Binding energies of five molecular pincers calculated by explicit and implicit solvent models

Molecular pincers or tweezers are designed to hold and release the target molecule. Potential applications involve drug distribution in medicine, environment technologies, or microindustrial techniques. Typically, the binding is dominated by van der Waals forces. Modeling of such complexes can significantly enhance their design; yet obtaining accurate complexation energies by theory is difficult. In this study, density functional theory (DFT) computations combined with dielectric continuum solvent model are compared with the potential of mean force approach using umbrella sampling and the weighted histogram analysis method (WHAM) with molecular dynamics (MD) simulations. For DFT, functional and basis set effects are discussed. The computed results are compared to experimental data based on NMR spectroscopic measurements of five synthesized tweezers based on the Tröger's basis. Whereas the DFT computations correctly provided the observed trends in complex stability, they failed to produce realistic magnitudes of complexation energies. Typically, the binding was overestimated by DFT if compared to experiment. The simpler semiempirical PM6‐DH2X scheme proposed lately yielded better magnitudes of the binding energies than DFT but not the right order. The MD‐WHAM simulations provided the most realistic Gibbs binding energies, although the approximate MD force fields were not able to reproduce completely the ordering of relative stabilities of model complexes found by NMR. Yet the modeling provides interesting insight into the complex geometry and flexibility and appears as a useful tool in the tweezers' design. © 2012 Wiley Periodicals, Inc.     

Oligo Tröger's bases--new molecular scaffolds

Oligo Tr?ger's bases are compounds containing two or more Tr?ger's base subunits (1,5-methanodiareno[b,f][1,5]diazocines) sharing one or more arene parts. Due to their interesting molecular shapes, these compounds are studied as chiral molecular tweezers, clips, cavitands, clefts, calixes, etc. This review includes all available data on oligo Tr?ger's bases, and introduces their preparation and properties to a wide audience.     

Simvastatin: structure solution of two new low-temperature phases from synchrotron powder diffraction and ss-NMR

Simvastatin is a substance used for the treatment of hypercholesterolemia. In addition to the already known room temperature structure of simvastatin (Čejka et al. in Acta Cryst C59:o428, 2003) two new low-temperature polymorphs were found by X-ray powder diffraction with the phase transition at 261 and 223 K (later confirmed by DSC to be 272 and 232 K). The main differences among three polymorphs consist in the side-chains conformation only and the phase changes are fully reversible. The structures of the polymorphs were studied by the powder diffraction based on synchrotron radiation as well as by solid-state NMR spectroscopy.     

New route to 1-thia-closo-dodecaborane(11), closo-1-SB11H11, and its halogenation reactions. The effect of the halogen on the dipole moments and the NMR spectra and the importance of spin-orbit coupling for the 11B chemical shifts

Reaction between nido-B10H14 (1) and elemental sulfur in CHCl3 in the presence of Et3N at room temperature, followed by treatment with Et3N.BH3 at 170-190 degrees C, resulted in the isolation of closo-1-SB11H11 (2) in 50% yield. Selected electrophilic halogenation reactions of compound led to the isolation of a series of monohalogenated derivatives of general constitution 12-X-closo-1-SB11H10 (12-X-, where X = Cl, Br, and I). The structures of 12-Cl- and 12-I- were determined by an X-ray diffraction analysis and the structures of all compounds were geometry optimised at the RMP2(fc)/6-31G* level. The constitution of all compounds is consistent with the results of mass spectrometry and multinuclear (1H and 11B) spectroscopy complemented by two-dimensional [11B-11B]-COSY and 1H{11B(selective)} NMR measurements. Experimental 11B chemical shifts generally show acceptable agreement with theoretical values calculated by GIAO methods, but spin-orbit coupling must be included for nuclei bearing heavy-atom substituents such as Br or I. The dipole moments determined for the B12-X bonds show similarities to those of aliphatic C-X bonds and confirm unambiguously the B12 --> S dipole moment orientation in the SB11 cage.     

Macropolyhedral boron-containing cluster chemistry. Cluster opening and B-frame rearrangement in the reaction of B(16)H(20) with [{(IrCl(2)(eta(5)-C(5)Me(5))}(2)]. Synchrotron X-ray structures of [(eta(5)-C(5)Me(5))(2)Ir(2)B(16)H(17)Cl] and [(eta(5)-C(5)Me(5))(2)Ir(2)B(16)H(15)Cl

Products from the reaction of + nido ten-vertex : nido eight-vertex, B(16)H(20) with [{(IrCl(2)(eta(5)-C(5)Me(5))}(2)] and tmnd show unanticipated rearrangement of the starting {B(16)} skeleton, as exhibited by + nido ten-vertex : nido ten-vertex, [(eta(5)-C(5)Me(5))(2)Ir(2)B(16)H(17)Cl] which has a {B(2)} edge conjunction and by + nido ten-vertex : nido eleven-vertex, [(eta(5)-C(5)Me(5))(2)Ir(2)B(16)H(15)Cl] which has a {B(3)} face conjunction.     

Overcoming regioselectivity issues inherent in bis-Tröger's base preparation

[structure: see text] Bis-Tr?ger's base derivatives are a new family of molecular tweezers. A major drawback to their study is a lack of commercially available precursors, ortho-nitrocarboxylic acids. A reverse synthetic strategy starting from known dinitrodicarboxylic acids, which circumvents this problem, is presented. Via this methodology regioisomeric bis-TB derivatives can be prepared selectively, using only common aromatic amines that are typically commercially available.     

Origin of ohmic behavior in Ni, Ni2Si and Pd contacts on n-type SiC

Ni, Ni₂Si and Pd contacts were prepared on n-type 4H-SiC and annealed in the temperature range of 750-1150 °C. The annealed contacts were analyzed before and after acid etching, and different features were found in unetched and etched contacts. Carbon left on the SiC surface after the acid etching of Ni₂Si contacts annealed at 960 °C was highly graphitized. In nickel contacts, the graphitization of interface carbon began at 960 °C and increased after annealing at higher temperatures. In palladium contacts, the onset of the interface carbon graphitization was observed after annealing at 1150 °C. For all three types of metallization, the minimal values of contact resistivity were achieved only when the sharp first-order peak at 1585 cm⁻¹ and distinct second-order peak at ∼2700 cm⁻¹ related to the presence of graphitized carbon were detected by Raman spectroscopy after the acid etching of contacts. The properties of unannealed secondary contacts deposited onto etched primary contacts were similar to the properties of the primary contacts unless carbon was selectively etched. The results show that ohmic behavior of Ni-based and Pd contacts on n-type SiC originates from the formation of graphitic carbon at the interface with SiC.     

[μ-6,9-Cl-8-(OMe)-6,9-(η5-C5Me5)2-arachno-6,9,5-Rh2SB7H7]

The title compound, μ-6,9-chloro-8-methoxy-6,9-bis­(η⁵-penta­methyl­cyclo­penta­dienyl)-6,9-dirhoda-5-thia-arachno-decaborane(7), [Rh₂(CH₁₀B₇OS)(C₁₀H₁₅)₂Cl], has a single Cl atom bridging the two remote rhodium `prow' vertices of an arachno ten-vertex dirhoda­thia­decaborane cluster, with Rh—­Cl distances of 2.3475 (11) and 2.3536 (11) Å, and an Rh—Cl—Rh angle of 106.82 (4)°.     

Synthesis of methyl methacrylate,styrene, and isobutylene multiblock copolymers using atom transfer and cationic polymerization

ABCBA‐type pentablock copolymers of methyl methacrylate, styrene, and isobutylene (IB) were prepared by the cationic polymerization of IB in the presence of the α,ω‐dichloro‐PS‐b‐PMMA‐b‐PS triblock copolymer [where PS is polystyrene and PMMA is poly(methyl methacrylate)] as a macroinitiator in conjunction with diethylaluminum chloride (Et₂AlCl) as a coinitiator. The macroinitiator was prepared by a two‐step copper‐based atom transfer radical polymerization (ATRP). The reaction temperature, ?78 or ?25 °C, significantly affected the IB content in the resulting copolymers; a higher content was obtained at ?78 °C. The formation of the PIB‐b‐PS‐b‐PMMA‐b‐PS‐b‐PIB copolymers (where PIB is polyisobutylene), prepared at ?25 (20.3 mol % IB) or ?78 °C (61.3 mol % IB; rubbery material), with relatively narrow molecular weight distributions provided direct evidence of the presence of labile chlorine atoms at both ends of the macroinitiator capable of initiation of cationic polymerization of IB. One glass‐transition temperature (T_g), 104.5 °C, was observed for the aforementioned triblock copolymer, and the pentablock copolymer containing 61.3 mol % IB showed two well‐defined T_g's: ?73.0 °C for PIB and 95.6 °C for the PS–PMMA blocks. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3823–3830, 2005