Intermolecular contacts in strong anion–π and halogen-bonded complexes follow frontier orbitals (instead of most positive or negative surface potentials) of reactants.
Halogen- and hydrogen-bonded complexes between trihalomethanes, CHX3, and (pseudo-)halide anions, A-, co-existing in acetonitrile solutions were identified and characterized via a combination of UV-vis and NMR spectral measurements with the results of X-ray structural and computational analyses. Halogen-bonded [CHX3, A-] complexes displayed strong absorption bands in the UV range (showing Mulliken correlations with the frontier orbital energies of the interacting species) and a decreased shift of the NMR signal of trihalomethanes' protons. Hydrogen bonding led to the opposite (increased) NMR signal shift and the UV-vis absorption bands of the hydrogen-bonded [CHX3, A-] complexes were similar in intensity to those of the separate CHX3 molecules. The simultaneous multivariable treatment of the results of UV-vis and NMR titrations of CHX3 with A- anions afforded formation constants of both halogen- and hydrogen-bonded complexes between these species, which existed side-by-side in the acetonitrile solutions. The relative values of the formation constants were consistent with the magnitudes of the positive potentials on the surfaces of the halogen or hydrogen atoms if the effects of the polarization of the trihalomethanes due to the presence of the anions were taken into account.
Pancake bonding between partially oxidized tetramethyltetrathiafulvalene (TMTTF) moieties was examined via X-ray structural, spectral, and magnetic susceptibility measurements of the salts comprising mono-and dicationic trimers, together with the computational analysis of their trimeric and dimeric associations. The salt with hexabromocarborane anions contained isolated dicationic (TMTTF) 3 2+ trimers, in which charge was residing mainly on the side moieties (rotated by 26°relative to the central TMTTF). Magnetic measurements revealed singlet ground states of these trimers with the antiferromagnetic coupling of 2J = −630 cm −1 . The salt with dodecamethylcarborane comprised TMTTF stacks consisting of distinct monocationic (TMTTF) 3 +• trimers with monomers shifted relative to each other along their main axes. The central TMTTF moiety in these trimers carried the largest fraction of charge, and intertrimer (TMTTF) 3 +• antiferromagnetic coupling was 2J = −290 cm −1 . A computational analysis demonstrated strong multicenter π-bonding within mono-and dicationic trimers. While a weakly covalent component was critical to the formation and properties of these complexes, the variation of their binding energies was related mainly to the interplay between electrostatic interaction between monomers and dispersion. Specifically, an electrostatic repulsion between cationic monomeric moieties in the dicationic trimers was close to that in the corresponding (TMTTF) 2 2+ dimers, but the dispersion energy in the former was roughly twice that in the dimers. As a result, the (TMTTF) 3 2+ trimers are thermodynamically stable in the gas and condensed phasesin contrast to the dicationic dimers, which are stable only if electrostatic repulsion is attenuated by the polar solvent and/or compensated by interaction with counterions in the solid state. Also, the weaker electrostatic repulsion between monomers in the monocationic trimers and dimers led to the higher stabilities of such complexes as compared to their dicationic analogues.
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