On the Electronic Origin of Strain Energy: QTAIM Study of Perfluorocycloalkanes

Estimating Ring Strain Energies of Highly Substituted Cyclohexanes with the Semi-homodesmotic Approach: Why Substantial Ring Strain Exists for Nominally Tetrahedral Ring Carbon Atoms

Estimation of ring strain energies (RSEs) of substituted cyclohexanes c-C6H(x)R(12-x) (R = F, Cl, Me; x = 0, 2, 4, 8, 10, 12) using homodesmotic reaction methods gives implausible results for highly substituted cases, particularly, c-C6R12. Prior work suggests that this stems from poorly canceled interactions between substituents on the acyclic reference molecules. We apply here our semi-homodesmotic approach that minimizes use of acyclic references and ensures cancellation of intramolecular substituent interactions. The approach provides RSEs that are more consistent with chemical intuition, although they are higher than expected for "strain-free" cyclohexanes. The RSE for c-C6Me12 is predicted to be 11.9 kcal mol(-1). RSEs for halogenated rings rise significantly from 8-9 kcal mol(-1) for c-1,1,2,2-C6H8R4 to 44-50 kcal mol(-1) for c-C6R12 (R = F, Cl). The increase, and accompanying observation of larger RSEs for "adjacent CR2" systems, can be tied to increased bond distances in the rings upon progressive substitution. The sizable RSE for perchlorocyclohexane suggests that it may be susceptible to ring-opening reactions, a facet of its chemistry that is currently unexplored.

Intermolecular potential for binding of protonated peptide ions with perfluorinated hydrocarbon surfaces

An analytic potential energy function was developed to model both short-range and long-range interactions between protonated peptide ions and perfluorinated hydrocarbon chains. The potential function is defined as a sum of two-body potentials of the Buckingham form. The parameters of the two-body potentials were obtained by fits to intermolecular potential energy curves (IPECs) calculated for CF4, which represents the F and C atoms of a perfluoroalkane chain, interacting with small molecules chosen as representatives of the main functional groups and atoms present in protonated peptide ions: specifically, CH4, NH3, NH4(+), and HCOOH. The IPECs were calculated at the MP2/aug-cc-pVTZ level of theory, with basis set superposition error (BSSE) corrections. Good fits were obtained for an energy range extending up to about 400 kcal/mol. It is shown that the pair potentials derived from the NH3/CF4 and HCOOH/CF4 fits reproduce acceptably well the intermolecular interactions in HCONH2/CF4, which indicates that the parameters obtained for the amine and carbonyl atoms may be transferable to the corresponding atoms of the amide group. The derived potential energy function may be used in chemical dynamics simulations of collisions of peptide-H(+) ions with perfluorinated hydrocarbon surfaces.

Understanding the electron density reorganization upon stacking vs. H-bonding interaction in methyl gallate–caffeine complexes

QTAIM analysis of selected subcomplexes contained in methyl gallate-caffeine crystal indicate: a) significant differences between electron density reorganization involved in H-bonds and stacking interactions; b) cooperative effects are only significant when bond paths associated to homomonomeric hydrogen bonds are found.