The Crystal Structure of Acridinium Bis(7,7,8,8-Tetracyanoquinodimethanide), ARD+(TCNQ)2− and Some Remarks on the Possibility of a Peierls Distortion of a Linear Chain of TCNQ

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Abstract
Crystals of acridinium-TCNQ(ARD(TCNQ)2) are triclinic, space group P\bar1 with a=3.841(2), b=13.018(6), c=18.370(12)Å, α=114.32(5), β=119.31(4), γ=90.57(7)° and Z=1. ARD and TCNQ, are separately stacked to form columns along the a axis. Each ARD column is surrounded by six TCNQ, columns. The orientation of ARD is disordered. Intermolecular distance between adjacent TCNQs is 3.246 Å and that between adjacent ARDs is 3.418 Å. The possibility of a Peierls distortion of TCNQ columns was discussed on the basis of the model of one-dimensional distorted lattice. Calculations of free energy show that alternation of intermolecular spacing disappears at Peierls transition temperature, Tp of about 200 K. For T>Tp, the one-dimensional band is not split (metallic), while it is split below Tp(semiconductive). The specific heat curve shows a peak at about 180 K and the heat of transition is calculated to be about 130 cal/mol.