The molecular structures and electron distributions of the 1,8-bis-(dimethylamino)-naphthalenes,studied by density functional and ab initio MP2 calculations

The equilibrium structure of 1,8-bis-(dimethylamino)-naphthalene (DMAN), has been investigated in some detail; only one minimum energy conformer was found, although a saddle point, close to the structure of a previously claimed second conformer (J. Am. Chem. Soc., 120, 4840 (1998)), was also identified. Alternative apparent minima are attributed to artefacts when small bases are used. Equilibrium structures of members of the DMAN series with chloro-, bromo-, nitro- and methoxy-substituents in the ortho(2,7-) and para(4,5-) positions, as well as 4-nitro-DMAN were also determined, and compared with crystallographic and previous theoretical data. There are close correlations between calculated and observed values for bond lengths, angles and dihedral angles across the known compounds, and this enables additional correlation with other members of the series where no experimental data so far exists. The MP2 and DFT methods used, with a triple zeta basis set including polarization (TZVP), lead to very similar results but the dihedral angles in particular show more variation. Substituent effects on the bond lengths and angles in the ring geometry are relatively small, and the intrinsic pattern of short αβ-bonds in the naphthalene system is largely preserved, but the C₃C₄ bond is shorter than that for C₁C₂. Steric effects arising from interaction of 1,8- and 2,7-substituents on the naphthalene ring leads to significant twisting of the naphthalene skeleton. In addition, considerable in-plane and out-of-plane bending of the substituents from the naphthalene ring system is apparent. The effect of adjacent substituents upon the dihedral angle between the dimethylamino-groups and the ring is also apparent. The ground state molecular electronic properties have been determined and compared with experiment. The electron distribution in the B3LYP methods lead to markedly higher dipole moments than the MP2 ones, and the latter are regarded as more reliable.