Complexation of a gaseous spin probe with cyclodextrins bound to the silica microspheres: Molecular dynamics of the complexed probes and the effect of aromatic hydrocarbon vapors on it

摘要

The formation of guest-host complexes of a gaseous radical TEMPO with cyclodextrins bound to the silica microspheres (MSs) either covalently with a chemical linker or by physical adsorption has been investigated. For the former complexes, two conformational states with strongly different rotational mobilities have been found similar to those described in [9] for aqueous MS suspensions. These states are assigned to the adsorbed (s) and desorbed (w) complexes, the fraction of s-states in the gaseous phase being noticeably higher than that in the suspensions. The binding of TEMPO to the copolymers of carboxymethyl β-CD (PCCD) or sulfated β-CD (PSCD) with epichlorhydrin adsorbed on MS results in dipole-dipole interactions (DDI) between the radicals. The local concentrations of the radicals have been estimated from the spectral parameters of DDI. The concentrations considerably exceed the average ones, indicating the preferred radical location in mesopores. The TEMPO rotational mobility in complexes with PCMCD is satisfactorily described by the model of anisotropic molecular rotation in the liquid-crystal-like orienting potential in the temperature interval of 206-273 K but deviates from this model at T > 290 K. The rotational diffusion coefficients are in the range of 10~(6.8)-10~(7.6) with an activation energy of 2.5 ± 0.15 kcal/mole and order parameter D_(20) in the interval of 0.85-0.55. The dependence of the portion of the s-states and rotational mobility of the w-states on the vapors of the aromatic hydrocarbons and on the CD structure was discovered for complexes with the covalently bound CD. The changes in molecular dynamics in the presence of benzene, toluene, and naphthalene were displayed for both types of sensor layers: covalently or noncovalently bound to MS. The character of these changes is specific for these hydrocarbons, so it may be used for their identification.