A STRUCTURAL STERIC ISOTOPE EFFECT IN DEUTERATED TETRACYANOANTHRAQUINODIMETHANE

Tetracyanoanthraquinodimethane (TCAQ, 1) is distorted from planarity into a bent ''butterfly'' shape due to steric repulsion between its cyano groups and the encroaching peri hydrogens. An X-ray structure determination of perdeuterated TCAQ (TCAQ-d8, 4) showed smaller b and c unit cell dimensions compared to TCAQ. The shortening along the b axis (which is roughly perpendicular to the central ring) from 10.022 angstrom for TCAQ to 9.992 angstrom for TCAQ-d8 indicates a combination of closer packing of the TCAQ-d8 molecules and/or flattening of the butterfly shape. A 0.39-degrees widening in the dihedral angle between the benzene rings in 4, relative to 1, is consistent with such a flattening, as is closer approach of the cyano group atoms to the benzene rings. Thus, the smaller effective size of carbon-deuterium bonds results in a steric deuterium isotope effect in the crystal structure of TCAQ.