Quantitative structure and retention relationships for gas chromatographic data: Application to alkyl pyridines on apolar and polar phases

Quantitative structure and retention relationships (QSRR) have been developed to model gas chromatographic retention data of alkyl pyridines on apolar (branched alkane) and polar (primary alcohol) stationary phases. The retention properties analyzed are Kovats retention index, I; partial molar enthalpy, ΔH; partial molar entropy, ΔS and partition coefficient, log K. Using the seven valence molecular connectivity indices (χ  ) calculated for the 18 alkyl pyridines, regression models are generated to predict the retention properties. The best model (model A) obtained with the descriptors χPV1, χPV3 and χCHV6 was unable to produce a satisfactory statistical performance and correct order of elution. The model has been modified (model B) by including steric parameter s, which has been empirically derived by considering the steric effects due to the presence of alkyl groups at the ortho and meta positions. The modified model predicts the correct order of elution for all the alkyl pyridines and good correlation coefficients, r. The r values obtained for I, ΔH, ΔS and log K are: r = 0.955, 0.975, 0.984 and 0.955 (model A) become r = 0.999, 0.996, 0.990 and 0.999 (model B) on the apolar stationary phase and r = 0.931, 0.926, 0.911 and 0.931 (model A) become r = 0.999, 0.992, 0.958 and 0.999 (model B) on the polar stationary phase.