摘要:

The author recalls his scientific career starting with electrochemistry and gas phase kinetics and leading, largely by serendipity, to atmospheric chemistry and measurements in the atmosphere using rockets and stratospheric balloons. The scientific problems met along the way required measurements with instruments that were not commercially available and required development of new methods and techniques. These included mass spectrometry, chemiluminescence, and tunable Diode Laser Absorption Spectrometry (TDLAS) to measure trace concentrations of gases in the lower and upper atmosphere. TDLAS is considered to be the method of choice where freedom from interferences, high sensitivity, and fast time response are required. Lead salt laser diodes operate in the near infrared where most molecules of atmospheric interest have strong fundamental absorptions. The research groups at York University and at Unisearch Associates have been pioneers in applying this powerful technique to atmospheric measurements in a variety of environments including the pristine air over the Pacific, at the top of Mauna Loa, at the Grand Canyon, and the polluted air of Los Angeles and other major cities. The results of recent measurements of the emissions of two strong greenhouse gases, CF4and C2F6, from Quebec aluminium refineries are described. More recently a family of instruments based on near-infrared laser diodes have been developed that can make remote-sensing measurements of a number of pollutants in ambient air and industrial stacks. Examples of passive, on-road, remote-sensing measurements of automobile exhaust emissions of CO and CO2, and of in-stack emissions of HF are presented. Another remote-sensing instrument that has been developed is an improved visible and UV Differential Optical Absorption Spectrometer (DOAS) capable of measuring, remotely and simultaneously, a number of important pollutants. Examples of measurements of NO, NO2, O3, benzene, and toluene are presented.Keywords: air measurements, instruments, remote sensing.

ISSN:0008-4042    

DOI:10.1139/v96-196

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

In aqueous sulfuric acid, aliphaticN-nitro amines decompose to N2O and alcohols. An excess acidity analysis of the observed rate constants for the reaction shows that free carbocations are not formed. The reaction is an acid-catalyzed SN2 displacement from the protonatedaci-nitro tautomer, the nucleophile being a water molecule at acidities below 82–85% H2SO4, and a bisulfate ion at higher acidities. Bisulfate is the poorer nucleophile by a factor of about 1000. Twelve compounds were studied, of which results obtained for nine at several different temperatures enabled calculation of activation parameters for both nucleophiles. The reaction appears to be mainly enthalpy controlled. The intercept standard-state rate constants are well correlated by the σ* values for the alkyl groups; the slopes are negative, with a more negative value for the slower bisulfate reaction. Interestingly them≠m* slopes also correlate with σ*, although the scatter is bad.Keywords:N-nitro amines, excess acidity, bisulfate, nucleophiles, acid-catalyzed, kinetics.

ISSN:0008-4042    

DOI:10.1139/v96-197

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Much attention has been paid to the base-catalyzed decomposition of nitramide, but despite this it was not certain that a corresponding acid-catalyzed reaction even existed before the work described in this paper. The excess acidity method has been applied to nitramide decomposition rate constants obtained in aqueous perchloric acid media, and to a lesser extent in aqueous hydrochloric and sulfuric acids. This analysis shows that the decomposition in dilute acid solution is actually a base-catalyzed reaction, with water acting as the base (perhaps in a cyclic process involving two water molecules), with rate constants in good agreement with previous estimates. However, in more concentrated acid media a true acid-catalyzed reaction can be dissected out. Activation parameters are given for both mechanisms, and it is shown that the acid-catalyzed process is similar to the acid-catalyzed decomposition of alkylnitramines. In sulfuric acid the reaction is slightly faster than it is in the other acids, due to the presence of a base-catalyzed process involving sulfate.Keywords: nitramide, excess acidity, acid-catalyzed, base-catalyzed, kinetics.

ISSN:0008-4042    

DOI:10.1139/v96-198

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Deuterium nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation times (T1) are used to investigate the guest molecular dynamics of tris(5-acetyl-3-thienyl) methane (TATM) inclusion compounds. The seven guests: acetonitrile, nitromethane, dimethyl sulfoxide, benzene, mesitylene,ortho-xylene, andpara-xylene show a wide variety of motional behaviour. The reorientation of acetonitrile in TATM and nitromethane in TATM were both modelled as precession on a cone, the base of which is more elliptical in shape for nitromethane, as would be expected considering their molecular symmetries. DMSO in TATM does not undergo any reorientation other than methyl rotation at the temperatures investigated. At low temperatures, the2H lineshape and the deuterium spin-lattice relaxation time both depend on the rate of methyl rotation. Activation barriers of 11.7 (±0.4) kJ/mol and 11.2 (±0.5) kJ/mol were found from the two techniques, respectively. Benzene undergoes sixfold reorientation about the principal molecular axis; however, the rate is still greater than 108 Hz down to 113 K. The spin-lattice relaxation time profile does not reach a minimum on decreasing temperature to 112 K, while the slope of this plot provides an activation energy of 4.1 (±0.4) kJ/mol for the sixfold reorientation. The deuterium NMR spectra of mesitylene-d9in TATM can be simulated using a model where the guest occupies two different sites in the TATM lattice, with the guest molecules performing in-planeC3rotation either very rapidly (k > 108 Hz) or very slowly (k < 103 Hz), with the relative populations of each changing with temperature. Another model proposed suggests the possibility that there is a continuous distribution of motional rates, the median of which is increasing with temperature. This second model is more realistic; however, too many parameters are present to consider a detailed fit. Finally, botho-xylene andp-xylene are rigidly held in the TATM clathrate, while rotation of the methyl groups is rapid.Keywords: inclusion compounds, solid state deuterium NMR spectroscopy, molecular dynamics.

ISSN:0008-4042    

DOI:10.1139/v96-199

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

MM3(94), AM1, and STO-3G calculations on racemic 2,2,2′,2′-tetramethyl-1,1′-biindanyl (1) show that seven minima are present on its conformational potential energy surface.1crystallizes in the space groupPbcn(no. 60) with cell parametersa = 11.190(7),b = 11.679(7),c = 13.19(1) Å,V = 1724(3) Å3,Z = 4,R = 0.0357,Rw = 0.0362 in the conformation predicted to be the global minimum by STO-3G and AM1, but not by MM3(94). Application of the Karplus equation and observation of a deshielded proton signal indicate that the same conformation is the major species present in solution, perhaps along with another conformation. Themesoisomer2shows evidence of slow exchange between conformers in its room temperature NMR spectra. Barriers to exchange were obtained from13C NMR coalescence experiments and from1H NMR line shape analysis of the exchanging AB pattern. The potential energy surface for2was calculated as for1and used to determine the nature of the process observed. The slow exchange was calculated to be between two enantiomeric conformers. The populated conformers for both1and2had the benzene rings from the two halves of the dimer stacked in an offset manner and the favourable interactions associated with this stacking may be a factor in the relative stabilities of these conformations.Keywords: MM3, molecular mechanics, hindered rotation, dialkyldiphenylethanes, X-ray crystal structure.

ISSN:0008-4042    

DOI:10.1139/v96-200

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

In an ABX high-resolution NMR spectrum the detection of combination peaks in the X region yields only the chemical shift of X,JAX + JBXand the positive quantities. However, the presence of an additional isotopic perturbation on νA − νB, the difference between the resonance frequencies of A and B, yields two X spectra; therefore four quantities,C. Hence all quantities in the surd become available from the composite X spectrum. A generalization to ABMRX spin systems, applicable to the1H and19F NMR spectra of the two isotopic molecules of 1-chloro-2,4-difluorobenzene, is possible. It turns out that, from the H-5(X) spectrum alone, the following spectral quantities are extractable; the two values of νA − νBwhere A and B are the19F nuclei;JAB;JAXandJBXwith their relative signs;JMXandJRXwhere M and R are H-3 and H-6. No37Cl/35Cl isotope effect is detectable on the1H shielding nor on any coupling constants. F-2 undergoes an isotope shift of −1.64(3) ppb. The isotope shift, over five formal bonds, of F-4 is −0.54(3) ppb (larger shielding in the presence of37Cl). This magnitude is three times larger than that over four formal bonds in another molecule, 2,6-dichloro-4-fluorophenol.Keywords:1H NMR, of 1-chloro-2,4-difluorobenzene, isotope effects in NMR, reflection of37Cl/35Cl isotope effects on19F shielding in the1H NMR spectrum of 1-chloro-2,4-difluorobenzene; isotope effects in NMR, over three and five bonds by37Cl/35Cl on19F shielding in 1-chloro-2,4-difluorobenzene;19F NMR,37Cl/35Cl isotope effects over three and five bonds in 1-chloro-2,4-difluorobenzene.

ISSN:0008-4042    

DOI:10.1139/v96-201

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The available data in the literature on vapour–liquid equilibria, excess Gibbs energies, and excess enthalpies for linear ketones + linear mono- or poly-ether mixtures are examined in terms of the DISQUAC group contribution model. Interaction parameters are reported. The quasichemical interchange coefficients are independent of the compounds in the systems; the dispersive interchange coefficients depend on the intramolecular environment of the carbonyl and (or) oxygen groups. Proximity effects, which seem to lead to an important increase of the interaction parameters, are briefly considered in treating systems including 1-methoxy-2-propanone or dimethyl carbonate molecules. DISQUAC consistently describes the experimental data of the mixtures investigated. The rather good representation obtained for vapour–liquid equilibria at high temperatures is noteworthy. A discussion in terms of effective and reduced dipole moments of binary mixtures containing carbonyl and oxygen groups in the same or different molecules is also presented.Keywords: liquids, mixtures, thermodynamics, dipole moments, group contributions.

ISSN:0008-4042    

DOI:10.1139/v96-202

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The dissociation energy of the Ti(OH2)+ion–molecule complex was calculated by the multiconfigurational self-consistent field theory, coupled cluster theory, and two density functional theory based methods, using both all-electron basis sets and effective core potentials. The calculations show that approximate density functional theory gives results in better agreement with experiment than either the multiconfigurational self-consistent field theory or the coupled cluster theory, with both all-electron basis sets and effective core potentials. Nevertheless, the optimized geometries and harmonic vibration frequencies are very similar, irrespective of the level of theory used. The interconfigurational energy ordering of the two valence electronic configurationsdn−1sanddn−2s2of the4Felectronic state of the titanium cation were also calculated and are discussed.Keywords: ab initio, dissociation energy, ion–molecule complex, effective core potentials, transition metals.

ISSN:0008-4042    

DOI:10.1139/v96-203

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

D-ice freezing temperatures and D-hydrate formation temperatures have been measured by the dynamic cooling method for deuterium oxide – ethylene oxide (EO) solutions containing from 0 to 95.5 mol% EO. The D-ice and the congruent EO D-hydrate freezing temperatures exhibited standard errors (SEs) on a single measurement of 0.004 °C and 0.017 °C, respectively. The D-ice–D-hydrate eutectic temperature was observed at 1.500 °C with standard error (SE) 0.002 °C and at composition 2.207 mol% EO with SE 0.010 mol% EO. The congruent EO D-hydrate was found to freeze at 13.242 °C with SE 0.007 °C and at composition 12.60 mol% EO with SE 0.07 mol% EO. The formula of the congruent EO D-hydrate was EO•6.93D2O with SE 0.045 mol D2O/mol EO. Only one type of D-hydrate was found over the whole composition range down to −23 °C: the shoulder of the D-hydrate freezing curve above 40 mol% EO resulted from the high activity coefficients of dilute deuterium oxide in concentrated EO solutions. Equations and best values for the D-ice freezing temperatures and the D-hydrate formation temperatures together with their SEs were evaluated by the method of least squares. Properties of EO D-hydrate are compared with those of EO hydrate.Keywords: clathrate D-hydrate of ethylene oxide, freezing of deuterium oxide – ethylene oxide, ethylene oxide D-hydrate, formula of ethylene oxide D-hydrate.

ISSN:0008-4042    

DOI:10.1139/v96-204

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

An enantioselective synthesis of (+)-8′-demethyl ABA (2) is described. The chiral intermediate7was prepared by yeast reduction of a substituted monoprotected cyclohexa-2,5-dien-1,4-dione (9) synthesized through a phenol oxidation. The scope and limitations of the phenol oxidation is described. 8′-Demethyl ABA shows ABA-like activity in wheat embryo germination inhibition, showing that the 8′-methyl group is not essential for biological activity.Keywords: abscisic acid, phenol oxidation, yeast reduction.

ISSN:0008-4042    

DOI:10.1139/v96-205

出版商:NRC Research Press    

年代:1996

数据来源: NRC