CONSISTENT POINT-CHARGES AT THE 18-CROWN-6 ATOMS FROM CORRELATED AB-INITIO CALCULATIONS

Ab initio energies of seven conformations of the ionophore 18-crown-6 are calculated at 10 different levels of approximation from HF/STO-3G up to MP2(full)/6-31G. The conformer with the symmetry point-group C(i) found in the solid state appeared to be the most stable among the considered conformers at the Hartree Fock levels from HF/4-31G up to HF/6-31++G. However, when the electron correlation is taken into account at the full MP2 level it decreases the relative energies of the three other conformers with symmetry S6, C(i) and C2, the most stable being a hitherto unknown structure with symmetry C2. The relative energy of the most symmetrical structure D3d is found to be about 80 kJ mol-1, which excludes its existence in the gas phase at room temperature. A procedure for calculating consistent point-charges from correlated ab initio energies is presented. The AMBER force field, widely used for 18-crown-6 simulations, reproduces ab initio relative conformation energies at the HF/STO-6G level only, which, however, underestimates the repulsive electrostatic interaction energy. Because of the extreme importance of this interaction the main problem is to find the optimal point-charges which, being incorporated into the AMBER force field, adequately describe intramolecular electrostatic interactions. Such consistent point-charges at the 18-crown-6 atoms are determined by least-square fitting and allow one to reproduce correlated ab initio energies by empirical force field calculations to a maximal degree. A recent electronegativity equalization method is found to yield very similar point-charges. Among the semiempirical methods such as MINDO/3, MNDO and AM1, only the latter gives acceptable relative conformation energies and atomic charges of 18-crown-6.