Absolute proton affinities of biphenyl and its derivatives

The spatial structure of biphenyl 1 is studied by the semiempirical AM1 and ab initio HF/6-31G* and MP2(fc)/6-31G* theoretical models. The resulting bond distances are in good agreement with the X-ray structure. The calculated dihedral angle is in accordance with the value observed by the electron diffraction technique (phi = 45 degrees). Its large value is a compromise between the steric hindrance effect and the pi-electron conjugation. The estimated barrier heights for the internal rotation are very low however. Theoretical values are in accordance with the available experimental evidence. The calculated proton affinity (PA) obtained by the scaled (AM1)(sc) model and by using the MP2 level of theory compares very well with the experimental value. It is some 13 kcal mol(-1) higher than the reference PA value f benzene, because of the strong resonance interaction between the two phenyl rings. The increase in the pi-electron conjugation energy triggered by protonation overcomes the steric repulsion between H atoms thus decreasing the twist angle by some 20 degrees. The apical carbon atom, placed para to the coannular CC bond, is most susceptible to the proton attack. On the other hand, the PA values for ipso and meta carbons are substantially lower since an amplification of the inter-ring conjugation interaction is then precluded. The PA increments for the CH, group and F atom monosubstituted biphenyls are determined by using the (AM1)(sc) approach. They are employed in estimating proton affinities of a number of polysubstituted biphenyls applying a very simple additivity formula based on the independent substituent approximation (ISA). It is shown that the performance of the additivity rule is very good. Variation in the PA of substituted biphenyls is rationalized in terms of the conjugation effect and repulsion between hydrogen atoms or substituents attached to the face-to-face ortho positions of the neighbouring rings (steric effect). Finally, the proton affinity of fluorene possessing a "frozen" planar biphenyl moiety is calculated and compared with that of the paradigmatic Mills-Nixon (MN) system-indan. It is found that PA values of the former compound are determined by the MN and resonance effects, the latter being predominant. The most basic site in fluorene is the C(4) atom where both effects act in a synergistic way.