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>Computer simulation studies on bgr;hyphen;quinol clathrates with various gases. Interdependence of host and guest molecules
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Computer simulation studies on bgr;hyphen;quinol clathrates with various gases. Interdependence of host and guest molecules
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机译：Computer simulation studies on bgr;hyphen;quinol clathrates with various gases. Interdependence of host and guest molecules
The equilibrium crystal structure of a bgr;hyphen;quinol clathrate occupied by various gases is determined through empirical force field calculations. The guests with van der Waals volumeVmle;27 cm3/mol affect the structure and rigidity of the lattice in a manner independent of their chemical nature: The guesthyphen;host attraction leads to a shrinking of the host cages. On the other hand, the inclusion of molecules of the volume 27 cm3/molle;Vmle;46 cm3/mol expands the cages. Guest molecules render the host lattice more rigid, except for large spherical rotor type molecules, which soften the lattice, eventually destroying it. The inclusion of molecules withVmgsim;46 cm3/mol is likely to be impossible. The energy of clathrate formation increases with increasingVm, with the polar guests forming more stable clathrates than the nonpolar ones with the sameVm. Interestingly, the optimal guesthyphen;cage compatibility occurs when the volume of guest molecules differs by less than 50percnt; from the volume of the empty cage. The orientation of a guest molecule in a cage is mainly determined by a competition of three types of interactions. (i) Van der Waals interactions tend to direct the long axis of the guests along thechyphen;axis of the cage. (ii) The electrostatic guesthyphen;host interaction tends to place a positively charged guest atom into the center of the cage. (iii) Electrostatic guestndash;guest interactions tend to align the dipole moments of the included molecules along thechyphen;axis. The different orientations are rationalized in terms of the complementary influences of the guest size, guest polarity and an lsquo;lsquo;atomic sizersquo;rsquo; factor. The calculated barriers hindering the reorientation movement of guest molecules in a cage are found to be in a qualitative agreement with experimental data except for the N2and CO molecules, which are supposed to be considerably polarized by the cage. The C3hyphen;reorientation motion of ethane is predicted to be accompanied by its synchronous rotation around the Cndash;C bond.
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