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 b ond distances are in good agreement with the X-ray structure. The calculate d dihedral angle is in accordance with the value observed by the electron d iffraction technique (phi = 45 degrees). Its large value is a compromise be tween the steric hindrance effect and the pi-electron conjugation. The esti mated barrier heights for the internal rotation are very low however. Theor etical values are in accordance with the available experimental evidence. T he calculated proton affinity (PA) obtained by the scaled (AM1)(sc) model a nd by using the MP2 level of theory compares very well with the experimenta l value. It is some 13 kcal mol(-1) higher than the reference PA value f be nzene, because of the strong resonance interaction between the two phenyl r ings. The increase in the pi-electron conjugation energy triggered by proto nation overcomes the steric repulsion between H atoms thus decreasing the t wist angle by some 20 degrees. The apical carbon atom, placed para to the c oannular CC bond, is most susceptible to the proton attack. On the other ha nd, the PA values for ipso and meta carbons are substantially lower since a n 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 estima ting proton affinities of a number of polysubstituted biphenyls applying a very simple additivity formula based on the independent substituent approxi mation (ISA). It is shown that the performance of the additivity rule is ve ry good. Variation in the PA of substituted biphenyls is rationalized in te rms of the conjugation effect and repulsion between hydrogen atoms or subst ituents attached to the face-to-face ortho positions of the neighbouring ri ngs (steric effect). Finally, the proton affinity of fluorene possessing a "frozen" planar biphe nyl moiety is calculated and compared with that of the paradigmatic Mills-N ixon (MN) system-indan. It is found that PA values of the former compound a re 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 i n a synergistic way.