J . Phys. Chem. 1988, 92, 6561-6566 of the parent species were not found to correlate with the shifts observed for complexes of the chromophore indole with the same solvents. This is taken as evidence that the primary hydrogenbonding site for the solvent in these clusters lies not on the indole ring but rather on the side chain functional group of tryptophan and its analogues. When this occurs, the conformer structure of the parent is found to be preserved in some cases such as the complexes of indolepropionic acid with water and methanol. In
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others, especially the water and methanol complexes of both tryptamine and tryptophan, the conformer distribution of the parent is radically altered.
Acknowledgment. This work was supported by the National Science l k n d a t i o n under Grant CHE-8311971. Registry No. TA, 61-54-1; IPA, 830-96-6; H 2 0 , 7732-18-5; MeOH, 67-56-1; CHC13, 67-66-3; dioxane, 123-91-1.
Structure and Stability of the X3- Systems (X = F, CI, Br, I ) and Their Interaction with Cations+ Juan J. Novoa,* Fernando Mota, Departament de Quimica Fisica, Facultat de Quimica, Universitat de Barcelona, Av. Diagonal 647, 08028- Barcelona, Spain
and Santiago Alvarez Departament de Quimica Inorghica, Facultat de Quimica, Universitat de Barcelona, Av. Diagonal 647, 08028- Barcelona, Spain (Received: February 2, 1988)
The molecular and electronic structure of the trihalide anions as well as their relative stability toward dissociation into a halogen molecule and a halide ion has been studied by full ab initio and pseudopotential methods including the electronic correlation at the MP2 and CIPSI level. In all cases, the X3- anions present a Dmhsymmetry in their ground state and are stable against dissociation to X- and X2 when the electronic correlation is included. Their relative stability with the exception of F3- increases along the group in agreement with the experimental trends. The electronic structure of these systems can be-explained by the formation of a three-center bond. Such a model gives a good explanation of the variation of the dissociation energy of X3- and other properties of these systems. The interaction of the X3- anions with a cation is studied for KBr3. The results show that this ionic interaction is directional, resulting in a stable structure with the cation in a side-on position where the X3- anion has lost the D.., symmetry and one of the Br-Br bonds is longer than the other by 0.5 A. Both the position and the asymmetry of the anion are in good agreement with various experimental crystal structures.
Introduction The halogen molecules are known to react with halide ions to form stable trihalide species, X3-. In the case of iodine, this reaction provides a practical way to obtain aqueous solutions of the halogen. These species also appear as [X3-], chains in many "organic metals" where partial charge transfer to an acceptor molecule A produces solids with A-X stoichiometry, which are usually best formulated as A(X3),13,hence producing partially filled bands of the A chains and potential pseudo-one-dimensional metals.' Higher polyhalides, such as X
