Frequency Domain Fluorescence Lifetime Study of Crude Petroleum Oils

Frequency domain (FD) fluorescence lifetime data was collected for a series of 20 crude petroleum oils using a 405 nm excitation source and over a spectral range of ~426 to ~650 nm. Average fluorescence lifetimes were calculated using three different models: discrete multi-exponential, Gaussian distribution, and Lorentzian distribution. Fitting the data to extract accurate average lifetimes using the various models proved easier and less time consuming for the FD data than with Time Correlated Single Photon Counting (TCSPC) methods however the analysis of confidence intervals to the computed average lifetimes proved cumbersome for both methods. The uncertainty in the average lifetime was generally larger for the discrete lifetime multi-exponential model when compared to the distribution-based models. For the lifetime distributions, the data from the light crude oils with long lifetimes generally fit to a single decay term. Heavier oils with shorter lifetimes required multiple decay terms. The actual value for the average lifetime is more dependant on the specific fitting model employed than the data acquisition method used. Correlations between average fluorescence lifetimes and physical and chemical parameters of the crude oils were made with a view to developing a quantitative model for predicting the gross chemical composition of crude oils. It was found that there was no significant benefit gained by using FD over TCSPC other than more rapid data analysis in the FD case. For the FD data the Gaussian distribution model for fluorescence lifetime gave the best correlations with chemical composition allowing a qualitative correlation to some bulk oil parameters.

Solid state water motions revealed by deuterium relaxation in 2H2O-synthesized kanemite and 2H2O-hydrated Na+-zeolite A

Deuterium NMR relaxation experiments, low temperature deuterium NMR lineshape analysis, and FTIR spectra are consistent with a new model for solid state jump dynamics of water in (2)H(2)O-synthesized kanemite and (2)H(2)O-hydrated Na(+)-Zeolite A. Exchange occurs between two populations of water: one in which water molecules are directly coordinated to sodium ions and experience C(2) symmetry jumps of their OH bonds, and a population of interstitial water molecules outside the sodium ion coordination sphere that experience tetrahedral jumps of their OH bonds. For both samples the C(2) jump rate is much faster than the tetrahedral jump rate. (2)H NMR relaxation experiments match well with the fast exchange regime of the model over a wide range of temperatures, including room temperature and above. For hydrated Zeolite A, the kinetic activation parameters for the tetrahedral and C(2) symmetry jumps are Delta H tet++=+17 kJ/mol, Delta S tet++=-109 J/(mol K), Delta H C2++=+19 kJ/mol, and Delta S C2++=-20 J/(mol K). For kanemite, Delta H tet++ =+23 kJ/mol, Delta S tet++=-69 J/(mol K), Delta H C2++ =+23 kJ/mol, and Delta S C2++ =-11 J/(mol K).     

Structure elucidation of cyclohexene (9Z)-octadec-9-enyl ethers isolated from the leaves of Uvaria cherrevensis (Annonaceae)

The phytochemical investigation of the leaf extract of Uvaria cherrevensis (Annonaceae) yielded three new cyclohexene (9Z)-octadec-9-enyl ethers, cherrevenols M-O (1–3), and a known fatty ester derivative (4). The structures of the isolated compounds were elucidated by spectroscopic and computer-aided molecular modelling methods. Ozone Induced Dissociation (OzID) mass spectrometry was employed to determine the C-9 position of the side chain olefinic double bonds, while ¹³C NMR spectroscopy indicated their (Z)-configurations. All isolated compounds were evaluated for their antimalarial and cytotoxic activities; all were inactive.     

An N‐Heterocyclic Carbene with a Saturated Backbone and Spatially‐Defined Steric Impact

The synthesis and coordination chemistry of a saturated analogue of a “bulky‐yet‐flexible” N‐heterocyclic carbene (NHC) ligand are described. “SIPaul” is a 4,5‐dihydroimidazol‐2‐ylidene ligand with unsymmetrical aryl N‐substituents, and is one of the growing class of “bulky‐yet‐flexible” NHCs that are sufficiently bulky to stabilize catalytic intermediates, but sufficiently flexible that they do not inhibit productive chemistry at the central metal atom. Here, the synthesis of SIPaul.HCl and its complexes with copper, silver, iridium, palladium, and nickel, and its selenourea, are reported. The steric impact of the ligand is quantified using percent buried volume (% V_bur), whereas the electronic properties are probed and quantified using the Tolman Electronic Parameter (TEP) and δ_Se of the corresponding selenourea. This work shows that despite the often very different performance of saturated versus unsaturated carbenes in catalysis, the effect of backbone saturation on measurable properties is very small.     

Rapid Assembly of Saturated Nitrogen Heterocycles in One‐Pot: Diazo‐Heterocycle “Stitching” by N–H Insertion and Cyclization

Methods that provide rapid access to new heterocyclic structures in biologically relevant chemical space provide important opportunities in drug discovery. Here, a strategy is described for the preparation of 2,2‐disubstituted azetidines, pyrrolidines, piperidines, and azepanes bearing ester and diverse aryl substituents. A one‐pot rhodium catalyzed N–H insertion and cyclization sequence uses diazo compounds to stitch together linear 1,m‐haloamines (m=2–5) to rapidly assemble 4 ‐, 5 ‐, 6 ‐, and 7 ‐membered saturated nitrogen heterocycles in excellent yields. Over fifty examples are demonstrated, including examples with diazo compounds derived from biologically active compounds. The products can be functionalized to afford α,α‐disubstituted amino acids and applied to fragment synthesis.     

Aryl Sulfonium Salts for Site‐Selective Late‐Stage Trifluoromethylation

Incorporation of the CF₃ group into arenes has found increasing importance in drug discovery. Herein, we report the first photoredox‐catalyzed cross‐coupling of aryl thianthrenium salts with a copper‐based trifluoromethyl reagent, which enables a site‐selective late‐stage trifluoromethylation of arenes. The reaction proceeds with broad functional group tolerance, even for complex small molecules on gram scale. The method was further extended to produce pentafluoroethylated derivatives.     

Preparation of Polyfunctional Arylzinc Organometallics in Toluene by Halogen/Zinc Exchange Reactions

A wide range of polyfunctional diaryl‐ and diheteroarylzinc species were prepared in toluene within 10 min to 5 h through an I/Zn or Br/Zn exchange reaction using bimetallic reagents of the general formula R′₂Zn?2 LiOR (R′=sBu, tBu, pTol). Highly sensitive functional groups, such as a triazine, a ketone, an aldehyde, or a nitro group, were tolerated in these exchange reactions, enabling the synthesis of a plethora of functionalized (hetero)arenes after quenching with various electrophiles. Insight into the constitution and reactivity of these bimetallic mixtures revealed the formation of highly active lithium diorganodialkoxyzincates of type [R′₂Zn(OR)₂Li₂].     

Investigation of molecular weight effects of polystyrene in ToF-SIMS using C60 and Au primary ion beams

In secondary ion mass spectrometry, polyatomic primary ion sources are known to enhance yields from many surfaces including polymers. In order to understand the fundamental causes for these increases, the enhancement as a function of material type and molecular weight needs to be delineated. In this article, we report results from a systematic investigation of polymeric films of polystyrene (PS) with varying molecular weights to examine the influence of the primary ion beam on the secondary ion yields in time of flight secondary ion mass spectrometry (ToF-SIMS). The masses of the polymers investigated ranged from 1000 to 20,000 Da, or from about n = 10 to 200 where n indicates the number of polymeric units in a polymer chain. The polymers had a narrow molecular weight range (PDI < 1.07). The multilayer polymeric films (10-30 nm) characterized by AFM were prepared by spin-casting onto silicon substrates and were analyzed using Au⁺ and C₆₀⁺ primary ion beams. The analysis with the two beams provided a useful comparison between atomic and polyatomic primary ion sources. Information gathered from this study provides insight into the role of molecular weight on the observed yield enhancement from polyatomic ion sources.