The C7-C10 cycloalkanes revisited.

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Organic Chemistry

Background:

• Understanding the conformational landscape of medium-sized rings is crucial in organic chemistry.
• Previous studies often relied on molecular mechanics, but theoretical methods offer higher accuracy.
• Discrepancies exist between different computational approaches for ring conformer energies.

Purpose of the Study:

• To investigate and compare the conformational energies of cycloheptane through cyclodecane.
• To evaluate the accuracy of B3LYP and MP2 theoretical levels against experimental data.
• To assess the performance of different computational methods in predicting ring structures.

Main Methods:

• Utilized B3LYP/6-311+G* and MP2/6-311+G* for theoretical calculations.
• Performed additional high-level calculations using CCD/6-311+G* and CCSD(T)/6-311++G**.
• Compared results with experimental electron diffraction data and molecular mechanics (MM4) calculations.

Main Results:

• MP2 and CCSD(T) identified the crown conformer of cyclooctane as higher in energy, agreeing with electron diffraction.
• B3LYP predicted different conformational minima for cyclononane compared to MP2, which found true minima.
• B3LYP systematically overestimated C-C-C bond angles compared to MP2 and CCD.

Conclusions:

• MP2/6-311+G* and higher-level methods provide more reliable predictions for medium-sized ring conformations than B3LYP.
• Molecular mechanics MM4 results generally align well with the more accurate MP2 calculations.
• Theoretical level choice significantly impacts the predicted conformational preferences and geometries of cyclic alkanes.