摘要:

The1H nuclear magnetic resonance spectra of 2-cyanobenzaldehyde (2CNB) and 3-cyanobenzaldehyde (3CNB) in CS2/C6D12and acetone-d6solutions at 300 K yield precise stereospecific long-range proton–proton coupling constants. These are used to establish the conformational population of theo-cisando-transconformers of these relatively polar molecules. For example, the fractionalo-cispopulation of 2CNB changes from 0.12(4) in CS2/C6D12to 0.46(6) in acetone-d6, whereas that of 3CNB is 0.48(2) in both solvents. Extrapolation to the vapor phase, using a dielectric model, implies a negligible concentration of theo-cisconformer of 2CNB and a roughly 50% abundance of each conformer of 3CNB. Computations at various levels of molecular orbital theory provide estimates of the rotational barrier of the aldehyde moiety and confirm the planar structure of each conformer. The geometries of three conformers are given as obtained from the 6-31G MO basis and may be useful to molecular spectroscopists. Theoretical and experimental dipole moments are interpolated to yield estimates of their magnitudes for the four planar conformers. Somewhat less precise1H nmr spectral parameters (than for the above solutions) are also obtained for dilute solutions in benzene-d6at 300 K. The conformational distributions based on these parameters are compared with their only other measurement, based on dipolar moments in benzene at 298 K. Good agreement between the results of the two methods is found for 3CNB but not for 2CNB. It is suggested that specific interactions occur between benzene solvent and solute molecules, particularly for 3CNB, for which these interactions stabilize the conformer having a low dipole moment. Remarkable changes in the intraring proton–proton coupling constants occur in going from CS2/C6D12to acetone-d6solution.Keywords: 2- and 3-cyanobenzaldehyde (2CNB and 3CNB):1H NMR, conformations, long-range spin–spin coupling constants, MO computations.

ISSN:0008-4042    

DOI:10.1139/v91-154

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Internal rotational potentials for rotation about the Csp2—O bond in C6H5OCF3and its 4-fluoro derivative are computed at the STO-3G and 3-12G levels of molecular orbital theory. As for anisole, the perpendicular conformer is calculated as least stable for C6H5OCF3but the height of the internal barrier is considerably lower than in anisole. The 4-fluoro substituent reduces the twofold component significantly, indicating reducedp …π conjugation, but does not much change the values of the higher terms in the theoretical potentials. The internal potentials are rather flat with minima between about 30° and 45°, these angles measuring the twist away from planarity. The1H,13C, and19F nuclear magnetic resonance spectra in polar and nonpolar solution yield long-range coupling constants,nJ(1H,19F) andnJ(13C,19F). These parameters are qualitatively consistent with relatively low barriers to rotation about the Csp2—O bonds in these molecules and with only a small dependence on solvent polarity.Keywords: C6H5OCF3and 4-F-C6H4OCF3:1H NMR,19F NMR, conformations, MO computations.

ISSN:0008-4042    

DOI:10.1139/v91-155

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Selenium-77 and phosphorus-31 spin-lattice relaxation times are reported for tri-tert-butylphosphine selenide in chloroform-d, at 303 K and at several different magnetic field strengths. At moderate fields the31P–1H dipole–dipole, spin-rotation, and chemical shift anisotropy mechanisms contribute significantly towards the31PT1. At high fields chemical shift anisotropy dominates. The selenium-77 nuclear spin relaxes almost exclusively by spin rotation at low to moderate fields and the chemical shift anisotropy contribution only becomes significant at very high fields. This is due to an unusually small77Se CSA. The contribution due to31P–77Se dipole–dipole interactions is small but significant.Keywords:77Se NMR, NMR relaxation, phosphine selenide.

ISSN:0008-4042    

DOI:10.1139/v91-156

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The rate constantk1of the reactionhas been investigated by discharge flow resonance fluorescence of OH in the fall-off pressure range. From systematic measurements at five pressures between 0.5 and 9.5 torr, the Troe parametersk0andk∞(withFc = 0.6) have been derived at two temperatures: room temperature and 353 K. For room temperature, (297 ± 3) K, these parameters arek0 = (1.7 ± 0.5) × 10−29 cm6molecule−2s−1,k∞, = (10 ± 2) × 10−13 cm3molecule−1s−1. Our experimental results are consistent with addition as the dominant path, in agreement with flash photolysis investigations and with the existence of a fast reaction with NO2of the (OH—benzene) radical adduct. A numerical simulation shows that this latter reaction should have a rate constant of (4 ± 2) × 10−11 cm3molecule−1s−1at 353 K.Keywords: gas phase kinetics, discharge flow, resonance fluorescence, reaction of OH with benzene, tropospheric chemistry.

ISSN:0008-4042    

DOI:10.1139/v91-157

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

The excess partial molar enthalpies of isobutyric acid (IBA),HmE(IBA), and those of H2O,HmE(H2O), were measured in aqueous solutions of IBA. The temperature dependence ofHmE(IBA) at the infinite dilution suggested that the structure enhancement of the solvent H2O by IBA is weaker than those bytert-butanol (TBA) or 2-butoxyethanol (BE). The concentration dependence ofHmE(IBA), and that of the enthalpic IBA–IBA interaction,N{∂HmE(IBA)/∂nB}, shows that there are two distinct mixing schemes bounded at aboutxB = 0.03, before reaching the two phase separation. Namely, the IBA–IBA interaction is repulsive below this boundary, while above this boundary it becomes attractive leading eventually to phase separation at a higher concentration. The transition between the two schemes is associated with a peak(negative) anomally in the fourth derivative of the free energy,N2{∂2HmE(IBA)/∂nB2}.Keywords: excess partial molar enthalpies, isobutyric acid – water, transition in mixing scheme.

ISSN:0008-4042    

DOI:10.1139/v91-158

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A new liposoluble porphyrin, 2,7,12,17-tetrahexyl-3,8,13,18-tetramethylporphin(THTMP), and its complexes with divalent (Mg, Co, Ni, Cu, Zn, Cd, Hg) and trivalent (Mn, Fe) metals have been synthesized. The electronic, infrared, and far-infrared spectra of these compounds were obtained in the same nonpolar, noncoordinating solvent, CCl4. After comparison with the experimental data and the results of normal coordinate analyses from the literature, the assignment of the vibrational bands of THTMP and its complexes is made. Some correlations between the shifts of the metal sensitive bands in the diffferent regions and the structural characteristics of the porphyrin complexes are reported. Some emphasis is made on the magnesium complex in relation to chlorophyll.Keywords: Metalloporphyrin, porphyrin, infrared spectroscopy, far-infrared spectroscopy, electronic spectroscopy, nitrogen-metal bond, covalent radius, reduced mass.

ISSN:0008-4042    

DOI:10.1139/v91-159

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Trisilver(I) amidoselenate-3-ammonia-2-water, Ag3SeO3N•3NH3•2H2O,Mr = 551.70, is orthorhombic, space groupP212121witha = 11.864(3),b = 14.550(3),c = 6.433(2) Å,Z = 4,Dm = 3.25,Dx = 3.30 Mg m−3, radiation, λ = 0.71069 Å, μ = 8.62 mm−1,F(000) = 1024,T = 295 K. The structure was solved by direct methods and refined toR = 0.036 for 1759 observed reflections. In the structure, only two independent Ag atoms are directly bonded to the N(Se) atom of the amidoselenic group (NSeO3) and these groups are joined by Ag into an infinite polymeric chain via [—N(Se)—Ag] bridges. The third Ag atom is present as a complex cation [Ag(NH3)2]+. Each silver atom is coordinated by two N atoms: Ag(1) by N(Se) and a NH3molecule, Ag(2) by two N(Se) atoms, and Ag(3) by two NH3molecules. Important blood lengths are Ag(1)—N(Se), NH32.111 and 2.110(8); Ag(2)—N(Se), N(Sei) 2.106 and 2.096(7); and Ag(3)—NH3, NH32.115 and 2.150(10); N(Se)—Se 1.727(8); and Se—O 1.641, 1.647, and 1.639(7) Å.Keywords: Silver amidoselenate, silver–nitrogen bond, nitrogen–selenium bond.

ISSN:0008-4042    

DOI:10.1139/v91-160

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

In this paper, we review a comparison of the empirical and chemiometrical approaches to the modelization of the effects of solvent on a number of NMR, IR and UV–visible spectral properties. The solvents have first been characterized by the π*, α and β scales, which describe respectively the polarity–polarisability, the acidity, and the basicity by hydrogen bond; the 17 most representative, with orthogonal parameters, have been chosen. An attempt has been made to rationalize the observed effects on the observed properties by using simple linear empirical models like the π*, α, and β parameters of Kamlet, Taft, and Abboud;of Reichardt; andAandBof Swain. Furthermore, an analysis of the main components has been applied to these effects and to the properties examined; it appears that the quality of the correlations obtained is generally satisfactory and can be compared to the quality that can be observed with the empirical approach.Keywords: solvent effect, empirical approach, chemiometrical approach. [Journal translation]

ISSN:0008-4042    

DOI:10.1139/v91-161

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

A new formulation for the analysis of low density multiple-ion swarms drifting, diffusing, and inter-reacting in a neutral gas is proposed on a transport theory basis. A set of coupled three-dimensional transport equations for an arbitrary number of ion species, which governs the number densities of the ion swarms as functions of time and position coordinates, is exactly solved using a Fourier transform in a matrix representation. A picture of dynamic equilibrium state for hypothetical four ion swarms in a neutral gas is numerically obtained. Also, experimental data of transport coefficients and reaction rates for (Li+, Li+•N2, Li+•2N2)flN2system are examined in a complete reversible cyclic reaction scheme and compared with a Green's function method. The initial and boundary conditions, the analysis in gas mixtures, and the inelastic process associated with the present formulation are briefly discussed.Keywords: ion swarm, cluster ion, transport equation, ion–molecule reaction.

ISSN:0008-4042    

DOI:10.1139/v91-162

出版商:NRC Research Press    

年代:1991

数据来源: NRC

摘要:

Two related stereoselective syntheses of 3,7-dimethylnonadecane, a sex pheromone of the alfalfa leafminer, are described to show that pulegone can serve as a useful starting material for the preparation of chiral aliphatic isoprenoid compounds. The schemes are designed to place the stereogenic center of pulegone at C.3 in one synthesis and at C.7 in the other so that the optical properties of the products can be compared with one another and with the values calculated using Brewster's rules.Keywords: chiral hydrocarbons, stereoselective synthesis, pheromone,Agromyzafrontella.

ISSN:0008-4042    

DOI:10.1139/v91-163

出版商:NRC Research Press    

年代:1991

数据来源: NRC