Self-diffusion coefficients have been measured for water, nitromethane, acetone, benzene, cyclohexane, isopentane, and neopentane. Temperature and pressure dependences have been determined for nitromethane, acetone, cyclohexane, and isopentane. The temperature dependence has been determined for neopentane and the pressure dependence has been determined for water and benzene. The results are discussed in terms of a semiempirical theory for transport in liquids and it is shown that semiquantitative predictions of self-diffusion, viscosity, and thermal conductivity coefficients can be made.
We have studied the high-resolution proton NMR spectrum of adenosine triphosphate (ATP) bound to the paramagnetic metal ions Mn2+, Ni2+, and Co2+ from 0° to 90°C at different concentrations of metal and ATP. In conjunction with our 31P study, we conclude that in the MnATP and NiATP complexes the metal ion is bound to the adenine ring all the time it is bound to the phosphates. At 298°K the average time the metal ion sticks to the adenine was measured to be (5±1)×10−6 and (1±0.15)×10−3 sec for MnATP and NiATP, respectively. The corresponding times measured for the phosphates were (5±1)×10−6 and (7±2)×10−4 sec. The excellent agreement between these sticking times indicates that the bonds to adenine and phosphates are made and broken simultaneously. The metal ion was located at a position in space between the amino group on C6 and N7 in the adenine ring by triangulation from the three protons H8, H2, and H1′. The triangulation was made from measurements of T1 under conditions where 1/T1∝ (μ/r3)2τc. The magnetic moment μ and correlation time τc of the metal ion with respect to the proton were determined independently. Therefore, r, the distances from the H8, H2, and H1′ protons to the metal ion, were determined. In CoATP the distances determined were the same as for the other metal ions. From this and from its similar behavior to CoITP (inosine triphosphate) where chemical evidence favors ring binding, we conclude that in CoATP as well as NiATP and MnATP the metal is bound to the ring.
The equatorial:axial ratio and rate of conformational isomerization of cyclohexyl fluoride has been measured by observation of the 19F resonance as a function of temperature (—87.6° to +29.5°) for a 25 vol.% solution in CCl3F. Rate information is readily obtained over a wide temperature range, since the equatorial and axial fluorines differ in chemical shift by 20.5 ppm; the equatorial and axial α protons differ in chemical shift by only 0.46 ppm. Interpretation of the spectra in terms of isomerization rates is complicated by the following: (a) the equatorial and axial conformers are present in unequal proportions; (b) their ratio changes with temperature; and (c) 19F–H coupling produces fine structure and/or peak broadening. The general expression for spectral line shape resulting from exchange between two unequal, chemically shifted spin populations is plotted (using the IBM 7090 computer), chemical shift and conformer ratio data being obtained from the low temperature 19F spectra. The observed spectra fit the calculated spectra very closely, and can be used with their aid to obtain the rates for axial-to-equatorial and equatorial-to-axial isomerization. Excess line broadening resulting from 19F–1H coupling was eliminated by double resonance. Modifications to the rate expressions arising from the differing entropies and enthalpies of the axial and equatorial conformers are presented. Theoretical expectations for the activation entropies and enthalpies are discussed and compared to observed values.
The spin-echo, magnetic resonance method for measuring self-diffusion coefficients is reviewed and the results obtained to date are summarized. Several uses of self-diffusion results are discussed.Es wird uber die Kernresonanz-Spin-Echotechnik zur Messung von Selbstdiffusionskoeffizienten berichtet. Die bis heute erhaltenen Ergebnisse werden zusammengestellt. Verschiedene Anwendungen dieser Ergebnisse werden diskutiert.
To confirm and extend further our previous prediction of a Curie transition in PVF2, we examined a copolymer of it with tetrafluoroethylene (81 mol % VF2). The tetrafluoroethylene units serve to render this copolymer equivalent to PVF2 of a slightly increased head-to-head content. Contrary to other reports, we have detected a Curie transition in this material using x-ray diffraction, dielectric, and calorimetric techniques. The ferroelectric-to-paraelectric transformation is barely perceptible because it begins at ∼122 °C (which is only slightly below the main melting point) and is rapidly overtaken by melting. However, the reverse (paraelectric-to-ferroelectric) transition is detected unequivocally in the vicinity of 110–115 °C because of the metastability of the paraelectric phase that crystallizes from the melt. The dielectric peaks associated with this Curie transition are clearly separated from any contribution of bulk conduction. The characteristics of this transition are analogous to those found in copolymers of VF2 with trifluoroethylene and thus indicate that such ferroelectric behavior is inherent to PVF2 and does not reflect influences of any particular comonomer.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.