Alkane conformational enthalpies are determined in real systems by bonding lengths, angles and dihedrals.  A simple ideal model assumes them to be constant and zero if the molecules can be superimposed upon a diamond lattice, with carbon and hydrogen atoms occupying the lattice sites, and in which all C-C bond angles are staggered.  Torsional strain can be incorporated if the molecules, or portions thereof, can be superimposed on a Lonsdaleite lattice, which contains eclipsed C-C bonds.  A van der Waals energy penalty can also be added to the conformational energy, and occurs when two H atoms occupy adjacent lattice sites or identical lattice sites.  This study describes a model for an alkane system that cannot be superimposed on a single lattice domain, cyclopentane.  The envelope conformer of cyclopentane is modeled as the junction of two lattices: a diamond lattice that contains three carbons and a Lonsdaleite lattice that contains the two carbons in the lone eclipsed bond.  Half-chair conformers of cyclopentaneare modeled in a similar fashion, with a single carbon in the Lonsdaleite lattice and four carbons in the diamond lattice.  The interactions of C and H atoms within each lattice domain are already well-defined, but the interactions between atoms across the lattice boundaries are enumerated and parameterized in this study.  To determine the validity of this model, and the magnitude of the few parameters required to model the conformational energies of substituted cyclopentanes, half-chair and envelope conformer energies were calculated by DFT methods.  These computed DFT conformational enthalpies were compared with those calculated with the simple lattice model.  The correlation between the two sets of energies was very good, with an R2 value of 0.98, an average error of 0.1 kcal/mol, and a RMS error of 0.2 kcal/mol over 13 different alkylcyclopentanes and ~200 conformers.

Modeling substituted cyclopentane conformational enthalphies using parameterized diamond and lonsdaleite lattice domains and interdomain boundaries

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.