Robert Melville Metzger

Crystal and Molecular Structure and Properties of Picolyltricyanoquinodimethan, the Zwitterionic Donor-Pi- Acceptor Adduct Beiween Li+Tcnq− and 1,2-Dimethyl-Pyridinium Iodide

Abstract The reaction of the lithium salt of 7,7,8,8-tetracyanoquinodimethan (TCNQ) with 1.2-dimethylpyridinium iodide in ethanol leads to a conventional charge-transfer salt. However, the same reagents dissolved in acetonitrile yield crystals which belong to space group P21/c, with unit cell constants a = 1493.5(2), b = 720.9(2) (stack axis), C = 1380.2(3) pm, β = 107.06(2)°. Z = 4 for Cl8H13N4. 1746 unique CAD-4F diffractometer Mo Ka data refined to R = 5.1 per cent, to yield the structure of the zwitterionic donor-pi-acceptor adduct Z-β-(N-methyl-2-pyridyl)-α-cyano-4-styryl dicyanomethids (trivial name: picolyl-tricyanoquinodimethan, P3CNQ). The pyridinium ring is inclined 30.13° from the quinodimethan 6-membered ring. The salt is diamagnetic, and the pyridinium N atom stacks above the dicyanomethide portion of the next molecule along the b axis.

Crystal Coulomb energies. VII. The electrostatic binding energy defect in Tetrathiofulvalinium 7,7,8,8,‐tetracyanoquinodimethanide

The Madelung energy EC of TTF TCNQ was computed by Ewald’s method using the 300 and 100 °K crystal structures and several charge models based on CNDO/2, PPP, and INDO atom charge densities, and as a function of charge transfer. In all cases EC compares unfavorably, by several electron volts, with the cost of ionizing the lattice. This had been noted previously for NMP TCNQ. This binding energy defect is too large to be explained by metallic binding or exchange forces, or by electron correlation, even in the Wigner crystal limit. We speculate that polarization forces, or covalent effects may play a large role in stabilizing the lattice. The Madelung site potentials in the TTF+1TCNQ−1 lattice are sufficient to explain the observed core level splittings in the photoelectron spectrum.

The enthalpy of formation and the experimental crystal binding energy of tetrathiofulvalenium 7,7,8,8‐tetracyanoquinodimethanide (TTF TCNQ)

By rotating‐bomb and static‐bomb combustion calorimetry, the standard enthalpies of formation at 298.15 K were measured for TTF (290.83±1.73 kJ mol−1), TCNQ (644.94±0.74 kJ mol−1), TTF+0.59TCNQ−0.59(918.37±2.05 kJ mol−1), and TMPD+TCNQ−(662.47±1.85 kJ mol−1). The inferred experimental crystal binding energy for the organic metal TTF+0.59 TCNQ−0.59 is Uρexp=−471±16 kJ mol−1 (−4.88±0.17 eV molecule−1).

CNDO/2‐FPP atom‐in‐molecule polarizabilities

The finite perturbation (FP) method of obtaining molecular and atom‐in‐molecule polarizabilities from the semiempirical CNDO/2 algorithm has been extended by the addition of parametrized polarization orbitals (PO) (2p for H, 3d for C, N, O, F, P, S, Cl, and 4d for Se and Br). The new parameters in CNDO/2‐FPP are a screening constant for the off‐diagonal core Hamiltonian terms, the PO orbital exponent, and, for the heavier elements, the PO electrongativities and the resonance integral. The parametrization preserves the charge distribution and the occupied molecular orbitals, and is designed to fit 13 model molecules; it obtains extra polarizability from the new hybrid contributions to the dipole moment and from reshuffling the virtual molecular orbitals. Excellent molecular and atom‐in‐molecule polarizabilities are obtained, for 47 other molecules, in agreement with available experimental data, and in competition with the best ab initio calculations.

Triplet Spin Excitons in a Sigma-Bonded TCNQ Dimer Salt: N-Ethylphenazinium TCNQ, (NEP+)2 (TCNQ−-TCNQ−)

Abstract The title compound NEP-TCNQ crystallizes with θ-bonded (TCNQ−-TCNQ−)-dimers. We show by model calculations and ESR spectroscopy, that these bonds can be broken by thermal activation. The reverse reaction occurs spontaneously. The excited states are characterized as quasi immobilized triplet spin excitons (TSE) localized on the two “parts” of the former TCNQ-dimer. Additional weaker TSE and doublet signals are observed.

Crystal Coulomb energies. V. The phase transition in Wurster’s blue perchlorate

In an extension of earlier work on Wurster’s blue perchlorate, it is shown that Madelung energy calculations by Ewald’s method account fairly well for the small observed enthalpy of transition at 186 °K; the Madelung energy differences are probably due only to observed rearrangements in the perchlorate geometry. The Madelung energy does not, however, favor the large extent of alternation in cation–cation distances determined in the low‐temperature crystal structure; this finding may shed some light on the mechanism of transition between the high temperature Curie–Weiss paramagnet and the low‐temperature spin‐paired diamagnet for this pseudo‐one‐dimensional organic crystal.

Crystal Coulomb energies. VI. Madelung energies of simple TCNQ salts. What is the electron affinity of TCNQ (7,7,8,8‐tetracyanoquinodimethan) ?

The Madelung energy EC (classical interionic crystal Coulomb energy in the fractional point‐charge approximation) has been calculated for Na+TCNQ−, Rb+TCNQ−(I), Rb+TCNQ−(II), and for NH4+TCNQ−. The values obtained are EC=−5.29, −4.71, −4.64, and −4.82 eV/molecule, respectively. When these values are coupled with available thermochemical data in a Born–Haber cycle, then the best recent experimental estimate of the electron affinity of TCNQ, 2.8 eV (obtained by Cs beam collisional ionization) is not verified, because apparently in these organic ionic crystals, as many as 2.5 eV of binding must be due to other interactions which our Madelung calculations ignore at present. It is likely that charge‐induced dipole interactions may supply the several electron volts that are missing from the calculated crystal binding energy.

MINDO/3-FP atom-in-molecule polarizabilities of TCNQ, TTF, TMPD, and of their radical ions

The MINDO/3‐FP method was used to obtain molecular polarizabilities a and atom‐in‐molecule polarizabilities ai for the neutral molecules 7,7,8,8‐tetracyanoquinodimethan (TCNQ), tetrathiofulvalene (TTF), and N,N,N′,N′‐tetramethylparaphenylenediamine (TMPD), and their radical ions TCNQ−, TTF+, and TMPD+. Except for the direction perpendicular to the molecular plane, the a and the ai describe fairly well the covalent bonding environment. The radical ions are more polarizable than their parent neutral molecules, but not spectacularly so. The ai appear to be covalent bond polarizabilities, and are the largest for the atoms that lie on the longest molecular axis.

Crystal and molecular structure and ESR spectra of the 1:1 salt 5‐(1‐butyl)phenazinium (NBP)‐2,2′‐(2,3,5,6‐tetrafluoro‐2,5‐ cyclohexadiene‐1,4‐diylidene)‐bispropanedinitrile (TCNQF4)

The title compound C28H17F4N6, Mr=513.48, crystallizes in the monoclinic space group P21/c, with a=10.972(2) A, b=17.557(3) A, c=13.523(4) A, β=111.88(2)°, V=2417.36 A3, z=4, and dc=1.411 Mg m−3. Final refinement yielded residuals of R=0.056 and Rw=0.046. The structure consists of (NBP+)2 and (TCNQF−4)2 dimers stacked in a DDAA sequence along the c axis. The NBP+ and TCNQF−4 ions are planar, with interplanar distances of 3.54(2) A for a donor pair and 3.15(3) A for an acceptor pair. The angle between the NBP+ and TCNQF−4 planes is 15.8°. In the ESR experiments, two equivalent species of thermally activated Frenkel triplet spin excitons (TSE), with differently oriented fine structure tensors, are observed. They are located on two TCNQF4 molecular pairs of different orientation. A motion of the TSE in the b direction can be excluded. Additional S=1/2 lines are due to immobile doublet spins on TCNQF−4 radical ions.

Partial ionicity, cohesion, and charge correlations in narrow‐band solids

A many‐electron, correlated‐state representation is introduced for narrow‐band solids of donors and acceptors. The electrostatic energy for partial ionicity 0⩽q⩽1 is related to zero bandwidth Wigner lattices and to wide‐band Hartree–Fock results for both simple orthorhombic lattices of point charges and for the organic conductor TTF–TCNQ at q = 1/2. Partial ionicity requires repulsive as well as attractive Coulomb contacts in the first coordination sphere, a condition that is met by organic conductors. The configuration interaction among correlated crystal states washes out the fixed neutral and ionic sites of a Wigner lattice, while retaining most of its electrostatic binding. Short‐range charge correlations in partly ionic lattices substantially improve the Hartree–Fock cohesion and rationalize the shallow minimum (relative to neutral solids) in representative partly ionic organic solids.