QUANTUM AND MOLECULAR MECHANICS STUDY OF THE TRIS(QUATERNARY AMMONIUM) CATION USED AS THE ZEOLITE ZSM-18 SYNTHESIS TEMPLATE

A combined first principles (both Hartree-Fock and local density funct ional) and force field study of the preferred conformations and confor mational flexibility of the tris(quaternary ammonium) examethyl-1H-ben zo[1.2-c:3,4-c':6-c'']tripyrrolium 'triquat' cation [(C4H4N(CH3)(2)))( 3)](3+) is presented. The potential energy surface of this molecular c ation is characterized by two local minima corresponding to C-3r and C -s symmetry, respectively, and a triple transition state with respect to pyrrolium ring inversion with D-3h symmetry. These structures were each successfully located by both first principles and force field met hodologies. Although the conformational energy differences obtained by Hartree-Fock and local density functional methods agree well, some va riability in the molecular mechanics results for different force field s is found, underlining the need for a careful calibration of force fi elds against experimental and/or ab initio data. The implications of t he calculated energy barriers for ring inversion in the primary applic ation of the triquat cation as a zeolite ZSM-18 templating agent are a lso discussed.