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Potential Systematic Errors (potential + systematic_error)

Distribution by Scientific Domains

Chemistry	75%

Selected Abstracts

Kinetics of the gas-phase reactions of cyclo-CF2CFXCHXCHX , (X = H, F, Cl) with OH radicals at 253,328 K

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2009
L. Chen
Rate constants were determined for the reactions of OH radicals with halogenated cyclobutanes cyclo-CF2CF2CHFCH2(k1), trans -cyclo-CF2CF2CHClCHF(k2), cyclo-CF2CFClCH2CH2(k3), trans -cyclo-CF2CFClCHClCH2(k4), and cis -cyclo-CF2CFClCHClCH2(k5) by using a relative rate method. OH radicals were prepared by photolysis of ozone at a UV wavelength (254 nm) in 200 Torr of a sample reference H2OO3O2He gas mixture in an 11.5-dm3 temperature-controlled reaction chamber. Rate constants of k1 = (5.52 ± 1.32) × 10,13 exp[,(1050 ± 70)/T], k2 = (3.37 ± 0.88) × 10,13 exp[,(850 ± 80)/T], k3 = (9.54 ± 4.34) × 10,13 exp[,(1000 ± 140)/T], k4 = (5.47 ± 0.90) × 10,13 exp[,(720 ± 50)/T], and k5 = (5.21 ± 0.88) × 10,13 exp[,(630 ± 50)/T] cm3 molecule,1 s,1 were obtained at 253,328 K. The errors reported are ± 2 standard deviations, and represent precision only. Potential systematic errors associated with uncertainties in the reference rate constants could add an additional 10%,15% uncertainty to the uncertainty of k1,k5. The reactivity trends of these OH radical reactions were analyzed by using a collision theory,based kinetic equation. The rate constants k1,k5 as well as those of related halogenated cyclobutane analogues were found to be strongly correlated with their CH bond dissociation enthalpies. We consider the dominant tropospheric loss process for the halogenated cyclobutanes studied here to be by reaction with the OH radicals, and atmospheric lifetimes of 3.2, 2.5, 1.5, 0.9, and 0.7 years are calculated for cyclo-CF2CF2CHFCH2, trans -cyclo-CF2CF2CHClCHF, cyclo-CF2CFClCH2CH2, trans -cyclo-CF2CFClCHClCH2, and cis -cyclo-CF2CFClCHClCH2, respectively, by scaling from the lifetime of CH3CCl3. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 532,542, 2009 [source]

Kinetics for the gas-phase reactions of OH radicals with the hydrofluoroethers CH2FCF2OCHF2, CHF2CF2OCH2CF3, CF3CHFCF2OCH2CF3, and CF3CHFCF2OCH2CF2CHF2 at 268,308 K

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2003
L. Chen
Rate constants were determined for the reactions of OH radicals with the hydrofluoroethers (HFEs) CH2FCF2OCHF2(k1), CHF2CF2OCH2CF3 (k2), CF3CHFCF2OCH2CF3(k3), and CF3CHFCF2OCH2CF2CHF2(k4) by using a relative rate method. OH radicals were prepared by photolysis of ozone at UV wavelengths (>260 nm) in 100 Torr of a HFE,reference,H2O,O3,O2,He gas mixture in a 1-m3 temperature-controlled chamber. By using CH4, CH3CCl3, CHF2Cl, and CF3CF2CF2OCH3 as the reference compounds, reaction rate constants of OH radicals of k1 = (1.68) × 10,12 exp[(,1710 ± 140)/T], k2 = (1.36) × 10,12 exp[(,1470 ± 90)/T], k3 = (1.67) × 10,12 exp[(,1560 ± 140)/T], and k4 = (2.39) × 10,12 exp[(,1560 ± 110)/T] cm3 molecule,1 s,1 were obtained at 268,308 K. The errors reported are ± 2 SD, and represent precision only. We estimate that the potential systematic errors associated with uncertainties in the reference rate constants add a further 10% uncertainty to the values of k1,k4. The results are discussed in relation to the predictions of Atkinson's structure,activity relationship model. The dominant tropospheric loss process for the HFEs studied here is considered to be by the reaction with the OH radicals, with atmospheric lifetimes of 11.5, 5.9, 6.7, and 4.7 years calculated for CH2FCF2OCHF2, CHF2CF2OCH2CF3, CF3CHFCF2OCH2CF3, and CF3CHFCF2OCH2CF2CHF2, respectively, by scaling from the lifetime of CH3CCl3. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 239,245, 2003 [source]

Further constraints on variation of the fine-structure constant from alkali-doublet QSO absorption lines

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2001
M.T. Murphy
Comparison of quasar (QSO) absorption-line spectra with laboratory spectra provides a precise probe for variability of the fine-structure constant, ,, over cosmological time-scales. We constrain variation in , in 21 Keck/HIRES Si iv absorption systems using the alkali-doublet (AD) method in which changes in , are related to changes in the doublet spacing. The precision obtained with the AD method has been increased by a factor of 3: . We also analyse potential systematic errors in this result. Finally, we compare the AD method with the many-multiplet method, which has achieved an order of magnitude greater precision, and we discuss the future of the AD method. [source]

Multipole electron-density modelling of synchrotron powder diffraction data: the case of diamond

ACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2010
H. Svendsen
Accurate structure factors are extracted from synchrotron powder diffraction data measured on crystalline diamond based on a novel multipole model division of overlapping reflection intensities. The approach limits the spherical-atom bias in structure factors extracted from overlapping powder data using conventional spherical-atom Rietveld refinement. The structure factors are subsequently used for multipole electron-density modelling, and both the structure factors and the derived density are compared with results from ab initio theoretical calculations. Overall, excellent agreement is obtained between experiment and theory, and the study therefore demonstrates that synchrotron powder diffraction can indeed provide accurate structure-factor values based on data measured in minutes with limited sample preparation. Thus, potential systematic errors such as extinction and twinning commonly encountered in single-crystal studies of small-unit-cell inorganic structures can be overcome with synchrotron powder diffraction. It is shown that the standard Hansen,Coppens multipole model is not flexible enough to fit the static theoretical structure factors, whereas fitting of thermally smeared structure factors has much lower residuals. If thermally smeared structure factors (experimental or theoretical) are fitted with a slightly wrong radial model (s2p2 instead of sp3) the radial scaling parameters (`,' parameters) are found to be inadequate and the `error' is absorbed into the atomic displacement parameter. This directly exposes a correlation between electron density and thermal parameters even for a light atom such as carbon, and it also underlines that in organic systems proper deconvolution of thermal motion is important for obtaining correct static electron densities. [source]