## Accurate Prediction of Cation−π Interaction Energy Using Substituent Effects

journal contribution

posted on 14.06.2012, 00:00 by Fareed
Bhasha Sayyed, Cherumuttathu H. SureshSubstituent 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,*E*_{M+}, showed a strong linear correlation with the molecular electrostatic potential (MESP) based measure of the substituent effect, Δ*V*_{min}(the difference between the MESP minimum (*V*_{min}) on the π-region of a substituted system and the corresponding unsubstituted system). This linear relationship is*E*_{M+}=*C*_{M+}(Δ*V*_{min}) +*E*_{M+}′ where*C*_{M+}is the reaction constant and*E*_{M+}′ 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*C*_{M+}^{}and*E*_{M+}^{}′ has been established, which facilitates the prediction of*C*_{M+}for unknown cations. Thus, a prediction of*E*_{M+}for any Φ–X···M^{+}complex is achieved by knowing the values of*E*_{M+}′ and Δ*V*_{min}. The generality of the equation is tested for a variety of cations (Li^{+}, Na^{+}, K^{+}, Mg^{+}, BeCl^{+}, MgCl^{+}, CaCl^{+}, TiCl_{3}^{+}, CrCl_{2}^{+}, NiCl^{+}, Cu^{+}, ZnCl^{+}, NH_{4}^{+}, CH_{3}NH_{3}^{+}, N(CH_{3})_{4}^{+}, C(NH_{2})_{3}^{+}), substituents (N(CH_{3})_{2}, NH_{2}, OCH_{3}, CH_{3}, OH, H, SCH_{3}, SH, CCH, F, Cl, COOH, CHO, CF_{3}, CN, NO_{2}), 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, [6]-cyclacene, and corannulene and found that*E*_{M}^{+}follows the additivity of substituent effects. Further, the substituent effects on cationic sandwich complexes of the type C_{6}H_{6}···M^{+}···C_{6}H_{5}X have been assessed and found that*E*_{M+}can be predicted with 97.7% accuracy using the values of*E*_{M+}′ and Δ*V*_{min}. All the Φ–X···M^{+}systems showed good agreement between the calculated and predicted*E*_{M+}^{}values, suggesting that the Δ*V*_{min}approach to substituent effect is accurate and useful for predicting the interactive behavior of substituted π-systems with cations.