摘要:

The photofragmentation of 2,3-dimethylbutene and 3,3-dimethylbutene has been studied at 147 and 184.9 nm in the gas phase. The main primary decomposition process at both wavelengths involves the rupture of a β(C—C) bond. The quantum yield for this process is higher than 0.7 at 147 nm and is probably even higher at 184.9 nm. All dimethallyl radicals formed at 147 nm in this process decompose at low pressure, but some of them isomerize from the α,β- to the α,α- structure (and vice versa) — via a 1,4-H transfer — before decomposition. At 184.9 nm, the same primary process is used to get a rough value for the lifetime of the photoexcited molecule, compared with the one made with RRKM calculations by assuming that all the photon energy resides in the vibrational framework of the fundamental electronic state. These lifetimes are about one nanosecond or less.

ISSN:0008-4042    

DOI:10.1139/v85-011

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Heat capacities and dissociation properties of trimethylene oxide and its structure I and structure II clathrate hydrates in the temperature range 85 to 270 K were determined using a Tian–Calvet heat flow calorimeter. The hydrates dissociate incongruently. The heat capacities of the hydrates are similar in magnitude to those observed for other structure I and structure II hydrates. For the structure I hydrate two thermal anomalies were observed. The one centered at 107 K can be identified as due to ordering of the trimethylene oxide dipoles along theaxes of the cages as suggested by the previous dielectric and nmr relaxation studies. The second centered at 168 K is interpreted as due to trimethylene oxide rich eutectic rather than due to some restructuring of the host lattice as has recently been suggested.

ISSN:0008-4042    

DOI:10.1139/v85-012

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

AbinitioMO calculations for Si2H2are reviewed and discussed in qualitative terms. New calculations are reported, by MNDO for Si2H2and Si2H4, and using both MNDO andabinitiomethods (including electron correlation at the MP2 level) for various possible structures for Al2H2(3). As with the analogous silicon dihydride, a dibridged structure3ais found to be superior to classical structures3b,3c, and3d. MNDO calculations for derivatives of Al2H2are reported. Finally, some implications of the calculations for the surface structures of partially-hydrogenated elemental aluminum and silicon are discussed.

ISSN:0008-4042    

DOI:10.1139/v85-013

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Spectroscopic properties of the polyene antibiotic Nystatin and its nitrobenzoxadiazole derivative have been characterized by lifetime and quantum yield measurements. The data suggest that Nystatin and other polyene antibiotics exhibit spectral features common to other linear polyenes such as diphenylhexatriene and parinaric acid, and support the conclusion that the strong absorption (ϵmax~70 000 M−1cm−1; λ = 306 nm) is due to a symmetry-allowed transition to a higher excited state while the red-shifted emission occurs from a lower lying excited state through a weak, symmetry-forbidden transition.Nitrobenzoxadiazole derivatives also exhibit efficient intramolecular fluorescence energy transfer from the polyene chromophore to the nitrobenzoxadiazole chromophore (λcm = 535 nm). The lifetime and quantum yield measurements suggest that the predominant mechanism of energy transfer is via a Dexter type transfer in a fraction of molecules with appropriate conformations.

ISSN:0008-4042    

DOI:10.1139/v85-014

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Nucleophilic attack of sodium azide on 2,4,6-triaryl-1,3-oxazinium species gives high yields of β-tetrazolo-trans-benzalacetophenones from the corresponding 2-azido-1,3-oxazines. The rearrangement of the azido oxazines likely proceeds via tautomerism of the intermediate iminoazides. If the formation of the 2-azido-1,3-oxazines is under kinetic control, these results are a rare example of high regioselectivity in nucleophilic attack at the C2 carbon of 1,3-oxazinium species.The reaction behaviour of the 1,3-oxazinium/N3−system is discussed, together with results obtained from the 2,4,6-triphenylpyrilium/N3−.

ISSN:0008-4042    

DOI:10.1139/v85-015

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Nematic and twisted nematic mesophases have been prepared from various mixtures of potassiumN-dodecanoyl-l-alaninate and its racemate. The induced pitch was determined as a function of optical purity as were nmr spectra from a variety of additives including several inorganic salts andd,l-alanine. Dipolar couplings from the CHCH3moiety of the alaninate headgroup were studied, as were quadrupole splittings from the N—D of the amido group and the D2O of the water. It was proposed that with development of cholesteric behaviour on proceeding in a stepwise fashion from thed,ltolamphiphile, the individual disk-like micelles twisted into a chiral shape, then aggregated to form domain-like structures. The major axis of these aggregates is the axis along which the helix propagates and this local axis serves as the director axis for the aggregates. The individual aggregates reorient as a unit when placed in the magnetic field of an nmr spectrometer. As a consequence of this, the diamagnetic anisotropy of the mesophase prepared from thelamphiphile is positive in sign while that of thed,lanalogue is negative. Despite this observed change in diamagnetic anisotropy, the alignment of the individual micelles is not different for the chiral and racemic materials.

ISSN:0008-4042    

DOI:10.1139/v85-016

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Secondary α-deuterium kinetic isotope effects have been determined for the elimination reactions of 2-phenylethyl halides withtert-butoxide intert-butyl alcohol at 40 °C in the presence and absence of the crown ether 18C6. The second-order rate constantk2and the normal (kH/kD)αeffect remained constant when thetert-butoxide concentration was varied for reaction of the iodo and bromo compounds. However, both the magnitude ofk2and the secondary α-deuterium isotope effect were significantly dependent on [t-BuO−] when chlorine and fluorine are the leaving groups. It is noteworthy that (kH/kD)αisinversefor the reaction of both the chloro and fluoro compounds at "low" base concentrations and normal at "high" base concentrations. These results are discussed in terms of bothsyn- andanti-elimination pathways promoted by various associated and dissociated base species. It is suggested that the (kH/kD)αeffect may be useful as a criterion for determining the stereochemistry of E2 elimination reactions.

ISSN:0008-4042    

DOI:10.1139/v85-017

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Conformational energies have been estimated for the segments of the bisphenol polycarbonate chain, using the Lennard–Jones and Hill's empirical force field type of functions. It is found that the conformation of the carbonate group, defined by the torsion angle ζ, is restricted to the range of 45° to 65°. The rotations χ and χ′ of the methyl groups also show similar limited flexibility. However, accessible conformations of the diphenyl propane (DPP) segment, defined by torsion anglesand ψ, span a wide area of thesurface, with the restriction that the rotationsand ψ be synchronized such thator 270°. These features explain the slow thermal crystallization behaviour of the polycarbonate chains. The variability of the conformations of the repeat unit is illustrated with a series of figures.

ISSN:0008-4042    

DOI:10.1139/v85-018

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

The title reactions are an important class of copper mediated nucleophilic aromatic substitution processes, which constitute a useful tool in the molecular design and synthesis of small molecules. We report the results of extensive investigation of these processes, primarily focussing on cyanodeiodination (ArI + CuCN → CuI + ArCN). Among the interesting features of these processes are: (a) an unusual rate equation involving autocatalysis by CuI product; (b) retardation by both excess nucleophile (as KCN) and excess leaving group (as KI), which compete with ArX to complex with CuNu; (c) only cuprous nucleophiles are active (ligand exchanged products from cupric salts arise from prior redox equilibria which form CuNu); (d) the halogen effect is large (kI~ 40–100kBr~ 300–5000kCl) but the Hammett ρ value is zero; (e)ortho-alkyl groups do not hinder the reaction (and actually cause mild acceleration by relief of steric strain). Finally, the introduction of anortho-COO−group accelerates the reaction by a factor of 104–105, but the general features of the accelerated reactions are also the same, again indicating a common mechanism, with entropic acceleration byortho-carboxylate. Both kinetic and thermodynamic factors were considered in detail, the latter apparently for the first time. Applications to practical syntheses are considered, and novel mechanistic models for these interesting processes are discussed.

ISSN:0008-4042    

DOI:10.1139/v85-019

出版商:NRC Research Press    

年代:1985

数据来源: NRC

摘要:

Kinetic studies are reported for the base hydrolysis to phthalamic acid anions (H) and ammonolysis to phthalamides (A) for seven phthalimides (P):1, unsubstituted;2, 4-NO2;3, 4-Cl;4, 4-tBu;5, 3-NO2;6, 3-Me;7, 3-Me3Si. The hydrolysis kinetics require two mechanisms, one which is first order in neutral imide and first order in hydroxide ion, and a second, which is important only in quite concentrated NaOH, which is first order in neutral phthalimide and second order in hydroxide ion. Ammonolysis kinetics for1–5revealed the rate law: Rate = kN[Unionized phthalimide] [NH3][OH−]. A mechanism is proposed with rate-determining breakdown of the anionic form of the tetrahedral intermediate derived by addition of NH3to the phthalimide. The ammonolysis is reversible. The phthalamide hydrolyzes to the phthalamic acid via cyclization to an intermediate phthalimide, which is detected in concentrated base where its formation from phthalamide is more rapid than its subsequent hydrolysis. Rate constants for the cyclization follow the rate law: Rate = kcyc[Phthalamide][OH−]. This reaction is the microscopic reverse of the ammonolysis, and the ratiokN/kcycprovides the equilibrium constantKeqfor the reaction P + NH3 = A. Values for1–5lie in the range 2 × 102 – 4 × 103. With 3-methylphthalimide, kinetics in aqueous ammonia do not obey a first-order relationship, but they could be analyzed by a scheme whereby the phthalimide is converted reversibly to the phthalamide and simultaneously undergoes an irreversible hydrolysis. The value ofKeqin the system is 1.8. With 3-trimethylsilylphthalimide the value ofKeqis further reduced to 0.01. The ammonolysis reaction does occur more quickly than hydrolysis but the equilibrium is so unfavorable that even in concentrated ammonia only a small amount of the phthalamide is ever formed.

ISSN:0008-4042    

DOI:10.1139/v85-020

出版商:NRC Research Press    

年代:1985

数据来源: NRC