Accurate Prediction of Cation−π Interaction Energy Using Substituent Effects
journal contributionposted on 14.06.2012, 00:00 by Fareed Bhasha Sayyed, Cherumuttathu H. Suresh
Substituent effects on cation−π interactions have been quantified using a variety of Φ–X···M+ complexes where Φ, X, and M+ are the π-system, substituent, and cation, respectively. The cation−π interaction energy, EM+, showed a strong linear correlation with the molecular electrostatic potential (MESP) based measure of the substituent effect, ΔVmin (the difference between the MESP minimum (Vmin) on the π-region of a substituted system and the corresponding unsubstituted system). This linear relationship is EM+ = CM+(ΔVmin) + EM+′ where CM+ is the reaction constant and EM+′ is the cation−π interaction energy of the unsubstituted complex. This relationship is similar to the Hammett equation and its first term yields the substituent contribution of the cation−π interaction energy. Further, a linear correlation between CM+ and EM+′ has been established, which facilitates the prediction of CM+ for unknown cations. Thus, a prediction of EM+ for any Φ–X···M+ complex is achieved by knowing the values of EM+′ and ΔVmin. The generality of the equation is tested for a variety of cations (Li+, Na+, K+, Mg+, BeCl+, MgCl+, CaCl+, TiCl3+, CrCl2+, NiCl+, Cu+, ZnCl+, NH4+, CH3NH3+, N(CH3)4+, C(NH2)3+), substituents (N(CH3)2, NH2, OCH3, CH3, OH, H, SCH3, SH, CCH, F, Cl, COOH, CHO, CF3, CN, NO2), and a large number of π-systems. The tested systems also include multiple substituted π-systems, viz. ethylene, acetylene, hexa-1,3,5-triene, benzene, naphthalene, indole, pyrrole, phenylalanine, tryptophan, tyrosine, azulene, pyrene, -cyclacene, and corannulene and found that EM+ follows the additivity of substituent effects. Further, the substituent effects on cationic sandwich complexes of the type C6H6···M+···C6H5X have been assessed and found that EM+ can be predicted with 97.7% accuracy using the values of EM+′ and ΔVmin. All the Φ–X···M+ systems showed good agreement between the calculated and predicted EM+ values, suggesting that the ΔVmin approach to substituent effect is accurate and useful for predicting the interactive behavior of substituted π-systems with cations.