Xiao-Na Xu

Hydrogen-bonding-mediated dynamic covalent synthesis of macrocycles and capsules: new receptors for aliphatic ammonium ions and the formation of pseudo[3]rotaxanes.

This paper describes a novel, highly efficient approach to the self-assembly of monomacrocycles and two-layered capsules by using dynamic covalent chemistry. Intramolecular hydrogen-bonding was used to preorganize aromatic amide-based monomers that contain aldehyde and tert-butoxycarbonylamino units. As a result, in the presence of an excess of trifluoroacetic acid (TFA), four monomers could self-couple to produce macrocycles selectively through the formation of three imine or hydrazone bonds. Three dipodal precursors were also prepared by connecting two hydrogen-bonded segments with a flexible linker. In the presence of TFA, these precursors could also self-couple, leading to the exclusive formation of two-layered capsules. As a result of intramolecular hydrogen-bonding, all the macrocycles and capsules were stable in solution and could be purified by simple recrystallization. The new capsules were able to form complexes with linear propylenediammonium derivatives to give unique two-layered pseudo[3]rotaxanes.

Folding-Induced Folding: The Assembly of Aromatic Amide and 1,2,3-Triazole Hybrid Helices

Folding-induced folding for the construction of artificial hybrid helices from two different kinds of aromatic sequences is described. Linear compounds 1 a, 1 b, and 2, containing one aromatic amide trimer or pentamer and one or two aromatic 1,2,3-triazole tetramers, have been designed and synthesized. The trimeric and pentameric amide segments are driven by intramolecluar N-H⋅⋅⋅F hydrogen bonding to adopt a folded or helical conformation, whereas the triazole segment is intrinsically disordered. In organic solvents of low polarity, the amide foldamer segment induces the attached triazole segment(s) to fold through intramolecular stacking, leading to the formation of hybrid helices. The helical conformation of these hybrid sequences has been confirmed by (1)H and (19)F NMR spectroscopy, UV/Vis spectroscopy, circular dichroism (CD) experiments, and theoretical calculations. It was found that the amide pentamer exhibits a stronger ability to induce the folding of the attached triazole segment(s) compared with that of the shorter trimer. Enantiomers (R)-3 and (S)-3, which contain an R- or S-(1-naphthyl)ethylamino group at the end of a tetraamide segment, have also been synthesized. CD experiments showed that introduction of a chiral group caused the whole framework to produce a strong helicity bias. Density-functional-theory calculations on (S)-3 suggested that this compound exists as a right-handed (P) helix.

Intramolecular C–H⋯F hydrogen bonding-induced 1,2,3-triazole-based foldamers

This paper reports the first series of artificial secondary structures that are induced by intermolecular C–H⋯F hydrogen bonding. Oligomers that contain two, four, six, and eight 1,2,3-triazole units have been designed and prepared by connecting the neighbouring 1,2,3-triazole units with 4,6-difluoro-m-phenylene linker(s). Two triphenylmethyl groups are appended at the ends of the backbones to suppress the stacking of the backbones and provide solubility. X-Ray analysis and 1H NMR and two-dimensional 1H–19F heteronuclear Overhauser enhancement spectroscopic (HOESY) experiments support that the 1,2,3-triazole backbones adopt folded or helical conformations due to the formation of continuous three-centred C–H⋯F hydrogen bonding. Quantum chemical calculations reveal that the longest 8-mer foldamer can form a one-turn helical cavity with a diameter of ca. 1.7 nm. Halide anion competition experiments show that the intramolecular C–H⋯F hydrogen bonding is more stable than the well-established intermolecular C–H⋯X− (X = Cl and I) hydrogen bonding.

A thermally stable pH-responsive “supramolecular buckle” based on the encapsulation of 4-(4-aminophenyl)-N-methylpyridinium by cucurbit[8]uril

Supramolecular building blocks that can respond to external stimuli are the basis for the fabrication of responsive materials. However, those that can be used in aqueous media and at elevated temperatures are extremely limited. In this work, a cucurbit[8]uril(CB[8])-based host–guest system has been developed. It exhibits excellent response to the change of pH by forming a thermally stable 1:2 host–guest complex under neutral conditions and transforming into a 1:1 complex by releasing one guest molecule in the presence of an acid. This transformation makes this system serve as a useful “supramolecular buckle”, which can fasten and unfasten two components. These features endow it with great potential in fabricating pH-responsive materials in aqueous media over a wide temperature range.

Analysis on the Colder Regenerator of 4.2K G-M Refrigerator

The real gas properties of helium, especially the enthalpy change between 30K and 4.2K for the 2.0MPa helium is lower than that for the 0.6MPa helium, result in that the temperature distribution in the colder regenerator of a multistage 4.2K G-M refrigerator forms a plateau, which is measured, and also verified by a numerical calculation. Based on the calculation result, a picture of helium mass in the regenerator void is presented in this paper. According to the movement of the helium, a three-cycle assumption is suggested.

A Numerical Simulation Method of a Two-Stage G-M Refrigerator and Comparison with Experiment

An improved numerical model and calculation method to simulate the dynamic process of a two-stage G-M refrigerator has been developed in this paper. A variety of factors, such as variable properties of regenerator materials and helium, existence of helium in a void space of a regenerator, were taken into account in the model. It can predict the distributions of temperature and mass flow, respectively with time, in the first and second stage regenerators, the regenerator efficiency and refrigeration capacity. Some analyses are made to study the effects of some parameters on refrigerator performance. The values predicted by computing agree with experimental results. The numerical method can provide useful information for the design of a high efficiency refrigerator.

Assessment of intermolecular N–H…F and N–H…Cl hydrogen bonding in stabilising hetero- and homodimers in solution

This paper describes the first assessment of intermolecular weak N–H…F and N–H…Cl hydrogen bonding in stabilising hetero- and homodimers in solution. Aromatic amide and urea monomers have been designed and synthesised. The association constants of the heterodimers formed by two complementary monomers and the homodimers formed by self-complementary monomers have been determined by using 1H titration and dilution experiments. The results show that both N–H…F and N–H…Cl hydrogen bonds are able to stabilise the corresponding dimers to a measurable extent, even though the stability of the dimers is generally low.