Pople-like SCF—LCAO—MO Treatment of 77′,88′-Tetracyanoquinodimethan and Its Univalent Anion

Roothaan closed- and open-shell SCF—LCAO—MO calculations were carried out for 77′,88′-tetracyanoquinodimethan (TCNQ) and its univalent anion (TCNQ−), respectively. The dominant integrals were evaluated semiempirically in the manner of Pariser, Parr, McWeeny, and Peacock, including the approximation of total zero-differential overlap (TZDO). Singlet and triplet spectra for TCNQ0 and doublet and quartet spectra for TCNQ− were calculated by means of a configuration-interaction treatment involving singly excited states. Good correspondence was obtained with the experimental spectra. In addition, also in the TZDO approximation, we calculated the same spectra based on a semiempirical evaluation of the dominant integrals in the manner of Mataga, Nishimoto, Popkie, and Moffat. The two sets of theoretical spectra can be taken to correspond equally well with experiment. Charge density and spin density in TCNQ− based on an approximation to a projected UHF wavefunction following Amos and Snyder were obtained. Comparisons made with SCF, Hückel, and McLachlan spin-density values show the usual correspondence, and there is reasonable agreement with an experimental value of the spin density at the unsubstituted ring carbon. Bond lengths obtained from bond orders agree very well with the x-ray results of Fritchie and Arthur on Cs TCNQ, where they observed two species of TCNQ and assigned one as TCNQ0 and the other as TCNQ−.