The structure-affinity relationships were studied for the guest inclusion parameters of solid tert-butylthiacalix-[4]arene (1) and tert-butylcalix [4]arene (2). The inclusion stoichiometry and inclusion free energy were calculated by the sorption isotherms obtained for guest vapor-solid host systems by the static method of headspace gas chromatographic analysis at 298 K. The obtained sorption isotherms have an inclusion threshold for guest thermodynamic activity corresponding to the phase transition between the initial host phase and the phase of inclusion compound. Unlike tert-butylcalix [4]arene, its thia analogue having a larger molecular bowl is able to bind only initial members of each studied homological series. All inclusion compounds of 1 formed upon host saturation by guest vapors have the same 1:1 stoichiometry, while for 2 the inclusion stoichiometry depends on the guest molecular size. A linear correlation between the inclusion free energy (standard state: infinitely dilute guest solution in toluene) and the guest size parameter (molar refraction) was observed for 1: ∆G trans (kJ mol -1 ) ) -12.24 + 0.568MR D (n ) 7, r ) 0.972, RSD ) 0.6). This correlation is regarded as a part of the V-like structure-affinity relationship with a minimum for a guest that is complementary to the host cavity.

No abstract

The parameters of stability, guest binding reversibility, and Gibbs energy of guest inclusion were determined for clathrates of adamantylcalix[4]arene (1). These data provide a new insight into the structure-property relationships in vapor sensor applications of clathrate-forming hosts. A thin layer of 1, used in the quartz microbalance (QCM) sensor, demonstrates a selectivity for organic vapors, which depends on the regeneration technique after the guest binding. Complete regeneration of 1 on the sensor surface was reached through the exchange of bound guest with ethanol vapor, which forms an unstable clathrate with 1. The efficiency of the used regeneration technique was proved by comparing the QCM data with the isotherms of guest vapor sorption by guest-free host 1 and with the data of simultaneous thermogravimetry and differential scanning calorimetry for the saturated clathrates of 1. In sensor, the extent of host regeneration without guest exchange depends on the guest molecular structure. This extent, or guest-binding reversibility parameter, being determined in a combination with the sensor responses of completely regenerated 1 to guest vapors, increases the recognition capability of single sensor device. Using this technique, 13 of 15 studied guests were discriminated. The structural hints on the suitable sensor properties of 1 were found in the determined X-ray monocrystal data for clathrate of this host with toluene.

An effective preparation procedure is offered for β-cyclodextrin (bCD) clathrates with volatile guests of moderate hydrophilicity, which otherwise require a finely tuned optimization of the bCD/water/guest ratio. The proposed procedure includes guest exchange in a water-free bCD matrix. As a result, more stable clathrates with a higher inclusion capacity can be prepared than by direct saturation of dried or hydrated bCD. To find an optimal preparation method, the structure-property relationships were studied for four different ways of bCD clathrate formation with guests of varying molecular structure. The study was much simplified by used procedures excluding direct contact of guest and water liquids with bCD. Besides, in clathrate preparation experiments, the thermodynamic activities of water and guest were varied independently, which makes explicit the role of water in this process. Hydration of bCD reduces its inclusion threshold for hydrophobic guests by their activity (relative vapor pressure), giving favorable hydration effect for their inclusion. Besides, water competes with hydrophilic guests for binding sites in bCD at high water activities. Together with bCD dehydration by excess of hydrophilic guests, these observations give a complete thermodynamic picture, which may be fruitful for elaboration of guest encapsulation techniques by cyclodextrins.