Structures and Energetics Study of Tetrathiafulvalene-Based Donors of OrganicSuperconductorsErsan Demiralp and William A. Goddard, III*Materials and Process Simulation Center, Beckman Institute (139-74), DiVision of Chemistry and ChemicalEngineering, California Institute of Technology, Pasadena, California 91125ReceiVed: May 16, 1997; In Final Form: July 30, 1997XThe donors of the best organic superconductors are all based on organic donor molecules (X) containing acore of tetrathiafulvalene (TTF) or a Se-substituted derivative. In this paper, we present ab initio quantummechanical calculations (HF, MP2, and DFT using the 6-31G** basis) for the optimized structures and otherproperties of TTF-based organic donors X and X+. We find that X+is planar but that X deforms to a boatstructure. The cases in which the boat is most stabilized with respect to the planar conformation are observedto be superconductors.1.0. IntroductionMost organic superconductors involve a tetrathiafulvalene(TTF)-like donor as in Figure 1 coupled with appropriateacceptors.1Bis(ethylenedithio)tetrathiafulvalene (denoted asBEDT-TTF or ET, see Figure 1b) is the best donor for theseorganic superconductors. The superconducting transition tem-peratures (Tc) have improved slowly over the year, now rangingup to 12.8 K.1This progress has been impeded because it isnot understood how superconductivity is related to fundamentalstructural quantities.The electronic structures of the molecular crystals involvingthese ET donors show a wide variety of electronic behaviorleading to semiconductors, metals, or superconductors,1-3depending on the anion and the packing. For metallic behaviorof ET salts, the intermolecular S‚‚‚S contacts seem important.3Hence, the conformations and packing of these donors play animportant role in determining the electronic behavior of thesematerials.We have shown4,5that the donors (X) for organic supercon-ductors all lead to a distorted boat conformation (see Figure 2)for the neutral X but a planar conformation for the ion X+. Theorganic superconductors tend to have about half the X oxidizedto X+, with a dynamic average of X+1/2. As an electron hopsfromXtoX+, the original X distorts from boat to planar whilethe original X+distorts from planar to boat. Thus conductionin this system leads to a coupling between charge-transfer andthe boat deformation phonon modes. We have suggested thatthis electron-phonon coupling is responsible for the supercon-ductivity.4,5Indeed the best organic superconductors have thelargest stabilization of the boat deformation for neutral X.4Thissuggests a criterion for determining new classes of donors andsuggests new TTF-based donors4(see Figure 1b).In this paper, we report structures and other properties of theseTTF-based organic donors that may be useful in establishingthe relation between boat distortions and superconductivity.2.0. Calculational DetailsWe used ab initio quantum mechanics6,13,14to examine thestructures of donors in both the oxidized (X+) and neutral (X)states. The methods used include Hartree-Fock (HF), second-order Moller-Plesset (MP2) perturbation theory, and densityfunctional theory (DFT). We used the Becke 1988 exchangefunctional and the Perdew gradient corrected correlated cor-relation functionals.6The changes in the properties of thesemolecules upon chemical modification (structure, ionization po-tential, shape and energy of the molecular orbitals) are importantfor understanding the electronic and crystal structures of mate-rials containing these organic donors. We used the 6-31G**basis for all atoms except for Se and Te where we used theHay-Wadt effective core potential with the LAV2P basis.* To whom correspondence should be addressed.XAbstract published in AdVance ACS Abstracts, September 15, 1997.Figure 1. (a) Parent TTF-like donors. (b) Modified TTF-like donorsof organic superconductors.Figure 2. (a) Top view of the optimum boat structure for TTF-Cl.The optimum angle is Θ ) 92.99° from MP2 calculation. (b) Sideview of the optimum boat structure for TTF-Cl. The optimum angleis Φ ) 24.4° from MP2 calculation. The bend is along the S-S axison each side. Here C is a solid circle, S is a crosshatched circle, andCl is an open circle.8128 J. Phys. Chem. A 1997, 101, 8128-8131S1089-5639(97)01654-X CCC: $14.00 © 1997 American Chemical Society3.0. Results3.1. Structures. It is often assumed that the TTF region ofthe TTF-related donors for organic superconductors are flat.7Some deviations from planarity have been suggested by theX-ray structure for the neutral ET crystal.8We find that X+always has a planar TTF region but that the neutral is distortedinto a boat form as in Figure 2 (for TTF-Cl) or a chair formwith equivalent energy.4,5Figure 2b shows the side view of the boat structures for TTF-Cl, which has C2Vsymmetry. For ET molecules the ethyleneof the six-membered rings must be nonplanar (to avoid eclipsedCH bonds). This leads to a staggered conformation with C2symmetry and an eclipsed conformation with Cssymmetry.These two conformations have almost same energy. Thus, atthe HF level the eclipsed conformation is lower by 0.000 005 2Hartrees ) 0.000 14 eV ) 0.0032 kcal/mol).5In this paperwe consider only the staggered conformation (with C2sym-metry). The boat distortion of the staggered conformation leadsto a degenerate pair of chiral molecules.3.2. Energetics. Table 1 compares the molecular symmetriesand total energies along with the orbital energies of the highestoccupied molecular orbital (HOMO) and lowest occupiedmolecular orbital (LUMO) from HF calculations. The deforma-tion angle (Φ), the C-X-C angle (Θ) in the pentagon ring,and the energy difference between boat and planar structures(∆Eboat) are reported in Table 2. In Table 3, we present themolecular symmetries and total energies along with the HOMOand LUMO energies of the key molecules from DFT and MP2calculations for understanding of the boat deformations. Table4 compares for several systems the deformation angles (Φ) andthe energy difference between boat and planar structures (∆Eboat)from HF, DFT, and MP2 calculations. We find that HF, DFT,TABLE 1: HF Energies for Organic Donors. All Results Are for the Optimized Geometries (Neutral and Cation) from HFCalculations (See Figure 1 for Structures)XX+speciesfconformation symmetrygtotal (H) HOMO (eV) LUMO (eV) total (H) HOMO (eV) LUMO (eV)TTF-H planar D2h-1819.5165 -6.806 3.070 -1819.3026 -11.641 -4.768-F boat C2Va-2214.8467 -7.833