The Molecular and Crystal Structure of Bis‐Dimethyl Pentamethine Cyanine Perchlorate, C₉H₁₇N₂Cl0₄

Bis‐dimethyl pentamethine cyanine perchlorate crystallizes in the space group P (2 1 / c ) with the lattice constants a = 6.230 Å; b = 14.970 Å; c = 14.005 Å; β = 83.54°. The carboncarbon bonds show the bond length compensation, typical of polymethines, towards a partial double bond (1.39 Å) between formally sp 2 ‐hybridized C atoms. The two CN bond lengths (mean value 1.32 Å) similarly correspond to partial NC double bonds. The CN bond lengths to the methyl carbon atoms (mean value 1.45 Å) correspond to normal CN single bonds. The size of the bond lengths confirms the assumption as to the characteristic features of the π electron system of such polymethines. The valence angles at the carbon atoms of the pentamethine chain clearly alternate and deviate systemically from 120°. This is also observed in other structures of a similar type. The bond lengths determined by experiment are compared with different π bond orders calculated on the basis of quantum chemistry. Here approximate agreement becomes apparent between all MO calculations and the results of the X‐ray crystallographic analysis. The perchlorate tetrahedra show disorder‐like behaviour in their crystal structure. The pentamethine chains approach each other as far as the contact spacing of 3.5 to 3.7 Å between unsaturated compounds and form two‐dimensional extended aggregates (“masonry” aggregation) within the crystal lattice.