Competition between N and O: use of diazine N-oxides as a test case for the Marcus theory rationale for ambident reactivity

The preferred site of alkylation of diazine N-oxides by representative hard and soft alkylating agents was established conclusively using the ¹H–¹⁵N HMBC NMR technique in combination with other NMR spectroscopic methods. Alkylation of pyrazine N-oxides (1 and 2) occurs preferentially on nitrogen regardless of the alkylating agent employed, while O-methylation of pyrimidine N-oxide (3) is favoured in its reaction with MeOTf. As these outcomes cannot be explained in the context of the hard/soft acid/base (HSAB) principle, we have instead turned to Marcus theory to rationalise these results. Marcus intrinsic barriers (ΔG^‡₀) and Δ_rG° values were calculated at the DLPNO-CCSD(T)/def2-TZVPPD/SMD//M06-2X-D3/6-311+G(d,p)/SMD level of theory for methylation reactions of 1 and 3 by MeI and MeOTf, and used to derive Gibbs energies of activation (ΔG^‡) for the processes of N- and O-methylation, respectively. These values, as well as those derived directly from the DFT calculations, closely reproduce the observed experimental N- vs. O-alkylation selectivities for methylation reactions of 1 and 3, indicating that Marcus theory can be used in a semi-quantitative manner to understand how the activation barriers for these reactions are constructed. It was found that N-alkylation of 1 is favoured due to the dominant contribution of Δ_rG° to the activation barrier in this case, while O-alkylation of 3 is favoured due to the dominant contribution of the intrinsic barrier (ΔG^‡₀) for this process. These results are of profound significance in understanding the outcomes of reactions of ambident reactants in general.

Marcus theory enables rationalisation and quantification of selectivities in reactions of ambident nucleophiles for which the HSAB principle cannot operate.     

Examination of complex oligosaccharides by matrix-assisted laser desorption/ionization mass spectrometry on time-of-flight and magnetic sector instruments

Matrix-assisted laser desorption/ionization (MALDI) spectra of underivatized oligosaccharides of the type attached to asparagine in glycoproteins (N-linked oligosaccharides) were examined with linear time-of-flight (TOF) and magnetic sector instruments using 2,5-dihydroxybenzoic acid (2,5-DHB), α-cyano-4-hydroxycinnamic acid, sinapinic acid, 1,4-dihydroxynaphthalene-2-carboxylic acid or 2-(4-hydroxyphenylazo)benzoic acid (HABA) as the matrices. All compounds formed abundant [M + Na]⁺ ions with the strongest signals being obtained from 2,5-DHB after recrystallization of the initially dried sample spot from ethanol. Only traces of fragmentation were detected from neutral oligosaccharides on the TOF system but more abundant fragment ions (about 5% relative abundance) were present in the spectra from the magnetic sector instrument. Fragmentation was dominated by Y-type glycosidic cleavages (Domon and Costello nomenclature) between all sugar residues yielding sequence and branching information. Sialic acid-containing oligosaccharides generally produced the sodium adduct of the sodium salt and gave much weaker signals than the neutral sugars in the positive-ion mode. There was also considerable loss of the sialic acid moleties as the result of fragmentation on the magnetic sector instrument. The least fragmentation of both neutral and acidic sugars was caused by 2.5 DHB, which proved to be the most appropriate matrix for examination of oligosaccharide mixtures. Much better resolution of the oligosaccharides was obtained than by traditional methods such as the use of Bio-Gel P-4 gel filtration column chromatography. It is worth noting also that the measurements were considerably faster (a few minutes as opposed to about 16 h). In addition, no radiolabelling was necessary as required for detection on the P-4 columns. Mixtures of oligosaccharides from several glycoproteins (ribonuclease B, human immunoglobulin G (IgG) transferrin, bovine fetuin and chicken ovalbumin) were examined and the patterns of the identified oligosaccharides were found to agree closely with the known compositions of the sugar mixtures. The mass spectrometric resolution on the magnetic sector instrument was very much better (up to 3000, FWHM) than could be obtained with the linear TOF systems (200–400). The technique was used as a detection system for the products of exoglycosidase digestion in experiments to determine the detailed structure of the oligosaccharide chains from human IgG.     

Ionisation and fragmentation of complex glycans with a quadrupole time-of-flight mass spectrometer fitted with a matrix-assisted laser desorption/ionisation ion source

This paper reports the use of an experimental matrix-assisted laser desorption/ionisation (MALDI) ion source fitted to a quadrupole time-of-flight (Q-Tof) mass spectrometer for the analysis of carbohydrates, particularly the N-linked glycans from glycoproteins. Earlier work on the Q-Tof instrument, using electrospray ionisation, gave excellent MS/MS spectra, particularly from the [M + Na]+ ions, but suffered from the major disadvantages that the signal was often split between singly and multiply charged ions and that sensitivity fell dramatically as the molecular weight of the carbohydrate rose. The MALDI ion source did not suffer from these problems and the instrument produced excellent MS and MS/MS spectra from small amounts of complex, underivatised glycans as well as those derivatised at the reducing terminus. Positive ion MS spectra of sialylated glycans recorded on the new instrument were much less complex than those recorded with a conventional MALDI-TOF instrument because of the absence of ions resulting from metastable (post-source decay, (PSD)) fragmentations occurring in the flight tube. However, considerable fragmentation by loss of sialic acid still occurred. MS/MS spectra of the [M + Na]+ ions from all compounds were almost identical to those recorded earlier with the electrospray-Q-Tof combination and far superior to MALDI-PSD spectra recorded with reflectron-TOF instruments. Spectra are shown for neutral and sialylated N-linked glycans from chicken ovalbumin, riboflavin binding protein, alpha1-acid glycoprotein, bovine fetuin and ribonuclease B, both as free glycans and as those derivatised at their reducing termini. The technique was applied to the structural determination of N-linked glycans from human secretory IgA and Apo-B 100 from human low-density lipoprotein.     

Measuring the thermal conductivity of heat transfer fluids via the modified transient plane source (MTPS)

Heat transfer fluids are often a critical performance component in industrial processes and system design. Fluids are used in heat dissipation to maintain stable operating temperatures in a variety of applications, such as diesel engines, chemical production, asphalt storage, and high-power electric transformers. A wide range of fluids specific to various applications are available, thus a reliable and accurate thermal conductivity characterization is extremely important. Thermal conductivity analysis of heat transfer fluids with traditional methods is time-consuming and error-prone due to the impact of convection. Convection often distorts effective thermal conductivity measurement as an additional source of heat transfer. The modified transient plane source method implemented in the C-Therm Technologies TCi Analyzer provides an easy way to accurately measure the thermal conductivity and distinguish this form of heat transfer in negating the impact of convection by (a) employing the shortest test time in commercially available sensors (0.8 s), (b) offering a minimal sample volume requirement (1.25 mL), and (c) employing a low-energy power flux to the specimen under test (approximately 2,600 W m^?2). This work presents thermal conductivity results generated on three types of heat transfer fluids over a wide temperature range and discusses the significance of the data in relevance to the application.     

High throughput liquid chromatography/mass spectrometric analyses using a novel multiplexed electrospray interface

A novel four- channel multiplexed electrospray liquid chromatography interface is described. This device has been used to analyse both single components and mixtures by liquid chromatography/mass spectrometry (LC/MS) as well as synthetic samples prepared by automated procedures. These data provided unambiguous molecular weight assignments to both major components and synthetic by-products in these samples. In this work particular attention has also been paid to the elimination of interchannel crosstalk. Copyright 1999 John Wiley & Sons, Ltd.     

X-ray emission from clusters of galaxies and cosmological parameters

Clusters of galaxies are excellent probes of cosmic structure and evolution. X-ray studies of clusters provide some of their key parameters, viz., temperature of the hot intra-cluster gas, its metallicity, X-ray luminosity and surface brightness giving mass distribution and mass-flow rate in the case of cooling flows. X-ray measurements for a large sample of clusters have lead to estimates of the total gravitating mass in them, which can be compared to the virial masses derived from dynamical considerations and gravitational lensing in some of them. X-ray derived total masses are consistent with masses obtained from the other methods after the effects due to the presence of cooling flows are taken into account in the analyses. Estimated virial masses, lack of evolution in X-ray properties, and detection of several very hot clusters at high redshifts indicate a Universe with a low value (≤ 0.3) for the Ω parameter.     

Photolytic processing of secondary organic aerosols dissolved in cloud droplets

The effect of UV irradiation on the molecular composition of aqueous extracts of secondary organic aerosol (SOA) was investigated. SOA was prepared by the dark reaction of ozone and d-limonene at 0.05-1 ppm precursor concentrations and collected with a particle-into-liquid sampler (PILS). The PILS extracts were photolyzed by 300-400 nm radiation for up to 24 h. Water-soluble SOA constituents were analyzed using high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) at different stages of photolysis for all SOA precursor concentrations. Exposure to UV radiation increased the average O/C ratio and decreased the average double bond equivalent (DBE) of the dissolved SOA compounds. Oligomeric compounds were significantly decreased by photolysis relative to the monomeric compounds. Direct pH measurements showed that acidic compounds increased in abundance upon photolysis. Methanol reactivity analysis revealed significant photodissociation of molecules containing carbonyl groups and the formation of carboxylic acids. Aldehydes, such as limononaldehyde, were almost completely removed. The removal of carbonyls was further confirmed by the UV/Vis absorption spectroscopy of the SOA extracts where the absorbance in the carbonyl n→π* band decreased significantly upon photolysis. The effective quantum yield (the number of carbonyls destroyed per photon absorbed) was estimated as ～0.03. The total concentration of peroxides did not change significantly during photolysis as quantified with an iodometric test. Although organic peroxides were photolyzed, the likely end products of photolysis were smaller peroxides, including hydrogen peroxide, resulting in a no net change in the peroxide content. Photolysis of dry limonene SOA deposited on substrates was investigated in a separate set of experiments. The observed effects on the average O/C and DBE were similar to the aqueous photolysis, but the extent of chemical change was smaller in dry SOA. Our results suggest that biogenic SOA dissolved in cloud and fog droplets will undergo significant photolytic processing on a time scale of hours to days. This type of photolytic processing may account for the discrepancy between the higher values of O/C measured in the field experiments relative to the laboratory measurements on SOA in smog chambers. In addition, the direct photolysis of oligomeric compounds may be responsible for the scarcity of their observation in the field.     

Review paper: Design of a mass and ion kinetic energy spectrometer for organic chemical work

The design of a mass spectrometer for the determination of the structural formulae of organic compounds is discussed. The ion-optical characteristics of electric and magnetic sectors and also of quadrupole mass analysers are considered and the additional information that can be gleaned when such components are combined in various ways is listed. The advantages of using collision cells for inducing fragmentation of selected ion species are listed including those that result when the collision cell is floated at an electrical potential different from that of the incident ion beam. Important performance characteristics are the resolution with which a particular ion may be selected and the resolution with which daughter ions formed from it can be separated. It is concluded, that for instruments comprising three analysing units together with the appropriate collision cells, the most versatile combinations and those with the highest performance are an arrangement consisting of a magnetic sector followed by an electric sector, this being followed either by a further magnetic sector or a quadrupole. The properties of these two systems are compared in detail.     

The Erdös-Fuchs theorem on the square of a power series

Erdös and Fuchs proved that if a₁, a₂,… is a sequence of nonnegative integers and R(n) is the number of ordered pairs (i, j) with a_i + a_j ≤ n, then it is impossible to have $\text{R(n) = An + o(n}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}\text{log}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}\text{n)}$ as n → + ∞, for some positive constant A. This paper gives a generalization of this result, in which An is replaced by a function of n whose second differences are nonnegative from some point on.