Molecular orbital studies of gas-phase interactions between complex molecules

Area of Science:

• Computational Chemistry
• Supramolecular Chemistry
• Polymer Science

Background:

• Interactions between large molecules like poly(ethylene oxide) (PEO) and cofactors are crucial for industrial flocculation systems.
• Experimental studies suggest association between PEO and phenolic cofactors, but theoretical descriptions remain challenging.
• Understanding these interactions requires advanced computational methods to model complex molecular behavior.

Purpose of the Study:

• To theoretically investigate gas-phase interactions between a poly(ethylene oxide) hexamer ((PEO)6) and phenolic cofactors (R-OH).
• To elucidate the bonding mechanisms and thermodynamic feasibility of PEO/cofactor complex formation using quantum chemical calculations.
• To model systems relevant to industrial PEO/cofactor flocculation applications.

Main Methods:

• Employed PM3 semiempirical molecular orbital theory to study gas-phase interactions.
• Analyzed delocalized molecular orbitals (DLMOs) to describe bonding and electron density distribution.
• Calculated Gibbs free energy, entropy, and enthalpy changes for complex formation.

Main Results:

• PM3 DLMOs revealed bonding between (PEO)6 and cofactors, with some delocalized over the entire complex.
• A distinct electron density decrease ('pinch') was observed between atoms involved in hydrogen bonds (R-OH...O or R-CH...O).
• Thermodynamic calculations indicated that PEO/cofactor complexes do not form at room temperature due to unfavorable entropy changes.

Conclusions:

• Despite structural indicators of hydrogen bonding, the enthalpy of association did not correlate with the number of intermolecular bonds.
• The bonding mechanism is characterized by the cumulative effect of numerous small contributions from delocalized orbitals.
• Thermodynamic limitations, primarily entropic losses, prevent complex formation under ambient conditions, challenging experimental observations.