X-ray crystal structure analysis and atomic charges of color former and developer: 5. Colored formers

The spirocarbon C6 of colorless forms of a fluoran compound is the base of the disengagement from the bond C(sp³)–O by the accessibility of the electrophilic H⁺, the phthalide ring is cleaved and this ring becomes a benzene ring D and a carboxyl group as colored forms. The crystal and molecular structures of colored forms 2-anilino-3-methyl-6-diethylamino-9-(o-carboxyphenyl) xanthene C₃₁H₂₉N₂O₃⁺·Cl⁻ (1) and 2-methyl-3-anilino-7-dibutylamino-9-(o-carboxyphenyl) xanthene C₃₅H₃₇N₂O₃⁺·Cl⁻ (2) complexes were determined by single-crystal X-ray diffraction analysis. The xanthene ring skeletons of colored forms 1 and 2 have planar geometries, and the ring junctions between the xanthene ring and the benzene ring D make a small angle compared to the colorless forms which are almost at right angles to the xanthene ring skeletons. 1 and 2 are just different from the ethyl and butyl groups in two side chains R¹ and R², but the rigid body and internal motions in molecules differ at R¹ and R⁵ (the ring F). The charges of the root atom N1 of the two side chains are increased by π-electron densities, those from N1 to C6 atoms show distinct odd alternant system, and two oxygens (O2 and O3) of the carboxyl group are decreased. The group charges of N1 of colored forms are positive, while those of the colorless forms are negative. Colored forms make networks of X–HY contacts with an anion Cl⁻ and the anion in the nucleus of network. It is explained here that delicate positive charges of atoms N2 and O2 cause ClH(N2)–N2 and ClH(O2)–O2 contacts.