THE CONSISTENT FORCE-FIELD .4. AN OPTIMIZED SET OF POTENTIAL-ENERGY FUNCTIONS FOR ALIPHATIC AND ALICYCLIC ETHERS AND ANOMERIC CARBON-ATOMS

A set of potential energy function parameters has been optimized for t he ether oxygen and anomeric carbon atoms. The force field is classica l except for the use of Morse functions for bond deformation. Optimiza tion was done on internal molecular structure of gases, unit cell dime nsions of crystals, and vibrational spectra of gases using the Consist ent Force Field program. A previous work led to optimization of 15 par ameters for alkanes. The ether parameter set contains 53 parameters, 3 8 of which were optimized on a database of 11 gas phase structures and 6 crystal structures, and vibrational spectra of 10 compounds. The pe rformance of the resulting force field was checked on substances and p roperties not included in the optimization: energy barrier data calcul ated with high-level ab initio methods, experimental thermodynamic dat a, and supplementary data on internal structure, rotational constants and dipole moments. Change of previous strategy from optimization of a tomic charge parameters on molecular dipole moments to the fitting of partial charges derived from ab initio studies improves the performanc e with respect to rotamer energies. With the simple classical force fi eld used and with parameters optimized exclusively on experimental dat a, we obtain rotational barriers conforming to recent ab initio studie s on 2-methoxytetrahydropyran (1), a model compound for carbohydrates in general and the exo-anomeric effect in particular.