The relative strength of noncovalent interactions between a thioether sulfur atom and various π systems in designed top pan molecular balances was determined by NMR spectroscopy. Compared to its oxygen counterpart, the sulfur atom displays a remarkable ability to interact with almost equal facility over the entire range of π systems studied, with the simple alkene emerging as the most powerful partner. With the exception of the O⋅⋅⋅heteroarene interaction, all noncovalent interactions of sulfur with π systems are favoured over oxygen.

A comparative study of molecular balances by NMR spectroscopy indicates that noncovalent functional‐group interactions with an arene dominate over those with an alkene, and that a π‐facial intramolecular hydrogen bond from a hydroxy group to an arene is favored by approximately 1.2 kJ mol−1. The strongest interaction observed in this study was with the cyano group. Analysis of the series of groups CH2CH3, CHCH2, CCH, and CN shows a correlation between conformational free‐energy differences and the calculated charge on the Cα atom of these substituents, which is indicative of the electrostatic nature of their π interactions. Changes in the free‐energy differences of conformers show a linear dependence on the solvent hydrogen bond acceptor parameter β.

A comparative study using NMR spectroscopy and designed top-pan molecular balances demonstrates that the noncovalent interaction of a hydroxy group with π-deficient pyrazine and quinoxaline units involves a lone pair-heteroarene interaction which is much stronger and solvent independent when measured relative to the classical π-facial hydrogen bond to a benzene ring. Alkyl fluorides also prefer the heteroarene rings over the benzene ring. The attractive interaction between a quinoxaline and a terminal alkyne is also stronger than the intramolecular hydrogen bond to an arene.

A comparative study of molecular balances by NMR spectroscopy indicates that noncovalent functional-group interactions with an arene dominate over those with an alkene, and that a π-facial intramolecular hydrogen bond from a hydroxy group to an arene is favored by approximately 1.2 kJ mol(-1). The strongest interaction observed in this study was with the cyano group. Analysis of the series of groups CH2CH3, CH=CH2, C≡CH, and C≡N shows a correlation between conformational free-energy differences and the calculated charge on the C(α) atom of these substituents, which is indicative of the electrostatic nature of their π interactions. Changes in the free-energy differences of conformers show a linear dependence on the solvent hydrogen bond acceptor parameter β.

The relative strength of noncovalent interactions between at hioether sulfur atom and various p systems in designed top pan molecular balances was determined by NMR spectroscopy. Compared to its oxygen counterpart, the sulfur atom displays aremarkable ability to interact with almost equal facility over the entire range of p systems studied, with the simple alkene emerging as the most powerfulpartner.With the exception of the O···heteroarene interaction, all noncovalent interactions of sulfur with p systems are favoured over oxygen.Noncovalent interactions involving aromatic systems, [1] such as p-p stacking [2] or cation-p interactions, [3] and the ability of an arene to act as ahydrogen-bond acceptor [4] are all firmly established as vital control elements in molecular recognition and consequently underpin vast areas of chemistry and molecular biology.More recently,asaconsequence of two virtually simultaneous theoretical studies predicting the strength of the related anion-p interaction, [5] this area has witnessed intense research activity. [6] In parallel, following on from the seminal observation of the attractive lone pair···p interaction in stabilizing the Z-DNAs tructure, [7] this force is also rapidly gaining recognition as anew area of supramolecular chemistry. [8] Detailed quantifiable knowledge of the relative strength of these weak forces is now therefore considered to be essential for the rational design of organocatalysts, [9] new drugs and supramolecular materials,aswell as the understanding of three-dimensional structure and function in biological systems.Thec onformational analysis of designed molecular balances [10] with limited degrees of freedom is ap articularly powerful tool for probing the strength, distance and angular dependence of such interactions,and also allows for exploration of all important solvation phenomena. [11] Whilst an increasing number of studies in recent years have focused on measuring the O···p interaction with electron-deficient arenes and heteroarenes, [12] we were surprised to note that quantifiable comparative information using such balances to probe noncovalent interactions involving as ulfur atom with p systems has been almost entirely neglected. Thev ital role played by sulfur in chemical and biological recognition and in drug development is well-recognized. [13] As summarized in an ongoing series of excellent reviews, [1] theinvestigation of the S···arene interaction is essentially based on extensive database mining of protein crystal structures.I nasignificant contribution by Dougherty and co-workers, [14] the strength of aS···arene interaction in the dopamine D2 receptor has been probed by progressive modulation of the electrostatic surface of the arene through fluorination. To the best of our knowledge,i ns pite of the fact that theoretical chemists continue to be intrigued by the noncovalent interactions of asulfur atom with p systems, [15] no quantitative experimental measurements have been reported for either simple alkenes or heteroarenes.I nt erms of ac...