Comparison of ab initio, semiempirical, and molecular mechanics calculations for the conformational analysis of ring systems

The potential energy surfaces of four cyclic alkanes have been examined using molecular mechanics, semiempirical, and ab initio methods to determine if they produce mutually consistent results and investigate the source of any errors between the methods. The C₅  C₈ cyclic alkanes were chosen since these structures present a finite set of conformations and transition‐state geometries and are still within the computational time and memory limits of the quantum mechanical approaches. We also examined several conformations of 1,2‐dideoxyribose to determine the effect of heteroatoms on the results for the 5‐membered ring. The molecular mechanics and ab initio calculations are consistent in the relative energies and geometries determined for the conformers of all ring systems. While the semiempirical calculations yielded geometries consistent with the other methods (except for 5‐membered rings), the relative energies often deviated substantially. A decomposition analysis of the semiempirical and molecular mechanics energies revealed that the disparities are mainly due to errors in the 1‐center energies of the semiempirical calculations. The 2‐center bonding and nonbonding energies followed reasonable trends for the conformers. The core‐repulsion function, however, is suspected of producing anomalies. A minimum in the attractive Gaussian of this term at 2.1 Å for HH interactions partly explains the propensity of the 5‐membered rings to optimize to near planarity (decreasing 1,2‐diaxial hydrogen distances to 2.3 Å) and the underestimation of the relative energy of the boat structure of cyclohexane.