Side‐Arm Effect of a Methyl α‐d‐Glucopyranoside Based Lariat Ether Catalysts in Asymmetric Syntheses

Methyl α‐d ‐glucopyranoside based monoaza‐15‐crown‐5 type lariat ethers with different heteroatom‐containing side arm attached to the nitrogen of the macrocyclic ring have been synthesized. These compounds were used as chiral phase transfer catalysts in a few asymmetric reactions, such as Michael additions, Darzens condensation, and epoxidation of chalcone. The side arms of the macrocycles had a significant impact on the chemical yields and the enantioselectivity. The effect of the lariat ethers with side arms having heteroatom (O, N, and S) was compared with the effect of the analogues having substituents without a heteroatom. The terminal allyl group also generated a significant enantioselectivity (79% enantiomeric excess) in one of the Michael additions. The application of crown ethers with substituents (CH₂)₃OH or (CH₂)₃OCH₃ leads to the best enantioselectivities 85% and 99%, respectively.     

Fluorescence quenching studies on the interaction of a novel deepened cavitand towards some transition metal ions

A novel ‘three-level’ deepened cavitand featuring a significantly sizable portal has been synthesized and its interaction with some transition metal ions has been investigated in THF/H₂O binary solvent using fluorescence quenching technique. The results suggest that among the used transition metal ions including Ag⁺, Cd²⁺, Cu²⁺, Fe³⁺, Cr³⁺, Hg²⁺, La³⁺, Mn²⁺, Ni²⁺, Zn²⁺ and Co²⁺, only Fe³⁺ and Cu²⁺ show good quenching ability. In order to interpret the quenching mechanism, the Stern–Volmer kinetics, and the presence of both the dynamic and static quenching have been discussed. It was found that the simultaneous presence of the sphere-of-action static quenching and dynamic quenching model agrees very well with the experimental results. The limits of detection for Fe³⁺ and Cu²⁺ were found to be 2.1 × 10⁻⁶ mol L⁻¹ (3σ) and 3.6 × 10⁻⁶ mol L⁻¹ (3σ), respectively. Cations with potential interference, such as K⁺, Na⁺, Mg²⁺, Ca²⁺, Co²⁺, La³⁺ and Mn²⁺ do not have significant effects on the determinations of Fe³⁺ and Cu²⁺. This cavitand can be potentially applied as optical sensor for the detection of Fe³⁺ and Cu²⁺.     

Synthesis of elongated cavitands via click reactions and their use as chemosensors

A very efficient modular reaction protocol was developed to attach various functionalities to a rigid cavitand scaffold. In this way, aryl, iodoaryl, benzyl, pyrrolidylmethyl groups, as well as a polyethylene-glycol chain were attached to the ‘triazol-level’ of the cavitand. The palladium-catalyzed ethynylation of the iodoarene moieties, followed by the copper(I)-catalyzed azide–alkyne cycloaddition produced novel cavitands with significantly elongated binding pockets. The dimensions of these molecules are calculated to be at least 9 Å×18 Å, which place them amongst the largest unimolecular hosts obtained by pure covalent synthesis. A cavitand-based click conjugate is used for selective complexation of Cu²⁺ and Fe³⁺, providing significant fluorescent quenching.     

Atomic Force Microscopy for Molecular Structure Elucidation

Using scanning probe microscopy techniques, at low temperatures and in ultrahigh vacuum, individual molecules adsorbed on surfaces can be probed with ultrahigh resolution to determine their structure and details of their conformation, configuration, charge states, aromaticity, and the contributions of resonance structures. Functionalizing the tip of an atomic force microscope with a CO molecule enabled atomic‐resolution imaging of single molecules, and measurement of their adsorption geometry and bond‐order relations. In addition, by using scanning tunneling microscopy and Kelvin probe force microscopy, the density of the molecular frontier orbitals and the electric charge distribution within molecules can be mapped. Combining these techniques yields a high‐resolution tool for the identification and characterization of individual molecules. The single‐molecule sensitivity and the possibility of atom manipulation to induce chemical reactions with the tip of the microscope open up unique applications in chemistry, and differentiate scanning probe microscopy from conventional methods for molecular structure elucidation. Besides being an aid for challenging cases in natural product identification, atomic force microscopy has been shown to be a powerful tool for the investigation of on‐surface reactions and the characterization of radicals and molecular mixtures. Herein we review the progress that high‐resolution scanning probe microscopy with functionalized tips has made for molecular structure identification and characterization, and discuss the challenges it will face in the years to come.     

The Chromatic Spectrum of Mixed Hypergraphs

 A mixed hypergraph is a triple ℋ=(X, ?, ?), where X is the vertex set, and each of ?, ? is a list of subsets of X. A strict k-coloring of ℋ is a surjection c:X→{1,…,k} such that each member of ? has two vertices assigned a common value and each member of ? has two vertices assigned distinct values. The feasible set of H is {k: H has a strict k-coloring}. Among other results, we prove that a finite set of positive integers is the feasible set of some mixed hypergraph if and only if it omits the number 1 or is an interval starting with 1. For the set {s,t} with 2≤s≤t−2, the smallest realization has 2t−s vertices. When every member of ?∪? is a single interval in an underlying linear order on the vertices, the feasible set is also a single interval of integers. Received: May 24, 1999 Final version received: August 31, 2000     

Sweet Sorghum as Feedstock for Ethanol Production: Enzymatic Hydrolysis of Steam-Pretreated Bagasse

Sweet sorghum is an attractive feedstock for ethanol production. The juice extracted from the fresh stem is composed of sucrose, glucose, and fructose and can therefore be readily fermented to alcohol. The solid fraction left behind, the so-called bagasse, is a lignocellulosic residue which can also be processed to ethanol. The objective of our work was to test sweet sorghum, the whole crop, as a potential raw material of ethanol production, i.e., both the extracted sugar juice and the residual bagasse were tested. The juice was investigated at different harvesting dates for sugar content. Fermentability of juices extracted from the stem with and without leaves was compared. Sweet sorghum bagasse was steam-pretreated using various pretreatment conditions (temperatures and residence times). Efficiency of pretreatments was characterized by the degree of cellulose hydrolysis of the whole pretreated slurry and the separated fiber fraction. Two settings of the studied conditions (190 °C, 10 min and 200 °C, 5 min) were found to be efficient to reach conversion of 85–90%.     

The cover pebbling number of graphs

A pebbling move on a graph consists of taking two pebbles off of one vertex and placing one pebble on an adjacent vertex. In the traditional pebbling problem we try to reach a specified vertex of the graph by a sequence of pebbling moves. In this paper we investigate the case when every vertex of the graph must end up with at least one pebble after a series of pebbling moves. The cover pebbling number of a graph is the minimum number of pebbles such that however the pebbles are initially placed on the vertices of the graph we can eventually put a pebble on every vertex simultaneously. We find the cover pebbling numbers of trees and some other graphs. We also consider the more general problem where (possibly different) given numbers of pebbles are required for the vertices.     

Optimisation of solid-state urea clathrate formation as a chemical separation method coupled to compound-specific stable carbon isotope analysis

Solid-state urea clathrate formation (SSUCF) as a chemical separation method prior to stable carbon isotope fingerprinting of diesel fuel contaminations was studied. The stable carbon isotope ratios (δ¹³C) of n-alkanes in diesel fuel can be used to trace the origin of a contamination. The accurate measurement of the stable isotopic composition of individual compounds requires baseline separation from any other co-eluting compounds. For this purpose silica gel column chromatography (SGCC) and SSUCF were applied. Detailed optimisation of SSUCF was performed: different activators, clathrate formation temperatures, activator volumes, clathrate formation times and sample capacity were investigated. The main benefits of the developed method are reduced clathrate formation time and increased recoveries for lower molecular weight n-alkanes. The recoveries of the developed SSUCF method ranged between 63 and 100% for C10–C24 n-alkanes with relative standard deviation no more than 7%. The precision of the gas chromatography-isotope ratio mass spectrometry measurement was acceptable with a standard deviation of the δ¹³C values ranging between 0.08 and 0.15‰. The absence of isotopic fractionation was also investigated.

The robustness of the method was tested within a model experiment. Nine different water samples including distilled water, tap water, river water, industrial wastewaters and groundwater samples were spiked with the same diesel fuel. The water samples were extracted with n-hexane and after purification with both SGCC and SSUCF n-alkanes were measured. The δ¹³C values of n-alkanes were found to be similar for all samples. The importance of sample purification prior to compound-specific isotope analysis (CSIA) was also demonstrated within this model experiment by analysing samples from different stages of the sample preparation.

Our results show that the proposed method can remarkably improve the precision of compound-specific stable carbon isotope analysis of n-alkanes originating from diesel contamination of the aquatic environment.     

Nonlinear Analysis of a Cantilever Pipe Containing Pulsatile Flow

In this paper we investigate the nonlinear dynamics of a cantilever elastic pipe that contains pulsatile flow. The equation of motion was derived by using Hamiltonian action function. We use Galerkin's technique to include only finite number of spatial modes in the solution.The stability chart of the time-varying system was computed in the space of the relative perturbation amplitude of the flow velocity and dimensionless forcing frequency using an efficient numerical method based on Chebyshev polynomials. In the near of some critical regions bifurcation diagrams were also computed which show secondary Hopf bifurcations and phase locking followed by chaotic motion.