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 ρ3M
= −355 and ρ3P = +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 ρ1M = +138, ρ1P = −140 deg mm-1, and ρ2M = −56, ρ2P
= +58 deg mm-1, much smaller than ρ3M and ρ3P . 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.