Characterizing cavities in model inclusion molecules: a comparative study.

We have selected fullerene-60 and -70 cavities as model systems in order to test several methods for characterizing inclusion molecules. The methods are based on different technical foundations such as a square and triangular tessellation of the molecule taken as a unitary sphere, spherical tessellation of the molecular surface, numerical integration of the atomic volumes and surfaces, triangular tessellation of the molecular surface, and a cubic lattice approach to a molecular space. Accurate measures of the molecular volume and surface area have been performed with the pseudo-random Monte Carlo (MCVS) and uniform Monte Carlo (UMCVS) methods. These calculations serve as a reference for the rest of the methods. The SURMO2 and MS methods have not recognized the cavities and may not be convenient for intercalation compounds. The programs that have detected the cavities never exceed 5% deviation relative to the reference values for molecular volume and surface area. The GEPOL algorithm, alone or combined with TOPO, shows results in good agreement with those of the UMCVS reference. The uniform random number generator provides the fastest convergence for UMCVS and a correct estimate of the standard deviations. The effect of the internal cavity on the accessible surfaces has been calculated.

Torsional effects on the molecular polarizabilities of the benzothiazole (A)-benzobisthiazole (B) oligomer A-B13-A.

We outline a method for the calculation of multipole moments and molecular dipole-dipole (alpha), dipole-quadrupole (A), and quadrupole-quadrupole (C) polarizabilities, which we have successfully applied to benzothiazole (A)-benzobisthiazole (B) oligomer A-B13-A. Three model rotational isomers have been characterized: (1) the fully planar (000) rotational isomer; (2) a conformation with each unit rotated 10 degrees in the alternate direction (+(-)+), and (3) a rotational isomer with each unit rotated 10 degrees in the same direction (+3). The dipole moment, mu, is smaller for isomers 000 and +(-)+ than for isomer +3. The calculation of alpha A, and C has been performed by use of the interacting induced dipoles polarization model, which calculates tensor effective anisotropic point polarizabilities (method of Applequist). The values of alpha, A, and C are in the same order of magnitude as reference calculations (PAPID) program). The values of A are rather sensitive to mu, which varies under rotation, explaining the greatest value of magnitude of Ax,xx for polar isomer +3. This rotational isomer has the maximum hydrophilic accessible surface, which would improve solubility in water. It is found that small torsional changes can enhance solubility by increasing the hydrophilic accessible surface without too much affecting the values of alpha and C. However, the torsion of the oligomer can vary the value of mu and so modify A.