No abstract

The recently reported NMR pulse sequence PENDANT, that enables the simultaneous detection of all insensitive nuclei (S), of a given type, in a compound and the enhancement of the signals arising from those nuclei coupled to sensitive nuclei (I), is developed further. The sequence has been modified to enable the detection of spin-spin coupled multiplets without phase distortions. Its normal form, for use with short range coupling constants, has been adapted to facilitate the S-N enhancement of insensitive nuclei through the use of long range coupling constants. The developments are illustrated by reference to {l H}-I3C and {1H}--29Si spectra.

The oxidation of C2H. has been studied in a shock tube by monitoring the infrared emissions from CO and CO2 and the visible emission from CH*. The characteristics of this oxidation closely resemble oxidations of Cili2 and C2H2+H2• The induction periods and exponential time constants for the early phase of CO formation are the same for all three oxidations over the temperature range 1500 0 -2300 o K and lead to a common activation energy of 17±1 kcal/mole. Above 1800 o K, the C2H.-D2 reaction, in its early stages, consists mainly of the pyrolysis of C2H., and the concurrent oxidation of pyrolysis products, Cili2 and H2. Later in the reaction, and more prominently so at lower temperatures, the reaction changes in character to a slower direct oxidation of C2H4• The relative rates of CO and CO 2 production in both C2H.-D2 and C 2 H 2 -D 2 reactions are reported as a function of temperature. The pyrolysis of C 2 H. has also been studied in the shock tube by an infrared technique and its rates determined between 1950° and 2250 0 K. The results are in good agreement with previous work and give an activation energy of 52 kcal/mole for the reaction. The oxidation results would indicate that C2H4, above 1800 o K, decomposes mainly by a molecular rather than a radical mechanism.

A new NMR pulse sequence that permits signal enhancement of insensitive nuclei by polarization transfer from sensitive nuclei is presented and is illustrated by its application to 13C NMR spectroscopy where resonancesfrom 13C in all chemical environments (including quaternary carbon nuclei) can be detected simultaneously.

A predictive method, based on artiÐcial neural networks (ANN) and equilibrium physical properties, has been developed for the viscosity, density, heat of vaporization, boiling point and PitzerÏs acentric factor for pure organic liquid hydrocarbons over a wide range of temperatures A committee ANN was (T reduced B 0.45È0.7). trained, using ten physicochemical and structural properties combined with absolute temperature as its inputs, to correlate and predict viscosity. A group of 281 compounds, of diverse structure, were arbitrarily ordered into a set of 200 compounds, which were used to train the committee ANN, and a group of 81 compounds, which were used to test the predictive performance of the committee ANN. The viscosity and input data for each individual compound was compiled on average at forty di †erent temperatures, ranging from the melting points to the boiling points for each of the chosen compounds. The mean average absolute deviation in viscosity, predicted by the committee ANN, was ^7.9% which reduces to ^6.5% when the correlated data is also considered. These values are almost a factor of 2 better than other predictive methods and are below the mean average absolute experimental deviation of approximately ^10%, quoted by the DIPPR reference database (AIChE, 1994). In a preliminary study a separate committee ANN was also used to predict the viscosity of the highly polar and hyrdogen bonding compounds, aliphatic acids, alcohols and amines. The predicted mean average absolute deviation for the amines, alcohols and aliphatic acids was ^8.9%. Although this paper deals predominantly with liquid viscosity the same methodology was applied to liquid density, heat of vaporization, boiling point and PitzerÏs acentric factor. The predicted mean average absolute deviation for these equilibrium properties was ^0.71%, ^1.04%, ^0.39% and ^5.6% respectively. An attempt has also been made to use the ANN to determine the hierarchical dependencies of viscosity on fundamental molecular and structural parameters.