Optical activity induced by helical arrangements of tryptamine and 4-chlorobenzoic acid in their cocrystal

Optical rotatory powers of chiral cocrystals formed from the achiral molecules tryptamine and 4-chlorobenzoic acid were determined by the HAUP (high accuracy universal polarimeter) method. These cocrystals belonged to space group P2₁2₁2₁, and their absolute configuration was confirmed by the Flack parameter. In the M-crystal, 2-fold helical arrangements are formed in a counterclockwise direction between the two components through the quaternary ammonium salt bridge, hydrogen bond, and the aromatic π-π interaction along the c axis, while clockwise helices alone exist in the P-crystal. Large rotatory powers ρ₃^M = -355 and ρ₃^P = +352 deg mm^-1 were obtained along the c axis in the M- and P-crystal, respectively, at 632.8 nm and 303 K. The magnitude was 10 to 100 times larger than those for ordinary organic crystals. Further, it was confirmed that the negative sign was induced by the counterclockwise helical structures and the positive sign by the clockwise helices. In contrast, the rotations along the a and b axis which are in perpendicular directions to the screw axis were ρ₁^M = +138, ρ₁^P = -140 deg mm^-1, and ρ₂^M = -56, ρ₂^P = +58 deg mm^-1, much smaller than ρ₃^M and ρ₃^P. The results revealed that the helically arranged aromatic π electrons as well as the helical ionic and hydrogen bond networks in the crystal contributed to the enhancement of the magnitude of these rotations.