Mapping the force field of a hydrogen-bonded assembly

Hydrogen bonding underpins the properties of a vast array of systems spanning a wide variety of scientific fields. From the elegance of base pair interactions in DNA to the symmetry of extended supramolecular assemblies, hydrogen bonds play an essential role in directing intermolecular forces. Yet fundamental aspects of the hydrogen bond continue to be vigorously debated. Here we use dynamic force microscopy (DFM) to quantitatively map the tip-sample force field for naphthalene tetracarboxylic diimide molecules hydrogen-bonded in two-dimensional assemblies. A comparison of experimental images and force spectra with their simulated counterparts shows that intermolecular contrast arises from repulsive tip-sample interactions whose interpretation can be aided via an examination of charge density depletion across the molecular system. Interpreting DFM images of hydrogen-bonded systems therefore necessitates detailed consideration of the coupled tip-molecule system: analyses based on intermolecular charge density in the absence of the tip fail to capture the essential physical chemistry underpinning the imaging mechanism

Xanthine quartets on Au(111)

The quartet of xanthine (X), a purine base ubiquitously distributed in most human body tissues and fluids, has been for the first time fabricated and visualized, as the first alternative purine quartet besides the known guanine (G)-quartet. The X-quartet network is demonstrated to be the most stable phase on Au(111). Unlike guanine, the fabrication of the X-quartets is not dependent on the presence of metal atoms, which makes it the first metal-free purine quartet. The X-quartet holds great promise to potentially construct artificial new DNA quadruplexes for genetic regulation and antitumor therapy. Moreover, both the X-quartet itself and the quartet networks favor homochirality, suggesting homochiral xanthine oligomers and the networks may have been formed as the precursors of the pristine oligonucleotides on primitive Earth

Mapping the force field of a hydrogen-bonded assembly

Hydrogen bonding underpins the properties of a vast array of systems spanning a wide variety of scientific fields. From the elegance of base pair interactions in DNA to the symmetry of extended supramolecular assemblies, hydrogen bonds play an essential role in directing intermolecular forces. Yet fundamental aspects of the hydrogen bond continue to be vigorously debated. Here we use dynamic force microscopy (DFM) to quantitatively map the tip-sample force field for naphthalene tetracarboxylic diimide molecules hydrogen-bonded in two-dimensional assemblies. A comparison of experimental images and force spectra with their simulated counterparts shows that intermolecular contrast arises from repulsive tip-sample interactions whose interpretation can be aided via an examination of charge density depletion across the molecular system. Interpreting DFM images of hydrogen-bonded systems therefore necessitates detailed consideration of the coupled tip-molecule system: analyses based on intermolecular charge density in the absence of the tip fail to capture the essential physical chemistry underpinning the imaging mechanism. © 2014 Macmillan Publishers Limited. All rights reserved

Chemical shielding of H2O and HF encapsulated inside a C60 cage

Molecular surgery provides the opportunity to study relatively large molecules encapsulated within a fullerene cage. Here we determine the location of an H2O molecule isolated within an adsorbed buckminsterfullerene cage, and compare this to the intrafullerene position of HF. Using normal incidence X-ray standing wave (NIXSW) analysis, coupled with density functional theory and molecular dynamics simulations, we show that both H2O and HF are located at an off-centre position within the fullerene cage, caused by substantial intra-cage electrostatic fields generated by surface adsorption of the fullerene. The atomistic and electronic structure simulations also reveal significant internal rotational motion consistent with the NIXSW data. Despite this substantial intra-cage interaction, we find that neither HF or H2O contribute to the endofullerene frontier orbitals, confirming the chemical isolation of the encapsulated molecules. We also show that our experimental NIXSW measurements and theoretical data are best described by a mixed adsorption site model

An Option Game Model of Supplier R&D Co-Competition under Uncertainty

In order to improve the market competitiveness of suppliers and their resilience to emergencies, it is of great significance to discuss the investment decision making of suppliers in developing new products under uncertain and competitive environments. In this paper, with the background of knowledge spillover, absorptive capacity, initial R&D investment, and innovation efficiency asymmetry, the uncertainty of price, sales, and cost are incorporated into the evaluation system as three important risk factors. On the basis of the existing real option investment evaluation model, a real option game model of R&D investment of supplier enterprises based on multiple random variables is established. The sensitivity analysis of parameters is carried out with an example of one enterprise’s monitor R&D project. The results depict that the probability of R&D success has a great impact on the value of enterprise options, which depends on the R&D investment, innovation efficiency, and R&D performance of enterprises. Secondly, the drift rate of price, sales, and cost also has a significant impact on improvement in enterprise option value

An Endergonic Synthesis of Single Sondheimer–Wong Diyne by Local Probe Chemistry

Recent advances in scanning probe microscopy on surface enable not only direct observation of molecular structures but also local probe reactions, in which unstable short-lived products have been synthesized and analyzed. Now, an endergonic reaction to synthesize a single Sondheimer–Wong diyne from 6,13-dibromopentaleno[1,2-b:4,5-b’]dinaphthalene by local probe chemistry on a ultra-thin film of NaCl formed on a Cu(111) surface at 4.3 K is presented. The structures of the precursor, two intermediates, and the final product were directly identified by the differential conductance imaging with a CO functionalized tip. DFT calculations revealed that the multiple-step reaction, being endergonic overall, is facilitated by temporal charging and discharging of the molecule placed in the nanometric junction between the Cu tip and the Cu substrate underneath the ultra-thin NaCl film. This local probe reaction expands possibilities to synthesize nanocarbon materials in a bottom-up manner.Ministry of Education (MOE)Nanyang Technological UniversityThis work was supported in part by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number19H00856, by Nanyang Technological University, by the Singapore Ministry of Education via Academic Research Fund Tier 1: 2018-T1-002-021, by the National Natural Science Foundation of China (Grant No. 21603086), and by China Scholarship Council (Grant No. 201608420186)

Going Nano with Confined Effects to Construct Pomegranate-like Cathode for High-Energy and High-Power Lithium-Ion Batteries

Pomegranate-like Li3V2(PO4)(3)@C (LVP@C) cathode materials are fabricated through confined effect helped by the vacuum-assisted capillary action. The performance of LixV2(PO4)(3) (x = 0-5) at an extended working voltage of 1.2-4.8 V has been studied by operando X-ray powder diffraction and hybrid functional density functional theory (DFT) calculation. The DFT calculation results suggest that Li3V2(PO4)(3) can be intercalated with another two Li+ with a stable crystalline structure, which improves the specific capacity of LVP significantly. The cathode exhibits a specific capacity of 320 mAh g(-1) with an energy density of 736 Wh kg(-1), which is one of the best performances for intercalation cathode materials for Li-ion batteries to our knowledge. Besides, the cathode showed excellent rate capability. In the working potential of 3.0-4.8 V, it exhibits a high specific capacity of 195 mAh g(-1) at 0.2 C, and even at a high rate of 30 C, it still delivers the specific capacity of 145 mAh g(-1) with a power density of 15.93 kW kg(-1). The good performance is mainly attributed to the unique pomegranate structure, which can provide continuous three-dimensional conductive networks for fast electron and Li-ion transfer. This paper provides a new strategy for synthesizing other cathode or anode materials with high energy and power density