Kinetic solvent isotope effects (KSIE) were measured for the hydrolyses of acetals of benzaldehydes in aqueous solutions covering the pH (pD) range of 1–6. For p-methoxybenzaldehyde diethyl acetal, k/k = 1.8–3.1, depending on the procedure used to calculate the KSIE and on the pH (pD) range used as the basis for k(k). It is shown that this variation is an experimental artifact, and is a characteristic of KSIE measurements in general. It is recommended that k be calculated from a least-squares fit of data to the equation kobs = k[L+], and that the KSIE be reported as k/k. The limitation remains, however, that the KSIE measured for a variety of substances over quite different pH (pD) ranges may not be comparable to more than ?20%. The source of these observations is discussed in terms of small changes in the activity coefficient ratios (a specific salt effect), including the solvent isotope effect on the activity coefficient ratio [eq. (3)]. 相似文献
Reactions of oxygen atoms with ethylene, propene, and 2-butene were studied at room temperature under discharge flow conditions by resonance fluorescence spectroscopy of O and H atoms at pressures of 0.08 to 12 torr. The measured total rate constants of these reactions are K = (7.8 ± 0.6)·10?13cm3s?1,K = (4.3 ± 0.4) ± 10?12 cm3 s?1, K = (1.4 ± 0.4) · 10?11 cm3 s?1. The branching ratios of H atom elimination channels were measured for reactions of O atoms with ethylene and propene. No H-atom elimination was found for the reaction of O-atoms with 2-butene. A redistribution of reaction O + C2 channels with pressure was found. A mechanism of the O + C2 reaction was proposed and the possibility of its application to other olefins is discussed. On the basis of mechanism the pressure dependence of the total rate constant for reaction O + C2 was predicted and experimentally confirmed in the pressure range 0.08–1.46 torr. 相似文献
Gel points in random polymerizations of the general type ΣiRA + ΣjRB in which A-groups react with A- and B-groups, and B-groups react only with A-groups are considered. (The symbols Σi and Σi signify that the A- and B-bearing reactants RA and RB can be mixtures of monomers of different functionalities, denoted generally as fai and fbj.) The usual case of A-groups reacting only with B-groups is a special case of the present theory. The effects of chemical kinetics, the competitive reaction of A- and B-groups, are separated from the generalized statistical condition for gelation. The former are used to define reaction curves and the latter, gelation curves. Both types of curve are represented as pa as a function of pb. For a given polymerization, gelation occurs when the reaction curve and the gelation curve intersect. When A-groups react only with B-groups, the gel points are those for the usual type of ΣiRA + ΣjRB polymerization, and, in the limit of A-groups only reacting with A-groups, the gel points are those for ΣiRA self polymerizations. 相似文献
The mass spectral fragmentation of twelve new 2-alkoxycarbonylalkylthio substituted derivatives of uracils is discussed and fragmentation pathways, elucidation of which were assisted by accurate mass measurements and metastable transitions are proposed. It has been found that the basic mass spectral fragmentation of these compounds is due to cleavage of S? C, O? C and C? C bonds of the alkoxycarbonylalkylthio group. 相似文献
Thermal Behaviour and Crystal Structure of YAl3Cl12 We determined the thermodynamic data of YAl3Cl12 ΔH = ?739.9 ± 3 kcal/mol and S = 136.1 ± 4 cal/K · mol by total pressure measurements and ΔH = ?739.1 ± 1.6 kcal/mol by solution calorimetry. Using DTA-investigations we established the phase diagram in the system AlCl3–YCl3. The crystal structure was refined on the basis of single crystal data (P31 12; Z = 3; a = 1 046.8(2); c = 1 562.3(3) pm). 相似文献
Hexafluoroacetone (HFA) and O2 were photolyzed at 147.0 nm to investigate their use in chemical actinometry. The products, CO for the former and O3 in the latter case, were monitored. For accurate comparison, both of these substances were irradiated by a single light source with two identical reaction cells at 180° to each other. The light intensities I were measured under the same integrated as well as instantaneous photon flux based on ? and ?CO (quantum yield) as 2 and 1, respectively. Optimum conditions for maximum product yield were 5.0 torr HFA pressure and an O2 flow rate of 200 ml/min at 1 atm pressure for a 20-minute photolysis period. For light intensity variations between 1.09 × 1014 and 2.10 × 1015 photons absorbed/sec, the ratio I/IHFA was found to be unity. Calibration with the commonly used N2O actinometer for a ? value of 1.41 showed that I/IHFA and I/I are unity. Both HFA and O2 are suitable chemical actinometers at 147.0 nm with ?CO and ? of 1 and 2, respectively. The light intensity determination in the first case involves the measurement of only one product which is noncondensible at 77°K, whereas wet analysis for O3, the only product, in the second actinometer is necessary. Both of these determinations are quite simple and are preferable over product analysis in N2O actiometry, wherein N2 separation from other noncondensibles at 77°K is required. 相似文献
An sp 2 /sp 3 get‐together : A novel and efficient method can be used to synthesize 3,3‐disubstitued oxindoles by the direct intramolecular oxidative coupling of an aryl C? H and a C? H center (see scheme; DMF=N,N‐dimethylformamide).
An ion-selective electrode (ISE) based on receptor 1 is highly selective for binding NH4+ over K+ (lg K=−2.6); the three imine nitrogen atoms in 1 are ideally positioned for hydrogen bonding with the tetrahedral NH4+ ion. This selectivity is considerably greater than that found for commercial ISEs based on nonactin (lg K=−1.0). 相似文献
The possibility of a trigonal bipyramidal structure for [Cu(tet b)X]+ (blue) (where X=Cl, Br, I) is supported by the observation of two distinct d-d bands, which are assigned as and d, dxy→d and dxz, dyz→d transitions respectively. The stability constants for the formation of [Cu(tet b)X]+ (blue) from [Cu(tet b)]z+ (blue) and X? were determined by spectrophotometric method at 25°, 35° and 45°C. The corresponding δH° and δS° values were obtained from the variations of the stability constants between 25° and 45°C相似文献
A cyclohexyl‐based POCOP pincer ligand (POCOP=cis‐1,3‐bis(di‐tert‐butylphosphinito)cyclohexyl) cyclometalates with nickel to generate a series of new POCOP‐supported NiII complexes, including the halide, hydride, methyl, and phenyl species. trans‐[NiCl{cis‐1,3‐bis(di‐tert‐butylphosphinito)cyclohexane}], [(POCOP)NiCl] ( 1 a ) and the analogous bromide complex ( 1 b ) were synthesized and fully characterized by NMR spectroscopy and X‐ray crystallography. Cyclic voltammetry measurements of 1 a and 1 b alongside their bis(phosphine) analogues [(PCP)NiCl] ( 2 a ) and [(PCP)NiCl] ( 2 a ) (PCP=cis‐1,3‐bis(di‐tert‐butylphosphino)cyclohexyl) indicate a reduced electron density at the metal center upon introducing electron‐withdrawing oxygen atoms in the pincer arms. The methyl [(POCOP)NiMe] ( 3 ) and phenyl [(POCOP)NiPh] ( 4 ) complexes were formed from 1 a by reaction with the corresponding organolithium reagents. 1 a also reacts with LiAlH4 to give the hydride complex [(POCOP)NiH] ( 5 ). The methyl complex 3 reacts with phenyl acetylene to give the acetylide complex [(POCOP)NiCCPh] ( 6 ). The reactivity of compounds 3 – 5 towards CO2 was studied. The hydride complex 5 and the methyl complex 3 both underwent CO2 insertion to form the formate species [(POCOP)NiOCOH] ( 7 ) and acetate species [(POCOP)NiOCOCH3] ( 8 ), respectively, although with a higher barrier of insertion in the latter case. Compound 4 was unreactive towards CO2 even at elevated temperatures. Complexes 3 – 8 were all characterized by NMR spectroscopy and X‐ray crystallography. 相似文献
The mechanism of the photolysis of formaldehyde was studied in experiments at 3130 Å and in the pressure range of 1–12 torr at 25°C. The experiments were designed to establish the quantum yields of the primary decomposition steps (1) and (2), CH2O + hν → H + HCO (1): CH2O + hν → H2 + CO (2), through the effects of added isobutene, trimethylsilane, and nitric oxide on ΦCO and Φ. The ratio ΦCO/Φ was found to be 1.01 ± 0.09(2σ) and (Φ + ΦCO)/2 = 1.10 ± 0.08 over the range of pressures and a 12-fold change in incident light intensity. Isobutene and nitric oxide additions reduced Φ to about the same limiting value, 0.32 ± 0.03 and 0.34 ± 0.04, respectively, but these added gases differed in their effects on ΦCO. With isobutene addition ΦCO/Φ reached a limiting value of 2.3; with NO addition ΦCO exceeded unity. The addition of small amounts of Me3SiH reduced Φ to 1.02 ± 0.08 and lowered ΦCO to 0.7. These findings were rationalized in terms of a mechanism in which the “nonscavengeable,” molecular hydrogen is formed in reaction (2) with ?2 = 0.32 ± 0.03, while the “free radical” hydrogen is formed in reaction (1) with ?1 = 0.68 ± 0.03. In the pure formaldehyde system these reactions are followed by (3)–(5): H + CH2O → H2 + HCO (3); 2HCO → CH2O + CO (4); 2HCO → H2 + 2CO (5). The data suggest k4/k5 ? 5.8. Isobutene reduced Φ by the reaction H + iso-C4H8 → C4H9 (20), and the results give k20/k3 ? 43 ± 4, in good agreement with the ratio of the reported values of the individual constants k3 and k20. 相似文献