Factor Analysis, Target Factor Testing and Model Designing of Aromatic Solvent Effect of the Formyl Proton Nuclear Magnetic Resonance Chemical Shift in Para Substituted Benzaldehydes

Problem statement: The variations of formyl proton Chemical Shifts (CS) of p-substituted benzaldehydes in aromatic solvents were investigated. The validity of several physical solvent and empirical solvent scales was examined. Also, to predict dipolarity-polarizability (n*) solvent scale for some aromatic solvents. Model designing was also achieved to rationalize the aromatic solvent effect on the formyl proton CS. Approach: The previously recorded formyl proton CS for p-X-benzaldehydes, with X were NMe2, OMe, OC3H7, H, Br, CHO and NO2 in benzene, toluene, p-xylene, m-xylene and mesitylene were subjected to Factor Analysis (FA). Target Factor Testing technique (TFT) was performed for several solvent scales namely: Unity, the intrinsic aromatic solvent induced shift of TMS (IASISTMS), f(n), f(d), (n2-1)/(n2+2), (d-1)/(d+2), ET(30) and n*. Iterative TFT was applied to predict unmeasured (n*) solvent scale for ethyl benzene, n-butyl benzene, sec-butyl benzene, tert-butyl benzene and isopropyl benzene. Results: It has been found that two factors were responsible for the variation in the formyl proton CS. The unity, f(n), (n2-1)/(n2+2), IASISTMS, ET(30) and n* were real factors. Model designing of the formyl proton CS in benzene, toluene, p-xylene, m-xylene and mesitylene were achieved. The models with lowest root mean square error (RMSE) have shown that Unity is a consistent term. The other term was either IASISTMS or n*. Iterative TFT predicted new n* values for ethylbenzene, n-butylbenzene, sec-butylbenzene, tert-butylbenzene and isopropylbenzene respectively. Conclusion: FA has revealed that two real factors are responsible for the variation of formyl CS in benzene, toluene, p-xylene, m-xylene and mesitylene solvents. TFT has shown to be a powerful technique in predicting new values of the n* solvent scale. Model designing for the formyl proton CS have revealed that the IASISTMS, n* and Unity are the best empirical solvent scales and were better than any physical solvent scales in reproducing the formyl CS. The IASISTMS reflects the dipolarity-polarizabilty of the aromatic solvent. The cofactor of the solvent scale was found to correlate with the ctp+ substituent parameter.