Parameter Linear Free Energy Relationships Research Articles

Combination of artificial neural network technique and linear free energy relationship parameters in the prediction of gradient retention times in liquid chromatography

In this work multiple linear regression (MLR) and artificial neural network (ANN) were used to predict the gradient retention times of diverse sets of organic compounds in four separate data sets. Descriptors which were used as inputs of these models are five linear free energy relationship (LFER) solute parameters including E, S, A, B and V. In the first step eight separate multiple linear regression and artificial neural network models were used to predict the gradient retention time for each gradient condition separately. Results obtained in this step reveal that there are significant relations between LFER parameters and gradient retention times of solutes in liquid chromatography. Then MLR and ANN were applied to develop more general models in which several different gradient elution conditions were used. The performances of these models are compared in terms of their standard errors and also correlation analysis. The results obtained reveal that although there are no significant differences between ANN and MLR in separate modeling of the gradient retention times, ANN has a significant superiority over MLR models in developing the general models for various gradient elution conditions. The results of sensitivity analysis on ANN models indicate that the order of importance for input terms in separate ANN models is V x > B > S > E > A and in the case of combined ANN model is V x > B > t g > S > E > A, which are in agreement with the order of percentage of significance terms that obtained from the MLR models.

Exploring organic compound interactions with organic matter: The thermodynamic cycle approach

Several approaches for studying sorption of organic compounds by natural organic matter (OM) include correlations of compound OM-water distribution coefficients ( K OM) with such properties as n-octanol–water distribution coefficients ( K OW), compound (liquid) aqueous solubilities ( S Aq), or multiple parameter linear free energy relationships (LFERs). However, such K OM-based comparisons of sorbate–OM interactions for different organic compounds may be obscured by compound solvation interactions in the bulk aqueous phase. To avoid this complication, we employ two alternative means for simultaneously eliminating the contribution from solute hydration in the aqueous phase, and for providing a single inert reference standard state. Both means utilize a thermodynamic cycle for replacing the aqueous reference state with an inert reference state (either gas phase or inert solvent), while maintaining the same final hydrated sorbed state. In this review, we describe both approaches and demonstrate their utility for elucidating sorption mechanisms, as well as discuss the rationale for choosing one approach (the gas phase reference state) or the other (inert solvent reference state). Specifically we demonstrate that: (i) specifically interacting compounds have an essentially greater ability to undergo interactions with hydrated soil organic matter as compared with non-specifically interacting compounds of the same molar refraction; (ii) in many cases, greater interaction ability is correlated with compound ability to undergo hydrogen bond formation with the hydrated soil organic matter phase; (iii) interactions of solute molecules with the bulk aqueous phase compensate for specific interactions between sorbate molecules and the soil organic matter phase, resulting in ‘apparent’ hydrophobic partitioning behavior of specifically-interacting solutes that masks compound specific interactions with soil organic matter; (iv) the overall trend in increasing strength of compound interactions with OM is as follows: aromatic and Cl-substituted aromatic hydrocarbons ≤ aliphatic hydrocarbons and Cl-substituted aliphatic hydrocarbons ≈ anisole ≈ o-Cl-nitrobenzene < nitrobenzene < acetophenone < 2,4-di-Cl-phenol ≈ atrazine ≈ benzyl alcohol < pyridine < phenol < m-nitrophenol; (v) only those compounds that interact most strongly with OM exhibit significant sorption isotherm nonlinearity; (vi) other commonly used standard states such as pure compound (liquid) state and solution in n-octanol are considerably less useful in deciphering sorption mechanisms; (vii) strength of compound interaction with hydrated OM is correlated with the effect of OM hydration on compound sorption. Thus, using a single inert reference state (either gas phase or inert solvent), is shown to be valuable for delineating the differences in organic compound interactions with OM.

Polyethylene-coated silica and zirconia stationary phases in view of quantitative structure-retention relationships

Retention properties of two new high-performance liquid chromatographic (HPLC) stationary phases, polyethylene-coated silica (PECSiO2) and polyethylene-coated zirconia (PECZrO2), were compared chemometrically. Reversed phase HPLC capacity factors were determined isocratically for a carefully designed series of 25 structurally diverse solutes. Linear free energy relationship (LFER)-based solute parameters (excess molar refraction, dipolarity-polarizability, hydrogen-bond acidity and basicity, McGowan volume) and solute structural descriptors determined by physico-chemical methods (water accessible van der Waals volume, dipole moment, atomic electron excess charge) were regressed as independent variables against log capacity factors. The quantitative structure-retention relationships (QSRR) obtained show the predominating partition mechanism of separation on both phases. Both the LFER parameters and the structural parameters obtained by molecular modelling produced QSRR equations of high and practically identical retention prediction ability. It has been concluded that intermolecular interactions of the dipolarity-polarizability type manifest themselves more strongly on the silica-based phase than on the zirconia-based one. This can be due to a role of active free silanols on the silica support.

An analysis of 13C nuclear magnetic resonance substituent chemical shifts in 3-substituted thiophene-2-carboxylic and 2-substituted benzoic acids by linear free energy relationships

The 13C n.m.r. chemical shifts of the title compounds and of the corresponding anions have been measured in methanol. The observed substituent chemical shifts have been analysed by means of cross-correlations and of single and dual substituent parameter linear free energy relationships. Different results have been obtained for thiophene and benzene derivatives using DSP analysis: for the first ring system the results have been rationalized in terms of separate resonance and inductive contributions of substituent effects (pointing out the occurrence of an apparent reverse polar effect); in contrast the same treatment of the data for the benzene system gave poor results, confirming once again the different proximity effects in six-and five-membered ring derivatives.

An analysis of 13C nuclear magnetic resonance substituent chemical shifts in 4- and 5-substituted thiophene-2-carboxylic acids by linear free energy relationships

The 13C n.m.r. chemical shifts of some 4- and 5-substituted thiophene-2-carboxylic acids and of the corresponding anions have been measured in methanol. The observed substituent chemical shifts have been compared with the calculated values and then analysed according to single and dual substituent parameter linear free energy relationships. The results have been rationalized in terms of separate resonance and inductive contributions of substituent effects. The resonance term has shown that no conjugation occurs between the ring and the carboxy group.

Substituent Effects on the N.M.R. Spectra of Carboxylic Acid Derivatives. IV. 13C N.M.R. Spectra of para-Substituted Phenols, Phenylureas and Phenyl Propionates

The 13C n.m.r. spectra of 9 para-substituted phenols, 12 para-substituted phenylureas and 11 para-substituted phenyl propionates have been measured in (CD3)2SO and the observed substituent chemical shifts have been found to be non-additive with respect to those of the corresponding monosubstituted benzenes. The 1H n.m.r. spectra of the phenols and phenyl propionates and the i.r . carbonyl stretching frequencies of the phenyl propionates have also been determined. The spectral data have been treated by single parameter and dual substituent parameter linear free energy relationships ( l.f.e.r .) and σR values of best fit have been identified. In general, treatment by single parameter l.f.e.r . Are at least as satisfactory as those by DSP l.f.e.r.

Substituent effects on the N.M.R. spectra of carboxylic acid derivatives. III. Correlation of 13C N.M.R. spectra of para substituted acetanilides and 4'-nitrophenyl 4-substituted benzoates with infrared carbonyl stretching frequencies, 1H N.M.R., rate and equilibrium data

The 13C n.m.r. spectra of 17 monosubstituted benzenes, 14 para substituted acetanilides and ten 4'-nitrophenyl 4-substituted benzoates have been measured in (CD3)2SO and the chemical shifts have been compared with the infrared carbonyl stretching frequencies of the substrates (in CCl4), the 1H n.m.r. chemical shifts of protons close to the reactive site of protonation, rates of hydrolysis or aminolysis and literature values of the corresponding basicity constants and rate constants. The observed substituent chemical shifts are non-additive and this result has been discussed in terms of the electronic properties of -NHCOCH3 and -COOC6H4NO2. The 13C n.m.r. and the i.r. data have been treated by single parameter and dual substituent parameter linear free energy relationships and σR values of best fit have been identified. Relative resonance and inductive contributions have been discussed.

The reactivity of p-nitrophenyl esters with surfactants in apolar solvents. VII. Substituent effects on the reactivity of 4'-nitrophenyl 4-substituted benzoates in benzene solutions of dodecylammonium propionate and butane-1,4-diamine bis(dodecanoate)

The rate of decomposition of a series of 4'-nitrophenyl 4-substituted benzoates has been measured at 341 K in benzene solutions of dodecylammonium propionate and butane-1,4-diamine bis-(dodecanoate). The bimolecular rate constant due to general acid-general base catalysis from the head-group components of the surfactants is dependent on the nature of the substituents, and the rate data have been correlated by Hammett σ and σ+, Tsuno-Yukawa, and Taft dual substituent parameter linear free energy relationships. The reactivity of the esters has also been compared with the 13C n.m.r. chemical shift of their carbonyl carbon atoms. The results are consistent with a mechanism involving a general-acid-catalysed aminolysis of the unprotonated ester.

A statistical approach to the blend of inductive and mesomeric contributions in dual parameter linear free energy relationships

A statistical approach to the blend of inductive and mesomeric contributions in dual parameter linear free energy relationships