Madelung Energy Systematics in the Kei'erofulvalene-TCNQ Charge-Transfer Salts

Abstract

The Madlung energies E_M of 15 charge-transfer salts analogous to TTF TCNQ have been calculated. In uniform as well as slightly dimerized segregated-stack crystals three (HMTSF TNAP, TWTSF TCNQ(black), TMTTF TCNQ) have an unusual titled structure with very poor counterion coordination and anomalously low E_M values; two (TTF TCNQ, TSF TCNQ) belong to a ‘normal-I’ structure type, with sheets of stacks of donors next to sheets of stacks of acceptors; seven (HMTTF TCNQ, HMTSF TCNQ, HMITF TCNQF₄, HMTSF TCNQF₄, DBTSF TCNQF₄, DBTTF TCNQF₄, ‘TTF TCNQF₂) have a ‘normal-II’ motif, with a checkerboard alternation of donor and acceptor stacks; the E_M values for normal-I structures are slightly lower than those for normal-II structures. Finally, three (DBTTF TCNQF₂, DBTTF TCNQ, TMTSF TCNQ(red)) have mixed-stack structures, with the highest Madelung energies.