Marie Madeleine Rohmer

Dynamic properties of a hexadecamolybdenum wheel: studies in solution and density functional theory calculations.

Variable temperature (1)H NMR studies of the host-guest complex [Mo(16)O(16)S(16)(OH)(16)(H(2)O)(4)(PDA)(2)](4-) (1 ; PDA(2-) = phenylenediacetate) previously carried out in D(2)O have revealed a complex behavior in solution, involving a gliding motion of both parallel phenyl rings of the PDA(2-) ligands. In the present work, we present new NMR spectra carried out in the aprotic solvent CD(3)CN, which allow the observation of the proton signals associated with the bridging hydroxo groups of the inorganic host. The new spectra provide detailed information about the concerted reorganization of the guest components, that is, PDA(2-) and water molecules. The existence of an equilibrium between two distinct isomers differing in the linking mode between the inorganic host and the two equivalent PDA(2-) ligands is evidenced. This equilibrium appears strongly dependent upon the temperature, leading to a complete inversion of the distribution between 300 and 226 K. The thermodynamic data related to the isomerization reaction have been determined (Delta(r)H = -50.5 kJ mol(-1) and Delta(r)S = -215 J mol(-1) K(-1)). Furthermore, at low temperature, one of the isomers exists in two conformations, only differing in the H-bond network involving the inner water molecules. Density functional theory calculations were carried out to push ahead the interpretations obtained from experiment, identify the isomers of 1, and specify the role and the positions of the guest water molecules. Among the various structures that have been calculated for 1, three fall in a narrow energy range and should correspond to the species characterized by variable-temperature (1)H NMR experiments in CD(3)CN. Finally, this study shows how the internal disposition of the ligands affects the ellipticity of the Mo(16) ring which varies from one isomer to the other in the 0.73-1 range and highlights solvation of the ring as one of the key parameters for the conformational design of these flexible host-guest systems.

Conformational changes in a flexible, encapsulated dicarboxylate: evidence from density functional theory simulations.

The dynamical behavior of the [Mo12O12S12(OH)12{O2C-(CH2)N-CO2}]2- complexes is analyzed via first-principles molecular dynamics simulations. Experimental X-ray data play the role of initial configurations for our search in the configuration space. We show that different stable and metastable conformers are possible, and these are thermally accessible at relatively low temperatures. This is the main outcome of our first-principles molecular dynamics approach in which the temperature allowing for thermal activation has been set to T = 500 K, which is consistent with the variable temperature 1H NMR spectra of these complexes in solution taken at comparable, although moderately lower, temperature. This implies that a relatively large manifold of folding configurations is available to the encapsulated guest species. Moreover, the high flexibility of both the host cage and the inserted guests allows for the accommodation of a rather wide variety of functional groups with potential applications in several fields.