Refinement of Molecular Mechanics Parameters for Biphenyl and Naphthalene

Abstract

A molecular mechanics calculation program MM 2' was applied to biphenyl and naphthalene. Parameters for the central C-C bond of biphenyl and C-C bonds of naphthalene were adjusted to reproduce experimental structures and barrier heights of internal rotation. A stretching force constant for the central C-C bond of biphenyl, ks, was changed from 8.067to 5.0 (mdyn.A^-1), a bond distance constant, r_o, from 1.3923 to 1.48 (A), and a torsional constant, V₂, from 9.0 to 1.0 (kcal. mol^-1). The calculated central C-C bond length and torsional angle of the energy minimum conformer in biphenyl were 1.4909A and 45°, respectively. Maximum energies were reached when the torsional angle was 0° and 90°, and were 1.7 and O.9 kcal. mol^-1 higher than that of the energy minimum conformer, respectively. In the calculation of naphthalene, a bond distance constant, r₀, for C₁-C₂ bond was changed from 1.3923 to 1.37 and for other bonds from 1.3923 to 1.41 (A). Calculated bond distances of C₁-C₂, C₂-C₃, C₁-C₉, C₉-C₁₀ bonds were 1.3737, 1.4134, 1.4150, 1.4168 (A). Satisfactory agreement with available experimental data was obtained. In these calculations, bond order calculation to adjust parameters for conjugated bonds, as performed in MMPI, was not carried out. So these parameters used for the calculation of biphenyl and naphthalene may not be applied to highly strained derivatives, but to lower strained biphenyl and naphthalene derivatives. This method might generally be applicable to a series of conjugated molecules of similar structures.