Quantum mechanics and molecular mechanics studies of the low-energy conformations of 9-crown-3

Quantum mechanics calculations and molecular mechanics calculations using the MMX force field have been performed on the low-energy conformations of 9-crown-3. A [144] conformation corresponds to the global minimum at the MP2/6-31G**//6-31G** level, with a [225] conformation only 0.72 kcal/mol higher in energy. The [333] conformation in which all three oxygen atoms are on the same side of the least squares plane passing through the ring, a structure particularly favorable as a tridentate ligand, is calculated to be 6.81 kcal/mol higher in energy. Electron correlation at the MP2 level and polarization functions on the hydrogen atoms have only minor effects on the relative conformational energies. However, polarization functions on the carbon and oxygen atoms are needed to obtain good results. MMX conformational energies are in good agreement with ab initio values if the electrostatic scale factor is set at 0.70. The role of transannular C-H ... O hydrogen bonding in the low-energy conformations of 9-crown-3 is discussed. Results are compared with those obtained from AM1, PM3, and AMBER calculations.