS-SCF, and MRD-CI correlation energy treatments yields an estimated energy difference of DELTAE = 0 +/- 3 kcal/mol between 1 and 2. Corrected DELTAE values obtained from various semiempirical treatments agree with this prediction. Additionally the lowest triplet and three of the lowest excited singlet states of 1 were considered using the MRD-CI method. The calculated excited-state energies are in reasonable agreement with the experimental UV spectrum of 1. On the basis of the calculational results the [4] paracyclophane molecule is classified as benzene-like due to the topological similarity to the parent compound.