STRUCTURE, STRAIN-ENERGY, AND MAGNETIC-SUSCEPTIBILITY OF [4]PARACYCLOPHANE AND THE ACTIVATION-ENERGY FOR ITS INTERCONVERSION WITH 1,4-TETRAMETHYLENE DEWAR BENZENE

Ab initio quantum mechanical methods were employed to study [4]paracyclophane and its Dewar benzene isomer at the self-consistent-field (SCF), two configurations SCF (TCSCF), configuration interaction with single and double excitations from two reference functions (TCSCF CISD) and second-order perturbation (MP2) methods. The quantities predicted include their equilibrium geometries, harmonic vibrational frequencies, the energy difference and activation energy for the interconversion between the two isomers, the evaluation and partition of the strain energy for [4]paracyclophane, and the magnetic susceptibility. Unlike previous results for the energy difference between [4]paracyclophane and its Dewar benzene isomer form (3.0 kcal mol(-1)), this energy difference is predicted to be 9 kcal mol(-1) at the DZ+d TCSCF CISD+Q level. The activation energy for the isomerization from Dewar benzene isomer is predicted to be 32 kcal mol(-1) at the DZ+d TCSCF CISD+Q level. The activation energy for the back isomerization from the benzene form to the Dewar benzene form is 21 kcal mol(-1) at the DZ+d TCSCF CISD+Q level, which explains the thermal stability of [4]paracyclophane against such a rearrangement. The strain energy for the [4]paracyclophane is estimated to be 85 kcal mol(-1) at the DZP MP2 level via the appropriate homodesmotic reaction. Our subpartitioning of the strain energy in benzene indicates that the conjugation energy may be lost. Therefore it is difficult to classify [4]paracyclophane as aromatic on the basis of conjugation energy. However, our study shows that the degree of bond alternation in the C-6 ring in [4]paracyclophane is only 0.02 Angstrom at both SCF and MP2 levels, and this may represent a lower limit. The extent of bond alternation increases to 0.087 Angstrom at the DZ+d TCSCF level, which is certainly the upper limit value for bond alternation. The small degree of bond alternation supports recent theoretical proposals that the driving force for the D-6h, benzene structure is a bending. An important finding in this study is that the boat-shaped benzene (with the same geometry as in [4]paracyclophane) has the same magnetic susceptibility as the hypothetical cyclohexatriene. Therefore a weak ring current is expected in [4]paracyclophane. However, whether [4]paracyclophane could be classified as aromatic on the basis of magnetic susceptibility depends on how one classifies the hypothetical cyclohexatriene.