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.

We study quantum ͑Shubnikov-de Haas and de Haas-van Alphen͒ oscillations and angular oscillations of the reluctance in the organic quasi-two-dimensional metal (BO) 2 Cl x (H 2 O) y. We show that the Fermi surface in this compound consists of a slightly corrugated cylinder with its axis perpendicular to the conducting plane. The cross section of the cylinder in this plane is a perfect circle of radius k F Ӎ3ϫ10 7 cm Ϫ1. The effective carrier mass associated with this cylinder is m*ϭ(1.65-2.0) m 0 in the conducting plane, while the Dingle temperature is T D ϭ3-4 K.