Carbon-13 nuclear magnetic resonance studies of coumarin and related compounds

Fourier-transform ¹³C NMR spectra of nine coumarinoid compounds of medicinal interest are reported. All of the carbon resonances are assigned with the aid of various spectral techniques and stable isotopic labeling. The substituent effects on the chemical shifts in several systems are also discussed.     

Finitely presented monoids with linear Dehn function need not have regular cross-sections

This paper shows that a finitely presented monoid with linear Dehn function need not have a regular cross-section, strengthening the previously-known result that such a monoid need not be presented by a finite complete string rewriting system, and contrasting with the fact that finitely presented groups with linear Dehn function always have regular cross-sections.     

Sequential Benzylic Oxidation of Naloxone 3-Methyl Ether

Naloxone 3-methyl ether was selectively oxidized by treatment with cerium ammonium nitrate in aqueous acetonitrile at ambient temperature to provide the 10-(S)-hydroxy adduct. The stereochemistry of the oxidation product was proven by single crystal X-ray structure determination. The Dess-Martin periodinane further oxidized the 10-hydroxy to the 10-keto analog. Deprotection formed 10-ketonaloxone as an analytical reference standard.     

Between d-stability and sign-stability: the 3 × 3 case

[2] introduced a decreasing sequence of sets of real n × n matrices, which begins with the D-stable matrices and stops at the sign-stable matrices. It is not clear how many of the n sets in the sequence are distinct. This article documents the disappointment that in the first case where the sequence could contain a set which is neither the D-stable matrices nor the sign-stable matrices(viz., the case n = 3) it doesn't.     

Radiative detection of NMR using the Mössbauer effect

NMR has been observed in ⁵⁷Co dissolved in iron by measuring a change in the Mössbauer effect spectrum at 0.07 K.     

Temperature derivatives at constant volume of the elastic constants of the alkali halides

Values of dC/dT)_V have been computed from experimental dC/dT)_P and dC/dP)_T for the three elastic constants (expressed as $\text{B}{}_{\mathrm{T}}\text{, C}{}_{44}\text{and}\text{(C}{}_{11}\text{?C}{}_{12}\text{)}\text{2)}$ of each of the 16 Li, Na, K, and Rb halides. The dC/dT)_V measure the explicit dependence of C on T, the effect of thermal expansion having been removed. The dC/dT)_V are all small (compared with dC/dT)_P), are all negative, and vary quite systematically and smoothly with anion, with cation and with the three elastic constants. Negative dB_T/dT)_V is accounted for both thermodynamically and by available lattice dynamical calculations. dC/dT)_V for shear constant is expressed in terms of two vibrational mode parameters, whose values can then be estimated from the observed value, and the trend of dC/dT)_V with Debye temperature.     

Gas and liquid phase kinetics of the unimolecular isomerization of 1-chloro-4-bromobicyclo[2.2.0]hexane: The radical stabilization energy of an α-bromine atom

The kinetics of the gas phase isomerization of 1-chloro-4-bromobicyclo[2.2.0]hexane to 2-chloro-5-bromohexa-1,5-diene have been measured in a static system over the temperature range of 135–215°C, with a variation in the total pressure from 0.6 to 400 torr. For these conditions the rate constants are well represented by the Arrhenius equation: where θ = 2.303RT kcal/mole. Transition state estimates for the biradical mechanism for the isomerization of bicyclo[2.2.0]hexanes are shown to be in good agreement with these Arrhenius parameters. By comparison of the activation energy with that for the isomerization of bicyclo[2.2.0]hexane and 1,4-dichlorobicyclo[2.2.0]hexane, the radical stabilization energy of an α-bromine atom is shown to be 1.0 ± 1.8 kcal/mole. Rates are also reported in the liquid phase at temperatures of 155°C and 175°C with diphenyl ether, nitrobenzene, and dimethylsulfoxide as solvents. The observed rate constants are all faster (by a factor of 1.1–1.7) than those measured in the gas phase and display no correlation between rate and solvent polarity.