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The 13C NMR chemical shift of sp2 carbon atoms in acyclic alkenes was estimated with multilayer feedforward artificial neural networks (ANNs) and multilinear regression (MLR), using as structural descriptors a vector made of 12 components encoding the environment of the resonating carbon atom. The neural network quantitative model provides better results than the MLR model calibrated with the same data. The predictive ability of both the ANN and MLR models was tested by the leave-20%-out (L20%O) cross-validation method, demonstrating the superior performance of the neural model. The number of neurons in the hidden layer was varied between 2 and 7, and three activation functions were tested in the neural model: the hyperbolic tangent or a bell-shaped function for the hidden layer and a linear or a hyperbolic tangent function for the output layer. All four combinations of activation functions give close results in the calibration of the ANN model, while for the prediction a linear output function performs ...

In chromatographic studies polar effect constants σ*c, similar to those of Taft and Ingold, can be obtained from an empirical additivity relationship which takes into account three structural increments.

lComparison of stationary-phase classification scales Comparison of the stationary-phase classifications demonstrates their relation to the nature of the forces interacting between the solvent and the solute. The ?* factor, which expresses the intensity of the chromatographic interaction, seems to be the most satisfactory basis for classification at present.

Behaviour of ethers. VII. Principle of determination of the polar effect constants It is pointed out that the simplified linear extrathermodynamic relationship where retention data are plotted versus the polar factor and the number of ramifications located on the first carbon atom neighbouring the chemical function, leads to a satisfactory result. This relationship may be used for enlargement of the σ* scale, when applied to chromatographic phenomena. On the other hand, enlargement of this scale can be carried out by incrementation.

Behaviour of ethers. I. Working out a “Taft”-like equation founded on retention data of saturated ethers Variations in retention data of a series of saturated aliphatic ethers have been studied with a view to express them in a Taft-like equation. The linear relationships pointed out show the influence of the ramifications located on the atom neighbouring the oxygen. The numerical values of ϱ * are determined for six stationary phases. They express the selectivity of the stationary phases towards the ethers and they can be considered as an evaluation of the polarity of these substances.

Application of a Taft-type equation to retention data. Additivity of substituent effects The previously established linear extrathermodynamic relationship can be applied to a chemical population the common functional group of which is characterized by an axis of symmetry. In all other cases it is possible to differentiate the particular contribution of each alkyl group Rl; consequently their effects must then be considered separately.

In order to predict retention data in gas-liquid chromatography, extension of the Taft scale of σ * polar effect constants is essential and can be carried out by means of the equation log t ′ R = ρ * σ * + h ( n H -3). Using retention data of alcohols, σ * of 42 substituents have been estimated, giving rise to a σ * c scale appropriated in the field of chromatography.