Molecular mechanics calculations on carbonyl compounds. III. Cycloketones
Department of Chemistry, University of Georgia, Атина, Georgia, United States Journal of Computational Chemistry
(Impact Factor: 3.59).
10/2001; 22(13):1451-1475. DOI: 10.1002/jcc.1100
Molecular mechanics (MM4) calculations were carried out on cycloketones for ring sizes ranging from 4 to 11 carbon atoms. The MM4 relative energies for the various conformations of the cycloketones were compared to density functional theory (DFT) calculations (B3LYP/6-31G*), which were also carried out in this work. For small ring sizes (n=4–6), calculated molecular geometries, dipole moments, moments of inertia, and vibrational spectra were compared to experimental data. The axial–equatorial energy differences in methyl-substituted cyclohexanones were also calculated by MM4 and compared to ab initio, DFT, and experimental results. The results of the MM4 studies on cycloketones showed significant improvement from those of MM3 calculations performed in parallel with the MM4 calculations. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1451–1475, 2001
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ABSTRACT: Oxygenated volatile organic compounds ((O)VOCs) contribute to ozone formation, affect the oxidising capacity of the troposphere and are sources of growth, and in some cases formation, of aerosols. It is therefore important to identify and quantify sources of (O)VOCs in the troposphere. In the late 1990s a unique technique for quantification of organic trace gas species, proton transfer reaction mass spectrometry (PTR-MS) was developed. PTR-MS potentially offers rapid response and high sensitivity without the need for sample pre-concentration. Concentrations can be derived from the PTR-MS either by calibration or can be calculated from measured ion count rates and kinetic considerations.
In this work, the methodology of PTR-MS application is critically assessed. The uncertainties and inaccuracies associated with each parameter employed in the calculation of concentrations are reviewed. This includes a critical appraisal of models for the calculation of the collisional rate constant currently applied in the field of PTR-MS. The use of a model to account for the effects of the electric field, available in the literature but not previously applied to the PTR-MS, is advocated. Collisional rate constants employing each of the models discussed have been calculated for the reactions of H3O+ with over 400 molecules for PTR-MS.
In PTR-MS it cannot be assumed that the product ion occurs only at the protonated non-dissociated mass. Few product distributions obtained from PTR-MS are cited in the literature, and even then the reaction chamber conditions (pressure, temperature and electric field strength) are not always specified. A large volume of product distributions for trace gases with H3O+ in select ion flow tube mass spectrometry (SIFT) exists in the literature and is reviewed. In SIFT, no electric field is applied to the reaction chamber and the extent and even nature of fragmentation can differ in PTR-MS. In addition to the application of an electric field, the energy in the reaction chamber can be increased by increasing the temperature or by variation of the reagent ion. In this work, the increase in energy via the three methods is approximated to enable a comparison of product distributions. The review of product distributions in PTR-MS, select ion flow drift tube mass spectrometry (SIFDT), variable temperature select ion flow tube mass spectrometry (VT-SIFT), SIFT, proton transfer reaction time of flight mass spectrometry (PTR-TOF-MS), proton transfer reaction ion trap mass spectrometry (PTR-ITMS) and electron ionisation mass spectrometry (EI-MS) is used alongside thermodynamic considerations to collate a list of potential contributors to a range of mass to charge ratios (m/z) in the PTR-MS. The need for further measurements of product distributions as a function of temperature, pressure and electric field strength for a wider range of (O)VOCs is highlighted. This enables dissociation to be better used as a tool for compound identification rather than being considered a hindrance.
The collation of likely product distributions is applied to identify possible contributors to m/z observed during PTR-MS measurements of emission from cattle slurry. Field measurements were made during fertilisation of a grassland site south of Edinburgh in 2004 and 2005 and in laboratory-based measurements in 2006. Contextual reasoning, previous measurements and isotope ratios are used to narrow the list of possible contributors. Large concentrations of m/z cautiously identified as alcohols followed by a latter peak in carboxylic acids were observed during laboratory measurements. Increases in the corresponding m/z were also observed during the fertilisations. Other tentatively identified compounds emitted included phenol, methyl phenol, trimethylamine, and various sulphur containing compounds.
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ABSTRACT: Syntheses and single crystal X-ray structural characterizations are reported for bis adducts (6) and (7) formed between Danishefsky's diene (8) with (respectively) the dienophiles 2,6- and 2,5-dichlorobenzoquinone. Formation of each adduct involves diastereocontrolled creation of six stereogenic centres.
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