An estimate of the global emissions of methyl bromide from automobile exhausts

An estimate of the annual global methyl bromide (CH3Br) emissions from automobile exhausts has been determined by extrapolating the results of a field study conducted in the United Kingdom (UK). A strong linear correlation was observed between the CH3Br and carbon monoxide (CO) concentrations of roadside air in three cities. This correlation and knowledge of the UK CO emissions was used to estimate the source strength of CH3Br from automobile exhausts in the UK (0.04 ktonnes yr−1). Further extrapolations lead to a value of 1.5 ktonnes yr−1 (with an upper limit of 3.0 ktonnes yr−1) of CH3Br released globally to the atmosphere from automobile exhausts.

Large carbon isotope fractionation associated with oxidation of methyl halides by methylotrophic bacteria

The largest biological fractionations of stable carbon isotopes observed in nature occur during production of methane by methanogenic archaea. These fractionations result in substantial (as much as 70) shifts in 13C relative to the initial substrate. We now report that a stable carbon isotopic fractionation of comparable magnitude (up to 70) occurs during oxidation of methyl halides by methylotrophic bacteria. We have demonstrated biological fractionation with whole cells of three methylotrophs (strain IMB-1, strain CC495, and strain MB2) and, to a lesser extent, with the purified cobalamin-dependent methyltransferase enzyme obtained from strain CC495. Thus, the genetic similarities recently reported between methylotrophs, and methanogens with respect to their pathways for C1-unit metabolism are also reflected in the carbon isotopic fractionations achieved by these organisms. We found that only part of the observed fractionation of carbon isotopes could be accounted for by the activity of the corrinoid methyltransferase enzyme, suggesting fractionation by enzymes further along the degradation pathway. These observations are of potential biogeochemical significance in the application of stable carbon isotope ratios to constrain the tropospheric budgets for the ozone-depleting halocarbons, methyl bromide and methyl chloride.

A method for carbon stable isotope analysis of methyl halides and chlorofluorocarbons at pptv concentrations

A pre-concentration system has been validated for use with a gas chromatography/mass spectrometry/isotope ratio mass spectrometer (GC/MS/IRMS) to determine ambient air 13C/12C ratios for methyl halides (MeCl and MeBr) and chlorofluorocarbons (CFCs). The isotopic composition of specific compounds can provide useful information on their atmospheric budgets and biogeochemistry that cannot be ascertained from abundance measurements alone. Although pre-concentration systems have been previously used with a GC/MS/IRMS for atmospheric trace gas analysis, this is the first study also to report system validation tests. Validation results indicate that the pre-concentration system and subsequent separation technologies do not significantly alter the stable isotopic ratios of the target methyl halides, CFC-12 (CCl2F2) and CFC-113 (C2Cl3F3). Significant, but consistent, isotopic shifts of -27.5 to -25.6 do occur within the system for CFC-11 (CCl3F), although the shift is correctible. The method presented has the capacity to separate these target halocarbons from more than 50 other compounds in ambient air samples. Separation allows for the determination of stable carbon isotope ratios of five of these six target trace atmospheric constituents within ambient air for large volume samples (10 L). Representative urban air analyses from Belfast City are also presented which give carbon isotope results similar to published values for 13C/12C analysis of MeCl (-39.1) and CFC-113 (-28.1). However, this is the first paper reporting stable carbon isotope signatures for CFC-11 (-29.4) and CFC-12 (-37.0).

Solvation effects on equilibria: Triazoles and N-methyl piperidinol

We have performed calculations of the solvation effects on a number of equilibrium constants in water using a recently proposed hybrid quantum classical scheme in which the liquid environment is modelled using classical solvent molecules and the solute electronic structure is computed using modern quantum chemical methods. The liquid phase space is sampled from a fully classical simulation. We find that solvation effects on both triazole tautomeric equilibrium constants and piperidinol conformational equilibrium constants can be interpreted in terms of subtle differences in the local environment which can be seen in probability densities and radial distribution functions. Lower level calculations were performed for comparison and we conclude that the solvation thermodynamics can be predicted from a good classical model of solvent and solute molecules, but the implicit models that we tried are less successful.

Neighboring Group-Controlled Hydrolysis: Towards “Designer” Drug Release Biomaterials

To give the first demonstration of neighboring group-controlled drug delivery rates, a series of novel, polymerizable ester drug conjugates was synthesized and fully characterized. The monomers are suitable for copolymerization in biomaterials where control of drug release rate is critical to prophylaxis or obviation of infection. The incorporation of neighboring group moieties differing in nucleophilicity, geometry, and steric bulk in the conjugates allowed the rate of ester hydrolysis, and hence drug liberation, to be rationally and widely controlled. Solutions (2.5 x 10-5 mol dm-3) of ester conjugates of nalidixic acid incorporating pyridyl, amino, and phenyl neighboring groups hydrolyzed according to first-order kinetics, with rate constants between 3.00 ( 0.12 10-5 s -1 (fastest) and 4.50 ( 0.31 10- 6 s-1 (slowest). The hydrolysis was characterized using UV-visible spectroscopy. When copolymerized with poly(methyl methacrylate), free drug was shown to elute from the resulting materials, with the rate of release being controlled by the nature of the conjugate, as in solution. The controlled molecular architecture demonstrated by this system offers an attractive class of drug conjugate for the delivery of drugs from polymeric biomaterials such as bone cements in terms of both sustained, prolonged drug release and minimization of mechanical compromise as a result of release. We consider these results to be the rationale for the development of 'designer' drug release biomaterials, where the rate of required release can be controlled by predetermined molecular architecture.

A critical evaluation of Raman spectroscopy for the analysis of lipids: fatty acid methyl esters.

The work presented here is aimed at determining the potential and limitations of Raman spectroscopy for fat analysis by carrying out a systematic investigation of C-4-C-24 FAME. These provide a simple, well-characterized set of compounds in which the effect of making incremental changes can be studied over a wide range of chain lengths and degrees of unsaturation. The effect of temperature on the spectra was investigated over much larger ranges than would normally be encountered in real analytical measurements. It was found that for liquid FAME the best internal standard band was the carbonyl stretching vibration nu(C = O), whose position is affected by changes in sample chain length and physical state; in the samples studied here, it was found to lie between 1729 and 1748 cm(-1). Further, molar unsaturation could be correlated with the ratio of the nu(C = O) to either nu(C = C) or delta(H-C = ) with R-2 > 0.995. Chain length was correlated with the delta(CH2)(tw)/nu(C = O) ratio, (where "tw" indicates twisting) but separate plots for odd- and even-numbered carbon chains were necessary to obtain R-2 > 0.99 for liquid samples. Combining the odd- ani even-numbered carbon chain data in a single plot reduced the correlation to R-2 = 0.94-0.96, depending on the band ratios used. For molal unsaturation the band ratio that correlated linearly with unsaturation (R-2 > 0.99) was nu(C = C)/delta(CH2)(SC) (where "sc" indicates scissoring). Other band ratios show much more complex behavior with changes in chemical and physical structure. This complex behavior results from the fact that the bands do not arise from simple vibrations of small, discrete regions of the molecules but are due to complex motions of large sections of the FAME so that making incremental changes in structure does not necessarily lead to simple incremental changes in spectra.

Conformations, vibrational frequencies and Raman intensities of short-chain fatty acid methyl esters using DFT with 6-31G(d) and Sadlej pVTZ basis sets

Density functional calculations, using B3LPY/6-31G(d) methods, have been used to investigate the conformations and vibrational (Raman) spectra of three short-chain fatty acid methyl esters (FAMEs) with the formula CnH2nO2 (n = 3-5). In all three FAMEs, the lowest energy conformer has a simple 'all-trans' structure but there are other conformers, with different torsions about the backbone, which lie reasonably close in energy to the global minimum. One result of this is that the solid samples we studied do not appear to consist entirely of the lowest energy conformer. Indeed, to account for the 'extra' bands that were observed in the Raman data but were not predicted for the all-trans conformer, it was necessary to add-in contributions from other conformers before a complete set of vibrational assignments could be made. Provided this was done, the agreement between experimental Raman frequencies and 6-31G(d) values (after scaling) was excellent, RSD = 12.6 cm(-1). However, the agreement between predicted and observed intensities was much less satisfactory. To confirm the validity of the approach followed by the 6-3 1 G(d) basis set, we used a larger basis set, Sadlej pVTZ, and found that these calculations gave accurate Raman intensities and simulated spectra (summed from two different conformers) that were in quantitative agreement with experiment. In addition, the unscaled Sadlej pVTZ, and the scaled 6-3 1 G(d) calculations gave the same vibrational mode assignments for all bands in the experimental data. This work provides the foundation for calculations on longer-chain FAMEs (which are closer to those found as triglycerides in edible fats and oils) because it shows that scaled 6-3 1 G(d) calculations give equally accurate frequency predictions, and the same vibrational mode assignments, as the much more CPU-expensive Sadlej pVTZ basis set calculations.

Local and semilocal density functional computations for crystals of 1-alkyl-3-methyl-imidazolium salts

The accuracy and reliability of popular density functional approximations for the compounds giving origin to room temperature ionic liquids have been assessed by computing the T=0 K crystal structure of several 1-alkyl-3-methyl-imidazolium salts. Two prototypical exchange-correlation approximations have been considered, i.e., the local density approximation (LDA) and one gradient corrected scheme [PBE-GGA, Phys. Rev. Lett. 77, 3865 (1996)]. Comparison with low-temperature x-ray diffraction data shows that the equilibrium volume predicted by either approximations is affected by large errors, nearly equal in magnitude (~10%), and of opposite sign. In both cases the error can be traced to a poor description of the intermolecular interactions, while the intramolecular structure is fairly well reproduced by LDA and PBE-GGA. The PBE-GGA optimization of atomic positions within the experimental unit cell provides results in good agreement with the x-ray structure. The correct system volume can also be restored by supplementing PBE-GGA with empirical dispersion terms reproducing the r-6 attractive tail of the van der Waals interactions.