Structure determination of sec-butylbenzene rotamers by UV spectroscopy and ab initio calculations

Sec-butylbenzene has been investigated using resonant two-photon ionization (R2PI) and UV–UV hole-burning spectroscopy aided by ab initio calculations. All three conformers predicted from theory are observed in the spectrum, and are assigned by rotational band contour analysis. The most strongly populated conformer (G1) has a gauche arrangement of the side chain dihedral angle τ₂(C₁C_αC_βC_γ). The populations of the anti (A) and the remaining gauche conformer (G2) are about 7% and 2%, respectively. The alpha methyl group is found to significantly affect the conformational preferences in sec-butylbenzene (sec-BB), compared to n-propylbenzene in which the anti conformer is favored.     

Liquid chromatography/tandem mass spectrometry for analysis of 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) and 1,2,5,6-tetrabromocyclooctane (TBCO)

Although the two flame retardants 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH) and 1,2,5,6-tetrabromocyclooctane (TBCO) have been widely used, a selective instrumental method of analysis for these compounds has not been developed to date. In this study, we demonstrate the feasibility to utilize liquid chromatography/tandem mass spectrometry (LC/MS/MS) for the separation and analysis of α- and β-TBCO and α-, β-, γ-, and δ-TBECH. Acetone was initially used in a tetrahedron solvent system for LC optimization. A simple isocratic elution allowed near-baseline separation of these compounds. Different ionization approaches and mechanisms were investigated. The mass spectrometric transition of [M + O(2)](-) => Br(-) (459.8 => 78.9) was a selective detection method for the target analytes. Good instrument detection limits (5 pg for γ-/δ-TBECH, 125 pg for α-/β-TBECH, and 30 pg for α-/β-TBCO with 2.0 μL injection) were obtained. Excellent linearity up to 50 ng/μL (R(2) >0.999) was also achieved. This method has been applied to environmental samples (surface water) for screening purposes with recoveries ranging from 76-92% (CV%: 5-8%). This method shows significant improvement over previous methods.     

Rheo-attenuated total reflectance infrared spectroscopy: a new tool to study biopolymers

Whilst rheology is the reference technique to study the mechanical properties of unspun silk, we know little of the structure and the dynamics that generate them. By coupling infrared spectroscopy and shearing forces to study silk fibroin conversion, we are introducing a novel tool to address this gap in our knowledge. Here the silk conversion process has been studied dynamically using polarized attenuated total reflectance Fourier transform infrared spectroscopy whilst applying shear, thus revealing silk protein conformation and molecular orientation in situ. Our results show that the silk conversion process starts with a pre-alignment of the proteins followed by a rapid growth of the β-sheet formation and then a subsequent deceleration of the growth. We propose that this tool will provide further insight into not only silk but any biopolymer solution, opening a new window into biological materials.     

Determination of decamethylcyclopentasiloxane in river and estuarine sediments in the UK

Robust analytical procedures for the measurement of decamethylcyclopentasiloxane (D₅) in river and estuarine sediments and their application in determining environmental concentrations in the UK are presented for the first time in this work. Novel approaches to minimise commonly reported artefacts are utilised, improving the confidence in the concentrations of D₅ reported. Accelerated solvent extraction (ASE) and liquid–solid extraction methods are compared. Both methods use on-column injection gas chromatography/mass spectrometry (GC/MS). Measurements of D₅ concentrations in sediments sampled from the river Great Ouse and from the Humber estuary (UK) are also reported. ASE was suitable to measure concentrations of D₅ in sediments obtained from the river Great Ouse, UK (186–1450 ng g⁻¹, dry weight) and octamethyltetracyclosiloxane (D₄, 12–24 ng g⁻¹, dry weight). C₁₂ linear alkybenzene (C₁₂ LAB), which can be used as a chemical marker for sewage effluent related emissions, was also measured in this analysis. Liquid–solid extraction was optimised to provide more confidence in the lower D₅ concentrations measured in the Humber estuary, UK (49–256 ng g⁻¹, dry weight). A Limit of quantitation (LOQ) for D₅ of 57–110 and 4 ng g⁻¹ dry weight was determined for ASE and liquid–solid extraction, respectively.     

Nanoscale polyoxometalate‐based inorganic/organic hybrids

The latest advances in the area of polyoxometalate (POM)‐based inorganic/organic hybrid materials prepared by self‐assembly, covalent modification, and supramolecular interactions are presented. This Review is composed of five sections and documents the effect of organic cations on the formation of novel POMs, surfactant encapsulated POM‐based hybrids, polymeric POM/organic hybrid materials, POMs‐containing ionic crystals, and covalently functionalized POMs. In addition to their role in the charge‐balancing, of anionic POMs, the crucial role of organic cations in the formation and functionalization of POM‐based hybrid materials is discussed. DOI 10.1002/tcr.201100002     

Utilizing Lifetimes to Suppress Random Coil Features in 2D IR Spectra of Peptides

We report that the waiting time delay in 2D IR pulse sequences can be used to suppress signals from structurally disordered regions of amyloid fibrils. At a waiting time delay of 1.0 ps, the random coil vibrational modes of amylin fibrils are no longer detectable, leaving only the sharp excitonic vibrational features of the fibril β-sheets. Isotope labeling with (13)C(18)O reveals that structurally disordered residues decay faster than residues protected from solvent. Since structural disorder is usually accompanied by hydration, we conclude that the shorter lifetimes of random-coil residues is due to solvent exposure. These results indicate that 2D IR pulse sequences can utilize the waiting time to better resolve solvent-protected regions of peptides and that local mode lifetimes should be included in simulations of 2D IR spectra.     

Identification of inorganic ions in post‐blast explosive residues using portable CE instrumentation and capacitively coupled contactless conductivity detection

Novel CE methods have been developed on portable instrumentation adapted to accommodate a capacitively coupled contactless conductivity detector for the separation and sensitive detection of inorganic anions and cations in post‐blast explosive residues from homemade inorganic explosive devices. The methods presented combine sensitivity and speed of analysis for the wide range of inorganic ions used in this study. Separate methods were employed for the separation of anions and cations. The anion separation method utilised a low conductivity 70 mM Tris/70 mM CHES aqueous electrolyte (pH 8.6) with a 90 cm capillary coated with hexadimethrine bromide to reverse the EOF. Fifteen anions could be baseline separated in 7 min with detection limits in the range 27–240 μg/L. A selection of ten anions deemed most important in this application could be separated in 45 s on a shorter capillary (30.6 cm) using the same electrolyte. The cation separation method was performed on a 73 cm length of fused‐silica capillary using an electrolyte system composed of 10 mM histidine and 50 mM acetic acid, at pH 4.2. The addition of the complexants, 1 mM hydroxyisobutyric acid and 0.7 mM 18‐crown‐6 ether, enhanced selectivity and allowed the separation of eleven inorganic cations in under 7 min with detection limits in the range 31–240 μg/L. The developed methods were successfully field tested on post‐blast residues obtained from the controlled detonation of homemade explosive devices. Results were verified using ion chromatographic analyses of the same samples.     

A general copper-BINAP-catalyzed asymmetric propargylation of ketones with propargyl boronates

An operationally simple copper-BINAP-catalyzed, highly enantioselective propargylation of ketones is presented. The methodology was developed as an enantioselective process for methyl ethyl ketone and shown to be applicable to a wide variety of prochiral ketones. The resulting homopropargyl adducts are versatile latent carbonyls from which γ-butyrolactones, β-hydroxy methyl ketones, and β-hydroxycarboxylates are readily obtained.     

Probing two-photon properties of molecules: large non-Condon effects dominate the resonance hyper-Raman scattering of rhodamine 6G

Experimentally measured resonance hyper-Raman (RHR) spectra spanning the S(1) ← S(0), S(2) ← S(0), and S(3) ← S(0) transitions in rhodamine 6G (R6G) have been recorded. These spectra are compared to the results of first-principles calculations of the RHR intensity that include both Franck-Condon (A-term) and non-Condon (B-term) scattering effects. Good agreement between the experimental and theoretical results is observed, demonstrating that first-principles calculations of hyper-Raman intensities are now possible for large molecules such as R6G. Such agreement indicates that RHR spectroscopy will now be a routine aid for probing multiphoton processes. This work further shows that optimization of molecular properties to enhance either A- or B-term scattering might yield molecules with significantly enhanced two-photon properties.     

Towards a universal product ion mass spectral library - reproducibility of product ion spectra across eleven different mass spectrometers

Product ion spectra produced by collision-induced dissociation (CID) in tandem mass spectrometry experiments can differ markedly between instruments. There have been a number of attempts to standardise the production of product ion spectra; however, a consensus on the most appropriate approach to the reproducible production of spectra has yet to be reached. We have previously reported the comparison of product ion spectra on a number of different types of instruments - a triple quadrupole, two ion traps and a Fourier transform ion cyclotron resonance mass spectrometer (Bristow AWT, Webb KS, Lubben AT, Halket JM. Rapid Commun. Mass Spectrom. 2004; 18: 1). The study showed that a high degree of reproducibility was achievable. The goal of this study was to improve the comparability and reproducibility of CID product ion mass spectra produced in different laboratories and using different instruments. This was carried out experimentally by defining a spectral calibration point on each mass spectrometer for product ion formation. The long-term goal is the development of a universal (instrument independent) product ion mass spectral library for the identification of unknowns.The spectra of 48 compounds have been recorded on eleven mass spectrometers: six ion traps, two triple quadrupoles, a hybrid triple quadrupole, and two quadrupole time-of-flight instruments. Initially, 4371 spectral comparisons were carried out using the data from eleven instruments and the degree of reproducibility was evaluated. A blind trial has also been carried out to assess the reproducibility of spectra obtained during LC/MS/MS.The results suggest a degree of reproducibility across all instrument types using the tuning point technique. The reproducibility of the product ion spectra is increased when comparing the tandem in time type instruments and the tandem in space instruments as two separate groups. This may allow the production of a more limited, yet useful, screening library for LC/MS/MS identification using instruments of the same type from different manufacturers.