NMR characterization of substituent effects in cation–π interactions

Towards a Molecular Understanding of Cation-Anion Interactions and Self-aggregation of Adeninate Salts in DMSO by NMR and UV Spectroscopy and Crystallography

Rare anionic forms of nucleic acids play a significant biological role and lead to spontaneous mutations and replication and translational errors. There is a lack of information surrounding the stability and reactivity of these forms. Ion pairs of mono‐sodium and ‐potassium salts of adenine exist in DMSO solution with possible cation coordination sites at the N1, N7 and N9 atoms of the purine ring. At increasing concentrations π‐π stacked dimers are the predominant species of aggregates followed by higher order aggregation governed by coordination to metal cations in which the type of counter ion present has a central role in the aggregate formation.

Substituent Effect Transmission Power of Alkyl, Alkenyl, Alkynyl, Phenyl, Thiophenyl, and Polyacene Spacers

The transmission of substituent effect through a variety of spacers, that is to say, alkyl, alkenyl, alkynyl, phenyl, thiophenyl, and polyacene has been studied by modeling Y-G-X type molecular systems (Y: reaction center; G: spacer moiety; X: substituent) using B3LYP/6-31G(d,p) density functional theory calculations. The reaction center is always kept as a C=C double bond and the molecular electrostatic potential (MESP) minimum (V_min ) observed for this bond showed subtle variation with respect to the changes in the spacer unit and the nature of substituent. Strong linear correlations are observed between Hammett substituent constants (σ_I and σ_p ) and V_min , which recommend the aptness of V_min as an electronic descriptor to quantify the substituent effect. Since V_min offers an alternative measure of substituent effect, the correlation between V_min and σ_p has been used for assessing the transmission of substituent effect through a variety of spacer moieties. The highest transmission coefficient (γ) is always observed for smaller spacer length. Among all the spacers, alkenyl showed the highest and alkyl showed the lowest transmission power. The study recommends the use of short chains of C=C double, C≡C triple or a combination of both as spacers for the effective transmission of substituent effect to the reaction center.

Accurate prediction of cation-π interaction energy using substituent effects

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, E(M(+)), showed a strong linear correlation with the molecular electrostatic potential (MESP) based measure of the substituent effect, ΔV(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(+))(ΔV(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 ΔV(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 ΔV(min). All the Φ-X···M(+) systems showed good agreement between the calculated and predicted E(M(+))() values, suggesting that the ΔV(min) approach to substituent effect is accurate and useful for predicting the interactive behavior of substituted π-systems with cations.