Remarks on the analysis of torsional energy surfaces of cycloheptane and cyclooctane by molecular mechanics

Abstract

Abstract A modified version ( MM 2′) of the Allinger's 1977 force field is checked against cycloheptane and cyclooctane. Cycloheptane is characterized by two pseudorotating itineraries, chair/twist‐chair and boat/twist‐boat, separated by a barrier of 8.5 kcal mol −1 . The activation energy in the C / TC pseudorotation is estimated to be 0.96 kcal mol −1 , while B and TB transform into each other freely at an energy level 3.8 kcal mol −1 above the global energy minimum ( TC ). With cyclooctane the lowest energy is calculated for the boat‐chair form which participates in a pseudorotational process with TBC through a saddle point lying 3.5 kcal mol −1 above BC . The chair/chair and boat/boat families contain only one local minimum, crown and BB , respectively, on the MM 2′ surface. The results are presented as an illustration for quick coverage of torsional energy surface by two‐bond driver calculation with the block‐diagonal Newton–Raphson minimization, followed by the force search of stationary points by full‐matrix Newton–Raphson optimization.